ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy makers.

April 14, 2000 -- Vol. 4 Number 15

-----------------------------------------------

The tendency has always been strong to believe
that whatever received a name must be an entity
or being, having an independent existence of
its own. And if no real entity answering to the
name could be found, men did not for that reason
suppose that none existed, but imagined that it
was something peculiarly abstruse and mysterious.
-- Stephen Jay Gould

-----------------------------------------------

Contents of This Issue:

1. Neurobiology:
On the Biological Basis of Memory
---------------------------------
Although the idea that processes underlying new memories
initially persist in a fragile state and then consolidate over
time still guides research on memory, neurobiologists do not yet
have a consensus theory concerning the biological basis of memory
formation. (Includes related background material.)

2. Neurobiology:
Functional Regeneration of Sensory Axons in Adult Spinal Cord
-------------------------------------------------------------
Natural repair of severed connections between the spinal cord and
spinal nerves does not occur in humans, but in the past decade
there has been much progress in understanding the mechanisms of
nerve fiber regeneration. There is now new evidence of functional
regeneration of dorsal root sensory axons in adult mammalian
spinal cord after treatment by local introduction of neurotrophic
factors. (Includes related background material.)

3. Medical Biology:
On New Approaches to Human Aging
--------------------------------
The resolution of all causes of death currently written on the
death certificates of those older than 65 will result only in an
increase in life expectancy of approximately 15 years. Any major 
increase in human life span will require knowledge of the
fundamental biology of the aging process. (Includes related
background material.)

4. Earth Sciences:
Ice-Core Evidence of Abrupt Climate Changes
-------------------------------------------
Records of abrupt changes in Earth's climate are particularly
clear in high-resolution ice cores, which can preserve histories
of local climate, regional climate, and broader climate -- all on
a common time scale. (Includes related background material.)

5. Astrobiology:
Evidence for Sufficient Energy for a Biosphere on Europa
--------------------------------------------------------
Disequilibrium chemistry in the ice cover of Europa's oceans,
driven by charged particles accelerated in Jupiter's
magnetosphere, should produce enough organic and oxidant
molecules to fuel a substantial Europan biosphere. (Includes
related background material.)

6. Astrophysics:
On Stars, Brown Dwarfs, and Superplanets
----------------------------------------
As recently as 1994, brown dwarfs were "theoretical" stars, with
no brown dwarfs considered to be unambiguously identified. During
the past few years, dozens of brown dwarfs have been discovered,
and it is now apparent they are as numerous as ordinary stars.
(Includes related background material.)

In Focus: On Carbon (Includes related background material.)

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

1. NEUROBIOLOGY:
ON THE BIOLOGICAL BASIS OF MEMORY
     Exactly 100 years ago, two psychologists, G.E. Mueller and
A. Pilzecker, proposed what came to be called the perseveration-
consolidation hypothesis of memory. In studies with human
subjects, Mueller and Pilzecker found that memory of newly
learned information was disrupted by the learning of other
information shortly after the original learning, and they
suggested that processes underlying new memories initially
persist in a fragile state and then consolidate over time. This
consolidation hypothesis still guides research, particularly
research in neurobiology on the time-dependent involvement of
neural systems and cellular processes enabling lasting memory.
     At the present time, the concept of "synaptic plasticity"
underlies nearly all theories of memories, the term referring to
changes in the behavior of the junction (synapse) between two
nerve cells resulting from past history. Two prominent aspects of
synaptic plasticity considered to be related to memory are 
"facilitation" and "potentiation". The term "facilitation" refers
to a progressive increase in the amount of *neurotransmitter
substance released at a synapse by successive nerve impulses
(action potentials), the increase occurring during an input
barrage consisting of repetitive stimulation (stimulus train).
The term "potentiation" refers to an increase in neurotransmitter
substance released by an action potential following repetitive
stimulation of a synapse. Both facilitation and potentiation can
be long-lasting, and "long-term potentiation" has been a focus of
much research on the cellular basis of memory, particularly in
the hippocampus, a brain cortex structure in the medial part
of the temporal lobe. In humans, among other functions, the
hippocampus is apparently involved in short-term memory, and
analysis of the neurological correlates of learning behavior in
the rat indicates that the hippocampus of the rat is also
involved in memory.
... ... James L. McGaugh (University of California Irvine, US)
presents a review of current research on memory, the author
suggesting the following caveats concerning the present state of
the field:
     1) The author points out that the idea that synaptic
mechanisms of long-term potentiation and long-term facilitation
underlie memory remains a hypothesis.
     2) The author points out that although studies of long-term
potentiation and memory have focused on the involvement of the
hippocampus, much evidence indicates that the hippocampus has
only a time-limited role in the consolidation and/or
stabilization of lasting memory.
     3) The author points out that there are forms of memory that
apparently do not involve the hippocampus and that may not use
any known mechanisms of synaptic plasticity.
     4) The author points out that despite theoretical
conjectures, little is known about system and cellular processes
mediating consolidation that continues for several hours or
longer after learning, consolidation that creates lifelong
memories.
     Concerning the above caveats, the author concludes: "These
issues remain to be addressed in this new century of research on
memory consolidation."
-----------
James L. McGaugh: Memory -- a century of consolidation.
(Science 14 Jan 00 287:248)
QY: James L. McGaugh [jlmcgaug@uci.edu]
-----------
Text Notes:
... ... *neurotransmitter substance: Neurotransmitters are
chemical substances released at the terminals of nerve axons in
response to the propagation of an impulse to the end of that
axon. The neurotransmitter substance diffuses into the synapse,
the junction between the presynaptic nerve ending and the
postsynaptic neuron, and at the membrane of the postsynaptic
neuron the transmitter substance interacts with a receptor.
Depending on the type of receptor, the result may be an
excitatory or an inhibitory effect on the postsynaptic nerve
cell.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THE BIOLOGICAL SUBSTRATES OF MEMORY FORMATION
The capacity of the nervous system to change (often referred to
as neural or brain "plasticity") is particularly prominent during
development, but the ability to learn new skills and establish
new memories clearly continues throughout life. The central
question is simply stated: How does the adult nervous system
mediate such changes? An understanding of the mechanisms
responsible for learning and other plastic changes in the adult
brain continues to be one of the most important challenges of
neuroscience, with a great deal of devoted research effort in
many laboratories in a number of associated disciplines. At the
present time, after nearly a century of research, there is a
consensus among investigators that the mechanisms of memory
formation involve carefully regulated changes in the strength of
existing connections between nerve cells (*synapses). Experiments
carried out in a variety of animals have demonstrated that
synaptic strength can be altered over periods ranging from
milliseconds to months, and that the cellular mechanisms
underlying these changes are transient modifications of the
transmission of activity from one nerve cell to another
(*neurotransmission) and, in the case of longer-lasting
alterations, changes in *gene expression.
... ... Jerry Chi-Ping Yin (Cold Spring Harbor Laboratories, US)
presents a review of current research on the localization of
memory functions, the author making the following points:
     1) Concerning investigations of memory localization at the
anatomical/systems level, Karl Lashley (1890-1958) used
anatomical lesions in the rat brain to search for the memory
"engram", the physical locus of long-term memory. Over a 30-year
period, Lashley performed surgical removal ("ablations") of
various regions in the rat *cerebral cortex, and came to the
disappointing view that no single well-defined lesion could
totally disrupt learning and memory formation [*Note #1]. This
resulted in the general hypothesis that memories are distributed
throughout the brain. Beginning in the late 1930s, however,
Wilder Penfield (1891-1976), in the course of neurosurgical
procedures for the treatment of human epilepsy, electrically
stimulated the temporal cortex of patients and caused them to
experience extremely vivid "memories" [*Note #2]. These
observations led Penfield to conclude that memories are
localized. Recent studies using non-invasive brain imaging,
coupled with refined animal ablation studies, have led to the
contemporary view that interacting widely distributed networks of
neurons participate in memory formation. A complication is the
apparent existence of functional redundancy ("backup" circuits)
and the possibility that different anatomical regions may be used
at different times after memory formation.
     2) Concerning investigations of memory localization at the
cellular level, neurophysiologists during the second half of this
century have developed a conceptual framework involving activity
dependent strengthening of neuronal connections. The search for
the loci of memory formation has become reduced to a search for
mechanisms that strengthen synaptic connectivity. The current
favorite cellular model for learning and memory formation is
"long-term potentiation", a physiological description of
increased synaptic efficacy following high-frequency stimulation.
A continuing controversy is whether the primary locus of changes
is on the pre- or post-synaptic side of the synapse. Proponents
of presynaptic change suggest that potentiation results from
changes in the amount of *transmitter release through one of many
possible mechanisms. Advocates of postsynaptic change propose
alterations in the efficiency of *receptor activation, perhaps
modulated through the "unmasking" of silent synapses.
     3) Concerning investigations of memory localization at the
molecular level, recent insights have been made into key
molecules whose activity apparently affects the process of memory
formation. These studies highlight two different uses of the term
"location": a) the various subcellular compartments where
important molecular entities are located; b) the changes in
specific protein amino-acid-residues produced by *post-
translational modification. In both cases, the activity and
interactions of important proteins are involved. At the present
time, there are at least 4 major *kinase systems believed to be
involved in memory formation: a) the *cyclic-AMP (cAMP)-dependent
protein kinase (protein kinase A); b) the *calcium-calmodulin
kinases; c) the *protein kinase C family; and d) the *mitogen-
activated protein (MAP) kinase pathway. The subcellular
localization of these kinases are apparently all regulated
through interactions with other proteins.
     4) The author suggests that the experimental results
gathered from different levels of analysis of learning and memory
formation can be integrated through the use of molecular
"reporters" -- molecular tags that allow monitoring of the
activity of important proteins and other molecular entities. For
example, a large amount of data collected from *Drosophila,
*Aplysia, the mouse, and the rat, have demonstrated the
importance of the *transcription factor cAMP-response-element-
binding protein (CREB) (and possibly its related family members)
in the process of consolidating long-lasting plastic changes.
CREB acts by binding to DNA sequences. This body of data,
collected from experiments involving a variety of behavioral
tasks and models of plasticity, supports the hypothesis that
learning-induced changes in gene transcription, at least
partially initiated through the activation of CREB family
members, are critical in the process of long-lasting changes in
plasticity.
-----------
Jerry Chi-Ping Yin: Location, location, location: The many
addresses of memory formation.
(Proc. Natl. Acad. Sci. US 31 Aug 99 96:9985)
QY: Jerry Chi-Ping Yin, Cold Spring Harbor Laboratory, Cold
Spring Harbor, NY 11724-2213.
-----------
Text Notes:
... ... *synapses: In general, nerve cells have a single long
extension (the "axon") that propagates the electrical output (the
action potential) of the cell. The term "synapse" refers to the
junction between the terminal of a neuron's axon and another
neuron. When studying the synapse, the first neuron is called the
"presynaptic" neuron, and the second neuron is called the
"postsynaptic" neuron.
... ... *neurotransmission: The term "neurotransmission" refers
to all the events at a synapse, particularly the release of
neurotransmitters and their action on the postsynaptic neuron.
(See main report notes for "neurotransmitters".)
... ... *gene expression: In general, the term "gene expression"
includes any gene activity, but particularly an activity that
produces the synthesis or activation of a specific protein.
... ... *cerebral cortex: (cortex) The cerebral cortex is a thin
surface layering of nerve cells of the brain, the region only
several millimeters thick but covering all of the brain surface.
This is the part of the central nervous system most intimately
involved with the so-called "higher faculties", although the
cortex operates in concert with other parts of the brain. The
structure is primitive in lower mammals, and is found
progressively more pronounced and with greater surface area in
primates and man.
... ... *Note #1: Lashley's failure to localize memory in a
specific region of the mammalian cerebral cortex was one of the
great puzzles of the middle part of the 20th century. In 1950,
Lashley wrote: "This series of experiments... has discovered
nothing directly of the real nature of the engram. I sometimes
feel, in reviewing the evidence on the localization of the memory
trace, that the necessary conclusion is that learning just is not
possible."
... ... *Note #2: The observations that came out of Penfield's
surgery and laboratory at McGill University had dramatic
theoretical consequences in psychology and neurobiology.
Penfield, a neurosurgeon with training in physiology who
specialized in therapeutic surgery in the treatment of certain
forms of epilepsy, used electrical currents to stimulate the
surface of the brain. The therapeutic objective was to make a
brain map for that particular patient prior to deciding exactly
which damaged parts of the brain could be safely removed without
producing problems more severe than the epileptic condition. The
technique had been developed in 1909 by the neurosurgeon Harvey
Cushing. Penfield's research on the neurological basis of
language and long-term memory, much of it in collaboration with
Herbert Jasper and Lamar Roberts, revolutionized the concepts of
brain maps that existed in the 1950s.
... ... *transmitter release: (neurotransmitter release) See
above: "neurotransmission".
... ... *receptor activation: In this context, the term
"receptor" refers to postsynaptic membrane receptors.
... ... *post-translational modification: In this context,
translation is protein synthesis, the process during which
polypeptides are synthesized on ribosomes in accordance with RNA
code. The term "post-translational modification" refers to a
modification of protein that occurs after synthesis of that
protein, i.e., the modification is not a result of changes in the
DNA or RNA coding for that protein.
... ... *kinase: In general, a "kinase" is any enzyme involved in
the transfer of a phosphate group.
... ... *cyclic-AMP (cAMP): Cyclic adenosine monophosphate (cAMP)
is an important postsynaptic intracellular substance activated by
incoming synaptic activity, a "messenger" involved in various
aspects of cell regulation and protein synthesis.
... ... *calcium-calmodulin kinases: Calmodulin is a calcium-ion-
binding protein that mediates many of the regulatory effects of
calcium ions in eukaryotic cells (cells with organelles such as
nuclei). A "calcium-calmodulin kinase" is a kinase enzyme whose
activity is dependent on the presence of calcium-calmodulin.
... ... *protein kinase C family: (PKC family) Any of a family of
protein kinase enzymes that require anionic phospholipid for
activity and are regulated by diacylglycerol and calcium ion.
These enzymes phosphorylate hydroxyl groups in substrate serine
and threonine residues.
... ... *mitogen-activated protein (MAP) kinase: A family of
protein kinases that perform a crucial step in relaying signals
from the plasma membrane to the cell nucleus. They are activated
by a wide range of proliferation- or differentiation-inducing
signals. (A "mitogen" is any compound that stimulates mitotic
cell division.)
... ... *Drosophila: A fruit fly genus.
... ... *Aplysia: A large gastropod mollusk with readily
identifiable individual nerve cells in its central nervous
system. It has been used extensively in neurobiological research
since the 1960s. (The class "gastropods" contains the snails,
slugs, limpets, and conchs.)
... ... *transcription factor: "Transcription" is the process by
which the genetic information in DNA is converted into RNA, and
transcription factors are a class of DNA-binding proteins that
regulate RNA transcription.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 1Oct99
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

2. NEUROBIOLOGY:
FUNCTIONAL REGENERATION OF SENSORY AXONS IN ADULT SPINAL CORD
     In vertebrates, the spinal cord is continuous with the
brain, and the two together constitute what is called the
"central nervous system". In addition to other functional
involvements, the spinal cord, and the nerves extending from and
leading into the spinal cord ("spinal nerves"), comprise neuronal
circuits that among other things mediate a number of fast
responses to environmental changes. For example, if you
inadvertently pick up a hot object, the grasping muscles in your
hand may relax and the object drop even before the sensation of
extreme heat or pain reaches your brain and your conscious
perception. This is an example of a "spinal cord reflex", a fast
automatic response to certain types of stimuli, the response
requiring only nerve fibers and nerve cells in the spinal nerves
and spinal cord. In addition to processing such reflexes, the
spinal cord also is the site for integration of nerve impulses
that originate locally in the spinal cord or that arrive from the
periphery and brain. Of great importance is that the spinal cord
is the "highway" traveled by sensory nerve impulses carrying
sensory information to the brain, and by motor nerve impulses
originating in the brain and destined for voluntary muscles via
the spinal nerves. In humans, there are 31 pairs of spinal nerves
arranged with bilateral symmetry to serve the two sides of the
body.
     Sensory input to the spinal cord (and to nerve cells in the
spinal cord) occurs via sensory neurons with a special
morphology. Ordinary neurons have a cell body with short (often
arborized) extensions (dendrites) to receive input, and a long
extension (axon) to propagate output away from the cell body to
either another neuron or to a muscle cell. But most sensory
neurons conveying input to the spinal cord are quite different:
such neurons have a long input extension, as much as 1 meter long
in humans, that propagates nerve impulses at high speed _toward_
the cell body, and a short or long (depending on the specific
type of sensory nerve cell) output extension into the spinal cord
from the sensory neuron cell body located just outside the spinal
cord.
     Spinal nerves are "mixed nerves", containing both input
(afferent) nerve fibers and output (efferent) nerve fibers. In
humans and other higher vertebrates, the anatomy is such that
near the spinal cord, just before joining it, each spinal nerve
bifurcates into a "dorsal root" and a "ventral root" (in humans,
posterior root and anterior root, respectively). The ventral root
contains output nerve fibers to "effector cells" (in muscles,
glands, etc.), while the dorsal root contains input nerve fibers
propagating peripheral sensory information to the central nervous
system. Each dorsal root, as seen in gross morphology, has a
bulge which contains the numerous cell bodies of the sensory
nerve fibers, and each of these bulges is called a "dorsal root
ganglion".
     When the human spinal cord is injured by physical trauma (as
in an automobile accident), one of common consequences is a
traction-caused ripping of the spinal nerves (spinal nerve roots)
out of the spinal cord at a particular location in the spinal
cord axis ("spinal root avulsion"). Root avulsion usually
produces complete paralysis of those regions of the body
controlled by those particular spinal nerves, with loss of local
motor control and loss of local sensation. Natural repair of
severed connections between the spinal cord and spinal nerves
does not occur in humans, but in the past decade there has been
much progress in understanding the mechanisms of nerve fiber
regeneration, and there is now some hope of defining
interventions that may possibly provoke regeneration in cases of
human spinal nerve avulsion.
... ... M.S. Ramer et al (3 authors at 2 installations, UK) now
report evidence of functional regeneration of sensory axons in
adult mammalian spinal cord. The authors point out that the
arrest of dorsal root axonal regeneration at the transition zone
between the peripheral and central nervous system (e.g., between
the spinal cord and the spinal nerves) has been repeatedly
described since the early 20th century. The authors report their
work indicates that with *neurotrophic support to damaged sensory
neuron axons, this regenerative barrier is surmountable. In adult
rats with experimentally injured dorsal roots, *intrathecal
treatment with *nerve growth factor, *neurotrophin-3, and *glial-
cell-line-derived neurotrophic factor, resulted in selective
regrowth of damaged axons across the dorsal root entry zone and
into the spinal cord, where neurons that ordinarily receive
sensory input (dorsal horn neurons) were found to be synaptically
driven by peripheral nerve stimulation in treated animals,
demonstrating functional reconnection. In behavioral studies,
rats treated with nerve growth factor and glial-cell-line derived
neurotrophic factor recovered sensitivity to noxious heat and
pressure. The authors report that the observed effects of
neurotrophic factors corresponded to their known actions on
distinct subpopulations of sensory neurons. The authors suggest
that neurotrophic factor intervention may serve as a viable
treatment in promoting recovery from root avulsion injuries. The
authors further suggest that apart from dorsal root injuries,
once the nature of traumatic injuries in general in the human
central nervous are better understood, neurotrophic treatment may
have vast therapeutic potential for such tissue damage.
-----------
M.S. Ramer et al: Functional regeneration of sensory axons into
the adult spinal cord.
(Nature 20 Jan 00 403:312)
QY: Matt S. Ramer [matt.ramer@kcl.ac.uk]
-----------
Text Notes:
... ... *neurotrophic treatment: (treatment with neurotrophins)
In general, neurotrophins are chemical entities apparently
essential for the viability of nerve cells. These substances are
polypeptides of 200 to 300 amino acids, and a number of different
neurotrophins have been identified.
... ... *intrathecal treatment: In general, treatment involving
injection into a local area surrounding the spinal cord:
injection beneath one or more of the protective sheaths that
cover the spinal cord.
... ... *nerve growth factor: A type of neurotrophin. The various
neurotrophins can be differentiated on the basis of tissue
specificities. Nerve growth factor has apparent specificity for
dorsal root ganglion cells.
... ... *neurotrophin-3: A specific type of neurotrophin: 257
amino acids, molecular weight 29.32 kilodaltons.
... ... *glial-cell-line-derived neurotrophic factor: Glial cells
are cells of the central and peripheral nervous system that
metabolically support neurons. Such cells also produce the
multiple membrane layers called myelin and enfold nerve cell
axons with it. The glial cells are found everywhere in the brain
and spinal cord, and one result of a localized injury to the
central nervous system is a local proliferation of glial cells to
form a scar matrix. In this context, the term "glial-cell-line"
refers to a line of laboratory cultured glial cells.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
NEUROTROPHIC FACTORS AND LONG-DISTANCE AXON DEVELOPMENT
One of the major essential challenges of the embryonic phase of
the development of the vertebrate nervous system is the
construction of accurate "hard-wiring" -- the formation of
anatomical connections between various nerve cell groups that
must physiologically work together. What are the mechanisms that
control the directed growth of neuron extensions (axons) to make
contact with target cells, both target cells that are close by
and target cells that are often at relatively enormous distances?
Classic studies in neurobiology have demonstrated that many
neuronal populations in vertebrates are initially generated in
excess numbers during embryonic development, and that excess
neurons are subsequently eliminated by naturally occurring
neuronal cell death. This massive death of excess neurons usually
occurs soon after the axons of these neurons reach their targets,
and appears to result from a dependence of the neurons on
"trophic substances" (*neurotrophic factors) secreted by their
target cells. The trophic substances are present in limited
amounts, and the incoming axons evidently compete for these
limited amounts. This competition is thought to provide a
mechanism for matching the size of the presynaptic neuronal
population to the size of the target population, and has also
been suggested to provide a mechanism for eliminating
misprojecting neurons (axons that have extended to the wrong
location), the misprojecting axons not having access to the
target-derived trophic support. With the identification of many
neurotrophic factors in recent years, evidence has started to
accumulate that neurons that have reached their targets may
receive trophic support not just from their target cells, but
also from other cellular sources such as those located near their
cell bodies or axons. There is also evidence that some
differentiated neurons may have trophic requirements before they
reach their target fields.
... ... H. Wang and M. Tessler-Lavigne (University of California
San Francisco, US) now report that rat spinal commissural
neurons, a group of long-projection neurons in the central
nervous system, in addition to trophic support from final
targets, are also dependent for their survival on trophic support
from one of their intermediate targets, the "*floor plate" of the
spinal cord. The authors report this dependence occurs during a
period of several days when the axons extend along the floor
plate, following which period they develop additional trophic
requirements. The authors suggest that a dependence of neuron
axon growth on trophic support derived "en passant" from their
intermediate targets provides a mechanism for rapidly eliminating
misprojecting neurons, which may help to prevent the formation of
aberrant neuronal circuits during the development of the nervous
system.
-----------
H. Wang and M. Tessler-Lavigne: En passant neurotrophic action of
an intermediate axonal target in the developing mammalian CNS.
(Nature 21 Oct 99 401:765)
QY: Marc Tessler-Lavigne [marctl@itsa.ucsf.edu]
-----------
Text Notes:
... ... *neurotrophic factors: See main report.
... ... *floor plate: A small well-defined area at the ventral
margin of the developing spinal cord. (The "ventral margin" is
the margin toward the abdomen.)
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 10Dec99
-------------------
Related Background:
REGENERATION OF MOTOR NEURONS: IDENTIFICATION OF A MITOGEN
Motor neurons are nerve cells that transmit nerve signals from
the brain or spinal cord to muscle or gland tissue, and sensory
neurons are nerve cells that carry signals from various parts of
the body to the brain or spinal cord. High signal propagation
velocities in motor and sensory neurons in vertebrates are
achieved by association of the nerve fiber with an enfolding
sheath called myelin. The myelin sheath consists of concentric
layers of electrically insulating lipid material, but the sheath
is periodically interrupted, and at the points where the sheath
is interrupted so is the electrical insulation interrupted. The
result, predictable from the classical physics of electrical
transmission lines and the electrical parameters of nerve fibers,
is that the propagation of an electrical pulse along such nerve
fibers occurs at a velocity much higher than that found in
unmyelinated fibers. Glial cells are the cells of the central and
peripheral nervous system that produce the multiple membrane
layers called myelin and enfold nerve cell axons with it, and
Schwann cells are a particular type of glial cell. A mitogen is
any compound that stimulates mitotic cell division.
... ... Livesey et al (6 authors at 3 installations, UK CA)
report the identification of an extracellular signaling molecule,
previously described as the pancreatic secreted protein Reg-2,
that is expressed solely in regenerating and developing rat motor
and sensory neurons, with Reg-2 a potent Schwann cell mitogen in
vitro. In vivo, Reg-2 is apparently transported along regrowing
axons, and inhibition of Reg-2 significantly retards the
regeneration of axons containing the protein. The authors suggest
that Reg-2 is an essential component of the neuron-glia
interactions underlying development and regeneration of mammalian
motor neurons.
QY: Frederick J. Livesey [rlivesey@mail.tcd.ie]
(Nature 11 Dec 97) (Science-Week 2 Jan 98)
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

3. MEDICAL BIOLOGY:
ON NEW APPROACHES TO HUMAN AGING
It is probably safe to say that research on the biological basis
of human aging will remain a central interest in the sciences as
long as there is any research at all. However, despite the
general public attention to the subject, it is only during the
past few decades that cell biologists have begun a vigorous
attempt to understand the aging process. A major reason for this
is that the new knowledge provided during the past 40 years by
molecular biology concerning fundamental cellular processes
suggests that an understanding of the biological basis of human
aging is indeed possible. The cell biologist Leonard Hayflick
(University of California San Francisco, US) first demonstrated
in the 1960s that the mortality of biological cells is either
directly or indirectly programmed in each cell, and that this is
an important aspect of the aging of human tissues (see related
background material below). Hayflick now presents a provocative
essay on the subject, the author making the following points:
     1) The author points out that in the past 100 years life
expectancy at birth in developed countries has increased from
approximately 48 years to 76 years, the same gain that occurred
over the previous 1900 years. But this progress has neither
advanced nor resulted from our understanding of aging. Instead,
it is the control of infectious diseases of the young that
explains the increase in life expectancy during the 20th century.
     2) The author suggests that the failure to distinguish
between the diseases of old age and the aging process is
widespread even in the scientific community. The virtual
resolution of various childhood diseases such as poliomyelitis
and iron-deficiency anemia did not increase our knowledge of
childhood development. Similarly, the resolution of the leading
causes of death in old age -- cardiovascular disease, stroke, and
cancer -- are unlikely to advance our knowledge of the aging
process.
     3) The author suggests that one example of the consequences
for science policy of the failure to distinguish research on age-
associated diseases from research on the fundamental biology of
aging is that "it is virtually impossible to raise funds for
research on aging, because in the minds of policy-makers and the
public no one suffers or dies from it." More than half of the
budget of the US National Institute on Aging is spent on
Alzheimer's disease, yet the elimination of this disease "will
have only a trivial impact on life expectancy and will not
advance our knowledge of the fundamental biology of aging." The
author suggests that greater attention must be given to a
question that is rarely posed: Why are old cells more vulnerable
to disease than young cells?
     4) The author concludes: "The resolution of all causes of
death currently written on the death certificates of those older
than 65 will result in an increase in life expectancy of only
about 15 years. An increase in our knowledge of how age changes
occur does not put a 15-year limit on what is possible."
-----------
Leonard Hayflick: New Approaches to Old Age.
(Nature 27 Jan 00 403:365)
QY: Leonard Hayflick, Univ. of Calif. San Francisco 415-476-4044.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
CELLULAR AGING: DONOR AGE AND CELLULAR REPLICATION LIFESPAN
Fibroblasts are a type of connective tissue cell, secreting
structural proteins such as collagen, the proteins forming a
matrix in which the fibroblasts become embedded. These cells can
be easily obtained from skin, and they can be easily cultured
outside the body. Normal human fibroblasts have a finite
replicative lifespan in vitro (i.e., they divide a finite number
of times), and this has been postulated to be a cellular
manifestation of aging of the human organism. Several studies
have indeed shown an inverse relationship between donor age (the
age of the persons from which cultured cells are derived) and
fibroblast culture replicative lifespan. But in all cases the
correlation was weak, and with few exceptions the health status
of the donors was unknown. Thus, the relationship between the
replicative lifespan and the age of the donor from which the
cells are derived has remained equivocal (*Note #1).
... ... V.J. Cristofalo et al (5 authors at 2 installations, US)
now report a study of the replicative lifespan of 124 skin
fibroblast cell lines established from donors of different ages.
All donors were medically examined and were declared "healthy"
(according to Baltimore Longitudinal Study of Aging protocols) at
the time the biopsies were taken. The authors report that both
long- and short-lived cell lines were observed in all age groups,
but no significant correlation between the proliferative
potential of the cell lines and donor age was found. A comparison
of multiple cell lines established from the same donors at
different ages also failed to reveal any significant trends
between proliferative potential and donor age. The authors
suggest their results clearly indicate that if health status and
biopsy conditions are controlled, the replicative lifespan of
fibroblasts in culture does not correlate with donor age.
-----------
V.J. Cristofalo et al: Relationship between donor age and the
replicative lifespan of human cells in culture: A reevaluation.
(Proc. Natl. Acad. Sci. US 1 Sep 98 95:10614)
QY: Vincent J. Cristofalo, Center for Gerontological Research,
Alleghany University of the Health Sciences, Philadelphia, PA
19129 US.
-----------
Text Notes:
... ... *Note #1: The possibility that the process of cell aging
and death is under genetic control was first suggested by Leonard
Hayflick in 1961. Hayflick reported that normal human fibroblasts
apparently have an intrinsic limit to the number of times they
can proliferate, with human fibroblasts removed from an embryo
and grown in culture dividing approximately 50 times before they
deteriorate and die. In contrast, human fibroblasts removed from
adults multiply only 15 to 30 times before dying. Also,
fibroblasts removed from young children suffering from Werner's
syndrome (a rare disease that causes premature aging) divide only
2 to 10 times in culture. Further evidence for a relationship
between aging and the replicative capacity of cells was provided
by the discovery that the number of replications in culture is
apparently related to the lifespan of organism. For example,
cultured cells of the Galapagos tortoise, whose maximum life span
is approximately 175 years, divide more than 100 times in
culture, whereas cells from the mouse, whose maximum life
expectancy is only a few years, divide fewer than 30 times in
culture. The correlation roughly holds for other species as well.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 16Oct98
-------------------
Related Background:
BIOLOGY OF AGING: ON TELOMERES AND REPLICATIVE SENESCENCE
Telomeres are defined ends of chromosomes that contain specific
repeated DNA sequences. They are essential for normal chromosome
replication, and since their length shortens a bit with each
replication, they are believed to be involved in the aging of the
cell. Telomerase is an enzyme that repairs damage to telomeres,
and it is thought by some that cancerous cells may have mutant
telomerase, the mutant enzyme conferring immortality on the
cancer cell. ... ... In a review of cell senescence, the aging of
cell cultures, and the immortalization of mammalian cells, John
M. Sedivy (Brown University, US) makes the following points: 1)
Finite replicative lifespan (senescence) of mammalian cells in
culture is a phenomenon that has generated much curiosity since
its description. The obvious significance of senescence to
organismal aging and the development of cancer has engendered a
long-lasting and lively debate about its mechanisms. 2) Three
classical observations are usually cited to argue that in vitro
replicative senescence is a phenomenon with biological
significance: a) the correlation of in vitro lifespan with the
age of the donor; b) the correlation of in vitro lifespan with
the average life expectancy of species; and, c) the reduced in
vitro lifespan of cells from patients afflicted with premature
aging syndromes. 3) Two major theories have been used to explain
limited replicative capacity. The first hypothesis invokes the
gradual accumulation of mutations, and the second hypothesis
invokes the existence of a molecular clock (or clocks) that can
keep track of cell divisions. The second theory is now believed
to be generally true. 4) It is known that cell senescence can be
overcome, because many cell lines in common laboratory use are
quite obviously immortal. Rodent cells can overcome senescence
spontaneously. Human, chicken, bovine, and horse cells rarely, if
ever, immortalize spontaneously. 5) Certain viral or biochemical
interventions in human cell cultures can overcome cell
senescence, typically by causing 20 to 30 extra population
doublings. At the end of this extended lifespan, there is a
decline and death of the culture in 4 to 6 weeks, which has been
termed "crisis". Senescent cells, on the other hand, can be
maintained in vitro in a viable non-proliferative state for very
long periods of time (reports of from 4 to 6 months, and up to 2
years). 6) The author suggests it is amazing that in spite of
very long periods of apparent "immortality", the senescent
program in cells remains intact in cells in which senescence has
been overridden, so that on removal of the overriding agent, the
program is capable of establishing rapid growth arrest. 7) The
current prevailing hypothesis for the nature of the molecular
clock involved in cell senescence is the attrition of telomeres.
*Germ cells, and some key *stem cells, are known to express
telomerase catalytic activity, whereas the majority of somatic
cells lack it. Murine (mouse) embryonic stem cells express
telomerase and are functionally immortal, and elimination of
telomerase eventually results in loss of proliferation. 8) The
author proposes that immortalization of human cells requires a
bypass of both cell senescence and crisis, whereas in rodent
cells cell crisis does not exist and culture lifespan is limited
only by senescence. 9) Evidence indicates that, at least in human
cells, telomere length appears to be linked critically to the
triggering of senescence. The author suggests that although it
remains to be rigorously demonstrated, this strongly implies that
activation of telomerase can result in one-step immortalization.
In conclusion, the author states the two most significant
questions in this field: a) Does cell senescence limit organismal
lifespan? And, b) Is telomerase expression necessary for cancer
progression in vivo?
QY: John M. Sedivy [john_sedivy@brown.edu]
(Proc. Natl. Acad. Sci. US 4 Aug 98 95:9078)
(Science-Week 4 Sep 98)
-----------
Text Notes:
... ... *Germ cells: Any cell from which gametes (sperm cells and
egg cells) are derived. All other cells are called "somatic"
cells.
... ... *stem cells: In general, a stem cell is any precursor
cell, a form prior to cell differentiation. E.g., stem cells in
bone marrow that give rise to blood cells.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 4Sep98
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

4. EARTH SCIENCES:
ICE-CORE EVIDENCE OF ABRUPT CLIMATE CHANGES
Records of changes in Earth's climate are particularly clear in
high-resolution ice cores, which can preserve histories of local
climate (as reflected in snowfall and temperature), regional
climate (as reflected in wind-blown dust, sea salt, etc.), and
broader climate (as reflected in trace gases deposited from the
atmosphere) -- all on a common time scale that can demonstrate
synchrony of climate changes over wide regions.
... ... Richard B. Alley (Pennsylvania State University, US)
reviews current ice-core research, the author making the
following points:
     1) Dating and accumulation: On some glaciers and ice sheets,
sufficient snow falls each year to form recognizable annual
layers that are marked by seasonal variations in physical,
chemical, electrical, and isotopic properties. These variations
can be counted to determine ages of the layers, and accuracy of
the determination can be assessed by a number of ways, including
comparison to the chemically identified fallout of historically
dated volcanoes.
     2) Paleothermometry: Ice cores are essentially local
paleothermometers. The classic paleothermometer is the stable
isotopic composition of water in the ice core. Natural waters
typically contain a fraction of 1 percent of isotopically heavy
water molecules, and the vapor pressure of this heavy water is
less than ordinary or "light" water. The result is that as an air
mass is cooled and precipitates, it preferentially loses heavy
water and must increasingly precipitate light water. Both
empirically and theoretically, isotopic composition of
precipitation and site temperature are strongly correlated in
time and space.
     3) Aerosols: Anything in the atmosphere can eventually end
up in an ice core. Some materials are reversibly deposited, but
most materials remain in the ice unchanged. Although details of
the air-snow transfer process are complex and not yet completely
elucidated, large changes in concentrations of most materials in
ice can with confidence be said to reflect changes in the
atmospheric loading of these materials.
     4) Gases: Trapped gases in ice-core bubbles are highly
reliable records of atmospheric composition, as indicated by
comparisons among cores from different ice sheets, and comparison
with instrumental records and the air in the *firn above the
bubble-trapping depth. The slight differences between bubble and
air composition caused by gravitational and thermal effects are
well understood and recognizable.
     5) Geographic coverage: The ice-core record of abrupt
climate changes is clearest in Greenland. Although no other
record is available that spans the same time interval with
equally high time resolution, it appears that ice cores from the
Canadian arctic islands, high mountains in South America, and
Antarctica also contain indications of the same abrupt changes.
Dating is considered secure for some of the Antarctic ice cores.
     6) The author suggests that as the world slid in and out of
the last ice age, the general cooling and warming trends were
punctuated by abrupt changes, and climate shifts up to half as
large as the entire difference between ice age and modern
conditions occurred over hemispheric or broader regions in mere
years to decades. Such abrupt changes have been absent during the
few key millennia when agriculture and industry have arisen.
     7) In summary, ice-core records indicate that climate
changes in the past have been large, rapid, and synchronous over
broad areas extending into low latitudes, with less variability
over historical times. These ice-core records come from high
mountain glaciers and the polar regions, including small ice caps
and the large ice sheets of Greenland and Antarctica.
-----------
Richard B. Alley: Ice-core evidence of abrupt climate change.
(Proc. Natl. Acad. Sci. US 15 Feb 00 97:1331)
QY: Richard B. Alley [ralley@essc.psu.edu]
-----------
Text Notes:
... ... *firn: The term "firn" refers to the transitional layer
between snow and glacier ice. The layer consists of snow that has
melted during one summer melt season, the layer in the process of
transforming to glacier ice as the temperature decreases.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON GLOBAL CLIMATE CHANGE
Environmental change involves jumps, fluctuations, and trends,
the environment changing through the operation of the internal
machinery of the *ecosphere (biosphere), and through the external
agencies of cosmic and geological forces. Evidence of past
environmental change, almost always incomplete, derives from
geochemical, physical, biological, historical, and instrumental
sources. In recent years, high-speed computers have allowed
researchers to manipulate complicated and reasonably realistic
models of environmental change, with modelling particularly
useful for studying changes in *sedimentary basins,
biogeochemical cycles, and climate. General circulation models,
run with appropriate boundary conditions, predict climates of the
past, and these predicted climates can be compared with
paleoclimatic indicators.
... ... R.B. Alley et al (3 authors 3 installations, US) present
a review of current research on global climate change, the
authors making the following points:
     1) Prediction of climate change requires observational
constraints on the current climate state, knowledge of the way
the coupled air-ocean-ice-earth-life system behaves, and
information on changing forcings such as solar variability.
Studies of past climate are also required to focus model-building
efforts on climate components that are likely to change, and to
allow testing of the ability of models to predict time-evolution
of the system.
     2) The last few million years have been generally cold and
icy compared with the previous hundred million years but have
alternated between warmer and colder conditions. These
alternations have been linked to changes over tens of thousands
of years in the seasonal and latitudinal distribution of sunlight
on Earth caused by features of Earth's orbit. Globally
synchronous climate change despite some hemispheric asynchrony of
the forcing is explained at least in part by lowering carbon
dioxide during colder times in response to changes in ocean
chemistry. We live in one of the warmer times of these orbital
cycles; the coolest times brought glaciation to nearly one-third
of the modern land area.
     3) Studies of past climate changes indicate that the Earth
system has experienced greater and more rapid changes over larger
areas that was generally believed possible, with jumping between
fundamentally different modes of operation in as little as a few
years. Most of the last 100,000 years or longer has been
characterized by large and abrupt regional-to-global climate
changes, and agriculture and industry have developed during
anomalously stable climatic conditions. New high-resolution
analysis of sediment cores indicates these past changes have been
caused by "*band jumps" between modes of operation of the climate
system. Recurrence of such band jumps is possible and might be
affected by human activities.
-----------
R.B. Alley et al: Global climate change.
(Proc. Natl. Acad. Sci. US 31 Aug 99 96:9987)
QY: Richard B. Alley [ralley@essc.psu.edu]
-----------
Text Notes:
... ... *ecosphere (biosphere): In general, the term "biosphere"
refers to the portion of the planet capable of supporting life.
It ranges from elevations of approximately 10,000 meters above
sea level to the deep ocean, and a few hundred meters below the
surface of the soil. The biosphere consists of the hydrosphere,
the lower atmosphere (troposphere), and the surface of the
*lithosphere, all three regions inhabited by metabolically active
organisms.
... ... *lithosphere: In current geology, the lithosphere is the
approximately 100 kilometer rigid upper layer of the crust and
upper mantle of the Earth.
... ... *sedimentary basins: The term "sedimentary basin" refers
to a subsiding area of the Earth's crust, which permits the net
accumulation of sediment, i.e., material derived from pre-
existing rock, from biogenic sources, or precipitated by chemical
processes.
... ... *band jumps: In this context, the term "band jump" refers
to an abrupt change from one range of variation to another.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 1Oct99
-------------------
Related Background:
ON THE POSSIBILITY OF RAPID CLIMATE CHANGE
Over the course of geologic history, the environment on Earth has
been far from static. Geologic evidence suggests that 600 million
years ago the atmosphere lacked sufficient oxygen to support
animal life. More recently, as indicated by sediments recording
conditions over the past 500,000 years, the climate of the planet
varied between at least two different states. The record from the
past 150,000 years is particularly well-preserved, offering
details concerning repeated climate changes. Between
approximately 131,000 and 114,000 years ago, a warm period
similar to the climate of today occurred. This was followed by
what is called the "Wisconsin ice age", which ended approximately
12,000 years ago when the current relatively warm *Holocene
period began. ... ... Kendrick Taylor (Desert Research Institute,
US) presents a review of the research of a large project to
develop a climate record for the past 110,000 years, the author
making the following points:
     1) The layerings of glacial ice record seasonal variations
of temperature, snowfall, concentrations of atmospheric gases,
and atmospheric circulation patterns. In general, the weight of
accumulating snow compresses the snow below it, trapping
atmospheric gases, dust, and chemicals, and a deep ice core thus
provides a sequential record amenable to analysis.
     2) The author reports that by examining ice cores from
Greenland, he and his colleagues have determined that climate
changes large enough to have extensive impacts on our society
have occurred in a time-frame of less than 10 years. The author
suggests that the climate of Earth could change significantly
during a lifetime, that we are still a long way from being able
to predict such a change, but we are getting closer to an
understanding of how it might occur. A pressing concern is
whether anthropogenic changes in the atmosphere of the planet
might perturb climate stability.
     3) The author points out that climate is the result of the
exchange of heat and mass between the land, ocean, atmosphere,
ice sheets, and space. As long as changes to the land, ocean,
atmosphere, and ice sheets stay below certain thresholds, climate
changes will occur slowly. But climate will change rapidly if
those thresholds are crossed. *Greenhouse warming, for example,
by altering ocean circulation and the flow of tropical heat to
the North Atlantic, could lead to rapid cooling in eastern North
America, Europe and Scandinavia. Altered ocean circulation could
lead to much larger changes. We have no experience predicting
climate switches between stable modes.
     4) The author suggests human ingenuity would most likely
allow us to adapt to a rapid change in climate, but we would pay
a larger price than our civilization has ever known. The author
poses a scenario: "Imagine the economic and social cost of
moving, in a 20-year period, most of our agricultural activities
500 miles south of their current locations. Imagine the social
cost and famine if agriculture could not be relocated quickly
enough."
     5) Although we do not know the critical level of greenhouse
gas concentration that would trigger a rapid climate change, we
do know that reducing the rate of greenhouse emissions would help
in two ways. First, the atmospheric concentration of greenhouse
gases would increase more slowly. Second, numerical models
predict that the climate threshold will occur at a higher
concentration of greenhouse gases if the concentration of
greenhouses increases slowly.
     6) The author suggests it will be another 20 years before
the climate changes that are predicted to be associated with the
greenhouse effect becomes large enough to be unambiguously
differentiated from naturally occurring variations in climate.
As a society we have the choice of ignoring the warning signs or
taking some action.
-----------
Kendrick Taylor: Rapid climate change.
(American Scientist Jul/Aug 1999 87:320)
QY: Kendrick Taylor [kendrick@dri.edu]
-----------
Text Notes:
... ... *Holocene period: The most recent epoch of the geologic
time scale, from approximately 10,000 years ago to the present.
... ... *Greenhouse warming: See notes to report #1, this issue.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Aug99
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

5. ASTROBIOLOGY:
EVIDENCE FOR SUFFICIENT ENERGY FOR A BIOSPHERE ON EUROPA
Jupiter's satellite system consists of at least 16 moons, the
four largest of which are called the Galilean moons, since they
were discovered by Galileo Galilei (1564-1642). The moons are Io,
Europa, Ganymede, and Callisto, in order of their orbital
distance from Jupiter. Europa, which is slightly smaller than
Earth's moon, has a thick icy crust, and may also have a liquid
water mantle beneath this crust. Very few craters are present on
Europa, which suggests an active surface that renews itself and
thus erases craters as fast as they form from impacts. The
surface also shows numerous lines approximately 30 kilometers
wide and 1000 kilometers long, and these have been interpreted to
be breaks in the crust where water from below has refrozen. The
possible existence of a liquid water mantle beneath the ice on
Europa is of great interest to astrobiologists, since such a
mantle might contain life forms.
... ... Christopher F. Chyba (Stanford University, US) presents
an analysis of energy on Europa available to fuel a biosphere.
The author proposes that disequilibrium chemistry in the ice
cover of Europa's oceans, driven by charged particles accelerated
in Jupiter's *magnetosphere, should produce enough organic and
oxidant molecules to fuel a substantial Europan biosphere. The
author suggests that microbial life could exist in concentrations
detectable by surface landers able to filter meltwater from
Europa's ice. The author suggests that his calculations indicate
that a particular radiation-driven ecosystem is plausible and
quantifiable using our current knowledge of Europa, but that
other radiation-driven ecosystems are possible. The author
concludes: "Neither photosynthesis nor *hydrothermal vents need
by postulated. But only direct exploration will reveal whether
life on Europa actually exists."
------------
Christopher F. Chyba: Energy for microbial life on Europa.
(Nature 27 Jan 00 403:381)
QY: Christopher F. Chyba [chyba@seti.org]
-----------
Text Notes:
... ... *magnetosphere: In general, a region surrounding a planet
in which charged particles are controlled by the magnetic field
of the planet rather than by the magnetic field of the Sun (or of
the star of which the planet is a satellite). The charged
particles originate in the *solar wind (or stellar wind, if the
planet is extrasolar), and they form a tenuous ionized gas
(*plasma) surrounding the planet.
... ... *plasma: In general, a fully ionized gas consisting of
ions and electrons moving freely. 
... ... *solar wind: The solar wind is the steady flow of charged
particles, consisting primarily of protons and electrons, from
the solar corona into interplanetary space. The solar-wind
particles have energies high enough to enable the particles to
escape the Sun's gravitational field, but the wind is influenced
by the Sun's magnetic field, and the particles can be trapped by
planetary magnetic fields.
... ... *hydrothermal vents: In general, hydrothermal vents are
hot springs occurring in volcanic regions of an ocean floor. On
Earth, it has been discovered that volcanically heated areas of
the ocean floor have a rich variety of biological organisms --
the hyperthermophiles, which thrive above 80 degrees centigrade.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
PLANETARY SCIENCE: THE OCEAN OF THE JOVIAN MOON EUROPA
... R.T. Pappalardo et al (3 authors 2 installations, US)
present a review of current research on Europa's apparent
subsurface ocean, the authors making the following points:
     1) In the 1960s, spectroscopic data indicated that the
Jovian satellite Europa, like many others in the outer Solar
System, is covered with ice. With surface temperatures of 110
degrees kelvin near the equator and 50 degrees kelvin near the
poles, the ice on Europa must form a rock-hard skin. Recently,
spectacular images radioed from visiting spacecraft have revealed
a young and tremendously deformed surface. Apparently, somewhere
under the icy shell must be a warm mobile interior. Is it glacial
ice? Or are the innards of Europa warm enough to sustain an ocean
of liquid water?
     2) Theory and observation have combined to provide a strong
self-consistent case for a global ocean within Europa. But the
existence of this ocean is not unequivocally proved. Warm
subsurface ice could mimic many of the effects of an internal
ocean. Although the surface of the satellite is sparsely cratered
and probably geologically young, searches for definitive evidence
of ongoing geological activity have been fruitless. Europa might
have had an ocean in the recent past that is now frozen solid.
The only way to be certain is to return a spacecraft to Europa
and this time go into orbit.
     3) Such a mission is being planned by the US National
Aeronautics and Space Administration (NASA). The Europa Orbiter
Mission could be launched as early as November 2003 and would
enter Jupiter's orbit 3 years later. Approximately 2 years after
that, the spacecraft would go into orbit around Europa at an
average altitude of just 200 kilometers. Precise tracking of the
position and altitude of the spacecraft would map the
gravitational field and shape of Europa in enough detail to track
the ebb and flow of tides as the moon trundles around Jupiter. If
Europa does have a subsurface sea, the surface of the moon should
rise and fall 30 meters every 3.6-day orbit. In this way, the
Europa Orbiter will provide the definitive test for an ocean.
-----------
R.T. Pappalardo et al: The hidden ocean of Europa.
(Scientific American October 1999)
QY: Robert T. Pappalardo, Brown University, US.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 3Dec99
For more information: http://scienceweek.com/search/search.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

6. ASTROPHYSICS:
ON STARS, BROWN DWARFS, AND SUPERPLANETS
Brown dwarf stars are formed by the contraction of a lump of gas
with a mass too small for ordinary stellar nuclear reactions to
begin in the core. Such a star has a relatively short-lived
luminosity (approximately 100 million years) as the result of
conversion of gravitational energy to radiation. The surface
temperature of a brown dwarf is below 2500 degrees kelvin. As
recently as 1994, brown dwarfs were "theoretical" stars, with no
brown dwarfs considered to be unambiguously identified.
... ... Ben Zuckerman (University of California Los Angeles, US)
presents a short review of current research on brown dwarfs, the
author making the following points:
     1) Although planets and stars were for centuries recognized
as different entities, it was only in the 20th century that the
physics underlying their differences became understood. In
general, stars fuse protons into helium in their hot interiors
and planets do not. But there is an enormous gap between the
relatively cool temperature of the interior of a large planet
such as Jupiter and the high temperature of a star of minimum
mass, approximately 75 times the mass of Jupiter. In the past,
astronomers often wondered whether any astronomical objects
between 1 and 75 Jupiter-masses existed, but only during the past
few years have dozens of such objects, the brown dwarfs, been
discovered, and it is now apparent they are as numerous as
ordinary stars.
     2) Astronomers currently divide the mass range between 1 and
75 Jupiter-masses into two domains: a) between 1 and
approximately 13 Jupiter-masses is the realm of the
"superplanets", and b) between 13 and approximately 75 Jupiter-
masses is the realm of the "brown dwarfs". The divide between
stars and brown dwarfs is sharply delineated: objects with masses
less than 75 Jupiter-masses do not achieve central temperatures
greater than the 2.7 million degrees kelvin necessary to fuse
protons into helium nuclei. In contrast, the dividing line
between brown dwarfs and superplanets is not so clearly
established, and various definitions have been suggested. One
such definition is based on the ability of sufficiently massive
substellar objects to fuse deuterium into helium: above
approximately 13 Jupiter-masses, the central temperature of an
object is approximately 1 million degrees kelvin or greater, and
at such temperatures deuterium will fuse into helium-3 nuclei.
     3) Another difference between minimum-mass stars and brown
dwarfs concerns surface temperature. The surface temperature of a
minimum mass star (75 Jupiter-masses) is approximately 2000
degrees kelvin, and that temperature is maintained for tens of
billions to trillions of years, as the star continues to fuse
protons into helium. In contrast, brown dwarfs, once they have
fused their relatively short supply of deuterium into helium,
begin to cool, and the surface temperature soon drops (and
continues to drop) below the 2000 degree kelvin mark.
     4) Most ordinary stars are apparently orbited by stellar
companions in "binary systems". But although a few brown dwarf
companions to nearby stars are known, the vast majority of brown
dwarfs are apparently freely floating among the stars. Indeed,
the contrast between the scarcity of companion brown dwarfs and
the apparent plentitude of free floating brown dwarfs was
unexpected and now constitutes one of the major unsolved problems
in astrophysics.
     5) Brown dwarfs have recently been considered as possible
major unseen constituents of the distant "*Halo" of our Galaxy,
and an intensive search for such objects as revealed by
*gravitational microlensing effects occurred in the past decade.
But from the microlensing studies, and from deep images obtained
with the *Hubble Space Telescope, it now appears that the bulk of
the unseen ("dark") matter that apparently exists in the Galaxy
Halo is due neither to brown dwarfs nor to very low-mass stars.
-----------
Ben Zuckerman: Brown dwarfs: At last filling the gap between
stars and planets.
(Proc. Natl. Acad. Sci. US 1 Feb 00 97:963)
QY: Ben Zuckerman [ben@astro.ucla.edu]
-----------
Text Notes:
... ... *Halo: A galactic halo, such as that associated with our
own Galaxy, is a spheroidal distribution of old stars and
globular clusters of old stars surrounding the galaxy. In the
case of our own Galaxy, the Galactic Halo has a radius of
approximately 50,000 light years.
... ... *gravitational microlensing: Gravitational lensing is the
bending of light and other radiation by a massive gravitational
entity such as a star, a black hole, a galaxy, or a cluster of
galaxies. The effect is predicted by Einstein's theory of
relativity and was first detected during a total solar eclipse by
Eddington in 1919. Large-scale gravitational lensing causes
multiple images of an object, the type and arrangement of the
images determined by the specifics of the lensing entity.
Gravitational "microlensing" is a small-scale lensing effect, the
gravitational field of the lensing object not strong enough to
form distinct images of the background source, but instead
causing an apparent brightening of the source.
... ... *Hubble Space Telescope: The Hubble Space Telescope was
launched from a space shuttle in 1990 into a 600-kilometer
low-Earth orbit and has been providing extensive imaging and
spectroscopic observations critical for the development of
astronomy and astrophysics. The new information has concerned hot
stars, stellar chromospheres and coronas, the interstellar
medium, galaxies and galactic clusters, quasars, etc. -- all of
it information uncorrupted by the Earth's atmosphere, which is
the problem for ground based telescopes.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON BROWN DWARF STARS
... C.G. Tinney (Anglo-Australian Observatory Epping, AU)
presents a review of recent observations of brown dwarf stars,
the author making the following points: 1) Most stars spend most
of their lives in a state of pressure balance maintained between
gravitational contraction and the energy generated by nuclear
reactions. In 1963, Kumar suggested there may exist a class of
star-like bodies with masses too low to create the central
temperature and densities required to ignite nuclear fusion
reactions. These "failed stars" became known to astronomers as
"brown dwarfs" [*Note #1]. 2) The lowest-mass ordinary stars can
theoretically maintain a quasi-equilibrium luminosity for almost
6000 billion years. Brown dwarf stars, in contrast, are expected
to fade throughout their lifetime, cooling to temperatures below
1000 degrees kelvin and becoming undetectable by direct
observation after just a few billion years. This has engendered
considerable interest in brown dwarf stars as possible candidates
for the *dark matter which apparently composes more than 90
percent of the mass our Galaxy. 3) The past 4 years have seen
success finally achieved in the hunt for brown dwarf stars. These
detections have confirmed predictions that both methane and dust
play an important role in determining the spectral behavior of
these objects. But the detection of brown dwarf stars in
significant numbers, when combined with results for the space
density of low-mass stars and *gravitational microlensing
results, allows us to conclude that brown dwarf stars do not make
a significant contribution to the dark matter of our Galaxy. The
author concludes: "No matter how nicely brown dwarfs would solve
the *baryonic dark matter problem, it appears we must look
elsewhere for a solution to this long-standing astronomical
quandary."
-----------
C.G. Tinney: Brown Dwarfs: The stars that failed.
(Nature 7 Jan 99 397:37)
QY: C.G. Tinney, Anglo-Australian Observatory, PO Box 296, Epping
NSW 1710, AU.
-----------
Text Notes:
... ... *Note #1: Present theoretical models predict a lower
mass-limit for fusion burning stars with the same element mix as
the Sun of 0.07 solar-mass, equivalent to 74 times the mass of
Jupiter.
... ... *dark matter: In general, in this context, the term "dark
matter" refers to material whose presence can be inferred from
its effects on the motions of stars and galaxies, but which
cannot be seen directly because it emits little or no radiation.
It is believed that at least 90 percent of the mass in the
Universe exists as some form or dark matter.
... ... *gravitational microlensing: See main report.
... ... *baryonic dark matter: Ordinary matter too dim to be
observed. A baryon is a nuclear particle, e.g., a proton, built
from 3 quarks (fundamental particles that combine to make up
protons, neutrons, and mesons).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 19Mar99
-------------------
Related Background:
THE DUSTY ATMOSPHERE OF A BROWN DWARF STAR
... C.A. Griffith et al (3 authors at 3 installations, US) now
report observations of the brown dwarf star Gliese 229B, which
exhibits certain unique characteristics. At 900 degrees kelvin,
the atmosphere of this object is too warm to contain ice clouds
like those on Jupiter and too cool to contain silicate clouds
like those on low-mass stars. These unique conditions (high
gravity and the lack of high clouds) permit spectroscopic
visibility of the atmosphere down to higher pressures (i.e.,
closer to the surface) than possible in cool stars or planets.
The authors investigated the structure of the atmosphere of
Gliese 229B by analyzing its optical spectrum in the interval
0.85 to 1.0 micron, the spectrum obtained at the *Keck 1
telescope. The authors report that the spectrum of Gliese 229B
indicates deep-atmosphere particulate matter with the optical
properties of neither ice nor silicates. The authors suggest the
reddish color of the particles indicates an organic composition
characteristic of aerosols in planetary stratospheres, and that
the *mass fraction of the particles agrees with a photochemical
origin involving incident radiation from its companion primary
star (Gliese 229A).
-----------
C.A. Griffith et al: The dusty atmosphere of the brown dwarf
Gliese 229B.
(Science 11 Dec 98 282:2063)
QY: Caitlin A. Griffith, Northern Arizona University 520-523-5511
-----------
Text Notes:
... ... *Keck 1 telescope: The Keck telescopes are a pair of twin
telescopes at the W. M. Keck Observatory on Mauna Kea, HI US,
each with a 10 meter mirror, the pair constructed 1992-1996. The
installation is managed by the University of California (US) and
the California Institute of Technology (US).
... ... *mass fraction: The mass fraction of aerosols is related
to the *eddy diffusion coefficient k, the mass density of the
atmosphere d, the net mass flux f, and the scale height of the
atmosphere h according to F = fh/kd.
... ... *eddy diffusion coefficient: (turbulent diffusion
coefficient) The exchange coefficient for the diffusion of a
conserved property by eddies in a turbulent flow. In general, an
"eddy" is a vortex-like motion of fluid running contrary to the
main current.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 12Feb99
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

IN FOCUS: ON CARBON
"Carbon, in fact, is a singular element: it is the only element
that can bind itself in long stable chains without a great
expense of energy, and for life on Earth (the only one we know so
far) precisely long chains are required. Therefore carbon is the
key element of living substance: but its promotion, its entry
into the living world, is not easy and must follow an obligatory,
intricate path, which has been clarified (and not yet
definitively) only in recent years. If the elaboration of carbon
were not a common daily occurrence, on the scale of billions of
tons a week, wherever the green of a leaf appears, it would by
full right deserve to be called a miracle. The atom we are
speaking of, [when] accompanied by its two satellites [two oxygen
atoms] which maintain it in a gaseous state, [may have] the good
fortune to brush against a leaf, penetrate it, and be nailed
there by a ray of the Sun. If my language here becomes imprecise
and allusive, it is not only because of my ignorance: this
decisive event, this instantaneous work _a tre_ -- of the carbon
dioxide, the light, and the vegetal greenery -- has not yet been
described in definitive terms, and perhaps it will not be for a
long time to come, so different is it from that other "organic"
chemistry which is the cumbersome, slow, and ponderous work of
man: and yet this refined, minute, and quick-witted chemistry was
"invented" two or three billion years ago by our silent sisters,
the plants, which do not experiment and do not discuss, and whose
temperature is identical to that of the environment in which they
live. If to comprehend is the same as forming an image, we will
never form an image of a happening whose scale is a millionth of
a millimeter, whose rhythm is a millionth of a second, and whose
protagonists are in their essence invisible. Every verbal
description must be inadequate, and one will be as good as the
next..."
-----------
Primo Levi: _The Periodic Table_
(Schocken Books, New York 1984)
(Science-Week 14 Apr 00)
-------------------
Related Background:
ON CARBON IN THE UNIVERSE
Carbon is a major factor in the evolutionary scheme of the
Universe because of its abundance and its ability to form complex
chemical entities. It is apparently also a key element in the
evolution of prebiotic molecules. The different forms of cosmic
carbon range from carbon atoms and carbon-bearing molecules to
complex solid-state carbonaceous structures, and evidence
gathered during the past decade has considerably enhanced our
understanding of the physical and chemical properties of carbon
materials in space. ... ...  Th. Henning and F. Salama (2
installations, DE US) present a detailed review of the subject,
the authors making the following points: 1) More than 75 percent
of the 118 *interstellar and circumstellar molecules identified
to date are carbon-bearing molecules, and one component of
interstellar dust is evidently carbonaceous. The cosmic evolution
of carbon from the interstellar medium into *protoplanetary disks
and *planetesimals, and finally into habitable bodies, is
intrinsic to the study of the origin of life. 2) Carbon plays an
important role in the physical evolution of the interstellar
medium because it is the main supplier of free electrons in
diffuse interstellar clouds, thus contributing to the heating of
interstellar gas. 3) The observation of unidentified ubiquitous
molecular and solid-state features in astronomical spectra, and
the realization that these features are linked to carbonaceous
materials, have resulted in major scientific progress in the past
decade. Laboratory and theoretical studies stimulated by these
astronomical observations have led to a better understanding of
the various forms of cosmic carbon such as polycyclic aromatic
hydrocarbons, carbon-chain molecules, carbon clusters, and
carbonaceous solids. These investigations have also led to the
detection of novel forms of carbon and laid the foundations for
the chemistry of *fullerenes. 4) The authors present the
following categorization of carbon in space:
... a) Carbon-rich circumstellar envelopes around *red giant and
*asymptotic giant branch (AGB) stars: CO, C(sub2)H(sub2), complex
hydrocarbons, gas-phase polycyclic aromatic hydrocarbons.
... b) Diffuse interstellar medium: C+, simple diatomic
molecules, gas-phase polycyclic aromatic hydrocarbons and carbon
chains.
... c) Dense interstellar medium: CO, complex hydrocarbons.
... d) Interstellar material in primitive meteorites: polycyclic
aromatic hydrocarbons.
5) The authors suggest that the widespread distribution of
complex organics in the interstellar medium has profound
implications for our understanding of a) the chemical complexity
of the interstellar medium, b) the evolution of prebiotic
molecules, c) the impact of this evolution on the origin and
evolution of life on early Earth through the exogenous delivery
(by cometary encounters and meteoritic bombardments) of prebiotic
organics.
-----------
Th. Henning and F. Salama: Carbon in the Universe.
(Science 18 Dec 98 282:2204)
QY: Th. Henning, Astrophysikalisches Institut und Universitats-
Sternwarte, Schillergabchen 2-3, D-07745, Jena DE.
-----------
Text Notes:
... ... *interstellar and circumstellar molecules: In this
context, an interstellar molecule is any molecule that occurs
naturally in clouds of gas and dust in space. In general, a
circumstellar molecule is any molecule that occurs in gas and
dust surrounding a star.
... ... *protoplanetary disks: These are dust disks surrounding
young stars; it is from these disks that planets presumably form.
... ... *planetesimals: Planetesimals are bodies with dimensions
of 10^(-3) to 10^(3) meters that are believed to form planets by
a process of accretion. The term "accretion" refers to an
aggregation, an increase in the mass of a body by the addition of
smaller bodies that collide and adhere to it, provided the
relative velocities are low enough for coalescence. As the mass
of the agglomerate increases, so does the rate of accretion, and
this accretion process is believed to generally occur in the form
of a disk. A stellar accretion disk is a swarm of dust grains
that evolve into planetesimals and then planets.
... ... *fullerenes: Fullerenes are large molecules composed
entirely of carbon, with the chemical formula C(sub n), where n
is any even number from 32 to over 100. They apparently have the
structure of a hollow spheroidal cage with a surface network of
carbon atoms connected in hexagonal and pentagonal rings.
... ... *red giant: A red giant star is a star in a late
stage of evolution. Having exhausted the hydrogen fuel in its
core, the star is burning elements heavier than hydrogen. It has
a surface temperature of less than 4700 degrees Kelvin and a
diameter 10 to 100 times that of the Sun.
... ... *asymptotic giant branch (AGB) stars: These are stars
that occupy a strip in the *Hertzsprung-Russell diagram that is
almost parallel to and just above what is called the "giant
branch" off the *Main Sequence. Stars evolve from the horizontal
H-R branch to the asymptotic giant branch when they have
exhausted the helium in their cores and are instead burning
helium in a shell.
... ... *Hertzsprung-Russell diagram: The Hertzsprung-Russell
diagram is a plot of stellar absolute magnitude against spectral
type, and is perhaps the most useful diagrammatic aid in
astrophysics. It allows the portrayal of the evolution of a star
as occurring along various paths in the diagram.
... ... *Main Sequence: The Main Sequence is a region on the
Hertzsprung-Russell diagram where most stars lie, including our
own Sun. The evolution of a star can be diagrammed as a movement
along the Main Sequence and an eventual branching off the Main
Sequence to regions associated with various types of old stars.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 26Feb99
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

NOTICES  
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

CHANGE OF EMAIL ADDRESS: If at any time you need to change the
Email address at which you receive SW, please send the
information to [request@scienceweek.com], and the change will be
made and confirmed the same day.  

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 

SCIENCE-WEEK SUBSCRIPTIONS: The subscription rate for ScienceWeek
(52 issues per year delivered via Email only) is US$20 for one
year. Subscriptions can be obtained with a credit card
(Visa, MC, Amex) at a secure website form accessed at:
http://scienceweek.com/subinfo.htm

Information concerning other methods of payment is available at
the above URL, or via Email at swsub@scienceweek.com

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

The first issue of SCIENCE-WEEK appeared May 1, 1997, and it has
been published regularly each week since that date. SW is
designed to cross existing conceptual and linguistic barriers
between the scientific disciplines. In general, the biology is
written for physicists and chemists, and the physics and
chemistry are written for biologists, with an attempt to retain
some exactitude in the particular science involved in the news.
These are the aims. Undoubtedly, we are not always successful,
and for that we apologize. In any case, what we hope is that our
readers are reading out of their fields more than in their
fields, since that is the essence of this publication.

We welcome comments, suggestions, and criticisms from our
subscribers. Public letters relevant to any report are also
welcome. Editorial contact: [editors@scienceweek.com].

Editor/Publisher: Dan Agin
Managing Editor: Claire Haller
Associate Editor: Joan Oliner

Copyright (c) 1997-1999 SCIENCE-WEEK/Spectrum Press Inc.
All Rights Reserved

---------------------------------------------
This publication is protected by U.S. and International Copyright
Laws, and no display, transmission, or duplication in any medium,
including BBS, Internet Email, website duplication, fax, or print
is permitted without the explicit consent of the holder of the
copyright. SCIENCE-WEEK is published by Spectrum Press Inc.,
3023 N. Clark Street #109, Chicago, 60657-5205 IL, USA.
---------------------------------------------