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.

November 26, 1999 -- Vol. 3 Number 48

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The particle scientist is more or less happy.
He has no home. All his ladders go straight down
and claim the nameless.
-- Anthony Piccione

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To our subscribers: This week the Editor of SW has been put flat
on his back by a nasty influenza virus, with the consequence that
3 of the reports in this issue are not new but are instead
reprints from the past. We apologize for the imposition; the
deficit will be made up as quickly as possible in future issues.

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Contents of This Issue:

1. On Cosmological Antigravity
2. Discovery of a Moon Orbiting an Asteroid
3. A Scientific Basis for Climate Forecasting
4. On Simple Ordering in Complex Fluids
5. Imaging the Functioning Human Brain
6. Carbon Monoxide Poisoning in Camping Tents

In Focus: On the Search for Life on Mars

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1. ON COSMOLOGICAL ANTIGRAVITY [from SW 5 Feb 99]
The branch of physics called "cosmology" deals with the structure
and evolution of the Universe, and during the past few years it
has become apparent that this field may be facing one of its most
significant transitions. In all the sciences, it is common for
observations to conflict with theory, with a resultant refinement
and even abandonment of old theory for new theory. Indeed, a case
can be made that this is the essence of science, and the basis
for the continued improvement in our understanding of natural
phenomena. In cosmology, a serious theoretical reformulation is
apparently imminent. At this point in its history, the science of
cosmology is evidently in the throes of a major overhaul of
theoretical constructs, an overhaul produced by mounting evidence
relevant for considerations of the structure of the Universe.
Central to these considerations are the distinctions between the
geometries of a "flat" (uncurved; infinite in both extent and
lifetime), "closed" (spherical; finite in both extent and
lifetime), and "open" (*hyperbolic; infinite and expanding
forever) Universe. An important quantity is the Omega parameter,
defined as the ratio of the density of matter (or energy) in the
Universe to the theoretical density required for flatness. An
Omega with a value of greater than 1 implies a closed Universe; a
value less than 1 implies an open Universe; a value equal to 1
implies a flat Universe. The problem for the past 60 years has
thus been to obtain an estimate of the mass density of the
Universe from observations. The current standard conception is
that the geometry of the Universe is flat. Recently, however, new
data have apparently indicated that the entire scheme upon which
models of the structure of the Universe are based may need
serious revision, and such revision may in turn force revision of
certain areas of fundamental physics. ... ... Lawrence M. Krauss
(Case Western Reserve University, US) presents a review of the
problem, the author making the following points: 1) The standard
cosmology of the 1980s, postulating a flat Universe dominated by
matter, is dead. The Universe is either open or filled with an
energy of unknown origin. 2) Although the visible contents of the
Cosmos are clearly not enough to make the Universe flat, analysis
of celestial dynamics indicates there is far more matter than we
can observe, with most of the material in galaxies and assemblies
of galaxies invisible to telescopes... An overwhelming body of
evidence now implies that even the unseen matter is not enough to
produce a flat Universe... If the Universe is open, the current
*inflationary theory of the evolution of the Universe immediately
following the *Big Bang must be modified or discarded. If the
Universe is indeed flat, the Universe must be composed largely of
an "ethereal" form of energy that inhabits empty space ("*vacuum
energy"). 2) The existence of vacuum energy is implied by quantum
mechanics and demonstrated experimentally by the *Casimir effect.
Physicists have thus corroborated the theory put forth by Dirac
(and later by Feynman, Schwinger, and Tomonaga) that space is
filled with fleeting "virtual particles". 3) The author
distinguishes 4 types of matter: a) visible matter, which is
ordinary matter composed mainly of protons and neutrons, and
which forms stars, dust, and gas; b) *baryonic dark matter, which
is ordinary matter too dim to be observed; c) nonbaryonic dark
matter, which consists of exotic particles such as "*axions",
*neutrinos with mass, or *weakly interacting massive particles;
d) cosmological dark matter, which consists of vacuum energy. 4)
Concerning the approximate quantitative contributions to the
dimensionless Omega parameter, the author presents the following:
visible matter 0.01; baryonic dark matter 0.05; nonbaryonic dark
matter 0.3; vacuum energy 0.6. The author states that of the 2
apparent alternatives, an open Universe or a flat Universe filled
with vacuum energy, "either scenario will require a dramatic new
understanding of physics."
-----------
Lawrence M. Krauss: Cosmological antigravity.
(Scientific American January 1999)
QY: L.M. Krauss, Case Western Reserve University 216-368-2000
-----------
Text Notes:
... ... *hyperbolic: This is a negative curvature, like the
surface of a saddle, and it is sometimes called a "saddle"
Universe. In such a geometry, the sum of the angles of a triangle
is less than 180 degrees. In a spherical (closed) geometry, the
sum of angles is more than 180 degrees; in a flat geometry, the
sum of angles is exactly 180 degrees.
... ... *inflationary theory: The inflationary model, first
proposed by Alan Guth in 1980, proposes that quantum
fluctuations in the time period 10^(-35) to 10^(-32) seconds
after time zero were quickly amplified into large density
variations during the "inflationary" 10^(50) expansion of the
universe in that time frame.
... ... *Big Bang: The Big Bang theory is the general
cosmological model that proposes that all matter and radiation in
the universe originated in an explosion at a finite time in the
past.
... ... *vacuum energy: The idea of "vacuum energy" is considered
by some theorists to be embodied in Einstein's relativity
equations as the so-called "*cosmological constant". Unlike
ordinary forms of mass and energy, the Einstein vacuum energy
adds gravity that is repulsive and can drive the Universe apart
at ever increasing speeds. At the level of quantum mechanics, the
existence of vacuum energy is based on considerations of zero-
point energy and the fact that according to quantum mechanics
nothing of interest (including a vacuum) can have zero energy. In
a vacuum, quantum fluctuations of a field produce a minimum
energy (zero-point energy), and through the relation of energy
and mass, it is possible to associate this vacuum energy with the
existence of "virtual particles", particles whose duration of
existence and non-existence is of the order of quantum
fluctuations.
... ... *cosmological constant: A mathematical term introduced by
Einstein into the equations of general relativity, the purpose to
obtain a solution of the equations corresponding to a "static
Universe" (i.e., flat Universe). The term describes a pressure
(if positive) or a tension (if negative) which can cause the
Universe to expand or contract even in the absence of any matter
("vacuum energy"). When the expansion of the Universe was
discovered, Einstein apparently began to regard the introduction
of this term as a mistake, and he described the cosmological
constant as the "greatest mistake of my life". But the term has
reappeared as the proposed source of apparent accelerated cosmic
expansion.
... ... *Casimir effect: This is the force between 2 macroscopic
conducting surfaces in a volume that contains only an
electromagnetic field (i.e., an electromagnetic field in a
vacuum). The zero-point energy of the electric field depends,
according to quantum mechanics, on the types of vibrations of the
field (mode frequencies), which in turn depend on the boundary
conditions on the field. This zero-point energy leads to a force
between the plates. The existence of this force was theoretically
predicted by H.B.G. Casimir and detected experimentally by M.J.
Sparnay in 1958. Both the sign and magnitude of the Casimir
effect depend on the geometry of the surfaces.
... ... *baryonic dark matter: 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).
... ... *axions:  A hypothetical elementary particle of very low
mass and zero charge, and one of the candidates for dark matter
in the Universe.
... ... *neutrinos with mass: An electrically neutral elementary
particle until recently considered to be always without mass.
... ... *weakly interacting massive particles: (WIMPS) This
refers to a hypothetical elementary particle that is a candidate
for cosmic dark matter, a stable neutral particle, somewhat
heavier than the neutron, that interacts only weakly with
ordinary matter.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Feb99
-------------------
Related Background:
COSMOLOGY: OPEN, CLOSED, OR FLAT UNIVERSE?
Marc Kamionkowski (Columbia University, US) reviews current
research in cosmology, making the following points: 1) Determin-
ation of the geometry of the universe has been a central goal of
cosmology ever since Hubble discovered its expansion 75 years
ago. 2) The central question is whether the universe is a multi-
dimensional equivalent of a 2-dimensional surface ("flat"), a
sphere ("closed"), or a saddle ("open"). The geometry, in the
context of current theory and observations, determines whether
the universe will expand forever or eventually collapse. 3) Until
now, most astronomers have pursued the geometry by attempting to
measure the mass density of the universe. According to general
relativity, if the density is equal to, larger than, or smaller
than a critical density fixed by the expansion rate, then the
universe is flat, open, or closed, respectively. 4) Another
possibility is to look directly at the predicted observational
effects of a curved (open or closed) universe versus a flat
universe, and in particular at the angular power spectrum of the
cosmic microwave background. The authors suggest that in the near
future a new generation of experiments will provide substantial
advances in these observations, enabling more definitive
statements about the geometry of the universe, and that these
results will in turn provide clues to the new particle physics
required to understand the inflation phase following the Big Bang
origin of the universe.
QY: Marc Kamionkowski (kamion@phys.columbia.edu)
(Science 29 May 98 280:1397) (Science-Week 19 Jun 98)
-------------------
Related Background:
ON THE NATURE OF DARK MATTER
Joel R. Primack (University of California Santa Cruz, US)
presents a commentary on a paper by E. Gawiser and J. Silk
(University of California Berkeley, US) ((Science 29 May 98
280:1405), Primack making the following points: 1) One of the
fundamental issues facing cosmologists concerns the evidence that
observable matter in the universe makes up only a fraction of
what is needed to explain the properties of the universe. A large
portion of matter in the universe must therefore be unobserved,
or "dark matter". 2) In current cosmology, "hot" dark matter is
defined as particles that were still moving at nearly the speed
of light at about a year after the big bang. "Cold" dark matter
is defined as particles that were moving sluggishly at that time.
Neutrinos are the standard example of hot dark matter, although
other more exotic possibilities have been discussed. 3) Gawiser
and Silk (ref. cited above) conclude that of all the currently
popular cosmological models, the only one whose predictions agree
with the data on the cosmic microwave background anisotropies and
the large-scale distribution of galaxies is the cold + hot dark
matter model, with 70% of the matter cold dark, 20% hot dark, and
10% ordinary matter (baryonic). 3) There are 3 species of
neutrinos, and there are mounting astrophysical and laboratory
data suggesting that neutrinos oscillate from one species to
another, which can only happen if they have nonzero mass. As
dark-matter candidates, neutrinos are entities with masses that
may be 10^(-5) of the mass of the electron, but with an expected
density more than 8 orders of magnitude greater than the density
of electrons and protons in the universe. Neutrinos, therefore,
can provide a substantial fraction of dark matter. 4) The success
of the cold + hot dark matter model in fitting the cosmic
microwave background and galaxy distribution data indicates that
this type of model should be investigated in more detail.
QY: Joel R. Primack (joel@physics.ucsc.edu)
(Science 29 May 98 280:1398) (Science-Week 19 Jun 98)
-------------------
Related Background:
A GRAVITATIONAL DIFFUSION MODEL WITHOUT DARK MATTER
R.J. Britten (California Institute of Technology, US) presents a
model that without dark matter quantitatively describes the flat
rotation curves of galaxies and the mass-to-light ratios of
clusters of galaxies. The hypothesis is that the agent of
gravitational force is propagated as if it were scattered with a
mean free path of about 5 kiloparsecs. As a result, the force
between moderately distant masses separated by more than the mean
free path diminishes as the inverse first power of the distance,
following diffusion equations, and describes the flat rotation
curves of galaxies. The force between masses separated by < 1
kiloparsec diminishes as the inverse square of the distance. The
excess gravitational force (ratio of 1/r:1/r^2) increases with
the scale of structures from galaxies to clusters of galaxies,
but there is reduced force at great distances because of the
approximately 12 billion years available for diffusion to occur.
This model with a mean free path of about 5 kiloparsecs predicts
a maximum excess force of a few hundredfold for galactic clusters
with dimensions of a few megaparsecs. With only a single free
parameter, the predicted curve for excess gravitational force vs.
size of structures fits reasonable well with observations from
those of dwarf galaxies through galactic clusters. Under this
diffusion model, no matter is proposed in addition to the
observed baryons plus radiation, and thus the proposed density of
the universe is only a few percent of that required for closure.
The author suggests that although the model does not follow from
present calculations based on the general theory of relativity,
it is not necessarily inconsistent with the general theory
because the diffusing gravitational elements might be interpreted
as spatial curvatures (e.g., distortions of the metric inducing
distortions in adjacent regions). The author further suggests
there is much at stake because of the scale of the intellectual
investment and the subtle arguments in cosmology that make use of
the general theory of relativity, and that the challenge of a
theory of intrinsic "beauty" may not be met at this time because
"beauty" is a subtle concept.
QY: Roy J. Britten (rbritten@etna.bio.uci.edu)
(Proc. Natl. Acad. Sci. US 31 Mar 98 95:3351)
(Science-Week 1 May 98)


2. DISCOVERY OF A MOON ORBITING AN ASTEROID
The possibility that some asteroids have satellites (moons) has
been debated since the 1970s, when observers reported anomalous
generally unconfirmed "additional" events during *occultations of
bright stars by asteroids, with these events sometimes
interpreted to be caused by unseen companions. Asteroidal
satellites have been suspected from unusual light curves, slow
rotation rates, and double impact craters on planetary surfaces.
The estimated prevalence of such satellites, inferred
observationally and theoretically, ranges from common to uncommon
to essentially absent. Theory indicates that stable orbits for
asteroidal satellites can indeed exist, even in the presence of
Solar or Jovian perturbations. The first asteroidal satellite to
be actually found was the satellite Dactyl orbiting the asteroid
Ida, discovered during the flyby of the Galileo spacecraft in
1993. Since then, modeling of the catastrophic, collisional
formation, tidal evolution, and longevity of asteroidal
satellites suggests that such satellites may be common, and
evidence for such moons has been sought with some effort because
the relative frequency of such satellites will bear on ideas
concerning the  collisional history of the asteroid belt and the
Solar System.
... ... W.J. Merline et al (10 authors at 9 installations, US DE
FR) now report the discovery of a satellite of the asteroid 45
Eugenia using a special optical system on a ground-based
telescope. The authors report the satellite has a diameter of
approximately 13 kilometers, and an orbital period  of
approximately 4.7 days, with an orbital radius of 1190
kilometers.
-----------
W.J. Merline et al: Discovery of a moon orbiting the asteroid 45
Eugenia.
(Nature 7 Oct 99 401:565)
QY: W.J. Merline [merline@boulder.swri.edu]
-----------
Text Notes:
... ... *occultations: The term "occultation" refers to the
temporary cutting off of the light from one celestial body as
another and nearer body passes in front of it.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 26Nov99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
GROUNDBREAKING OBSERVATIONS OF SMALLEST SOLAR SYSTEM ASTEROID
Asteroid 1998 KY26, discovered last year, is considered the
smallest Solar System object ever studied in detail. The 30-meter
object passed 800,000 kilometers from Earth in the period June
2-8, 1998. A report of observations of the asteroid will be
published in the journal _Science_ this week by S.J. Ostro et al.
Ostro is quoted as saying, "Enormous numbers of objects this
small are thought to exist very close to Earth, but this is the
first time we've been able to study one in detail. Ironically,
this asteroid is smaller than the radar instruments we used to
observe it." The asteroid is spinning with a rotation period
calculated at 10.7 minutes, compared to rotation periods of at
least several hours for the approximately 1,000 asteroids
measured to date. In addition to these findings, the minerals in
1998 KY26 are believed to probably contain approximately 1
million gallons of water. Ostro says, "This asteroid is quite
literally an oasis for future space explorers. Its optical and
radar properties suggest a composition like carbonaceous
chondrite meteorites, which contain complex organic compounds
that have been shown to have nutrient value. These could be used
as soil to grow food for future human outposts. And among the
25,000 or so asteroids with very reliably known orbits, 1998 KY26
is in an orbit that makes it the most accessible to a
spacecraft."
(NASA 22 Jul 99) (Science-Week Bulletin 23 July 99)


3. A SCIENTIFIC BASIS FOR CLIMATE FORECASTING [from SW 20 Nov 98]
At the beginning of the 20th century it was believed that it
should be possible to predict weather by solving the mathematical
equations that describe the physical laws that govern the motion
of air. It took several decades to develop an appropriate set of
equations and numerical and computational techniques to solve
these equations, and by 1960, routine weather predictions using
global observations, complex mathematical equations, and fast
computers seemed immediately at hand. The promise, however, was
short lived, because in the early 1960s it was discovered that
the mathematical equations for weather forecasting represent a
forced *dissipative nonlinear dynamic system that exhibits
*chaotic behavior, which means that even an infinitesimally small
uncertainty in the initial conditions will grow exponentially to
make the forecast useless after a finite amount of time.
... ... J. Shukla now presents a study involving analyses of
weather observations coupled with model simulations. The author
reports that although the Earth's atmosphere is generally
considered to be an example of a chaotic system that is
sensitively dependent on initial conditions, certain regions of
the atmosphere are an exception, with wind patterns and rainfall
in certain regions of the tropics so strongly determined by the
temperature of the underlying sea surface that they do not show
sensitive dependence on the initial conditions of the atmosphere.
The author suggests it should therefore be possible to predict
the large-scale tropical circulation and rainfall for as long as
the ocean temperature can be predicted, and that if changes in
tropical Pacific sea-surface temperature are quite large, even
the extratropical circulation over some regions, especially over
the Pacific-North American sector, is predictable. In particular,
the author suggests that for all future major *El Nino events, it
should be possible to predict large-scale changes in the winter
season mean circulation over North America several months in
advance, provided we can predict tropical sea surface
temperatures.
-----------
J. Shukla (George Mason University, US)
Predictability in the midst of chaos: A scientific basis for
climate forecasting.
(Science 23 Oct 98 282:728)
QY: J. Shukla, George Mason University 703-993-2400
-----------
Text Notes:
... ... *dissipative: In general, a dissipative system
is a system that loses energy by conversion of energy into heat.
... ... *chaotic behavior: In the study of physical systems, the
term "chaotic behavior" has a specific meaning: the behavior of a
system is said to be "chaotic" if its final state is so sensitive
to the system's precise initial conditions that the behavior of
the system is in effect unpredictable and cannot be distinguished
from a random process, even though the behavior of the system is
strictly determinate in a mathematical sense. In other words, a
deterministic system characterized by extremely sensitive
instabilities, despite the system being determinate, can exhibit
behavior that is unpredictable, and the system is then called
"chaotic". During the past several decades, the analysis of such
chaotic systems has intrigued both physicists and mathematicians.
... ... *El Nino: El Nino is an aperiodic intermittent (2 to 10
years) flow of unusually warm surface water along the western
coast of South America, the flow capable of causing abnormally
high rainfall in usually dry areas and severe local ecosystem
dislocations -- what is termed an El Nino "event". El Ninos are
regional phenomena, but they have global consequences. The name
"El Nino" ("The Child") arose because the phenomenon usually
occurs around Christmas. In 1986, M.A. Cane and S.A. Zebiak
proposed a model for making forecasts of El Nino several seasons
ahead by applying Newton's equations of motion and the laws of
thermodynamics to the dynamics of the ocean and atmosphere of the
tropical Pacific.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 20Nov98


4. ON SIMPLE ORDERING IN COMPLEX FLUIDS [from SW 1 Jan 99]
A colloid is a system of particles 1 to 1000 nanometers in
diameter dispersed in another phase. A colloidal crystal is a
periodic array of suspended colloidal particles, the array
arising spontaneously in a "monodisperse" colloidal system under
proper conditions. A monodisperse colloidal system is simply a
colloidal system in which the suspended particles have identical
size, shape, and interaction. ... ... A.P. Gast and W.B. Russel
(2 installations, US) review current research on ordering in
complex fluids and 2-dimensional crystals, the authors making the
following points: 1) One important feature of colloidal
suspensions is that the sub-micron particles are subject to
constant Brownian motion from the thermal fluctuations in the
surrounding solvent. Thus, to some degree, the particles can be
considered as effective molecules and treated according to the
theories of statistical mechanics. Because the solvents often
contain -- in addition to the colloidal particles -- dissolved
ions, polymer molecules, surface-active molecules, and other
small solutes, colloids are referred to as a general class of
complex fluids. 2) Perhaps one of the most important answers to
the dreams of physicists has been the development of colloidal
particles that interact by means of hard-sphere repulsions...
Entropy is usually thought to bring about disorder. But in a
system of hard spheres, particles gain entropy by arranging
themselves equidistantly from one another to maximize the space
in their vicinity, and thus they are compelled to order. 3) The
addition of a "soft" (i.e., long-range) repulsion to colloidal
particles can keep them sufficiently separated that the *van der
Waals attraction is negligible, rendering the suspension stable
against aggregation under a variety of conditions. Most commonly,
aqueous suspensions are stabilized by the *screened electrostatic
repulsion between charges imparted by the *Debye length, which
scales inversely with the square root of the *ionic strength of
the suspension. 4) The disorder-order transition for particles
having purely repulsive interactions persists when attractive
interactions are added... In aqueous systems, adding electrolytes
to screen the electrostatic repulsion between charged particles
can induce aggregation by means of van der Waals attraction. 5)
The complex structure of proteins can often be deciphered using
the power of crystallography -- but only if the proteins can be
crystallized. Some proteins form 2-dimensional arrays when
attached to a lipid monolayer floating on top of an aqueous
solution, and the monolayer of proteins can then be transferred
to an electron microscope grid for imaging and study by electron
diffraction... Although proteins remain complex in their detailed
structure and interactions, they provide ample opportunity to
study the general phenomenon of crystallization at both colloidal
and molecular scales.
-----------
A.P. Gast and W.B. Russel: Simple ordering in complex fluids.
(Physics Today December 1998)
QY: Alice P. Gast, Stanford University 415-723-3058.
-----------
Text Notes:
... ... *van der Waals attraction: (also spelled Van der Waals)
Considering molecules that have permanent dipoles, and molecules
that can have dipoles induced by the electric fields of other
molecules, there are three possible mechanisms recognized in the
formation of the van der Waals bonds: 1) the orientation effect,
in which molecules rearrange themselves in their mutual
electrical fields, the rearrangements involving reorientations of
whole molecules; 2) the static induction effect, in which
molecules that are static monopoles (ions) or dipoles may induce
a static rearrangement of the electron distribution of other
molecules; 3) the dynamic induction effect, or "dispersion"
effect, in which any molecule, polar or nonpolar, may induce in
other molecules transient electron distribution rearrangements
that are time-variant. All these mechanism involve interaction
energies, and they are "bonds" in the sense that they all involve
energetic couplings between molecules.
... ... *screened: In general, screening is a reduction of the
effective electric field at a point, the reduction due to the
space charge of ambient charged particles of sign opposite to the
source of the field.
... ... *Debye length: (Debye shielding length, Debye-Huckel
screening radius) A characteristic distance in a system of
particles beyond which the electric field of a charged particle
is shielded by particles having charges of the opposite sign. 
... ... *ionic strength: A measure of the average electrostatic
interactions among ions in an electrolyte. Quantitatively defined
as one-half the sum of the terms obtained by multiplying the
molality of each ion by its valence squared.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 1Jan99
-------------------
Related Background:
ATTRACTION BETWEEN LIKE-CHARGED SPHERES IN A CHARGED PORE
A colloid is a system of particles 1 to 1000 nanometers in
diameter dispersed in another phase, and such systems, 
particularly systems of electrically charged colloids, have
important practical significance and are also of considerable
theoretical interest. The existence of long-range attractive (as
opposed to the expected repulsive) electrostatic forces between
particles of like charge is one of the current major
controversies of colloid science. The established classical
theory (Derjaguin-Landau-Vervey-Overbeek) of colloidal
interactions predicts that an isolated pair of like-charged
colloidal spheres in an electrolyte should experience a purely
repulsive *screened electrostatic (coulombic) interaction. Direct
measurements of such interactions have shown quantitative
agreement with the classical theory, but recent experiments have
provided evidence that the effective interparticle potential can
have a long-range attractive component in more concentrated
suspensions and for particles confined by charged glass walls.
This long range attraction in concentrated systems is apparently
due to multi-body interactions. Theoretical explanations have
been proposed but remain the subject of controversy.
... ... Bowen and Sharif (University of Wales, UK) now present a
quantitative theoretical explanation of the attractive forces
between confined colloidal particles, the theory based on direct
solutions of the classical nonlinear Poisson-Boltzmann equation
for two like-charged spheres confined in a cylindrical charged
pore. The calculations show that the attraction may be explained
by the redistribution of the electric double layer of ions and
counterions in solution around the spheres, owing to the presence
of the wall. The authors suggest there is thus no need to revise
the established concepts of underlying theories of colloidal
interactions. [Editor's Note: The theoretical result in this
paper is unequivocal: the calculation shows that for the given
boundary conditions, the force between two particles of like
charge dips below zero (i.e., becomes attractive) before
returning to zero at infinite distance.]
QY: W. Richard Bowen [r.bowen@swansea.ac.uk]
(Nature 18 Jun 98 393:663) (Science-Week 3 Jul 98)
-------------------
Related Background:
COLLOIDAL CRYSTAL FILMS AS CHEMICAL SENSORS
In chemistry, a ligand is any molecule that is bound by another
molecule. A colloidal crystal is a periodic array of suspended
colloidal particles, the array arising spontaneously in a
"monodisperse" colloidal system under proper conditions. A
monodisperse colloidal system is simply a colloidal system in
which the suspended particles have identical size, shape, and
interaction. Osmotic pressure is the pressure exerted by a
dissolved material in a solution on a semipermeable membrane or
phase separating the solution from another solution or from pure
solvent. John H. Holtz and Sanford A. Asher (University of
Pittsburgh, US) report the preparation of a material that changes
color in response to selective binding of ligands (analytes). The
material is a crystalline colloidal array of polymer spheres
(approximately 100 nanometers in diameter) polymerized within a
hydrogel that swells and shrinks reversibly in the presence of
certain analytes (for example, ions and glucose). The basis for
the response is the diffraction of light determined by the
crystalline lattice spacing, which changes as the gel swells or
shrinks in response to osmotic pressure changes.
QY: S. Asher [asher@pitt.edu]
(Nature 23 Oct 97) (Science-Week 14 Nov 97)
-------------------
Related Background:
UNEXPECTED SHAPES FORMED BY COLLOIDS IN MICROGRAVITY CONDITIONS
Colloids are extremely small phases (e.g., solid particles) in
the range one nanometer to one micron dispersed in a larger phase
of a different substance, for example in water. They can be
bubbles, globules, microcrystals, etc. In the 1930s, many cell
biologists were excited by the study of colloids because the
interior of the living cell, protoplasm, has many properties of a
colloidal suspension. These days the study of colloids is almost
exclusively the province of the physical chemist. Although
colloidal particles are small, they are much larger than
molecules, large enough to perhaps be affected by gravitational
forces. This week the results of experiments with uniform
colloidal polymer particles carried out in microgravity
conditions aboard the space shuttle Columbia in November 1995
were reported by Jixiang Zhu et al (Princeton University, US;
NASA Lewis Research Center, US; University of Bristol, UK;
Johnson Space Flight Center, US). It was found that the colloidal
crystallization properties of the particles studied are indeed
significantly different under microgravity conditions.
Theoretical models of the behavior of colloidal suspensions
therefore need to take into account the gravitational forces
acting on the particles.
(Nature 26 June) (Science-Week 3 July 97)


5. IMAGING THE FUNCTIONING HUMAN BRAIN
For most of this century, research on the human brain concerned
with relating structure to function depended in general on
clinical studies (both postmortem and in living patients),
investigations that correlated specific brain traumas to specific
behaviors and behavioral defects. In addition to these studies,
there were related studies in animal models in which various
surgical and pharmacological interventions were correlated with
animal behavior, but animal models have limited utility when the
so-called "higher functions" in humans are the focus of
investigation. Then, approximately 20 years ago, new "imaging"
technology became available, methods that allow non-invasive
identification of those parts of the human brain activated by
specific behaviors. These new technologies have revolutionized
the field of brain physiology, and neurobiologists now anticipate
a near-future acceleration of our understanding of the relation
between the brain and behavior.
... ... X. Weng et al (3 authors at 3 installations, CN US)
present a short review of recent work on imaging the functioning
human brain, the authors making the following points:
     1) Functional brain imaging techniques have made significant
advances during the past decade. These techniques allow the
identification of the functional anatomy of cognitive processes,
the characterization of temporal correlations of brain activity
and behavior, and the examination of regional changes of
physiological and biochemical processes. Among various imaging
modalities, *functional magnetic resonance imaging (fMRI) and
*positron-emission tomography (PET) have seen the most technical
advances.
     2) Functional magnetic resonance imaging measures
oxygenation-level-dependent activity. Recently, thanks to
advances in fast imaging techniques and a refinement in our
understanding of the relationship between neuronal activity and
associated cerebrovascular hemodynamics, a new method -- event-
related fMRI -- has been developed. This new method is
significant because the procedure allows for selective averaging
of individual trials in mixed task paradigms, which permits
exploration of brain function with experimental paradigms similar
to those used in traditional behavioral and electrophysiological
studies. Event-relate fMRI has quickly led to a number of
applications in cognitive neuroscience.
     3) Positron-emission tomography is a sensitive imaging
method that uses radiotracers labeled with short-lived positron-
emitting isotopes to track chemical transformation in a living
system. The technique measures radioisotope concentrations in the
nanomolar-picomolar range. Another unique feature of PET is its
biochemical selectivity to molecular targets such as cell
receptors, transporters, or enzymes involved in the synthesis or
metabolism of neurotransmitters. PET has thus been extensively
used for biochemical and pharmacological imaging of the human
brain, in addition to its wide use to identify the anatomical
correlates of cognitive processes.
-----------
X. Weng et al: Imaging the functioning human brain.
(Proc. Natl. Acad. Sci. US 28 Sep 99 96:11073)
QY: Xuchu Weng [wengxc@psych.ac.cn]
-----------
Text Notes:
... ... *functional magnetic resonance imaging (fMRI): We must
first distinguish between magnetic resonance imaging (MRI) and
"functional" magnetic resonance imaging (fMRI) as applied to the
brain. The former is essentially a technique for examining
morphology, while the latter is a technique for examining
activity of brain tissue. Both techniques involve computerized
analysis of data. In general, MRI involves magnetic coils
producing a static magnetic field parallel to the long axis of
the patient or subject, combined with inner concentric magnetic
coils producing a static magnetic field perpendicular to the long
axis. A radio-frequency coil specifically designed for the head
perturbs the static fields to generate a magnetic resonance
image. The interaction physics in this technique is that between
the magnetic fields and atomic nuclei in brain tissue. "Sliced"
views can be obtained from any angle, and the resolution is quite
high and on the order of millimeters for current magnetic field
strengths of 1.5 tesla. Functional magnetic resonance imaging
(fMRI), the variant of MRI discussed here, is based on the fact
that oxyhemoglobin, the oxygen-carrying form of hemoglobin, has a
different magnetic resonance signal than deoxyhemoglobin, the
oxygen-depleted form of hemoglobin. Activated brain areas utilize
more oxygen, which transiently decreases the levels of
oxyhemoglobin and increases the levels of deoxyhemoglobin, and
within seconds the brain microvasculature responds to the local
change by increasing the flow of oxygen-rich blood into the
active area. This local response thus leads to an increase in the
oxyhemoglobin-deoxyhemoglobin ratio, which forms the basis for
the fMRI signal in this technique. Because of its high spatial
resolution (millimeters) and high temporal resolution (seconds)
compared to other imaging techniques, fMRI is now the technology
of choice for studies of the functional architecture of the human
brain.
... ... *positron-emission tomography (PET): This is a technique
for producing cross-sectional images of the body after ingestion
and systemic distribution of safely metabolized positron-emitting
agents. The images are essentially functional or metabolic, since
the ingested agents are metabolized in various tissues.
Fluorodeoxyglucose and H(sub2)O(sup15) are common agents used for
cerebral applications, and in cerebral applications of central
importance to the technique is the fact that changes in the
cellular activity of the brains of normal, awake humans and
unanesthetized laboratory animals are invariably accompanied by
changes in local blood flow and also changes in oxygen
consumption.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 26Nov99
[For more information: http://scienceweek.com/search/search.htm]


6. CARBON MONOXIDE POISONING IN CAMPING TENTS
Carbon monoxide is a clear, odorless, poisonous gas produced by
the incomplete combustion of carbon-based fuels, e.g., in
gasoline engines. If carbon monoxide is released in a small
enclosed space, death can occur within minutes. The toxicity of
carbon monoxide has been recognized since the mid 1800s, when
Claude Bernard (1813-1878) began his pioneering work on the
elucidation of its mechanism of action, and the fundamentals of
the toxicity mechanism are now well known. Carbon monoxide
competes reversibly and successfully with molecular oxygen for
the ferrous heme binding sites on hemoglobin because the affinity
of those sites for carbon monoxide is almost 250 times greater
than that for oxygen, and the result is that sufficient oxygen is
not transported by hemoglobin to the tissues of the body.
Therefore, the substance is extremely toxic at low
concentrations, and its lack of odor, color, or irritant effect
gives no warning of exposure and makes it especially dangerous.
Traditionally, the danger of a release of carbon monoxide into
closed indoor spaces is well-known to the public. What is not so
well-known is that carbon monoxide released into outdoor spaces
by portable gas stoves and charcoal grills can also be extremely
hazardous. A recent report by the US Centers for Disease Control
and Prevention describes two incidents among campers in Georgia
(US), the two incidents resulting in 6 deaths as the result of
carbon monoxide poisoning in outdoor settings.
     1) In the first incident, on the afternoon of 14 March 1999,
a 51-year-old man, his 10-year-old son, and a 7-year-old girl
were found dead inside a zipped up, 10-foot by 14-foot, two-room
tent at their campsite in southeast Georgia (a pet dog also
died). A propane gas stove, still burning, was found inside the
tent, the stove having apparently been brought inside to provide
warmth. The occupants had died during the night, and postmortem
analysis revealed carboxyhemoglobin levels of 50, 63, 69, and 63
percent, respectively. (In the general US population,
carboxyhemoglobin concentrations average 1 percent in nonsmokers
and 4 percent in smokers.)
     2) In the second incident, on 27 March 1999, a 34-year-old
man and his 7-year-old son were found dead inside their zipped up
tent at a group camping site in central Georgia. They were
discovered by other campers just before 9 a.m. A charcoal grill
was found inside the tent, the grill apparently brought inside to
provide warmth after it had been used outside for cooking.
Postmortem carboxyhemoglobin levels in the two campers measured
68 and 76 percent, respectively.
-----------
US Centers for Disease Control and Prevention: Carbon monoxide
poisoning deaths associated with camping -- Georgia, March 1999.
(Morbidity and Mortality Weekly Report 1999 48:705)
(J. Amer. Med. Assoc. 13 Oct 99 282:1326)


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

IN FOCUS: ON THE SEARCH FOR LIFE ON MARS
"There will be people on Mars long before the end of the twenty-
first century. It's inevitable, and irresistible. It might happen
before 2020. It could happen by 2011. Mars is our next frontier.
The plans are being laid now, the missions designed. The
technology exists. The latter-day equivalents of Magellan,
Columbus and Cook, and the other explorers of the age of European
expansion, are preparing themselves. The motives are many, as
always, but central for scientists is the search for life, or
former life. At stake is the issue, are we alone in the
Universe?... The first people on Mars will find the sky pink from
suspended bright red dust. The sunrises and sunsets may be even
more beautiful than ours, though they might resemble those that
follow wildfires in the Australian bush. Just before dawn on some
nights there will be water-ice clouds, high in the sky; these
will dissipate shortly after sunrise. The ground will be dry and
barren, and strewn with grey rocks. Instead of soil there will be
red dust. On many days, light winds will ripple the ground, and
during the northern winter, great storms may develop, enveloping
the planet in dust. In summer, temperatures near the equator will
rise as high as 17 degrees centigrade during the day, but at
night will plummet to minus 80 degrees, or less. Perhaps
somewhere, on the flank of a volcano, or deep in Valles
Marineris. our explorers will find a place where the ground is
damp, maybe in a fresh landslip, in the heat of summer. On that
summer's day, in that Martian spring, they might just find the
microbes that will show that Earth is not the only inhabited
planet. And you and I could still be alive to contemplate that
moment."
-----------
Malcolm Walter: _The Search for Life on Mars_
(Perseus Books, Cambridge MA 1999, p. 1 and 154)
-----------
[Malcolm Walter is at the University of Sydney (AU), and is
associated with the US National Aeronautics and Space Agency
(NASA) program directed to searching for life on Mars.]
-------------------
Related Background:
MARS PATHFINDER MISSION: A SUMMARY
One year has passed since the Mars Pathfinder mission, and
various assessments of the mission have recently appeared. Jim
Bell (Cornell University, US), a member of the Mars Pathfinder
science team, reviews the mission with a particular focus on the
question of whether Pathfinder, as an instance of "faster and
cheaper" solar system exploration, also produced "better"
science. Bell makes the following points: 1) The Pathfinder
mission was a phenomenally successful demonstration of new
technologies and sheer engineering nerve -- a high-stakes gamble
that traded high risk for low cost and hit the jackpot. 2)
"Better" data means higher-quality data, not just more data, and
data that provides more definitive results than previous data.
The mission satisfies this definition of "better", and is viewed
by most scientists involved as a major scientific success in its
own right, and not merely a test flight. 3) The measurements
performed by Pathfinder fall into 5 categories: geology,
mineralogy/geochemistry, surface-material properties, atmospheric
science, planetary rotational dynamics. 4) Geology: Many of the
features observed at the landing site area are similar to
formerly flooded plains on Earth. But other processes involving
wind, volcanism, or impact could be responsible for many of the
observed landforms. A major geologic finding was that wind has
been an extremely important, and perhaps dominant, geologic
process on this part of Mars, and may have been the only major
agent of change for a huge span of time, conceivably several
billion years. 5) Mineralogy and geochemistry: The most striking
result was the finding by the Sojourner Alpha Proton X-Ray
Spectrometer that most of the rocks analyzed have a high silicon
content, higher than that of the Martian meteorite samples
gathered on  Earth. If these Pathfinder site rocks are indeed
volcanic, there constituents suggest a more active subsurface
volcanic system than previously proposed. It was also found that
the dust of Martian dust storms has a high magnetic component,
and the data support the conclusion that the magnetic dust grains
consist at least partly of the iron oxide mineral maghemite. This
mineral is rare on Earth, usually forming in iron-rich water
solutions, which suggests that Martian dust may have formed in a
much wetter and perhaps warmer environment than exists today. 6)
Surface materials: The Pathfinder evidence indicates the
uppermost surface layer of soil at the site is extremely fine-
grained, like flour, consisting of particles only a few microns
in diameter. This top layer was probably formed by the gentle
settling of atmospheric dust. 7) Rotational dynamics: The data
from Pathfinder appear most consistent with Mars possessing a
relatively large metallic core, from 1300 to 2000 kilometers in
radius, which is approximately 40 to 60 percent of the overall
radius of the planet. In contrast, Earth's core occupies only 19
percent of its radius. 8) Atmospheric science: The Martian
atmosphere is complex and dynamic. For example, temperature at
the landing site increased by more than 20 degrees centigrade
after sunrise, and in the morning the air at the surface was more
than 10 degrees centigrade warmer than the air only a meter off
the ground. 9) The mission returned nearly 300 megabytes of data,
including more than 17,000 images. Only a fraction of the data
has yet been analyzed in detail or published in the peer-reviewed
literature. Analysis and interpretations are likely to continue
for some time.
QY: Jim Bell, Cornell University 607-255-2000.
(Sky and Telescope July 1998) (Science-Week 3 Jul 98)


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