ScienceWeek

ASTRONOMY: ON THE NUCLEI OF COMETS

The following points are made by Harold A. Weaver (Science 2004 304:1760):

1) New in situ observations of a comet are demonstrating once again how little we understand about these dark and mysterious planetesimals. Just when a consensus was being reached that cometary nuclei are gravity-dominated "rubble piles" (1), stunning images of the nucleus of comet Wild 2 (pronounced "vilt 2") taken by the camera onboard NASA's Stardust spacecraft (2) are challenging that paradigm. The analysis of the many jets detected in the images (3) and results derived from the Stardust instruments that sampled dust in the coma of Wild 2 (4,5) are also providing valuable new insights into the nature of comets.

2) Brownlee et al (2) have presented evidence that at least some of the depressions on the surface of Wild 2 are indeed craters. They identify two different types: "pit-halo" craters, which have a rounded central pit surrounded by an irregular region of excavated material, and "flat-floor" craters, which do not have halos and are bounded by steep cliffs. Comparisons with laboratory cratering experiments suggest that the pit-halo features are consistent with impact into a homogeneous, cohesive, brittle material in a microgravity environment. The flat-floor features resemble craters produced during experiments on weakly cohesive and porous silicate targets. In both cases, the implication is that the nucleus of Wild 2 has substantial strength and that gravity plays little role in the shaping of the features, which is contrary to the conventional wisdom that cometary nuclei are gravity-dominated rubble piles (1). The presence of nearly vertical ( more than 70 deg) cliffs making sharp contact with their floors provides further evidence that Wild 2's nucleus has substantial internal cohesiveness.

3) Undoubtedly, these results will spark renewed debate within the planetary science community about the physical structure of minor bodies in the Solar System. The rubble pile proponents can still point to the tidal disruption of comet Shoemaker-Levy 9 during its close approach to Jupiter in 1993, and to the frequent and apparently spontaneous disruptions of many other cometary nuclei, as strong circumstantial evidence that at least some cometary nuclei are weak agglomerations of smaller units held together only by gravity. Perhaps one lesson to be learned from the Stardust encounter with Wild 2 is that different cometary nuclei may have different physical structures, just as some asteroids may be rubble piles while others are probably not.

References (abridged):

1. P. R. Weissman, E. Asphaug, S. C. Lowry, in Comets II, M. Festou, H. U. Keller, H. A. Weaver, Eds. (Univ. of Arizona Press, Tucson, AZ, in press)

2. D. E. Brownlee et al., Science 304, 1764 (2004)

3. Z. Sekanina et al., Science 304, 1769 (2004)

4. A. J. Tuzzolino et al., Science 304, 1776 (2004)

5. J. Kissel, F. R. Krueger, J. Silun, B. C. Clark, Science 304, 1774 (2004)

Science http://www.sciencemag.org

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Related Material:

ASTROPHYSICS: ON COMETS AND THE OORT CLOUD

The following points are made by S. Alan Stern (Nature 2003 424:639):

1) Comets are small bodies with characteristic sizes of 1 to 15 kilometers that orbit the Sun. They are usually detected as they approach the Sun because their near-surface volatiles sublimate under the increasing insolation, in turn generating an extensive, highly visible gas-and-dust atmosphere, called the "coma". Because of the small size of the solid nucleus of the coma, a comet's gravity is too weak to retain these constituents, so the coma expands to great distances and is lost to space.

2) As first recognized decades ago, the cometary nucleus is the source of the escaping gas and dust that make up both the coma, and its extension, called the "tail". Strong circumstantial evidence, based on the ease with which comets split and fragment, points to the inherent mechanical weakness of cometary nuclei; in fact, many comets may essentially be strengthless, gravitationally bound "piles of rubble".

3) Comets consist of approximately equal proportions of nonvolatile solids (silicates, refractory organics) and volatile ices. Cometary ices are dominated by water ice, but CO2 and CO are also present at significant levels (in extreme cases having combined abundances as high as 15-20% that of the water ice). Other volatiles, notably including H2S, CH3OH, H2CO, NH3, HCN, CH4 and S2, have also been detected in the atmospheres of comets. The presence of such a wide array of high-volatility species strongly suggest that comets (1) originated in the cool, outer regions of the Sun's protoplanetary nebula and (2) have ever since been stored only in cold conditions.

4) Most comets have very long-period orbits that extend from thousands to several tens of thousands of astronomical units (AU) from the Sun. This, and the related observation that the orbits of such comets are nearly isotropically oriented relative to the plane of the Solar System, caused Jan H. Oort (1900-1992) to conclude in 1950 that such comets must be derived from an essentially spherical reservoir surrounding the Sun at these very great distances. This reservoir is now known as the "Oort cloud". Modern estimates place the number of Oort cloud comets in the range 10^(11) to perhaps 5 x 10^(12), corresponding to a total Oort cloud mass of order 1 M (where M is the mass of the Earth) to perhaps 50 M (depending also upon the presently ill-determined typical masses of cometary nuclei). Models of Solar System and Oort cloud formation have repeatedly shown that the formation of an Oort cloud is a natural by-product of the clearing and ejection of debris from the giant planet region some 3.5 to 4.5 Gyr ago.

5) In summary: Comets are remnants from the time when the outer planets formed, 4 to 4.5 billion years ago. They have been in storage since then in the Oort cloud and Kuiper belt-distant regions that are so cold and sparsely populated that it was long thought that comets approaching the Sun were pristine samples from the time of Solar System formation. It is now recognized, however, that a variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine.

Nature http://www.nature.com/nature

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ON THE ORIGIN OF COMETS

Notes by ScienceWeek:

Composed of ice and dust, comets are relatively small objects in orbit around the Sun. They are believed to exist in large numbers in regions beyond the planets (the *Oort Cloud and the *Kuiper Belt), where they are perturbed by the gravitational influence of passing stars into new orbits that bring them into the inner Solar System. When a comet is far from the Sun, its nucleus is frozen solid and shines only by reflected light; as the nucleus nears the Sun, its temperature increases and it releases gas and dust.

Like certain meteorites, comets are apparently vestiges of the origin of the Solar System, with comets believed to be icy *planetesimal remainders from the formation of the outer planets. The total population of the Oort Cloud and Kuiper belt may be 10^(12) objects, with a combined mass greater than the Earth. The main component of cometary ice is apparently frozen water, plus some methane, carbon monoxide, and carbon dioxide. Also detected in comets are formaldehyde, hydrogen cyanide, and methyl cyanide. All of these molecules, detected by spectroscopy, are also found in interstellar nebulae similar to the original "solar nebula" (see below) from which the Sun was formed.

The current consensus view of the origin of the Solar System proposes that its formation began with the gravitational collapse of part of an interstellar cloud of gas and dust, the cloud with an initial mass only 10 to 20 percent larger than the present mass of the Sun and approximately spherical in shape. As the cloud revolved about the Galactic center, its collapse caused it to rotate, the speed of rotation increasing as the cloud contracted, the increase in accordance with the conservation of angular momentum. As the cloud contracted, it flattened to form a disk around a central condensation, this configuration called the "solar nebula". As gas and dust were pulled in toward the central condensation, potential energy was converted to kinetic energy and the temperature of the material rose until ultimately the temperature became great enough in the interior of the condensation for nuclear reactions to begin and give birth to a star -- our Sun. Meanwhile, the material of the rotating disk collided, coalesced, and gradually formed larger and larger objects by accretion.

A chondrule is a small rounded particle embedded in certain meteorites (chondrites). Chondrules are usually approximately 1 millimeter or less in diameter and consist for the most part of the silicate minerals olivine and pyroxene. From textural and chemical relationships, it is apparent that chondrules were formed at high temperatures as dispersed molten droplets, which subsequently solidified and aggregated into chondrite masses. This process evidently occurred in the solar nebula before the accretion of the planets. However, how the chondrules were melted is not understood. It is believed that dust particles already in existence were melted by high-energy events such as high-velocity collisions, the melts splashed about as droplets that quickly cooled and crystallized. In general, it is believed that the formation of chondrules required temperatures of approximately 1500 kelvins.

The following points are made by Joseph A. Nuth III (American Scientist 2001 89:228):

1) The author points out that comets are apparently the most primitive bodies in the Solar System, with some of the material inside a comet believed to be preserved in nearly the same state the material was in when the Solar System was just forming and before the Sun and the planets were fully constituted. The author suggests each comet is effectively a "grab bag" sample of the building blocks present in the solar nebula at the time the comet was formed, approximately 4.5 billion years ago. Although the nebular material may have undergone considerable processing before incorporation into the comet, very little has been altered since.

2) The author points out that at a basic level, a comet is simply a collection of silicate dust and a smattering of organic molecules coated with ice made primarily of water. Some of the ice-coated grains may have been present in the giant *molecular cloud that partly collapsed to form the solar nebula, but other ice-coated grains must have formed in the solar nebula itself. In general, it is believed that comets begin to form by an accreting "snowball" effect in which the icy dust grains stick together to form fractal-like aggregates. According to current models, this process begins at some considerable distance from the center of the solar nebula, perhaps as far as 100 astronomical units (AU) away from the center. (As an indication of scale, Pluto is 40 AU from the Sun; the distance from the Earth to the Sun is 1 AU) At this stage, the movements of the dust grains and small aggregates are coupled to the movements of the ambient nebular gas. Over time, however, as the aggregates accumulate into compact boulder-sized snowballs (cometesimals), they are slowed down by drag in the ambient gas, and they begin to drift inward as their orbits decay. As the cometesimals fall closer to the center of the solar nebula, they continue to grow by the accretion of dust and ice grains, as well as by merging with other aggregates in their path. In due course (10,000 to 100,000 years), this pile of rubble becomes a comet, perhaps 10 to 20 kilometers in diameter, the comet containing a collection of materials from a wide swath of its orbital radius.

3) The author suggests that strong convection ("winds") in the solar nebula may have been responsible for the mixing of "hot" and "cold" components found in both meteorites and comets. Meteorites contain calcium-aluminum-rich inclusions (formed at approximately 2000 kelvins) and chondrules (formed at approximately 1650 kelvins), which were created near the proto-Sun and then blown several astronomical units away into the asteroid region between Mars and Jupiter, where they were embedded in a matrix of temperature-sensitive carbon-based "cold" components. The "hot" component in comets -- tiny grains of annealed silicate dust (olivine) -- would be vaporized at approximately 1600 kelvins, suggesting that this component never reached the innermost region of the accretion disk before it was transported out beyond the orbit of Pluto, where it was mixed with ices and unheated silicate dust ("cold" components). Vigorous convection in the accretion disk may have contributed to the transport of materials.

4) In summary, the view of the author is that comets are not simply collections of unaltered presolar grains and ices formed in the precollapse molecular cloud, but are instead aggregates of materials representative of the building blocks then present in the nebula at the time of their accretion.

American Scientist http://www.americanscientist.org

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Notes by ScienceWeek:

Oort Cloud: The Oort cloud is an apparent spherical shell of comets 10,000 to 100,000 astronomical units (AU) from the Sun and the proposed source of comets that orbit the Sun. The cloud is at the extreme edge of the Sun's influence, halfway to the nearest star, and it is believed that when the cloud is perturbed by passing stars, comets may be sent into a solar orbit. The size and structure of the Oort cloud have been deduced from statistical studies of the orbits of comets; there is no direct evidence for the cloud's existence. Approximately 900 comets are known. The cloud is named after Jan Hendrik Oort (1900-1992). Oort first proposed the existence of the cloud in 1950. In 1927, Oort calculated the mass and size of the Galaxy, and the distance of the Sun from its center, from the observed movements of the stars around the center.

Kuiper Belt: In 1951 the astronomer Gerard P. Kuiper (1905-1973) postulated the existence of a belt of objects beyond the orbit of Pluto. Both the existence and nature of the objects were matters of speculation for decades, until finally in 1992 Jewitt and Luu identified the first Kuiper object. The current estimate is that as many as 10^(8) objects larger than 10 kilometers in diameter may exist in what is called the "Kuiper belt", a disc that hugs the plane of the planetary system and lies between 35 and 1000 *AU from the Sun. Observations to date have yielded some 55 trans-Neptune bodies with radii on the order of 100 km or larger, and Pluto is considered by some astronomers to be a member of this population.

planetesimal: 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.

molecular cloud: In this context, the term "molecular cloud" refers to a cool and dense region of interstellar matter within which atoms tend to be combined into molecules. Such clouds are composed principally of molecular hydrogen, with between 300 to 2000 molecules per cubic centimeter. Such clouds also contain an admixture of "cosmic dust" comprising approximately 1 percent of the mass, with gas temperatures between 10 and 20 degrees kelvin.

ScienceWeek http://scienceweek.com