Carbonic acid in the gas phase and its astrophysical relevance

In outer space, high-energy irradiation of cryogenic ice mixtures of abundant water and carbon dioxide is expected to form solid carbonic acid. Experiments and thermodynamic analyses show that crystalline carbonic acid sublimates without decomposition. Free-energy considerations based on highly accurate molecular quantum mechanics, in combination with vapor pressures resulting from experimental sublimation rates, suggest that in the gas phase, a monomer and dimer of carbonic acid are in equilibrium, comparable to that of formic acid. Gaseous carbonic acid could be present in comets, on Mars and outer solar system bodies, in interstellar icy grains, and in Earth's upper atmosphere.     

The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets

Determining the source(s) of hydrogen, carbon, and nitrogen accreted by Earth is important for understanding the origins of water and life and for constraining dynamical processes that operated during planet formation. Chondritic meteorites are asteroidal fragments that retain records of the first few million years of solar system history. The deuterium/hydrogen (D/H) values of water in carbonaceous chondrites are distinct from those in comets and Saturn's moon Enceladus, implying that they formed in a different region of the solar system, contrary to predictions of recent dynamical models. The D/H values of water in carbonaceous chondrites also argue against an influx of water ice from the outer solar system, which has been invoked to explain the nonsolar oxygen isotopic composition of the inner solar system. The bulk hydrogen and nitrogen isotopic compositions of CI chondrites suggest that they were the principal source of Earth's volatiles.     

EPOXI at comet Hartley 2

Understanding how comets work--what drives their activity--is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO(2), which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus.     

The Kuiper belt and the solar system's comet disk

Our planetary system is embedded in a small-body disk of asteroids and comets, vestigial remnants of the original planetesimal population that formed the planets. Once formed, those planets dispersed most of the remaining small bodies. Outside of Neptune, this process has left our Kuiper belt and built the Oort cloud, as well as emplacing comets into several other identifiable structures. The orbits in these structures indicate that our outer solar system's comet disk was shaped by a variety of different physical processes, which teach us about how the giant planets formed. Recent work has shown that the scattered disk is the most likely source of short-period comets. Moreover, a growing body of evidence indicates that the sculpting of the Kuiper belt region may have involved large-scale planetary migration, the presence of other rogue planetary objects in the disk, and/or the close passage of other stars in the Sun's birth cluster.     

Whence comets?

Recent advances in cometary science have indicated the importance of mixing of materials in the disk where the planets of our solar system formed. Now, the results from the Stardust Discovery Mission unambiguously show that even more extensive and earlier mixing of the material took place, raising new challenges for theories of the protoplanetary disk and the formation of comets.     

Interstellar chemistry recorded in organic matter from primitive meteorites

Organic matter in extraterrestrial materials has isotopic anomalies in hydrogen and nitrogen that suggest an origin in the presolar molecular cloud or perhaps in the protoplanetary disk. Interplanetary dust particles are generally regarded as the most primitive solar system matter available, in part because until recently they exhibited the most extreme isotope anomalies. However, we show that hydrogen and nitrogen isotopic compositions in carbonaceous chondrite organic matter reach and even exceed those found in interplanetary dust particles. Hence, both meteorites (originating from the asteroid belt) and interplanetary dust particles (possibly from comets) preserve primitive organics that were a component of the original building blocks of the solar system.     

X-ray emission from comets

The discovery of x-ray emission from comet Hyakutake was surprising given that comets are known to be cold. Observations by x-ray satellites such as the R?ntgen Satellite (ROSAT) indicate that x-rays are produced by almost all comets. Theoretical and observational work has demonstrated that charge-exchange collisions of highly charged solar wind ions with cometary neutral species can explain this emission. X-ray observations of comets and other solar system objects may be used to determine the structure and dynamics of the solar wind.     

The mass disruption of Oort cloud comets

We have calculated the number of dormant, nearly isotropic Oort cloud comets in the solar system by (i) combining orbital distribution models with statistical models of dormant comet discoveries by well-defined surveys and (ii) comparing the model results to observations of a population of dormant comets. Dynamical models that assume that comets are not destroyed predict that we should have discovered approximately 100 times more dormant nearly isotropic comets than are actually seen. Thus, as comets evolve inward from the Oort cloud, the majority of them must physically disrupt.     

Oxygen isotope composition of subglacially precipitated calcite: possible paleoclimatic implications

Isotopic analyses of subglacially precipitated calcite from near a modern temperate glacier show that the delta(18)O (= (18)O/(16)O relative to standard mean ocean water) of the calcite records the delta(18)O of the ice from that glacier. It may therefore be possible to determine the delta(18)O of Pleistocene ice sheets on the basis of isotopic analyses of calcite formed under that ancient ice. This, in turn, would allow estimation of the delta(18)O of Pleistocene oceans and correction of the paleotemperature scale based on foraminiferal oxygen isotopic analyses.     

Water vapor from a lunar breccia: implications for evolving planetary atmospheres

The exposure of a typical complex lunar breccia to hydrogen after a thorough outgassing produces a fully reduced surface state. Subsequent outgassing over a wide temperature range results in the production of water vapor formed from the chemisorbed hydrogen and oxygen from the lunar sample; the proposed mechanism has been confirmed in terms of the chemisorption of deuterium and the release of heavy water. Since the conditions of the experiments are consistent with those on the lunar surface, it is postulated that water vapor will be produced on the moon through the interaction of the solar wind with lunar soil. It is also proposed that such a process could play an important role in the early history of many planets where an oxygen-rich soil is exposed to a reducing atmosphere.     

ESSAYS ON SCIENCE AND SOCIETY: Of Comets and Meteors

Fred L. Whipple was born in Iowa in 1906. During his career as an astronomer, he discovered six comets and advanced the study of comets and meteors. He has encouraged amateur astronomers and advised many U.S. governmental and scientific agencies. He deduced that comets consist of ice and dust from observing that their orbits and timing change slightly through the gas vaporization from the ices. In this essay, he recounts highlights of his career, including developing radar-disrupting chaff and the promise of the global positioning system.     

Detection of water in the LCROSS ejecta plume

Several remote observations have indicated that water ice may be presented in permanently shadowed craters of the Moon. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to provide direct evidence. On 9 October 2009, a spent Centaur rocket struck the persistently shadowed region within the lunar south pole crater Cabeus, ejecting debris, dust, and vapor. This material was observed by a second "shepherding" spacecraft, which carried nine instruments, including cameras, spectrometers, and a radiometer. Near-infrared absorbance attributed to water vapor and ice and ultraviolet emissions attributable to hydroxyl radicals support the presence of water in the debris. The maximum total water vapor and water ice within the instrument field of view was 155 ± 12 kilograms. Given the estimated total excavated mass of regolith that reached sunlight, and hence was observable, the concentration of water ice in the regolith at the LCROSS impact site is estimated to be 5.6 ± 2.9% by mass. In addition to water, spectral bands of a number of other volatile compounds were observed, including light hydrocarbons, sulfur-bearing species, and carbon dioxide.     

Venus: new microwave measurements show no atmospheric water vapor

Two sets of passive radio observations of Venus-measurements of the spectrum of the disk temperature near the 1-centimeter wavelength, and interferometric measurements of the planetary limb darkening at the 1.35-centimeter water vapor resonance-show no evidence of water vapor in the lower atmosphere of Venus. The upper limit of 2 x 10(-3) for the mixing ratio of water vapor is substantially less than the amounts derived from the Venera space probes (0.5 x 10(-2) to 2.5 x 10(-2)). This amount of water vapor cannot produce dense clouds, and it is doubtful that it may contribute significantly to a greenhouse effect.     

Comet yanaka (1988r): a new class of carbon-poor comet

As part of a program to determine the chemical composition of a sample population of comets, a very unusual comet, Yanaka (1988r), was observed in January 1989. Although the comet showed the usual emissions of Ol and NH(2), it did not display any hint of C(2) or CN emission. The comet is depleted in C(2) by at least a factor of 100 and in CN by a factor of 25 relative to typical comets. If comets originate from interstellar clouds, Yanaka (1988r) could be an interloper from a cloud of different composition. If Yanaka (1988r) was formed within our solar system, the solar nebula was less uniform than assumed by most present models of formation.     

Mars: occurrence of liquid water

In the absence of juvenile liquid water, condensation of water vapor to ice and subsequent melting of ice are the only means of producing liquid water on the martian surface. However, the evaporation rate is so high that the available heat sources cannot melt pure ice. Liquid water is therefore limited to concentrated solutions of strongly deliquescent salts.     

Organic composition of C/1999 S4 (LINEAR): a comet formed near Jupiter?

In the current paradigm, Oort cloud comets formed in the giant planets' region of the solar nebula, where temperatures and other conditions varied greatly. The measured compositions of four such comets (Halley, Hyakutake, Hale-Bopp, and Lee) are consistent with formation from interstellar ices in the cold nebular region beyond Uranus. The composition of comet C/1999 S4 (LINEAR) differs greatly, which suggests that its ices condensed from processed nebular gas, probably in the Jupiter-Saturn region. Its unusual organic composition may require reevaluation of the prebiotic organic material delivered to the young Earth by comets.     

Comparison of comet 81P/Wild 2 dust with interplanetary dust from comets

The Stardust mission returned the first sample of a known outer solar system body, comet 81P/Wild 2, to Earth. The sample was expected to resemble chondritic porous interplanetary dust particles because many, and possibly all, such particles are derived from comets. Here, we report that the most abundant and most recognizable silicate materials in chondritic porous interplanetary dust particles appear to be absent from the returned sample, indicating that indigenous outer nebula material is probably rare in 81P/Wild 2. Instead, the sample resembles chondritic meteorites from the asteroid belt, composed mostly of inner solar nebula materials. This surprising finding emphasizes the petrogenetic continuum between comets and asteroids and elevates the astrophysical importance of stratospheric chondritic porous interplanetary dust particles as a precious source of the most cosmically primitive astromaterials.     

The role of the tropical super greenhouse effect in heating the ocean surface

Measurements made by a Fourier transform infrared (FTIR) spectroradiometer operating in the middle infrared (5 to 20 micrometers, with a spectral resolution of one inverse centimeter) imply that there is an anomalously large greenhouse effect over equatorial oceans that is caused by water vapor. As sea-surface temperature increased from 297 to 303 degrees kelvin, the net infrared cooling at the surface decreased by 30 to 50 watts per square meter. Thus, according to the FTIR data, the super greenhouse effect that had been inferred from satellite measurements contributes directly to radiative heating of the sea surface. The data demonstrate that most of this heating occurs in the middle infrared by means of the continuum emission window of water vapor and that tropical deep convection contributes substantially to this super greenhouse effect.     

A microphysical connection among biomass burning, cumulus clouds, and stratospheric moisture

A likely causal chain is established here that connects humidity in the stratosphere, relative humidity near the tropical tropopause, ice crystal size in towering cumulus clouds, and aerosols associated with tropical biomass burning. The connections are revealed in satellite-observed fluctuations of each quantity on monthly to yearly time scales. More aerosols lead to smaller ice crystals and more water vapor entering the stratosphere. The connections are consistent with physical reasoning, probably hold on longer time scales, and may help to explain why stratospheric water vapor appears to have been increasing for the past five decades.     

From stars to dust: looking into a circumstellar disk through chondritic meteorites

One of the most fundamental questions in planetary science is, How did the solar system form? In this special issue, astronomical observations and theories constraining circumstellar disks, their lifetimes, and the formation of planetary to subplanetary objects are reviewed. At present, it is difficult to observe what is happening within disks and to determine if another disk environment is comparable to the early solar system disk environment (called the protoplanetary disk). Fortunately, we have chondritic meteorites, which provide a record of the processes that operated and materials present within the protoplanetary disk.