Origin and Evolution of Outer Solar System Atmospheres

Constraints from Comets on the Formation and Volatile Acquisition of the Planets and Satellites

Comets play a dual role in understanding the formation and evolution of the solar system. First, the composition of comets provides information about the origin of the giant planets and their moons because comets formed early and their composition is not expected to have evolved significantly since formation. They, therefore serve as a record of conditions during the early stages of solar system formation. Once comets had formed, their orbits were perturbed allowing them to travel into the inner solar system and impact the planets. In this way they contributed to the volatile inventory of planetary atmospheres. We review here how knowledge of comet composition up to the time of the Rosetta mission has contributed to understanding the formation processes of the giant planets, their moons and small icy bodies in the solar system. We also discuss how comets contributed to the volatile inventories of the giant and terrestrial planets.

Acetylene as fast food: implications for development of life on anoxic primordial Earth and in the outer solar system

Acetylene occurs, by photolysis of methane, in the atmospheres of jovian planets and Titan. In contrast, acetylene is only a trace component of Earth's current atmosphere. Nonetheless, a methane-rich atmosphere has been hypothesized for early Earth; this atmosphere would also have been rich in acetylene. This poses a paradox, because acetylene is a potent inhibitor of many key anaerobic microbial processes, including methanogenesis, anaerobic methane oxidation, nitrogen fixation, and hydrogen oxidation. Fermentation of acetylene was discovered approximately 25 years ago, and Pelobacter acetylenicus was shown to grow on acetylene by virtue of acetylene hydratase, which results in the formation of acetaldehyde. Acetaldehyde subsequently dismutates to ethanol and acetate (plus some hydrogen). However, acetylene hydratase is specific for acetylene and does not react with any analogous compounds. We hypothesize that microbes with acetylene hydratase played a key role in the evolution of Earth's early biosphere by exploiting an available source of carbon from the atmosphere and in so doing formed protective niches that allowed for other microbial processes to flourish. Furthermore, the presence of acetylene in the atmosphere of a planet or planetoid could possibly represent evidence for an extraterrestrial anaerobic ecosystem.

Absolute Intensities and Pressure-Broadening Coefficients of 2-mum CO(2) Absorption Features: Intracavity Laser Spectroscopy

The high detection sensitivity available from intracavity laser spectroscopy (ILS) is extended into the near infrared by solid-state laser systems operating with relatively narrow (~0.002 mum) bandwidths for three CO(2) absorption features of importance to an understanding of planetary atmospheres. The absolute intensities and pressure-broadening properties of the P(12), P(14), and P(16) lines of the ?-? band (12 degrees 1-00 degrees 0) of CO(2) (at 2.0129, 2.0136, and 2.0143 mum) are measured quantitatively by ILS with a Tm:YAG laser operating near 2.0 mum. The temperature dependencies of these absolute intensities and collisional-broadening parameters for these three CO(2) features are also measured over the 110-300 K range. The 3.0-km equivalent absorption path length available from the ILS Tm:YAG system is used to enhance detection sensitivity by more than a factor of 1.5 x 10(4) while maintaining a physical sample cell path length of ~20 cm. The enhanced detection sensitivity of ILS permits absolute intensities and collisional-broadening parameters to be measured from <1-Torr CO(2) over a series of temperatures, conditions that emulate those found in the atmospheres of Mars, Triton, and Venus.

Exobiology, the study of the origin, evolution and distribution of life within the context of cosmic evolution: a review

The primary goal of exobiological research is to reach a better understanding of the processes leading to the origin, evolution and distribution of life on Earth or elsewhere in the universe. In this endeavour, scientists from a wide variety of disciplines are involved, such as astronomy, planetary research, organic chemistry, palaeontology and the various subdisciplines of biology including microbial ecology and molecular biology. Space technology plays an important part by offering the opportunity for exploring our solar system, for collecting extraterrestrial samples, and for utilizing the peculiar environment of space as a tool. Exobiological activities include comparison of the overall pattern of chemical evolution of potential precursors of life, in the interstellar medium, and on the planets and small bodies of our solar system; tracing the history of life on Earth back to its roots; deciphering the environments of the planets in our solar system and of their satellites, throughout their history, with regard to their habitability; searching for other planetary systems in our Galaxy and for signals of extraterrestrial civilizations; testing the impact of space environment on survivability of resistant life forms. This evolutionary approach towards understanding the phenomenon of life in the context of cosmic evolution may eventually contribute to a better understanding of the processes regulating the interactions of life with its environment on Earth.

Radar Reflectivity of Titan

The present understanding of the atmosphere and surface conditions on Saturn's largest moon, Titan, including the stability of methane, and an application of thermodynamics leads to a strong prediction of liquid hydrocarbons in an ethane-methane mixture on the surface. Such a surface would have nearly unique microwave reflection properties due to the low dielectric constant. Attempts were made to obtain reflections at a wavelength of 3.5 centimeters by means of a 70-meter antenna in California as the transmitter and the Very Large Array in New Mexico as the receiving instrument. Statistically significant echoes were obtained that show Titan is not covered with a deep, global ocean of ethane, as previously thought. The experiment yielded radar cross sections normalized by the Titan disk of 0.38 +/- 0.15, 0.78 +/- 0.15, and 0.25 +/- 0.15 on three consecutive nights during which the sub-Earth longitude on Titan moved 50 degrees. The result for the combined data for the entire experiment is 0.35 +/- 0.08. The cross sections are very high, most consistent with those of the Galilean satellites; no evidence of the putative liquid ethane was seen in the reflection data. A global ocean as shallow as about 200 meters would have exhibited reflectivities smaller by an order of magnitude, and below the detection limit of the experiment. The measured emissivity at similar wavelengths of about 0.9 is somewhat inconsistent with the high reflectivity.