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Project Progress

Recent simulations of planetary accretion by J. Chambers and collaborators have shown that the amount of mass accreted by Earth from the asteroid region depends sensitively on the formation time and the early orbital evolution of the giant planets. In particular, the mass of volatile materials delivered to Earth depends sensitively on the giant planets’ orbital eccentricities, since these control the width and strength of the unstable resonances in the asteroid region. Most of the planets recently discovered in orbit around other Sun-like stars have orbits that are substantially more eccentric than those of Jupiter and Saturn. Therefore, even if these stars developed protoplanetary disks with similar masses and thermal histories to the Sun’s nebula, and even if terrestrial planets formed in these disks, the volatile inventories of these planets will differ widely depending on the characteristics of the giant planets in these systems.

J. Kasting and graduate student H. Justh have completed a study of the effects of methane on early Mars’ climate. It is found that methane mixing ratios of 10^-3 (0.1% by volume) could have raised the surface temperature of early, 3.5-Ga Mars by 30 K (from 220 K to ~250 K). This is not by itself enough to explain the existence of fluvial features on Mars’ surface, but it could have been a contributing factor. Greenhouse warming by CO₂ ice clouds could have provided the additional warming needed to bring global surface temperatures up to near the freezing point of water. Surprisingly, CH₄ mixing ratios higher than 10^-3 would have cooled the surface, rather than warming it. This is because CH₄ absorbs visible and near-infrared (IR) radiation from the sun; thus, high CH₄ concentrations lead to stratospheric warming and an anti-greenhouse effect. A CH₄ mixing ratio of 10^-3 could probably not have been sustained abiotically. Thus, life must have been present on the Martian surface if this explanation for surface warming is correct.

During the previous year the group led by Brian Toon and Margaret Tolbert at the University of Colorado has made progress in a number of areas. Through a combination of laboratory studies and numerical modeling, they were able to show that carbon dioxide clouds are unlikely to have provided a significant greenhouse warming in the early history of Mars. Therefore, the greenhouse model to explain the ancient river valleys on mars is seriously flawed. They were also able to use the lab data and a model to explain observed clouds on current day Mars. Building on this work they also suggested that the Martian rivers were not formed under a more equitable climate, but rather are the results of impacts early in Mars’ history. This finding means that there may never have been an extended period with warm temperatures and flowing water on Mars. Instead there may have been high temperature bursts followed by decades or centuries of rainfall interspersed with millions of years of cold temperatures such as those currently found on Mars. This is a far different environment than envisioned previously for early Mars and one that is not conducive to the origin of life at the surface.

Further work in progress has shown what types of particles may have formed in the early terrestrial atmosphere {Trainer, MG et al., Astrobiology yearly meeting 2003, and 2002 A. G. U. meeting.}. The group at UC is presently writing a paper on the detailed chemistry of these particles, which is quite interesting from the point of view of sustaining atmospheres with large amounts of methane in them.