M.: Early Mars Climate Models

Abstract. It is often stated that Mars and Earth had similar environmental conditions early in their history and that life might therefore have originated on Mars as well as on Earth. However, the atmospheric conditions required to produce and sustain a warm, wet climate on early Mars remain uncertain. State-of-the-art greenhouse models predict global mean surface temperatures early in Mars' history that differ little from today's, unless special conditions are invoked. The greatest difficulty the models have is coping with a faint early Sun. Introduction In spite of the failure of the Viking landers to detect life of any kind, Mars remains a high-priority target for exobiological investigations. The reason for this is the possibility that Mars and Earth experienced similar climatic conditions early in their history and that because life started on Earth, it might also have started on Mars. Then as the planets evolved, life flourished on Earth but became extinct on Mars. Certainly, the evidence for possible Martian microbes found in ALH84001 has heightened interest in this scenario However, in spite of a great deal of work on the subject, our understanding of early Mars' climate history remains fuzzy. While the evidence for flowing liquid water in Mars' ancient terrains is compelling, a widely accepted mechanism for producing warm, wet conditions through climate change has yet to emerge. The main difficulty is dealing with the faint young Sun and finding a way to generate and sustain an atmosphere capable of producing a significant greenhouse effect. As will become evident, our thinking about how the early Mars atmosphere could have produced a significant greenhouse effect has alternated between views concluding it could have, and those concluding it could not have. It is the classic modus operandi of scientific thinking; theories are proposed, scrutinized, revised, or abandoned altogether. The purpose of this paper is to review the history of this fascinating subject to provide a context for where we presently stand on this issue. The paper draws heavily from related reviews by Cart [1996], Squyres and Kasting [1994], and Fanale et al. [1992] but emphasizes the climate models themselves and the processes they represent. The presentation is chronological, so that the reader can follow how our thinking has evolved with time. Background At the turn of the century the possibility that Mars had liquid water flowing on its surface was popularized in the writings of Percival Lowell, who envisioned vast canal systems constructed by intelligent beings struggling to survive on a dying planet. This romantic view of the red planet was convincingly dispelled This paper is not subject to U.S. copyright. Published in 1998 by the American Geophysical Union. Paper number 98JE01396. in the 1960s by the early Mariner spacecraft, which found no evidence for canals and, more important, near-surface temperatures and pressures that were simply too low for liquid water to be in equilibrium with the atmosphere With Mariner 9, perspectives changed. Evidence for widespread fluvial activity, particularly in the older terrains, was found in images returned from the first Mars orbiter. Later, the Viking orbiters would reveal similar features. Liquid water was (and still is) thought to be the erosive agent that sculpted these features and while some could form under present climatic conditions (outflow channels), others could not (valley networks). Furthermore, the valley networks were found almost exclusively in the highly cratered ancient southern highlands, which date back to the end of the heavy bombardment period some 3.5-3.8 Gyr ago. While the precise mechanism by which the valley networks formed remains debatable (e.g., runoff versus sapping), the logical conclusion was that Mars must have had a warmer and possibly wetter climate very early in its history than it does today. The easiest way to achieve warmer conditions early in Mars' history is through the greenhouse effect. In its simplest form, the greenhouse effect operates when solar radiation penetrates the atmosphere and is absorbed directly at the surface, which then radiates in the infrared. Greenhouse gases in the atmosphere absorb some of this upwelling radiation and reradiate i