The discovery of ancient riverbeds on Mars, and the possibility of ancient oceans, raises exciting questions about Mars and our own planet. For rivers and seas to exist, the climate must have been different in the past. Why did the climate change? What was the ancient Martian climate like? Was Mars habitable and did it in fact host biology? Could the same thing happen to our planet? These are compelling question to address with data from the current set of orbiters and rovers.
Mars Global Climate Zones, based on temperature, modified by topography, albedo, actual solar radiation. Click here for original source URL
The Martian surface now is cold and dry — a frozen desert. The temperature of the thin air ranges from -123° F (187 K) at night to -20° F (244 K) on a typical afternoon. But the red soil is warmer, because it absorbs sunlight, which mostly passes through the atmosphere without being absorbed by the thin air. Soil temperatures can get well above freezing on a typical summer afternoon. Liquid water cannot exist on present day Mars. The air pressure today is so low in most areas that liquid water is not stable. The Martian atmosphere is so thin that the air pressure on the surface is only about 0.7% of that on Earth at sea level. A pan of liquid water would either evaporate immediately (or actually boil away at low temperature, just as it would do at 100,000 feet above Earth), or freeze. If substantial amounts of liquid water existed in the past on Mars, it probably means that the air pressure was greater at that time, and the temperatures may have been warmer.
No one knows why the Martian climate changed. The number of impact craters superimposed on the riverbeds implies that the riverbeds are probably a billion years old or more. The atmosphere today is extremely thin, composed of mostly carbon dioxide (95% CO2 by volume). But chemical measurements suggest that during the early history of Mars, volcanoes probably emitted much more CO2 than is present today, enough to make an atmospheric pressure as much as 30% or more of ours, rather than the present-day value of only 0.7%. Thus, Mars may have had a denser atmosphere early in its history, which was slowly lost as gases leaked into space due to the low gravity of the small planet. Unfortunately, the story of a warmer and wetter Mars in the past doesn’t hang together yet. Even with a thicker atmosphere, Mars has less surface heating that the Earth. Climate models are unable to produce an average temperature above 0° C at any point in Mars history. It’s not clear if this is just a limitation of the models, a problem of insufficient data, or a flaw in the hypothesis.
Scientists have proposed several theories to account for climate change on Mars. Some theories suggest that the "wet period" may have been transient and minor. Geothermal activity or thermal energy from impacts might have melted local ice on Mars and created short-lived rivers and lakes. Other theories, which are based on analysis of the planet’s orbital motion, note that the tilt of Mars’ polar axis is quite variable. Under some conditions, the tilt reaches as much as 46° or more. Perhaps during brief ancient periods, when the pole was tilted farthest toward the Sun, the polar ice caps melted completely during summer. This melt-off would have released large amounts of water onto the surface and carbon dioxide into the air, possibly creating rivers and a thicker, warmer atmosphere. A moderate greenhouse effect caused by the CO2 may have increased temperatures, and perhaps prolonged liquid water flow. A solution to the mystery of the ancient Martian rivers and the ancient climate will probably require future missions to Mars, and it could tell us a great deal about planetary climate evolution.
For centuries, people believed that a planet’s environment is unchanging. For instance, Shakespeare wrote, “All the world's a stage, and all the men and women merely players”. The Bard was referring to the Earth as a forever-fixed backdrop. We learned from planetary science that a planet's climate is not constant over the history of the solar system. The primitive Earth was so hot that it was shrouded by steam. Oceans could not condense until the planet slowly cooled. Another drastic global change in the Earth’s climate occurred when primitive life evolved and put large amounts of oxygen into the atmosphere. We also know that the Earth's climate has changed during ice ages, and more recently, under the influence of human technology. Mars’ climate has changed even more dramatically. Perhaps planets are not such stable backdrops to life as Shakespeare thought — the planet itself may be an active player in the story.