Despite previous studies to the contrary, Mars may have been cold and icy, not warm and wet. A new attempt to model conditions on the Red Planet 3-4 billion years ago favors a planet far too cold to support widespread surface water.
Mars presents planetary scientists with a major puzzle. Today, it is too cold for liquid surface water, at least in any sustained fashion. Everything we know about stellar evolution indicates that the sun should have been putting out 25% less heat early in its life than it does nowadays. Consequently, Carl Sagan argued, Mars should have been even colder back then.
Despite this, we keep finding signs that the planet once had oceans. Indeed, the time 4.1-3.7 billion years ago has been dubbed the Noachian Era, a reference to Noah's flood. Attempts have been made to explain the contradiction through a major greenhouse effect, but some problematic assumptions are required to make it work.
Thus, Harvard's Dr. Robin Wordsworth looked at the problem from a new angle, creating 3D atmospheric circulation models of how water would behave on Mars if the average temperature was a warm 10°C (50°F) or a cold -48°C (-54°F). The average temperature on Mars today is a colder -63°C (-81°F).
Using our estimates for sunlight and Martian orbital tilt at the time, the latter model looks a lot more likely. In the Journal of Geophysical Research – Planets, Wordsworth concludes that the cold model also better explains the erosion features seen on Mars.
Wordsworth argues that during that era, Mars lay almost on its side—similar to Uranus today—with its polar axis pointed towards the sun. Consequently, the ice caps lay at the equator, not the poles. This would explain the drainage channels seen at the equator, which were formed when volcanoes or meteorite strikes melted the ice, or possibly even by brine during the summer.
The cold scenario allows for a thicker atmosphere, warming northern low-lying regions of Mars a little and increasing the temperature gradient with altitude. Since Mars' equatorial regions are much higher than its northern polar latitudes, it would be permanently cold at the equator, while the north pole would get to experience a limited summer.
While Wordsworth was also able to come up with a scenario that gave a warm and wet outcome, he had to force the model on several accounts to explain the heat and still got something that doesn't fit observations. Rainfall was heaviest in places like the Hellas Basin, Wordsworth concluded, parts of which show few drainage features. Meanwhile, the Margaritifer Sinus, considered an example of a water-carved valley, got almost no rain at all. Similarly, Wordsworth found there to be more rain on the western side of the volcanic plateau known as the bulge of Tharsis, yet the erosion channels we see are to the east, which he thinks would have been a rain shadow.
"There is lots of work to be done,” says Wordsworth. “But our results show that the cold/icy scenario matches the surface distribution of erosion features more closely. This strongly suggests that early Mars was generally cold, and water was supplied to the highland regions as snow, not as rain."