Mars was wet once, but wet with what? Mars was a planet with lakes and rivers. Satellites map washes and basins left behind when the planet dried up. There are deltas where river dumped into lakes. Robots find rocks rounded by eons of tumbling down rivers and streams. Early Mars was a wet and wild place. Curiosity is descending into a dry lakebed to take a closer look at the remains of that time long ago.
What liquid flowed across the Martian plains? Mars has never been warm enough for liquid water. It could never have been an Earth-like ocean planet. Conditions on the cold moons of Jupiter and Saturn are more representative of what early Mars must have been like.
Saturn’s giant moon Titan has an ecosphere based on methane rather than on water. Its rivers and lakes are filled with liquefied natural gas. It rains methane on Titan. Mars was probably cold enough for methane oceans once. The early sun was not as hot as it is today. Venus may have had liquid water then, and the Earth might have been an ice planet. The colder Mars could have been hospitable to liquid methane. As the sun warmed over billions of years, the Goldilocks zone-where temperatures are right for liquid water-expanded. Venus became Hell, and Earth grew into the Garden of Eden. The Goldilocks zone for methane expanded along with the one for water. Mars became too warm for liquid methane and its seas evaporated.
Alternatively, Mars might have hosted liquid water under a thick mantle of ice. The moons Enceladus and Europa store vast quantities of liquid water under miles of ice. This configuration is surprisingly common throughout the solar system. Data from the New Horizons probe suggest it may occur even on the distant dwarf planet Pluto. Closer to home, much of Earth’s fresh water lies in Antarctica under miles of ice. It’s there in rivers and lakes. The water there is moving like the flows on Mars might have at one time. Could Mars have hosted liquid water under a mantle of ice? Where did all that water go?
We know Mars had rivers and lakes. We don’t know what flowed in them yet. There are traces of both methane and water in today’s Martian atmosphere. One or the other may be the remnant of those ancient seas. What life might have evolved in them? What surprises are in store for us there? Mars has a lot to teach us.