Four and a half billion years ago, Mars was a very different place than it is today. Now Mars is a cold, dry desert. But, early in the history of the solar system, Mars was probably endowed with enough heat leftover from its formation, and from unceasing meteor and comet barrages, to melt water ice at the planet’s surface.
Judging from the many rivulets, channels, and canyons we see on Mars today, especially in and around the planet’s volcanic regions, abundant water must once have flowed across Mars and penetrated its subsurface. Craters on the martian highlands, which date back more than 3. 5 billion years, clearly show signs of water erosion. And as recently as 250 million years ago, volcanic eruptions on Mars may have melted ground ice, extruding ground water to the surface.
If water still exists on Mars in any great quantity — and many planetary scientists believe that it does — it is most likely to be found now in subsurface layers. Evidence for subsurface water was revealed by the Viking orbiters in the 1970s. In addition to the countless water-caused erosion features imaged by the orbiters’ cameras, many impact craters were seen with features that appear suspiciously similar to what you would get by dropping a stone into wet mud.
Studies of the other martian meteorites found on Earth have also brought to light tantalizing evidence suggesting that hot springs may once have bubbled up on Mars’ surface. It has long been known that tiny veins of calcium carbonate form when water flows through hot rocks on Earth. Similar deposits, a few made as recently as 1.3 billion years ago, have been found in some of the martian meteorites. Planetary scientists speculate that some martian hot springs may still exist today, perhaps providing oases for martian life.
If Mars were as wet and warm three to four billion years ago as the evidence appears to indicate, then it is possible that primitive forms of single-cell life could have evolved there just as they did on Earth. But that is where the similarities between the two planets’ biologic development end. We know now that Mars did not stay wet and warm. Instead, the planet cooled and lost most of its atmosphere to space, leaving only a rarefied carbon-dioxide envelope. Surface water froze out and subsurface water became locked in as ground ice. All these conditions combined to condemn Mars to become the (apparently) dead planet that we see today.