A cold desert today
Mars today is a cold, dry world. Its average temperature is far below freezing, and liquid water is not stable at the surface for long. The air is thin, mostly carbon dioxide, with a surface pressure well under one percent of Earth’s. Fine reddish dust covers much of the planet, which gives it the color seen from Earth and its nickname.
Yet the landscape tells a different story. Dry riverbeds, ancient lake basins and layered rocks suggest the planet’s climate has changed dramatically. Understanding how and why is one of the central questions in planetary science.
Signs of ancient water
Evidence for a wetter past comes from several kinds of observation, and many of them were first made from orbit.
- Valleys and channels. Branching valley networks and enormous channels cut across the surface. Some look like they were carved by flowing water, perhaps over long stretches of time, and others by sudden floods.
- Minerals. Orbiting instruments found clays in ancient terrain. On Earth, clays typically form when rock reacts with water for a long time. Salts and sulfate minerals in other places suggest that water was present but evaporated or became more acidic as conditions changed.
- Lake and river deposits. Rovers have studied layered sedimentary rocks and landforms that look like river deltas. One landing site was chosen precisely because it appears to be an old delta flowing into a crater lake.
- Rounded pebbles and mineral veins. Rock fragments shaped like stream gravel and veins of minerals deposited by fluids point to water moving through the crust.
Taken together, these findings suggest that liquid water was at least occasionally present on the surface in the first billion years or so of the planet’s history. How long such episodes lasted, and how warm the planet was in between, are debated.
Warm and wet, or cold with brief thaws?
Scientists do not agree on one picture. In a “warm and wet” scenario, a thicker atmosphere kept the planet mild enough for rain and persistent lakes. In a “cold and icy” scenario, the planet was mostly frozen, with meltwater produced during warmer spells caused by volcanic activity, impacts or changes in its orbit.
The problem is that the young Sun was fainter than it is now, so a very thick greenhouse effect would have been needed to keep early Mars warm. Models struggle to produce one with carbon dioxide alone, and researchers have suggested other gases might have helped. This remains an open research area.
Where did the air go?
If Mars once had a thicker atmosphere, it needs to have gone somewhere. Most scientists think a large share was lost to space, with some possibly locked into rocks or frozen at the poles.
One important factor involves the planet’s magnetic field. Early Mars appears to have had a global magnetic field, shown by magnetized patches of ancient crust. That field later faded. Without it, the solar wind could strip gas from the upper atmosphere more easily. Spacecraft orbiting Mars have measured this loss happening today and find it is faster during solar storms. Mars is also smaller than Earth, so its gravity is weaker, making escape easier.
This does not mean the loss is fully explained. Estimates of how much atmosphere was lost, and how much went into the ground, vary. Researchers combine orbital measurements, rover chemistry and meteorites to narrow the possibilities.
Ice, seasons and cycles
Present-day Mars still has a climate, just a harsh one. The polar caps contain water ice, and in winter a layer of carbon dioxide frost builds up on top. Some of the atmosphere freezes out and returns each year, so surface pressure rises and falls with the seasons. Dust storms are common, and occasionally one expands to cover the planet for weeks.
Mars has an axial tilt similar to Earth’s, which gives it seasons. Unlike Earth, though, Mars’s tilt is thought to swing widely over hundreds of thousands to millions of years. Those swings may have shifted ice between the poles and lower latitudes, creating a kind of long-term cycle comparable to ice ages. Radar has also detected large amounts of buried ice at mid-latitudes. Some features, such as streaks on slopes, once seemed to suggest flowing water but are now more often explained by dry processes, so interpretations of seasonal features have changed over time.
Why it matters
Mars is the best-studied example of a rocky planet whose climate changed profoundly. Comparing it with Earth and Venus helps test models of how atmospheres evolve. It also bears on the question of extraterrestrial life: if early Mars had stable water, life might have started there, and ancient rocks could preserve traces. No confirmed evidence of life has been found, and rovers are searching for chemical signs in old sedimentary rocks. Some are collecting samples, with the aim of returning them to Earth for laboratory analysis, although the plans for doing so have faced changes in schedule and cost.
The same observations inform discussions about settling Mars. Thin air, low temperatures, radiation exposure and a lack of liquid water at the surface make the planet a very difficult place for people, whatever its past.
What we know and what we do not
The basic outline is widely accepted: early Mars had liquid water at least at times, its atmosphere later thinned, and the planet became the cold desert seen now. The details, such as how warm it was, how long wet periods lasted and how much atmosphere was lost to space versus locked underground, are still being worked out. New data from space robotics, including rovers and orbiters, keep adding pieces to the puzzle, and a few conclusions may change as further samples and measurements arrive.