Mars
The red planet is cold, dry and nearly airless today, but rivers once ran across it. Here is what the rovers have found, and why we keep going back.
Mars has always looked different. To the naked eye it is an unmistakable orange-red point, and the ancients named it for gods of war and blood. Through telescopes it showed polar caps that grew and shrank with the seasons and dark markings that some astronomers, as late as the 1900s, took for canals built by a dying civilisation. The truth is less romantic and more interesting: Mars is a world that was once far more like Earth, and we are learning what happened to it.
A small, cold desert
Mars is the fourth planet from the Sun, orbiting about one and a half times farther out than Earth. It is small, about half Earth's diameter and a tenth of its mass, so you would weigh about a third of what you do here. The air is almost all carbon dioxide and so thin that the pressure at the surface is less than one percent of Earth's; liquid water exposed to it would boil and freeze at the same time. The average temperature is around minus 60 degrees Celsius, though a summer afternoon at the equator can reach a pleasant 20.
The famous colour comes from rust. Iron minerals in the soil have oxidised over billions of years, coating the planet in fine orange dust that the wind whips into storms that occasionally engulf the entire globe for weeks.
Giants of rock
Because Mars has low gravity and, for most of its history, no moving crustal plates, its volcanoes stayed in one place and grew enormous. Olympus Mons rises about 22 kilometres above the surrounding plains and is some 600 kilometres across, so broad and gently sloped that standing on it you would not know you were on a mountain. Nearby, Valles Marineris splits the planet's face: a system of canyons 4,000 kilometres long, as wide as Australia and up to seven kilometres deep, probably a crack that opened as the volcanic region beside it swelled.
The two hemispheres look oddly different. The south is high, old and cratered; the north is low, smooth and young. One idea is that a colossal impact early on blasted away the northern crust.
The water that was
Everywhere orbiters look, they see the handwriting of water. Winding valley networks drained ancient highlands. Deltas fan out where rivers met standing water. Minerals that only form in liquid water, clays and sulphates, are common. Rovers have driven across what were once lake beds. In Gale Crater, Curiosity found mudstones laid down in a lake that persisted for perhaps millions of years, with the chemical ingredients life would need. In Jezero Crater, Perseverance is collecting rock samples from a river delta for eventual return to Earth.
Three and a half billion years ago, then, Mars had rivers, lakes and possibly a northern ocean, under a thicker, warmer atmosphere. What went wrong was size. A small planet cools fast; Mars's core stopped generating a strong magnetic field, and without that shield the solar wind gradually stripped the atmosphere away into space. As the air thinned, the water froze into the ground or escaped. Mars still holds a great deal of water as ice: in the polar caps, in glaciers buried under dust, and in frozen soil just below the surface across much of the planet.
Visitors from Earth
More spacecraft have been sent to Mars than to any other planet, and about half have failed. The successes have been spectacular. Viking landed in 1976. The little rover Sojourner rolled out in 1997. Spirit and Opportunity, designed for ninety days, lasted six and fourteen years. Curiosity has driven more than 30 kilometres since 2012. Perseverance landed in 2021 carrying Ingenuity, a small helicopter that made 72 flights in the thin air, the first powered flight on another planet. Orbiters from the United States, Europe, India, the United Arab Emirates and China circle overhead.
Life, past or present?
Mars is the best place in the solar system to look for evidence that life ever began anywhere besides Earth. Its early lakes were habitable by any reasonable definition, and because the planet has been so quiet since, their sediments are far better preserved than Earth rocks of the same age. No mission has yet found convincing signs of life, but none has been equipped to settle the question either. Returning samples to laboratories on Earth, planned for the 2030s, may finally do so.
What we still don't know
How long Mars stayed warm and wet is debated; some models struggle to keep it warm at all under a fainter young Sun. Methane appears and vanishes in the atmosphere for reasons nobody can explain. Whether liquid brines exist today beneath the surface, and whether anything could live in them, remains open. And the largest question of all is still unanswered: are we alone?
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Test yourself
4 questions · answers explained at the end
1.Why is Mars red?
2.What is Olympus Mons?
3.What evidence shows Mars once had liquid water?
4.Why did Mars lose most of its atmosphere?