Mars — the red planet, watched by many
Half of Earth's diameter and a third of its gravity, wrapped in a carbon-dioxide atmosphere so thin it barely deserves the name. A world of extremes preserved from an ancient, warmer age — and the only planet in the solar system currently being watched by spacecraft from six different national programmes at once.
The name and the red planet
Every ancient culture that watched the sky picked out Mars quickly. It moves against the fixed stars faster than most planets, it can outshine everything but Venus at opposition, and — uniquely among the naked-eye planets — it is unmistakably red.
To the Sumerians it was Nergal, the god of war and pestilence. To the Babylonians, the same association: a red star tied to conflict. The Greeks called it Ares. The Romans took the same god and gave him the name modern astronomy kept — Mars. In Sanskrit it is Mangala, in Chinese Huǒxīng ("fire star"), in Egyptian Har Décher ("the red one"). Across four continents and thousands of years, the reading is nearly the same: a small red thing overhead, associated with heat, blood, or violence.
The colour itself is not violence. It is iron oxide — rust — coating the basaltic surface and suspended in the thin atmosphere as fine dust. Under all of it Mars is a rocky planet like Earth, built from the same broad chemistry. What made Mars red, and Earth blue, was the water.
A world with a third of the gravity
Mars is half of Earth's diameter and about a tenth of its mass. Its surface gravity is 38 percent of Earth's — enough that things fall in a recognisable arc, but not enough for a human body to keep its bone density without daily resistance work.
The atmosphere is the more startling number. Surface pressure averages about 600 pascals — under one percent of Earth's sea-level pressure. The composition is 95 percent carbon dioxide, with trace nitrogen and argon, and effectively no free oxygen. That thin air holds no heat: at the equator, noon can reach a mild +20 °C, and by nightfall the same latitude drops to −73 °C. Winter poles fall past −125 °C, cold enough that atmospheric carbon dioxide freezes directly onto the caps as dry-ice frost. The white polar caps you see growing and shrinking on Mars are partly water ice, partly seasonal CO₂.
Two other environmental facts define the modern Martian surface. First, Mars has no global magnetic field — the internal dynamo froze billions of years ago — which leaves the surface unshielded from galactic cosmic rays and solar particle events. Second, every few Martian years, local dust storms merge into a global storm that shrouds the entire planet in fine, electrostatic, iron-oxide haze. In June 2018 one of those storms drove enough dust into the atmosphere to end the Opportunity rover's mission by starving its solar panels.
The water that was here
Modern Mars is a hyper-arid desert. Everything interesting about its water is in the past — and the surface still carries the fingerprints.
Orbital and rover data have accumulated an unmistakable picture: for a long stretch of the planet's first billion years, Mars was warmer, wetter, and geologically active. Ancient river valleys, sedimentary layering, deltas at the mouths of former channels, clay minerals that only form in the presence of standing water — every one of these has been mapped from orbit and, in the case of Gale and Jezero craters, walked over by rovers on the ground.
One of the strongest recent results came from China's Zhurong rover, which touched down in the vast northern plain of Utopia Planitia in May 2021. Spectral analysis of hard crust rocks near the landing site identified water-bearing minerals, evidence that liquid water was active in Utopia as recently as one billion years ago — supporting the long-standing hypothesis that Mars's northern lowlands once held a substantial ocean.
Under the surface, water is still there. Radar has mapped underground ice deposits, and there are indications of subsurface aquifers. What happened to the rest is another open question: some was lost to space when the magnetic field died and solar wind stripped the atmosphere away; some is bound up in polar caps and subsurface ice; some may have gone into the crust as hydrated minerals. Between them, they explain a planet that was blue once and is red now.
The great volcano and the great canyon
Mars holds the record for both the largest volcano and the deepest canyon in the solar system. Both are on the same hemisphere, and the two things are almost certainly connected.
Both features sit on or beside the Tharsis dome — a vast volcanic bulge on Mars's western hemisphere, the largest single volcanic structure in the solar system. It is so massive that, around three to three-and-a-half billion years ago, it physically rotated the entire outer shell of Mars — crust and mantle — by twenty to twenty-five degrees relative to the core. Modelling at France's Laboratoire de Météorologie Dynamique showed this "great tilt" (in French, the grand basculement) is the clean explanation for three otherwise puzzling patterns: why so many ancient Martian river valleys sit along a tropical band, why underground water ice appears far from the current poles, and why Tharsis itself sits exactly at the equator today. A planet reshaped by the weight of its own volcanoes.
Phobos and Deimos
Both moons are tiny by planetary-satellite standards — irregular, unspherical, dark, and low-density. The likeliest explanation is that they are captured asteroids, pulled into Martian orbit from the nearby main belt, though the mechanics of that capture (and the alternative that they formed from debris after a giant impact on Mars) are not fully settled. Japan's MMX mission — Martian Moons eXploration — is scheduled to arrive in the late 2020s, land on Phobos, and return the first surface sample of another world's moon since Apollo. That sample should settle the argument.
The most-visited planet
More spacecraft have been sent to Mars than to any other planet except Venus, and — for the first time in the space age — the fleet at Mars right now is genuinely international. Rovers from two nations are on the ground; orbiters from five programmes are overhead. What follows is not the full list, only the representatives.
More spacecraft from more nations are watching Mars right now than have ever watched a single planet other than our own.