Posted by On 15 July 1965, Mariner 4 flew past Mars and sent back 21 grainy pictures, transmitted so slowly that engineers at the Jet Propulsion Laboratory got impatient and hand-coloured a printout of the numbers with pastel crayons before the computers caught up. The images showed craters. Lots of them. A world that looked as dead as the Moon, and decades of hopeful talk about canals and vegetation collapsed in a week. What makes the Mars planet story so interesting is that the verdict of 1965 turned out to be only half right. Mars is a frozen desert today. It was not always one.
Why the Red Planet is red, mostly
The colour is rust, more or less. Iron in Martian rocks and dust has been oxidized, and the fine dust that blankets much of the surface spreads that orange tint everywhere, including into the sky. For a long time the culprit was assumed to be hematite, a dry iron oxide. A 2025 study suggested instead that much of the dust may be ferrihydrite, a mineral that forms in the presence of cool water, which would push the reddening back to a wetter era. That’s not settled. Up close, rover cameras show plenty of grey basalt, dark sand and pale layered rock that isn’t red at all.
A landscape built on a giant scale
Mars is about half Earth’s diameter, yet some of its features dwarf anything at home. Olympus Mons rises roughly 22 kilometers above the surrounding plains, about two and a half times the height of Everest above sea level, and is so broad that someone standing on its flank would not see the summit for the curve of the planet. It sits on the Tharsis bulge, a vast volcanic plateau carrying three more giant volcanoes in a neat row.
Next to Tharsis runs Valles Marineris, a system of canyons about 4,000 kilometers long and up to around 7 kilometers deep. Put it on Earth and it would stretch across the continental United States. It probably formed as the crust cracked under the weight and uplift of Tharsis, then widened through collapse and erosion, rather than being cut by a river the way the Grand Canyon was.
The two hemispheres look strikingly different, too. The south is high, ancient and pocked with craters, including the Hellas basin, one of the largest impact scars in the solar system. The north is low and smooth, so smooth that some researchers think it once held an ocean. That idea remains contested.
Reading Martian history in layers
Geologists divide Martian history into three broad eras. The Noachian, more than about 3.7 billion years ago, was a time of heavy bombardment and, apparently, plenty of water. The Hesperian brought widespread volcanism and catastrophic floods that carved huge outflow channels. The Amazonian, which covers the last three billion years or so, is the long, cold, dry era we see now, with wind and ice doing most of the work.
Water’s fingerprints are all over the older terrain. Mariner 9, the first spacecraft to orbit another planet, arrived in 1971 during a planet-wide dust storm and had to wait for the air to clear; when it did, it revealed dry riverbeds and branching valley networks. Orbiters since then have found clay minerals and sulfates that form in water, ancient deltas and fans of sediment, and chloride deposits where salty lakes may have evaporated.
The rovers confirmed it on the ground. In 2004, Opportunity found tiny hematite spheres, nicknamed blueberries, that formed in groundwater. Curiosity, which landed in Gale Crater in 2012, has spent more than a decade climbing Mount Sharp through layer after layer of mudstone that settled at the bottom of a lake system lasting for long stretches of time, perhaps millions of years. It also found organic molecules preserved in that rock. Organics are not life, but they are the kind of raw material life could use, or leave behind.
Where the air and water went
If Mars was wet, it must once have had a thicker atmosphere. Today’s surface pressure is under one percent of Earth’s, and the reasons it thinned are partly understood. Mars lost its global magnetic field early in its history, leaving the upper atmosphere exposed to the solar wind. Its weaker gravity, about 38 percent of Earth’s, made it easier for gas to escape. Some carbon dioxide ended up locked into carbonate rocks.
Water didn’t all vanish. Large amounts remain frozen in the polar caps and in ice buried under the mid-latitudes. In 2008 the Phoenix lander scraped the ground near the north pole and exposed bright ice that slowly sublimated over a few days. Radar from Mars Express hinted in 2018 at a lake of salty liquid water under the south polar ice, though other teams have argued the signal may come from clays or other materials. And in 2024, a team analyzing seismic data from NASA’s InSight lander suggested there could be a reservoir of liquid water trapped in fractured rock some 11 to 20 kilometers down. Interesting, if true. Nobody is drilling that deep any time soon.
Mars the living planet, geologically speaking
For a long time Mars was described as geologically dead. InSight, which operated from 2018 to 2022, recorded more than 1,300 marsquakes, including one in May 2022 with a magnitude of about 4.7. Its data showed Mars has a large liquid metal core, bigger and less dense than expected. Many of the quakes came from Cerberus Fossae, a region of young fractures where one dark volcanic deposit has been estimated at only around 50,000 years old. Not dead, then. Very, very quiet.
The question everyone actually asks
Was there life? The honest answer is that nobody knows, and the evidence so far is tantalizing rather than decisive. The first attempt to find out came in 1976, when the twin Viking landers scooped soil into onboard labs and fed it nutrients. One experiment, the labeled release test, gave a response that looked at first like microbial metabolism. The other instruments found no organic molecules at all, and most scientists concluded the reaction was chemical, probably driven by reactive oxidants in the soil. Gilbert Levin, who designed the experiment, argued for the rest of his life that he had detected life. Few agreed with him, but the episode taught a lasting lesson about how hard a clean answer would be. Curiosity has measured methane in Gale Crater that seems to rise and fall with the seasons, while Europe’s Trace Gas Orbiter, looking from above, has seen almost none. The two results have not been fully reconciled.
Perseverance, exploring an ancient river delta in Jezero Crater since 2021, found a rock in 2024, nicknamed Cheyava Falls, with small light spots ringed by dark material that on Earth are often associated with microbial chemistry. NASA has described the features as a potential biosignature, while stressing they could also have formed without life. To know for sure, scientists want the sealed samples Perseverance has collected brought back to Earth labs, and the plan for doing that has been tangled up in cost problems and redesigns.
This is how Mars research tends to go: one careful step at a time, with every exciting claim followed by a round of skeptical checking. The Mars planet we know now is very different from the dead cratered ball of 1965, and from the canal-covered fantasy before that. It is probably different again from the one we will know in twenty years.
If you’d like to follow the Mars planet story as new results come in, from Jezero to InSight’s data to whatever the next orbiter finds, SETIworld covers it regularly. Come read more, and join other readers in arguing over what those spots on Cheyava Falls really mean.
