Posted by Mars is the only place besides Earth where you can look at four-billion-year-old ground in something close to its original condition. No plate tectonics has recycled the crust, no oceans have buried it, no forests have grown over it. The Mars landscape visible from orbit today is largely the surface that existed when the planet was young, freeze-dried and left out. That is why so much effort goes into reading it: the rocks are a record of a climate that no longer exists, and possibly of conditions where something could have lived.
Two halves that do not match
The first thing anyone notices on a global map is that the planet has two faces. The southern hemisphere is high, rough and saturated with craters. The north is low, smooth and comparatively young, sitting several kilometres beneath the southern terrain. This hemispheric dichotomy has been argued over since the Viking orbiters mapped it, and the leading explanation — one enormous impact early in the planet’s history — remains a hypothesis rather than a settled result.
Then there is the Tharsis bulge, a continent-sized swelling of volcanic rock carrying Olympus Mons, which rises roughly 22 kilometres and would swallow most of France at its base. Next to it runs Valles Marineris, a canyon system stretching about 4,000 kilometres with floors up to seven kilometres below the surrounding plateau. It is not a river canyon. Most models treat it as a tectonic rift associated with the loading of Tharsis, later widened by landslides and possibly by water in places.
Mariner 4 flew past in July 1965 and sent back 21 images of nothing but craters, which convinced a generation that Mars was a dead, Moon-like rock. Mariner 9 arrived in 1971, waited out a global dust storm, and then revealed volcanoes, the canyon and channels. The planet has been getting more interesting with every mission since.
Counting craters to tell time
Geologists divide Martian history into three long chapters, each named after a type of terrain. The Noachian runs from roughly 4.1 to 3.7 billion years ago and covers the heavily cratered highlands. The Hesperian, from about 3.7 to 3 billion, is the era of huge lava plains and catastrophic flooding. The Amazonian covers everything since — cold, dry, dominated by wind and dust.
The dating comes from counting craters. A surface accumulates impacts at a roughly known rate, so a densely pockmarked plain is old and a smooth one is young. It works, but the absolute numbers depend on calibration borrowed from the Moon, where Apollo brought samples home. Nobody has yet returned a dated rock from Mars, so every age quoted for the ancient Mars landscape carries an error bar wider than the press releases usually admit.
Minerals as a climate archive
The mineralogy is where the story gets specific. The OMEGA spectrometer on Mars Express and CRISM on the Mars Reconnaissance Orbiter mapped what the surface is made of, and Jean-Pierre Bibring’s team proposed a sequence that has held up reasonably well.
Oldest first: clays. Phyllosilicates form when rock sits in contact with water that is roughly neutral in pH, and they show up in Noachian terrain across the southern highlands. Above them come sulfates, which need water too but acidic water, formed as volcanic sulfur worked its way into a drying system. Youngest are the anhydrous iron oxides — the rust that gives the planet its colour and requires no liquid water at all.
Read in order, that stack describes a world that started wet and comparatively benign, turned sour, then dried out for good. Clays are the target minerals for anyone interested in habitability, which is why they drive landing site selection.
What Curiosity found in Gale
Curiosity set down in Gale crater on 6 August 2012 and reached mudstone within months. At a site called Yellowknife Bay the rover drilled into fine-grained sediment that had settled out of standing water, and the chemistry came back containing carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur — the elemental shopping list for terrestrial biology — along with clays and a mix of oxidation states that a microbe could in principle draw energy from.
The team’s conclusion, announced in 2013, was carefully worded: Gale once held a habitable environment. Not that anything lived there. Habitability is a statement about the setting, not the tenant.
Since then the rover has been climbing Mount Sharp, the five-kilometre mound in the middle of the crater, driving upward through layer after layer that records the transition from lakes to sulfate-rich drying conditions. In 2024 it happened to crack open a rock and expose crystals of elemental sulfur, which nobody had predicted and which still lacks a settled explanation.
Where the air went
None of this works without a thicker atmosphere. Liquid water is unstable on the surface today because the pressure sits below one percent of Earth’s — a puddle would boil and freeze simultaneously.
Mars Global Surveyor found the first piece of the explanation in the late 1990s: strips of magnetized crust in the southern highlands, the fossil signature of a global magnetic field that shut down early. Without it, the solar wind has direct access to the upper atmosphere. MAVEN, in orbit since 2014, has been measuring that loss in progress, watching ions get stripped away and finding that the rate spikes sharply during solar storms. Multiply a modest ongoing loss by four billion years and much of an atmosphere goes missing.
Some of the carbon dioxide is also locked in the ground as carbonate and in the polar caps rather than lost to space. The accounting is not finished.
There is a further complication that rarely makes it into summaries. The young Sun was around thirty percent fainter than it is now, which makes warming early Mars above freezing harder, not easier. Climate modellers have thrown carbon dioxide, methane, hydrogen and high clouds at the problem and still struggle to produce a sustained warm and wet planet. Some now argue for a mostly cold Mars punctuated by episodes of melting. The geology says water flowed; the physics has trouble explaining how.
Ice, dust and the modern surface
The present-day Mars landscape is not entirely static. Dust devils leave dark tracks across the plains, avalanches slump off the polar layered deposits each spring, and radar instruments have traced buried ice through the mid-latitudes, including cliffs where thick, comparatively clean ice is exposed in cross-section.
Wind has had four billion years to work as well, and it shows. The Medusae Fossae Formation, a vast soft deposit near the equator, has been carved into ridges and grooves that look sandblasted because they were, and orbiters have watched individual dunes creep across crater floors from one year to the next. Erosion of that kind is a mixed blessing for anyone hunting old chemistry: it strips away the radiation-damaged upper layers, and it also erases the fine sedimentary detail worth reading.
Those deposits are geologically recent, laid down when the planet’s axial tilt swung to values that moved ice away from the poles. Mars wobbles far more than Earth does, having no large moon to steady it, so its climate has oscillated repeatedly.
What remains unanswered is whether any of the wet intervals lasted long enough for chemistry to become biology. Perseverance is caching samples in Jezero for a return mission whose schedule and budget keep shifting, and until those tubes reach a laboratory the argument stays open. SETIworld follows the missions, the sample return politics and the disputed results as they come in, which is where this particular question is going to be settled or abandoned.
