Mars exploration has produced a remarkable catalog of discoveries over the past few decades. Rovers, landers, and orbiters have revealed a planet that was once warm and wet, with lakes, a thick atmosphere, and a magnetic field. Today it’s cold, dry, and bathed in radiation, but the evidence of its more hospitable past is written across its surface and beneath it.
Water: Past and Present
The single biggest discovery on Mars is that liquid water once flowed across its surface in enormous quantities. Orbital cameras have photographed ancient river channels, lake beds, and mineral deposits that only form in the presence of water. The Curiosity rover, exploring Gale Crater since 2012, found that the crater once held a lake with chemically complex water. Layers of rock there contain iron oxide minerals and silica signatures that point to waters with different oxygen levels at different depths, much like stratified lakes on Earth. The chemistry of those ancient waters included the key ingredients for microbial life as we know it.
Water hasn’t entirely vanished. Ground-penetrating radar from orbit detected a massive subsurface ice deposit beneath a region called Utopia Planitia. That single deposit covers an area roughly the size of New Mexico and holds about as much water as Lake Superior. Seasonal frost and ice caps at the poles add to the inventory. Mars still has water; it’s just locked away as ice rather than flowing on the surface.
Organic Molecules in Martian Rock
Finding organic molecules on Mars was a major milestone, and multiple missions have now confirmed they’re there. The Perseverance rover detected several types of aromatic organic molecules (ring-shaped carbon compounds) in rock formations on the floor of Jezero Crater. These molecules persist on the surface despite the harsh radiation and oxidizing conditions, which means they’re either being replenished or are protected within the rock itself.
Organic molecules aren’t proof of life. They can form through geological and chemical processes that have nothing to do with biology. But they are a necessary ingredient for life, and finding them on Mars means the raw chemical building blocks were available. The diversity of organic compounds found so far suggests Mars had, and may still have, a more complex carbon chemistry than scientists once expected.
The Methane Mystery
Curiosity has repeatedly detected methane in the thin Martian atmosphere, and nobody can fully explain where it comes from. Background levels of the gas rise and fall with the seasons, hovering near or below 1 part per billion. But the rover has also recorded sudden spikes, including one burst of about 21 parts per billion, the largest it has ever measured.
On Earth, methane is produced in huge quantities by microbial life. It can also be generated by chemical reactions between water and rock, a process called serpentinization. Either explanation would be significant: one would mean biological activity, the other would mean liquid water is interacting with rock somewhere underground. Scientists haven’t found a clear pattern to the sudden plumes, and they don’t yet know how long the spikes last or what triggers them. It remains one of the most tantalizing open questions in Mars science.
What’s Beneath the Surface
NASA’s InSight lander, which operated from 2018 to 2022, placed a seismometer directly on the Martian surface and listened to “marsquakes” for years. By analyzing how seismic waves traveled through the planet’s interior, scientists mapped its internal structure for the first time. They found that Mars has a liquid iron core that is smaller and denser than earlier models predicted. About a fifth of the core is made up of lighter elements like sulfur, oxygen, carbon, and hydrogen mixed in with the iron.
This matters because the core is tied to another major discovery: Mars once had a global magnetic field, much like Earth’s. Ancient rocks in the southern highlands retain intense magnetization, a fossil imprint from billions of years ago when the planet’s molten core was generating that protective shield. At some point, the internal dynamo shut down. Without a magnetic field, Mars lost its primary defense against the solar wind, which began stripping away its atmosphere.
A Disappearing Atmosphere
The MAVEN orbiter, which has been studying Mars’s upper atmosphere since 2014, measured exactly how fast that stripping is happening. Currently, Mars loses about 1 to 2 kilograms of atmospheric gas to space every second. Hydrogen and oxygen escape at rates sufficient to remove 2 to 3 kilograms per second during active periods. Those numbers sound small, but over billions of years they add up to a dramatic transformation. Mars once had an atmosphere thick enough to support liquid water on the surface. Today it’s less than 1% as dense as Earth’s.
The loss accelerates during solar storms, when the sun bombards Mars with energetic particles that the planet can no longer deflect. MAVEN’s data confirmed that this process, compounded over the age of the solar system, is enough to account for the vast majority of the atmosphere Mars has lost.
Radiation on the Surface
Curiosity carries a radiation detector that has been measuring the surface environment continuously. An unshielded person standing on Mars would absorb about 0.7 millisieverts of radiation per day. For context, that’s roughly equivalent to getting a chest X-ray every day, or about 10 times the average daily dose a person receives on Earth from natural background radiation. Over a year, that adds up to about 255 millisieverts, well above the annual limits set for radiation workers on Earth.
This radiation comes from two sources: a constant rain of galactic cosmic rays from deep space, and occasional bursts of energetic particles from the sun. Without a thick atmosphere or a magnetic field to block them, these particles reach the surface largely unimpeded. The radiation measurements have been critical for planning future human missions and for understanding how likely it is that organic molecules or microbial life could survive near the surface.
Samples Waiting to Come Home
Perseverance has been doing something no rover has done before: collecting and sealing rock and soil samples in small titanium tubes for a future return to Earth. As of its latest count, the rover has gathered 33 sample tubes total. That collection includes 27 rock cores drilled from different geological formations in Jezero Crater, two samples of regolith (loose surface rock and dust), and one sample of Martian atmosphere.
These samples span a range of rock types, from volcanic formations on the crater floor to sedimentary rocks in the ancient river delta that once fed a lake in Jezero. If and when they reach Earth-based laboratories, scientists will be able to analyze them with instruments far more powerful and sensitive than anything a rover can carry. The search for definitive biosignatures, chemical patterns that could only be produced by living organisms, is one of the primary goals of getting those tubes home.
A Planet That Was Once Habitable
Taken together, the discoveries paint a coherent picture. Roughly 3 to 4 billion years ago, Mars had a magnetic field protecting a thick atmosphere that kept the surface warm enough for liquid water. Lakes and rivers carved the landscape. The water had the right chemistry, with the energy sources, nutrients, and organic building blocks needed for life. Then the magnetic field collapsed, the atmosphere began leaking into space, the water froze or evaporated, and the surface became the cold, irradiated desert we see today.
Whether life ever actually arose during that habitable window is the question everything else orbits around. We haven’t found it yet. But every major discovery, from the organic molecules to the methane to the ancient lake chemistry, keeps that possibility firmly on the table.