What if Mars had oceans again?

Mars looks red and dusty. But long ago, water flowed there. It made rivers and lakes.

What if Mars had oceans again? Let's find out what scientists know.

From space, Mars looks like a rusty desert. But NASA missions have found ancient river valleys, deltas where rivers once fed lakes, and rocks that could only have formed in liquid water. Some features suggest Mars had huge floods about 3.5 billion years ago.

So what if Mars had oceans again? What would it take, and what might a blue Mars look like? This is a thought experiment. Choose a path to explore a watery Mars, the air it would need, and a possible clue to past life that a rover has found.

NASA describes abundant evidence that Mars was wetter and warmer, with a thicker atmosphere, billions of years ago: river valley networks, deltas, lakebeds and minerals that form only in liquid water. Ancient features led scientists to think rivers, lakes and perhaps even oceans once existed.

This thought experiment reverses the clock and asks what renewed oceans would require. The evidence comes from orbiters, rovers and measurements of Mars's air and its loss to space. Any picture of a future ocean-covered Mars is an idea: the lab does not model Mars's climate, and no source here predicts that oceans will return.

Mars is very cold. Its air is very thin.

There is water on Mars, but it is frozen into ice near the poles.

NASA calls Mars a dusty, cold desert world with a very thin atmosphere, made mostly of carbon dioxide, nitrogen and argon. The average temperature is about −59 °C, and it can drop to about −153 °C.

There is water on Mars today, but mostly as ice just under the surface near the poles. NASA says the air is too thin for liquid water to last long on the surface. Mars's air presses down with less than 1 percent of the push of Earth's air.

Mars's thin atmosphere is mostly carbon dioxide, nitrogen and argon. NASA's fact sheet gives a mean surface pressure of 6.36 millibars, varying from 4.0 to 8.7 with the seasons, against 1,014 millibars on Earth, and an average temperature of about 214 K (−59 °C). Temperatures range from about 20 °C down to about −153 °C.

Water exists mainly as ice just beneath the surface in the polar regions. Because the atmosphere is so thin, NASA notes, liquid water cannot persist on the surface for long, and solar heat escapes easily: at the equator at noon, your feet might feel like spring while your head felt like winter.

To have oceans again, Mars might need much thicker air. It might need to be much warmer too.

Then water could stay liquid and fill the low places.

Imagine pouring oceans back into Mars's great basins. Under today's thin air, that water would not last. A study led by NASA scientists explains that Martian temperatures and pressures are mostly so low that liquid water will boil and/or freeze on its own.

For oceans to stay, Mars might need much thicker, warmer air, like it seems to have had billions of years ago. It might also need a way to keep that air from leaking back into space. These are ideas built on real measurements, not predictions.

Water's triple point is 273 K and 6.1 millibars. A NASA Ames-led study of Martian gullies (Heldmann and colleagues) notes that current Martian temperatures and pressures are generally below it, so liquid water will spontaneously boil and/or freeze. Mars's mean pressure of 6.36 millibars sits right at that edge.

Renewed oceans might therefore need a substantially denser, warmer atmosphere and some way to retain it against continuing loss to the solar wind. How much air, and how it could be supplied or kept, is beyond this lab, which models nothing about Mars's climate; these are ideas, not forecasts.

Mars once had thicker air. Scientists think lots of it drifted off into space over a long time.

The Sun's wind helped carry it away.

Mars has no global magnetic field today. NASA's MAVEN spacecraft measured the solar wind, a stream of particles from the Sun, stripping about 100 grams of gas from the top of Mars's air every second. The loss sped up during solar storms.

Scientists think the Sun was younger and more active long ago, so the loss may have been much faster then. NASA says that losing air to space was likely a major reason Mars changed from warm and wet to the cold, dry planet we see today.

MAVEN identified solar-wind stripping as a process that appears to have played a key role in Mars's transition from an early warm, wet climate to today's cold, arid one. It measured a present loss of about 100 grams per second, increasing significantly during solar storms such as those of March 2015. Principal investigator Bruce Jakosky, of the University of Colorado Boulder, compared it to stealing a few coins from a cash register every day.

Because erosion rises with solar activity, the team thinks loss rates were much higher billions of years ago when the Sun was young. Mars also lacks a global magnetic field, though magnetized southern crust records one from about 4 billion years ago.

The Perseverance rover landed in a crater on Mars. Long ago, the crater was a lake!

The rover collects rock samples to study for signs of tiny, ancient life.

NASA's Perseverance rover landed in Jezero Crater in February 2021. MIT researchers on the team say its first images confirmed that the crater was once a quiet lake, fed by a small river about 3.7 billion years ago. Later, flash floods swept in boulders up to 1 meter across.

In July 2024, Perseverance came upon a rock called Cheyava Falls and took a sample from it. The sample, Sapphire Canyon, holds what NASA calls a potential biosignature: something that might have been made by life but needs more study. It is not proof of life, and scientists say it might also have formed without life.

Perseverance's first image analysis, published in Science, confirmed a fan-shaped outcrop as a river delta that fed a calm lake about 3.7 billion years ago; boulders up to 1 meter wide in its upper layers record late flash floods flowing up to 9 meters per second, MIT reported.

In 2025 a Nature paper led by Joel Hurowitz of Stony Brook University reported potential biosignatures in the Sapphire Canyon core, one of 27 the rover had collected. Leopard-spot reaction fronts carry vivianite and greigite, minerals that can form through microbial activity but also abiotically. JPL project scientist Katie Stack Morgan stressed that abiotic explanations, while less likely, cannot be ruled out. It is not a confirmed sign of life: NASA released the data so other scientists can work to confirm or refute its biological potential.