We keep finding water on Mars – here are all the places it might be

Mars isn’t as arid as it may seem. Billions of years ago, the surface of the Red Planet rippled with oceans and rivers of liquid water, but now it seems that all of that fluid has disappeared, leaving behind a dusty wasteland. However, as we have explored the planet with orbiters, landers, rovers and even telescope images from afar, traces of water keep popping up.

Each hint tantalises researchers because of how crucial water is for living organisms and how helpful it could be for future exploration. Water has now been discovered all over Mars, in many different forms – here are five places it has been spotted.

1. Buried underground

Just beneath Mars’s parched surface lies a wonderland of water ice. These deposits are kept insulated by the layers of dust on top of them, but erosion and meteorite impacts can expose them to the prying eyes of our orbiters. A single ice deposit recently identified using data from the Mars Express orbiter seems to contain enough water to cover the entire surface of Mars in an ocean 1.5 to 2.7 metres deep.

It isn’t just ice buried under the shifting orange sands. Hints of a huge lake beneath the planet’s south pole have been controversial – it may simply be wet silt or volcanic rock. But a new study using data from the InSight lander has revealed another possible reservoir of water near the planet’s equator. InSight found this water buried 11.5 to 20 kilometres underground by feeling for marsquakes and measuring how fast those seismic waves travelled. This revealed that the rocks those quakes were propagating through seemed to be saturated with water.

2. Frosting over the poles

Getting at the buried water on Mars would be difficult, so perhaps a more promising reservoir for future explorers is right on the surface. The poles on Mars have ice caps just like on Earth, and we have known about them for decades. Many craters on Mars also have smaller ice sheets inside them. These are the only places on Mars’s surface that stay cold enough for ice to stick around.

However, some transient frost also forms at high latitudes on Mars, where the air tends to be colder and more humid. On some frigid Martian mornings, volcanic peaks frost over as well, which is probably due to water vapour freezing out of the atmosphere.

3. Floating in the atmosphere

Because of the bitter cold and tenuous atmosphere on Mars, any liquid water on the surface would sublimate away, turning directly into gas and floating up into the air. Water vapour in the atmosphere is a sign of water and ice migrating across the planet’s surface to form frost, but it is only present in minuscule amounts. Occasionally, there is enough water vapour in one area to generate a few wispy clouds, but for the most part, it is nearly negligible.

4. Running downhill

Perhaps the most controversial of the possible signs of water on Mars are recurring slope lineae, which are dark streaks that sporadically appear running down the sloped edges of craters. They were first discovered in 2011 and there has been lively debate among researchers since then about how they form. They occur primarily in the warmest parts of the year, so they could be caused by ice melting and running downhill before evaporating away – which would make them the only liquid water ever spotted on the surface of Mars. Or, they could be simple sand flows. Over time, the latter hypothesis has gained support, but some researchers hold out hope that there could be a trickle of liquid water on the Red Planet.

5. Trapped in rocks

If Mars used to be covered in water and now all that is left is a bit of ice and a whole lot of dust and stone, where did all that water go? One possible solution is that it got slurped up into the rocks themselves. Mars rovers have found no shortage of minerals with water molecules incorporated into their chemical structures all over the planet.

This process is irreversible, so there is no way for us to get all that water back, but accounting for where all the water went is crucial to understanding what Mars was like before it dried out. That may be our best chance of knowing whether Mars ever really was hospitable to life.