Scientists think that water flowed on the surface of Mars around 1.25 million years ago - in the relatively recent past. Their latest study suggests water carved out a system of gullies in Mars' mid-latitudes, sculpting a geological feature known as an alluvial fan.
The topic will be discussed at the 40th Lunar and Planetary Science Conference (LPSC) in Houston, US, this week.
Two of the study's authors, James Head III and Samuel Schon, talked to BBC News science reporter Paul Rincon about the implications for understanding Mars and its potential for supporting life.
How would you describe the significance of this evidence from Mars' Promethei Terra region?
The alluvial fan provides really strong evidence for multiple distinct episodes of (water) flow. We're talking about a rate of cubic metres per second. It's not a trickle but not a torrent either.
We work in the Antarctic Dry Valleys, which is a really good Martian analogue because it's so cold and dry. We actually see this kind of activity there. Wind blows a limited amount of snow across the surface and it gets caught in little troughs. In the peak summertime sunlight, it approaches the melting point of water for a few hours a day.
When the snow melts, it comes down as a little trickle which freezes at night. It's not a huge torrent like a fire hose, but multiple small phases of melting and erosion.
When we look at these gullies using the HiRise and CTX imaging cameras on the Mars Reconnaissance Orbiter spacecraft, we see bright material that has the characteristics of CO2 and water ice. And this accumulates in the protected areas of the gullies.
The alcoves are protected from the Sun, so if (snow) blows in there, it's already cold and it doesn't get hit by the sunlight for as much of the day as does the rest of (the gully).
It's much colder on Mars than it is on Earth. How can water stay liquid on the surface?
On Mars, ice usually wants to sublimate - go straight back to the vapour phase. In certain conditions, while the ice is sublimating, you can generate limited amounts of melt water, which can flow in the gullies, evaporate, freeze and melt out later.
This is also supported by evidence in the recent past for variations in the obliquity of Mars ("obliquity" measures how far the planet's rotational axis tilts away from a perpendicular line). Ice was re-mobilised from the poles to more mid-latitude locations. This provided the ice for this kind of dynamic melting.
Over tens to hundreds of millions of years, the ice has been transported to lower latitudes. We have found evidence for huge tropical mountain glaciers where the sides of big volcanoes at the equator have these huge deposits - 170,000 sq km - on their north-west flanks that are caused by big changes in Mars' obliquity.
On Earth, obliquity variations actually caused the Ice Ages that we experienced over the last tens of thousands of years. But changes in Mars' obliquity have been significantly greater. So we're seeing evidence for ice having been transported all the way down to the equator.
It's very exciting because we can learn a lot about climate change on Mars and really understand how it works on both Earth and Mars.
The Earth's Moon plays a pivotal role is stabilising the Earth's obliquity variations
without that Moon for stabilisation, Mars goes through much bigger changes in obliquity.
It has been calculated that Mars' obliquity can go to 80 degrees. That's virtually like taking the polar cap and pointing it at the Sun.
Early on in its history, Mars is supposed to have had a lot of water on its surface. Where did that water go?
One of the co-authors from Brown University, Caleb Fassett, mapped out 210 open basin lakes in the earliest history of Mars. These are large craters with valley networks going in and valley networks coming out. The implication is that the crater was filled with water.
Our own planet has many spectacular alluvial fan deposits
So there is a lot of evidence there was a significant amount of water early in Mars' history, and indeed, it has been lost since then.
Some of it has gone to make the polar caps. Some amount, certainly, has left the planet through dissociation in the upper atmosphere. But what we're finding is that a significant amount may have been sequestered in glacial deposits.
Data from the Sharad (SHAllow RADar) instrument (on Mars Reconnaissance Orbiter), document that some of these glacial-like features, which you find at Mars' mid-latitudes, have a significant volume of ice left below a surface of rock debris.
So some of the water is being taken out of the system, not by being lost from the planet, but by being sequestered - or hidden away - in these glacial deposits. That's very exciting because these occur at latitudes that humans - or robotic explorers - could get at.
There's no question that being able to drive a rover up and dig into these things
will have a big influence in the future. It's very exciting because I think this is going to open up a whole new area for exploration.
Imagine being able to use a scoop or a shovel on ice that may be 100 million years old, something that represents the earlier history of the atmosphere. We get the gas bubbles out of ice in Antarctica to study ancient atmospheres. The same should be possible on Mars.
Do these episodes of flowing water have any implications for the habitability of Mars?
It's very interesting and exciting from several different points of view. We don't have any direct evidence at all. But it is also clear that on Earth there are organisms that are in stasis for significant periods of time until liquid water becomes available. We see these in the Antarctic Dry Valleys.
These are areas in which it is extremely dry and extremely cold - so there's very little melt water. But in places where we find gully-like features in the Antarctic Dry Valleys, I have watched the melting begin, releasing stream water that comes down and hits this dried-up "crud".
Antarctica's Dry Valleys show some similarities to Mars
This turns out to be algae that had been sitting there in the extreme radiation, because of the ozone hole. All of a sudden, the water hits the algae and it springs to life. This is the bizarre part - when it dries up again, the algae curls up and wind blows it all over the place
it has a survival and spreading strategy: it is very resistant to radiation, and wherever the water is, there's probably a little piece of that algae waiting.
I can't say that that is going on at all on Mars. But it's clear that on Earth, organisms have adapted to very periodic availability of water and thrive in Antarctica under those conditions.
James Head III and Samuel Schon are based in the department of geological sciences at Brown University, in Rhode Island, US. James Head is Louis and Elizabeth Scherck Distinguished Professor at Brown. The research was funded by Nasa