Q&A: Liquid water on Mars

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.

Ice may have been transported from the Martian poles to lower latitudes Mars had 'recent running water'

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.

PR: How would you describe the significance of this evidence from Mars' Promethei Terra region?

SS: 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.

JH: 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.

There is a lot of evidence there was a significant amount of water early in Mars' history Professor James Head, Brown University

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).

PR: It's much colder on Mars than it is on Earth. How can water stay liquid on the surface?

The gullies may extend the history of running water on the surface of Mars Enlarge Image

SS: 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.

JH: 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.

SS: The Earth's Moon plays a pivotal role is stabilising the Earth's obliquity variations