The gullies may extend the history of running water on the surface of Mars
Mars appears to have had running water on its surface about one million years ago, according to new evidence.
Images from a Nasa spacecraft orbiting the Red Planet show fan-shaped gullies on the surface which seem to be about 1.25 million years old, the study says.
They believe the channels were sculpted by surface water from melting ice.
It may represent the most recent period when water flowed on the planet, a team from Brown University in Rhode Island, US, report in the journal Geology.
Gullies on the Red Planet are known to be young features, but scientists have found it difficult to pin down their precise ages.
But Samuel Schon and colleagues from Brown were able to do this using impact craters on a gully system in Promethei Terra, an area of cratered highlands south of the Martian equator.
"You never end up with a pond that you can put goldfish in," Mr Schon explained.
"But you have transient melt water. You had ice that typically sublimates. But in these instances it melted, transported, and deposited sediment in the fan. It didn't last long, but it happened."
The researchers say the discovery of a gully system, even an isolated one, that supported running water as recently as 1.25 million years ago greatly extends the time that liquid water could have been active on the Red Planet.
It also adds to evidence that Mars experienced a recent ice age in which polar ice is thought to have been transported toward the planet's equator, where it settled in mid-latitude deposits.
From afar, the gully system looks like one entity several hundred metres wide.
But detailed study of images from Nasa's Mars Reconaissance Orbiter (MRO) spacecraft shows there were four intervals in which water-borne sediments were carried down the steep slopes of nearby features called alcoves and laid down in a deposit called an alluvial fan.
In order to estimate the age of the system, the scientists used a method of counting craters.
Because impacts occur with some regularity, this has become an established way of dating planetary surfaces. More cratered surfaces are deemed older, while smoother surfaces are considered younger.
The researchers accept that because the rate of cratering may shift up and down over time there is a degree of uncertainty in this calculation. But they say this is within acceptable limits.
Mr Schon and his colleagues were able to distinguish four individual "lobe" features which make up the alluvial fan, and determine that each of these lobes must have been deposited in separate events.
One of the lobes was pockmarked with small craters; the scientists identified this as the oldest component of the fan. The other lobes, meanwhile, were unblemished, suggesting they had to be younger.
"We think there was recent water on Mars," said co-author James Head III, a professor of geological sciences at Brown University.
"This is a big step in the direction to proving that."
Using a comparison with another cratered surface 80km to the south-west, the team was able to date the oldest lobe of the fan to about 1.25 million years ago.
This established a maximum age for the younger, superimposed lobes.
The researchers suggest the formation developed when ice and snow deposits formed in the alcoves during the most recent ice age on Mars.
About half a million years ago, ice in the mid-latitudes began to melt or, in most instances, changed directly to vapour - a process called sublimation.
The team considered other options, such as groundwater bubbling up to the surface. But they say the most likely mode of formation for the gullies was the melting of snow and ice deposits that created "modest" flows of water.
The finding follows the discovery of water-bearing minerals such as opals and carbonates on Mars.