Water is present on Mars today, but it is entirely bound up in ice because the surface is too cold for liquid water.
But evidence has been mounting that shows water once flowed across the Martian surface, potentially supporting life. While water does not mean there was life, it's a key prerequisite.
A new study of a system of gullies worn into the surface of Mars suggests the most recent period of water flow on the red planet was only 1.25 million years ago.
But throughout the more than 4 billion-year history of our neighbor, its climate has cycled back and forth between warmer and cooler periods as the planet wobbled on its axis.
Like Earth, Mars' axis is tilted with respect to its orbital plane and the degree of tilt changes over thousands of years.
But Mars' tilt changes more over time, alternately heating up or cooling down parts of the planet as the amount of sunlight falling on them changes.
Over the past decade, scientists have found numerous geological features, such as gullies and possible lakebeds, that indicate water was once present on the surface.
They have also found water-bearing minerals, such as opals and carbonates that show that water has reacted with the Martian regolith, or dirt.
Gullies are known to be young surface features on Mars, but pinning a precise date to them has been difficult.
The new study, detailed in the March issue of the journal Geology, did just that with a gully system located on the inside of a crater in Promethei Terra, showing that water flowed on Mars more recently than previously thought.
"We think there was recent water on Mars," said study team member Jim Head of Brown University in Providence, R.I. "This is a big step in the direction to proving that."
The gully system the team examined shows that water-borne sediments were carried down steep slopes of nearby alcoves and deposited in a fan-like shape during four different intervals.
"You never end up with a pond that you can put goldfish in," said study team member Samuel Schon, a graduate student at Brown. "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."
Viewed from far away, the fan looks like one continuous feature, but close-up images taken by the NASA Mars Reconnaissance Orbiter HiRISE camera show four distinct lobes in the alluvial fan.
Schon was able to determine that the lobes were created at different times and could tell which was the oldest because it was pockmarked with craters, while the younger lobes were left relatively unblemished. (The longer a surface has been exposed, the more meteorites have had a chance to leave their mark.)
Schon linked the craters in the oldest lobe of the fan to a rayed crater more than 50 miles (80 kilometers) to the southwest.
He dated the crater to about 1.25 million years, which meant the younger lobes of the fan could not be any older than that.
The team also determined that ice and snow deposits formed in the alcoves when Mars was tilted so that it plunged into an ice age and ice could form in the mid-latitude areas, instead of being confined to the poles, as it is today.
About half a million years ago, the planet's tilt change and the ice began to melt or sublimate.
Schon said that other explanations for the water's presence in the gully were ruled out: Groundwater bubbling up seemed unlikely to have occurred multiple times, and dry mass wasting (for example, a rockslide) also didn't seem to fit the pattern.
The findings add more evidence that Mars underwent a recent, geologically-speaking, ice age that moved water closer to the equator of the planet.
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