Apart from two wandering rovers, there's not much going on the martian surface these days. In fact, scientists believe the planet has been relatively dead for the past 3.5 billion years. But new research suggests that in at least one place, water gushed over Mars's surface less than 1 billion years ago. The finding increases the likelihood that life may have existed relatively recently there.
In 2004, the twin Mars rovers, Spirit and Opportunity, confirmed scientists' suspicions that water covered much of the martian surface from about 4.5 to 3.75 billion years ago, when it was a much warmer planet (ScienceNOW, 16 December 2004). After that time, Mars cooled down and dried up. Scientists have found evidence of flowing water, as revealed by gullies that formed over the past few million years (ScienceNOW, 22 June 2000), but the amount of water has been relatively small, and where the water came from is still unknown. Now researchers have evidence that rivers, tens of kilometers in length and about 250 meters across, carved valleys up to 20 meters deep across the martian landscape.
Planetary geologist Jay Dickson of Brown University and colleagues looked at a 7000-meter-deep depression in the martian surface known as the Lyot crater. Situated at about the same latitude as southern Canada in Mars's northern hemisphere, the crater contains buried glaciers and has a current average temperature of –16°C. But its location in the mid-latitudes could have seen temperatures up to 15°C during times when Mars was tilted more toward the sun than it is today, Dickson says. And because the crater is so deep, the atmospheric pressure is high enough to allow ice to melt, instead of turning into a vapor as it typically does over the rest of the planet.
Using high-resolution images from the Mars Reconnaissance Orbiter, a spacecraft that orbits the planet, Dickson's group zoomed in on the eastern half of the crater and discovered about 20 winding valleys and a number of deposited materials typically found at the end of rivers. Counting the number of impact craters on the surface (the older the surface, the more impact craters it will have), the researchers managed to date the surface to about 1 billion years old. That means that the rivers must have formed after this time, the team reports in the last issue of Geophysical Research Letters. Because water was there relatively recently, the river valleys might also be a good place to look for evidence of life, Dickson says.
"We've traditionally thought that Mars has been inactive in terms of fluvial features [features shaped by flowing water] throughout the last couple billion years," says Alan Howard, a planetary scientist at the University of Virginia, Charlottesville. This work "brings [liquid water] activity on Mars to a much younger age." Planetary scientist Jeffrey Kargel of University of Arizona in Tucson agrees. "It's a beautiful case," he says, because the findings don't just point to water, but make a compelling case for where it came from, which has not necessarily been the case for other evidence of recent water on Mars.
For more on the Mars rover, Spirit, which may be close to expiring, click here.