Science —

Parts of Mars used to be Earth-like

Curiosity is climbing a martian mountain that used to be a lake bottom.

These sands look to have been laid down in a martian river delta over 3 billion years ago.
These sands look to have been laid down in a martian river delta over 3 billion years ago.

One of the joys of geology is painting a picture of a different world—a long-ago world otherwise completely lost to us. If you’re a planetary geologist, you have the chance to pull off that trick with a properly different world, one not currently named Earth. Courtesy of Curiosity, the robotic geologist currently exploring Mars, we have a new picture of the history of the crater the rover is roving in.

Gale Crater was excavated by an impact event roughly 3.7 billion years ago. Rising over five kilometers from the floor of the crater is Mount Sharp (officially Aeolis Mons). How’d it get there? That question is, in a way, interesting enough that we went there to find out.

After landing safely on the plain surrounding Mount Sharp, Curiosity has made its way to the base of the mountain, stopping to smell the rocks along the way. Climbing Mount Sharp takes the rover on a trip through martian history, as the mountain is basically a layer cake of sedimentary rock. A new study published this week by 47 researchers back on Earth focuses on what we’ve seen so far in the two layers at the base of the cake.

In the lower part of the basal layer, there are sandstones as well as some gravelly layers. The sandstones show a pattern called “cross-bedding”—repeated diagonal lines between the top and bottom of a layer. These are the remains of crawling ripples or dunes. In this case they are short ripples deposited by something flowing toward the center of the crater. Together with the presence of the gravels, this points to the action of streams.

The flowing water kind.

The upper part of the basal layer is different, but equally intriguing. Long, linear little ledges of rock were apparent from orbital imagery, but it wasn’t clear what they were. They turned out to be sandstones deposited in layers shaped like a long, stretched out “S.” But some of the particles in the sandstone are too large to have been carried by wind. One place this commonly occurs on Earth is at river deltas. Sediment gets draped over the gentle mound of the delta—almost flat on top, ramping downward at the front, and then flat again.

The next layer up in the cake is a little more uniform. It’s made of fine, mud-sized grains of sediment stacked in many thin little layers—the phyllo dough of sediments. Where do you find that kind of sediment? At the bottom of a lake.

Weighing all the details, the researchers think they can rule out wind as the transporter of these sediments. Instead, it looks like streams carried sediment from the crater rim toward a lake in the center, building a delta at the boundary between the two.

The sequence of events the researchers lay out goes like this: first, the crater obviously forms, with a small peak in the center ringed by a lake at least a few meters deep. As the rim of the crater erodes, water carries sediment into the lake, where it piles up as lake-bottom mud, delta sediment, and stream bed sediment. The next big chunk of the story is waiting to be read in the rocks farther up Mount Sharp, but the crater would have eventually pretty much filled with sediment. The weight of all that sediment compressed the stuff at the bottom, helping it harden into rock.

But at some point, we have to excavate the material around Mount Sharp, or Curiosity wouldn’t have a crater to drive around in. That erosion would have been done by the martian winds back when the atmosphere was still thicker.

The plains around Mount Sharp are estimated to be over 3.1 billion years old, so that leaves a few hundred million years after the crater was created for all this to play out. This would straddle two periods of Mars’ geologic history: the water-rich Noachian and the colder, drier Hesperian. But at least so far, Curiosity hasn’t run into any signs that the sediment formed under freezing temperatures.

Whatever the climatic details, the researchers say it seems there must have been pretty stable lake systems for thousands to millions of years—what they call a “geologically and perhaps biologically relevant period of time.” As University of Utah researcher Marjorie Chan puts it in an article accompanying the paper in Science, “The more the geology looks like Earth, the more likely it seems that some life-form(s) could have developed in the martian waters.”

Science, 2015. DOI: 10.1126/science.aac7575, 10.1126/science.aad0902  (About DOIs).

Channel Ars Technica