Reading the Rock Record on Mars

Mars Rocks!

While we’re waiting for more news from New Horizons, I wandered over to the Mars Curiosity web site to look at what our intrepid rover has been doing since scientists regained communications with it. They didn’t have much contact with it when Mars was on the “other side” of the Sun during much of June, so the images slowed to a halt for a short time.

Curiosity is now exploring a region where the rocks are telling us a story about the environmental conditions on Mars in the distant past. The image here shows an area where two different rock types meet. One type is made up of light-colored mudstone, which indicates that the region was once under water that wasn’t moving very fast (if at all). The other type is a sandstone that appears to have been laid down in multiple layers as water moved across the surface.

What does that tell us about the long-term history of the planet? For now, it means that water once clearly existed in the region where Curiosity is studying the rocks. The mudstone was laid down by standing water, perhaps in a lake or a pond or a deep-ocean region. Mudstone is made of fine particles of rock that wafted to the bottom of the body of water and was allowed to rest before the water went away and the mud hardened to rock.

The sandstone is larger grained than the mud and silt, and it was likely carried along by a slow-moving current in a lake or sea, or by a quiet river. It had time to settle into layers and harden, so it’s unlikely that there were many catastrophic floods at the time the sandstone was laid down. If there had been floods, we’d see larger rocks embedded in the sand.

Rocks can tell us many stories about the environment, if we know how to read what they have to say. That’s the job of geology — to help us make sense of how the rocks were made, deposited, and perhaps carried along by water and wind.

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