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The Mars Curiosity rover recently accidentally cracked through the unremarkable exterior of a rock and found a surprise.
When the rover rolled its two-thousand-pound body over the rock, the rock broke open and revealed yellow crystals of elemental sulfur. Sulfates are fairly common on Mars. However, this is the first time sulfur has been found on the red planet in its pure elemental form.
The Gediz Vallis Channel, where Curiosity found the rock, is littered with rocks that look suspiciously similar to the sulfur rock before it got fortuitously crushed. This suggests that, somehow, elemental sulfur may be abundant there in some places.
Ashwin Vasavada is a Curiosity project scientist at NASA's Jet Propulsion Laboratory. He said, “Finding a field of stones made of pure sulfur is like finding an oasis in the desert. It shouldn't be there, so now we have to explain it. Discovering strange and unexpected things is what makes planetary exploration so exciting.”
Sulfates are mineral salts that form when sulfur, usually in compound form, mixes with other minerals in water. When the water evaporates, the mixture of minerals dries, leaving the sulfates behind. These sulfate minerals can tell us a lot about Mars. They reveal the water history of Mars, and how it has weathered over time.
Pure sulfur only forms under a very narrow set of conditions, which are not known to have occurred in the region of Mars where Curiosity made its discovery.
There are a lot of things we don't know about the geological history of Mars. However, the discovery of scads of pure sulfur just hanging about on the Martian surface suggests that there's something pretty big that we're not aware of.
Sulfur is an essential element for all life. It is usually absorbed in the form of sulfates, and used to make two of the essential amino acids living organisms need to make proteins.
We've known about sulfates on Mars for some time and the discovery doesn't tell us anything new in that area. We're yet to find any signs of life on Mars. But we do keep finding the remains of bits and pieces that living organisms would find useful, including chemistry, water, and past habitable conditions.
We're fairly limited in how we can access Mars. Curiosity's instruments were able to analyze and identify the sulfurous rocks in the Gediz Vallis Channel. However, if it hadn't taken a route that rolled over and cracked one open, it could have been sometime until we found the sulfur.
The next step will be to figure out exactly how that sulfur may have come to be on Mars. That may possibly involve some detailed modeling of Mars's geological evolution.
Curiosity will continue to collect data on Mars. The Gediz Vallis channel is an area rich in Martian history. It is an ancient waterway whose rocks now bear the imprint of the ancient river that once flowed over them, billions of years ago.
Curiosity has drilled a hole in one of the rocks to take powdered samples of its interior for chemical analysis. The rover is now trundling its way deeper along the channel, to see what other surprises might be waiting just around the next rock.