Geologically, there are ways in which minerals move around and get deposited with rock. A common phenomenon is for a crack to form due to cooling of molten rock or an earthquake or something, and then this space gets filled in. Stuff might just fall into it. Liquidizer rock (magma) might intrude into it. Hot gasses containing residue might build up a deposit within it, or liquid water might flow through it leaving behind minerals, which fill the crack. The thing is, geologists have studied these processes and have a pretty good idea of what they are, how they work, and what they look like.
NASA’s Mars Exploration Rover Opportunity has found bright veins of a mineral, apparently gypsum, deposited by water. Analysis of the vein will help improve understanding of the history of wet environments on Mars.
“This tells a slam-dunk story that water flowed through underground fractures in the rock,” said Steve Squyres of Cornell University, Ithaca, N.Y., principal investigator for Opportunity. “This stuff is a fairly pure chemical deposit that formed in place right where we see it. That can’t be said for other gypsum seen on Mars or for other water-related minerals Opportunity has found. It’s not uncommon on Earth, but on Mars, it’s the kind of thing that makes geologists jump out of their chairs.”
Geologists are pretty staid and conservative folk. Jumping out of their chairs is a very, very big deal.
The vein examined most closely by Opportunity is about the width of a human thumb (0.4 to 0.8 inch, or 1 to 2 centimeters), 16 to 20 inches (40 to 50 centimeters) long, and protrudes slightly higher than the bedrock on either side of it. Observations by the durable rover reveal this vein and others like it within an apron surrounding a segment of the rim of Endeavour Crater. None like it were seen in the 20 miles (33 kilometers) of crater-pocked plains that Opportunity explored for 90 months before it reached Endeavour, nor in the higher ground of the rim.
They believe the mineral is calcium sulfate, which makes up a lot of different rock types. When calcium sulfate forms in a water-rich context, it forms gypsum. And this looks like gypsum.
Gypsum had been found on the planet before, but its point of origin was unclear. This could help explain that, although the structural context for the formation of this mineral could take on a lot of forms. For instance, if a crater is periodically filled with water, which then evaporates, various minerals will form in the crater, and sometimes, that will be gypsum.
I don’t want to get your hopes up, but gypsum also has, under certain conditions, an interaction with bacteria. Bacteria is not necessary to form gypsum, but I suspect that if gypsum was formed in association with bacteria, there would be a fossil or isotopic signature. Likely, the instrumentation on mars would not be able to detect that at this point, but in the future it seems like a possibility.
The other thing that veins of gypsum within rock indicate is groundwater relationships. This is part of a larger phenomenon: As water accumulates, evaporates, or flows both over the surface or underground, minerals get moves around and form deposits in situ which indicate where the action (or stagnation) is. These veins of gypsum are so far only known on a craters’ slope, not on the plains below the slope. This may give a picture of the movement of ground water from depth after the crater impact disrupted the surface, or as evaporative conditions changed with climate shifts. Or something .