Placing a Bet on the Surface of Mars
Mars has been on my radar for a very long time, since the astonishing day back in 1965 when Mariner 10 first sent back a picture of craters on its surface. So I'm not a Johnny-come-lately to the red planet. I've followed the news from every Mars mission, orbiters and landers alike. But Curiosity, the most recent robot rover, has especially piqued my curiosity as a geologist. I think there are two reasons: the darn thing has finally become a decent field assistant, and NASA is sending it to some of Mars' most Earthlike places. So let me channel the late Huell Howser here and share some of what makes me go, "That's amazing!"
Unlike most previous landers, Curiosity has decent vision, about as good as my pocket camera. Its pictures actually look good on my desktop display, no longer like a frame grab from an old videocassette. Curiosity is a lot sturdier too—big, quick on its feet—and smarter.
It's got a nice hand lens, better than mine, that offers almost-microscopic closeups. It has a shovel, like lots of its predecessors did. One of my worst frustrations in watching Mars robots over the years was wishing I could lean in and just blow the dust off of things. Lo and behold, this rover packs a broom!
And while previous rovers had little grinding tools to function like the hammer and chisel in my field pack, Curiosity has a proper rock drill. They'll be testing it for the first time in coming weeks, somewhere in this car-sized piece of landscape named John Klein.
Until they can send a Gigapan outfit to Mars for a really huge, zoomable picture, the full-size version of this image on the NASA website at 3483 by 2651 pixels will be the state of the art.
The rocks in this part of Mars, on the floor of Gale crater, are full of minerals and features that testify to the chemical action of water. Here's a closeup from an outcrop called Sheepbed.
To all appearances it's a fine-grained sandstone, shot with veins of gypsum (like those I showed you last year) and tiny concretions of hematite, a hydrated iron oxide. Larger concretions lie in the surrounding dirt.
Elsewhere, the rover has shown us clear examples of crossbedding.
Crossbeds testify to not just the presence of water, but its physical action—rushing rivulets that sent large ripples of sand down their streambeds. Each crossbed represents the root of a ripple, spared from erosion in a setting where sediment was brought in faster than it was taken away. (Wind-blown sand dunes also make crossbeds, but the particles involved are much smaller.)
So, back to the drilling site at John Klein. I look at this detail and have several questions about it.
I see curving veins cropping out of the surface. The Curiosity team claims that "some of the veins have two walls and an eroded interior." I think they look more like double veins, but the rover will help us decide. Why are they protruding, and why is the surface around them so flat? The whole surface gives the impression of having been gently swept for a very long time—not strongly enough to streamline anything, but enough to winnow away the finest material as it works loose under Mars' 100°C daily temperature swings. What do the veins consist of, and why do they curve so tantalizingly?
Curiosity's camera is too far away to confirm or deny that curvature, and its x-ray instrument cannot yet tell us whether the veins are gypsum. So for now I can indulge in the hypothesis that we may be seeing the curving forms of liesegang structures, which you've probably noticed many times without knowing their name.
Liesegang (LEEZ-gahng) structures are thin waves of iron-oxide minerals found in porous rocks where chemically active groundwater has come and gone. In Earth rocks they may or may not affect a stone's strength, or the difference may not matter in the abrasive environment of a riverbed, like this example. But we already know that the right minerals exist on Mars, making up the concretions. Perhaps, under the utterly different conditions of Mars, these homely features can emerge to display their thin, curving, multiple form to Curiosity's eyes and toolkit.