THE Mars rover Opportunity has been providing ever more spectacular images and data as it carries out its first close-up surveys of an outcrop of bedrock that has tantalised geologists since the rover landed on 25 January.
Opportunity鈥檚 initial study of the finely layered bedrock has already revealed important details. Its images show the rock is made up of very fine parallel layers. This had excited scientists, as it raised the possibility that it was sedimentary rock formed in deep water. But the latest images show that the rock is made of extremely fine-grained material, which means this is unlikely.
And after finding a scattering of tiny spheres a few millimetres in diameter near its initial landing site, last weekend Opportunity found a mother lode of these spheres (main picture). They appear to be heavily concentrated in and around the bedrock, with hundreds of them spread across the surface. In at least one spot a single layer of the rock appears as a band of uniform spheres and in others, they are clearly weathering out of the rock.
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Lead scientist Steve Squyres says that the thinness of the layers, along with the way the spheres are embedded in them and the fact that the rock appears well cemented together, mean there are now two main possibilities for how the bedrock formed.
One is that the rock is made of layers of very fine volcanic ash. The spheres could have formed separately, as droplets of molten rock that were ejected skyward by a volcanic eruption or the blast of an impact and which solidified into spherical shapes as they rained back down. If they turn out to be glassy in composition, this could clinch the case for volcanic composition.
The other main possibility is that the spheres formed as water percolated through layers of fine sediment, originally laid down by either wind-blown dust or volcanic ash, and slowly built up spherical shapes as minerals precipitated out onto grains, layer by layer. If signs of concentric layering are found within the Martian spheres, it would be strong evidence that they formed by such an accretion process.
The images have revealed several broken spheres, providing an opportunity to test this idea by closer examination. Science team member Tim Parker told 快猫短视频 that there are already some 鈥渢antalising hints鈥� of such layering in some of these broken spheres. But the signs are preliminary and will require much closer examination before they can be confirmed.
This second scenario does not necessarily rule out a role for standing water. It is possible that the fine-grained material of the bedrock was laid down in shallow water. A detailed analysis of the layering itself might still point to formation in water, if it shows ripples or certain kinds of cross-bedding.
More clues should come when the rover gets a chance to take a close look at the haematite, an iron oxide usually formed in water. Its unusual concentration in the area was the main reason it was selected as the landing site. An infrared image of the outcrop area taken with the miniature thermal emission spectrometer (mini-TES) has revealed a very uneven distribution of haematite. It is the first geological map made of the surface of another planet (see Graphic), and will guide the rover鈥檚 survey.
Most of the haematite seems to be in the material above the bedrock, and it may be some time before the rover takes a close-up look at this. But Philip Christensen of Arizona State University, designer of the mini-TES, thinks spectral analyis has the potential to rule out any process for the formation of haematite that does not include water.
Squyres says the study of the site is unfolding like a detective novel, with lots of clues and probably some red herrings. But he is confident it will be possible to figure out exactly how the bedrock formed.