Spirit Finds Stromatolite Fossils
Primitive organisms are probably the first multi-celled life
On Earth, single-celled organisms dominated the planet for billions of years. The same was probably true on Mars. Once blue-green algae evolved, they changed the atmosphere from carbon dioxide, nitrogen, and methane to something more like what we have today. In fact, the oxygen they produced "rusted" the free iron in our soil and helped to produce the iron deposits we mine today.
On Mars, the "great rusting" of the Pre-Cambrian happened and turned the planet red, but it apparently had lost so much atmosphere that the rust remained where it was due to poor rainfall. This is an important clue about Mars' deep history and when its biosphere became very unlike our own.
Stromatolites typically are a form of blue-green algae that forms colonies- they grow layers of algae in a form something like cabbage, in thin sheets that are rounded and layered. Many types of stromatolite lived in hypersaline lakes, and the extremely salty Martian soil is the remnant of such hypersaline brine. But what makes them different from other algae is that they also grow layers of mineral in them, like a seashell. Calcium carbonate makes these layers hard as rock and supports the algae cells so they can get the most sunlight and nutrients.
These images show very clear cabbage-like layering and structure and are exactly identical to stromatolites here on Earth. No geological process that does not take life into account can produce this structure. This is perfect and indisputable evidence of past life on Mars.
| This is from Spirit,
Sol 105. In this picture there are two stromatolites. One
is smaller and located to the left, and one is large and located to the
right.
I assembled this color picture from left filters 4, 5, and 6. There are also pictures from L7 and R1. The R1 picture allows the assembly of a stereo image since it was taken from a different angle. Click the image for a full-sized picture. |
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| See the concentric
rings? It looks like a sawed-off log. Stromatolites grew in
rounded layers, and each layer left a thin shell of limestone
behind. Click the image for a larger view.
This structure is undoubtedly made of calcium carbonate or some other simple carbonate mineral. By comparing the optical and infrared spectra available in the images, we can see that it matches those of many other rocks, leading us to conclude that they, too, are carbonate minerals. Click to compare with terrestrial stromatolites. Here is another terrestrial stromatolite. And here is a third stromatolite image showing how they tend to form in groups, just like these images show. Now, could those flakes and flat pieces actually be in the rounded, "dome" shape that we expect, or is this just an illusion? That is very simple to check with a stereo picture. I assembled this one from the one color image and one right channel of the panoramic imager. |
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Since there is no color data in the right filter 2 frame, I have adjusted its color to nearly match that of the true color image. This makes visualizing the three dimensional image much easier. Otherwise, one eye would see color and the other monochrome- an unsettling experience for some.
This is a cross-eyed stereo image. Lean back from the screen and slightly cross your eyes to get the stereo view.
What this reveals is that the thin plates are absolutely in a perfect concentric pattern, and that they are indeed very thin and convex. Now we know what all those thin, flat plates of rock came from. These are not layers of sediment (which in itself would be strong evidence of liquid water) but instead are the plates from a crumbling fossil colony of algae. And now we know what the "hollow" rocks are that show up all over Gusev Crater, which is a dried out lake bed.
Here is a link showing living stromatolites and the conditions they consider favorable. It also shows that flat plates of rock can be stromatolites, and it explains the odd textured flat sheets we find all over Gusev Crater. They too are likely the remains of ancient stromatolites.