Stem Formation From Geyser Action
Even better images of the geyser erosion, and mysterious clay "fins"
Examination of the Sol 164 images from inside Endurance Crater show that geysers are forming stems even now. It also reveals good close-up images of the mystery "fins" that have shown up in Gusev as well as here and there at Meridiani Planum. First, let's look at a stereo view of the mystery fins.
Cross-eyed stereo view- this shows the wafer-thin "fins" that have been seen by both rovers in clear detail. Clay material is being struck by high-speed water droplets from an active geyser and carried up and away from the impact. On Earth, gravity might cause these structures to collapse, but on Mars they are rigid enough to stand. Thin atmosphere ensures that they dry quickly, remaining in the upright position.
Notice the thin standing wall of material at the back edge of the rock slab? This appears to be formed by water. A geyser sprays high speed, tiny droplets of water that strike the edge of the rock slab and push a thin layer of clay upwards. On Earth, our gravity would tend to make such a thin wall collapse, but on Mars, with only 38% of the gravity we have on Earth, the wall can easily stand without collapsing. Click here for an anaglyph.
Once the geyser stops ejecting spray, the thin air allows the water in the clay to evaporate quickly. This hardens the thin wall and leaves it for the next spray episode. The most interesting point is that Opportunity showed the presence of large amounts of magnesium and sulfur, and the presence of silicon is a known. There is a mineral called palygorskite (classed as a "sheet silicate" mineral) that is made of hydrated magnesium aluminum silicate that easily forms sheets and cloth-like structures. It would not be surprising to find that many of these odd, "drapery-like" rock structures on Mars could be made of palygorskite or some similar mineral. (The third specimen down on this page shows the structure and characteristics of palygorskite.)
On Earth, thin clay walls also form under some circumstances. In particular, they form when damp, claylike soil is compressed under rock slabs, and can be driven by freezing and thawing or by rock sinking slowly into damp ground, for example. This is called frost heave and it can also explain the presence of all the spherules on top of the soil.
Also, the phenomenon known as "mud volcanoes" produces eerie and odd looking mud outflows, driven apparently by the escape of natural gas. Methane and water can produce compressed pockets of mud that can drive spouts of water, mud, and gas. On Mars, the geyser activity could easily be driven by geothermal heating or by the escape of natural gas from underground.
Stem formation and geyser action, part two
When a geyser erupts and sprays its water on Mars, the droplets encounter almost no air resistance and continue at high speeds before striking a surface. This means that much more energy is presented by the water flowing from geysers on Mars. The result is that much more erosive action occurs if this high speed water carries small amounts of sand or silt. Click here for a large anaglyph.
A geyser from the left of the image has sprayed parallel with the crater wall, eroding the rocks in characteristic manner. Notice how all the stems point to the left- they point directly at the geyser fumarole. Also see how the "wedge" of rock on the ground is worn into points and edges that also point to the left? The soil is undercut from the rock slab, showing how water has washed it out. And, a large rounded channel has been worn in the rock slab at the left. The original image is here at the NASA/JPL web site.
The end result is the sculpting of rock substrates in this manner. Note how the harder fossil spherules show little wear compared to the rock substrate around them. The water, carrying sediment and dissolved minerals, washes and wears the softer sedimentary rock, leaving the spherules and the rock behind them untouched.
On drying, minerals in the water soak into the stems and might harden them, although at this point it is only speculation on my part. However, it is expected that subsurface water on Mars would contain significant amounts of dissolved carbon dioxide, like seltzer water. There will also be salts and minerals in solution, as is common on Earth. For Mars, however, the amount of salt in the water would likely be much higher, as the salt was left behind when the oceans and lakes evaporated.
A stereo view of the eroded rock and newly formed stems
This view is a cross-eyed stereo image of the eroded rock and the resulting stems. Look closely at the curved "knife-edge" of rock at the lower left of the image, just above Opportunity's solar panel. See how the fine, sharp points jut into the geyser vent? The larger rock to the right and top (the back of the image) also shows long stems with fossil spherules on the tips. Behind it (to the far right) is the washout area where the resulting water has flowed back downhill (which is to the left) and through the channel between the two stone slabs. Click here for an anaglyph.
Finally, the flow of water and the erosion patterns are shown in this close-up.
| The path of the geyser's
spout is shown in large red arrows. The most prominent stems are
shown circled in green. The runoff path and channels are shown in
blue.
This picture shows that the hydrological processes on Mars today are dynamic and ongoing. The presence of liquid water on the surface is mostly short-lived, unless it is soaked into the soil, or seasonal air pressure is higher. However, the presence of large reservoirs of groundwater is clearly demonstrated through the stems and their formation. |
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