This clear indication comes from recent soil experiments
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Damp soil has packed inside the cleats of the rover's wheels, and now it cannot be dislodged. My recent in-depth experiments with Martian soil simulant reveals that the cause is moisture that is drawn from the ground and atmosphere by the salts in the soil. This makes the soil sticky and when you combine this with the lower gravity of the planet, there is not enough force to free the soil from the wheels. This destroys the traction and the rover sits bogged down.
JPL and NASA scientists have been creating Mars soil knockoffs in a series of attempts to get Opportunity free. The test rover here on Earth can crawl out of the mess every time. The problem is, their efforts are crippled by their failure to accept the presence of water. This one crucial factor dooms the efforts to failure.
On Earth, dry packed soil, even with very fine dust (such as diatomaceous earth) easily dislodges because it weighs over two and a half times as much as it would on Mars. In other words, the soil's own weight helps to drag it out of the wheel cleats. But on Mars, this packed soil will not fall out. Add to that the fact that it is clinging due to the salts it contains, and you have a hopeless situation.
Here is the set of images that proves that Mars is wet now, and that they will have to find another solution to their problem.
| This is the wheel track from Sol 447.
Notice that it is covered in the nearly white material?
This is a mixture of salts that is contained in the
soil. When the soil contains enough water, the rover
wheels literally squeeze the moisture out. The wheels
are relatively hot, and they flash-evaporate the water,
leaving the white, shiny crystals behind. Some are
genuine ice crystals, but others are not.
But there are other signs that the soil is wet, and I will go into them in detail shortly. This is a false color image from R1/R2 data. The original image is here at the NASA/JPL web site. |
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| This is the exact same track on Sol 461.
Notice anything different?
All the white crystals are gone! But how can that be? If that was salt, where did the salt go? If it wasn't salt, then it would have to be ice crystals. The reality is this- a mixture of ice crystals and salts was left in the tracks, as we have seen many times in the past. Now, 14 sols later, the crystals have vanished. This image is from R1-R2 data, and the original image is not yet published on the NASA/JPL site. |
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| In this close up view from Sol 447, we can see
the complex cleat pattern is sharp and well defined. We
can also see the fine line of crystals that fills each
line. The salt crystals are very sharp and easy to
distinguish. Contrast has been enhanced to help pick out
the differences.
The following image has undergone exactly the same processing, including the contrast enhancement. |
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| But here, in this close up from Sol 461, the
lines are dark. The salt crystals are gone and the tread
pattern is starting to erode away. In other words, we
are seeing how fast a marking in the soil erodes on
Mars. The answer is, it takes about two weeks for the
signs of erosion to be clear.
This proves a very important point- soil erosion on Mars occurs in two weeks or less. We now know that it takes just days to wear down fine features based on this simple observation. features pressed into the soil will start to break down that rapidly, even without a dust storm or other overt weather. |
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Dry soil on Earth will not take an imprint. The sample
on the left is an example of this.
Dry soil on Mars is identical and will not take on imprint. The image on the right illustrates this. The image is here on the NASA/JPL web site. |
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Damp soil on Earth takes a sharp imprint. The image on
the left shows this.
Damp soil on Mars also takes an imprint. The image from Sol 061 on the right shows that the damp soil even takes the imprint of screw heads in fine detail.. The original image is here at the NASA/JPL web site. |
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Damp soil on Earth "lenses" when stressed.
Chunks break free and move as a single piece. See the damp sand image on the left.
Damp soil on Mars also lenses. See the image on the right to prove this. The original image is here at the NASA/JPL website. |
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Wet soil on Earth bulges when you compress it. This is
nearly mud. The left image shows the results.
Wet soil on Mars also bulges when compressed. The rover's instrument head was pressed against this mud on Mars. In this case, it is even reflecting sunlight. See the image on the right. The image is here on the NASA/JPL web site. |
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| Now, let's put all the evidence together with this Sol 461
image of the rover track, 14 sols after it became stuck.
A - sharp, well-defined imprints, razor-sharp edge of the track and the cleats, such as damp soil will produce B - darker, richer soil color, indicating salts and moisture C - white salt crystals have vanished, due to the action of dew, subsurface moisture, or both D - soil lensing where pressure is applied, making the soil break into coherent clumps E - crust on the outer surface that sticks together, typical of high-salt soils that have been dampened and then dried again All indications show that this soil is wet on a regular basis. The chemical analysis shows the presences of salts, which draw moisture from their environment. The microscopic images show a crust on the soil. The missing crystals have dissolved back into the soil due to moisture. Conclusion: Opportunity is stuck in mud, and liquid water is present on Mars at this instant. |
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So clearly, moisture had to be responsible for the disappearance of the salts in the soil. This is why the dune is so difficult to drive out of. Water present in the soil has created a sticky mixture, and the freezing of the ground makes ice crystals fluff the soil back up every time it gets cold enough. This makes it soft and impossible to drive through.
The solution is to "jog" the wheels, one at a time. This is done by rapidly shaking the wheels back and forth. If the motors can be made to cycle rapidly enough from right to left, it may shake enough of the soil loose to enable the rover to make some progress.