Spirit's Mysterious "Moat" Rocks
Another strong indicator of subsurface water activity
Recent images from Spirit have shown some odd sights. There are numerous rocks that seem to be identical to the other rocks except that they exhibit a trench or moat surrounding them. Most of the rocks are very ordinary in their placement and are sunk into or laid atop the sand. These rocks, however, appear to have some sort of landscaping done to them. The explanation turns out to be very simple.
Examples of moat rocks in Gusev Crater
| Let's start out with a
look at some of these mystery rocks and what makes them stand out.
This color thumbnail shows a typical moat rock. It was imaged by
Spirit on Sol 344.
The rock is identical in any respect to the other surrounding rocks, but those rocks seem to be worked into the soil and not disturbing it in any particular way. Notice that this rock has a "bowl" around it in the sand. Click the image for a larger view. Click here for a stereo view, here for NASA data. |
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| Here is another of the
moat rocks. This is from the Sol 335 navigation images.
At first glance, it too shows nothing really unusual, but when you compare it to the rest of the scenery, it stands out quite sharply in contrast to the normal rocks in sand. All the other rocks are partially buried and the sand "ramps" up onto them. Clearly, some influence other than wind has created the moat around the moat rocks. Click the image for a larger view. Click here for a stereo view, here for NASA data. |
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| This was also imaged on
Sol 335, a short distance from the above moat rock.
It appears that the size of the rock is about average, not very small and not very large. All of the rocks should have something in common and by comparing them we might be able to find out what that common factor is. Click the image for a larger view. Click here for a stereo view, here for NASA data. |
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| This is perhaps the single
most striking example of the moat rock phenomenon. It clearly shows
a ring in the sand around a rock, and the other rocks scattered about the
area show no such feature. Some influence has operated here that is
not immediately apparent to the eye.
Once again, the rock is of average size and does not appear to have any special composition or difference from the other rocks. This image is from Sol 337 and was also imaged on Sol 350. Click the image for a larger view. Click here for a stereo view, here for NASA data. |
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| This image shows a pair
of moat rocks next to each other, yet in the larger picture (click the
image to see it) all other rocks in the area are normal and do not exhibit
moats. This is a crucial piece of the puzzle. It demonstrates
that the rock itself is not the cause of the moat phenomenon.
Rather, location is the key. This is from Sol 339.
Click the image for a larger view. Click here for a stereo view, here for NASA data. |
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| There are numerous other
moat rocks, and one image from Sol 340 shows three of them in a rough
line. This is one of them, and while this is not meant to be an
exhaustive list, the examples presented here show that something is
happening with these rocks and excluding their neighboring rocks.
To see an example of an extensive rock field from Sol 340 with a single moat rock centered in the frame, click here. |
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As we can see from these different moat rocks, the size is unimportant. There are various sizes of moat rock in the images. We can also rule out rock type because some of the rocks are relatively smooth while others are quite rough. And, we can rule out the shape of the rocks- some are round and others are irregular. We can also rule out wind since the surrounding rocks would have similar features sculpted around them if wind were responsible.
Any reasonable investigation should include a control or baseline to compare the phenomenon in question to. This is an image of the rocks in the immediate vicinity of the moat rocks, showing that virtually every rock is surrounded by sand in the expected manner. Only a handful of the moat rocks show themselves, as can be seen here.
Here is a crop of an image field from Sol 347 to show the regular appearance of those rocks.
The action of wind has spread the sand over the surface and up onto the rocks. In no case does the action of wind create a moat or ring around any of these rocks. Since wind cannot select which rock to affect, there must be a force other than wind capable of creating the moats.
What is causing this odd phenomenon?
Since neither the rock nor the wind is responsible, there must be some other explanation. The only consistent difference is the location of the rocks- and that is borne out by the pair of side-by-side moat rocks in the image from Sol 339. In that image, we can see two rocks in nearly the same location show the same effect. The question now becomes, "what is special about those particular locations?"
As it happens, there is a very simple explanation. It is related to the geysers in Meridiani Planum. As subsurface water leaks toward the soil from deep underground, it begins to boil. This releases quantities of low temperature (but relatively high pressure) steam. This steam would normally vent into the atmosphere or soil and be unnoticed.
But moat rocks happen to rest on top of these vents and cap them off, thereby increasing the steam pressure and allowing it to vent from beneath the rock. This steam literally blows the sand away from the rock. Since most rocks do not cap off a steam vent, they do not have the sand blown from beneath them and are partially buried. But the periodic venting of this steam keeps the base of the moat rocks clear of sand and creates the ring that so precisely outlines these stones.
The bottom line is that Gusev, like Meridiani Planum, is home to a large number of subsurface water vents and in this case, the venting creates steam pressure. Technically, these vents are called fumaroles. This is the cause of the moat rock phenomenon.
This can be verified by imaging the rocks closely just at sunrise or sunset and attempting to locate the source of the fog or ice crystals that commonly form on Mars, particularly in the mornings.