RJ Emery wrote:
I was wondering what percent of the Martian days does the sun cast a shadow.
In fact, virtually all Martian days have sharp shadows. There is little to obscure them, as opposed to the traditional image of a smoggy, dusty sky. Check the front and rear hazcam images for confirmation of this. Only early mornings when there is fog, or when there is in fact a dust storm, will you see any softening of the shadows. Take note of the fact that no dust storms have been observed since the rovers landed.
RJ Emery wrote:
Are there many cloudy days on Mars, and if so, what are the clouds made of that obscure the sun?
There are quite a few cloudy days on Mars, but the clouds are typically fairly high altitude types, showing wispy structures similar to cirrus or cirrostratus clouds on Earth.
There is little or no obscuration of the Sun due to the thinness of the clouds, but they consist of water in the form of droplets or ice crystals. Hubble polarized light studies of the Martian clouds reveal that roughly half of them do not polarize the light while the other half do. Ice crystal clouds do not polarize light but water droplet clouds do. This shows us that half of the clouds are made of liquid water droplets or mist.
RJ Emery wrote:
By what mechanism do these clouds form?
The mechanisms are identical to those in our own atmosphere. Water evaporates from the ground from brine present in the soil, or sublimates from ice in the soil. This vapor is transported to the upper atmosphere where it nucleates droplets on fine dust or salt crystals, or those same particles nucleate ice crystals in colder circumstances.
In either case, you would be unable to distinguish between Martian clouds and terrestrial ones except for one feature: the very thin air means very little drag, and this places a sharp limit on droplet or crystal size. Thicker air such as we have can support larger droplets or ice crystals. Thinner air, like the Martian atmosphere, can only support very small droplets or crystals, regardless of the weaker gravitation.
RJ Emery wrote:
Mars is known for dust clouds, and dust is beginning to cover the solar panels, reducing their efficiency.
True enough, but we have not yet seen any of these ubiquitous dust clouds since the rovers landed. The deposition of dust is actually quite slow, which provides us with a good amount of data on how little dust there really is in the Martian atmosphere.
If the amounts were anything like NASA seems to protray, then the solar panels would have been useless within a few weeks. As it is, we see that the dust is taking three years in the case of Spirit to cover the panels. Again, we see how sharp the shadows are, and when we combine this with the fact that it takes years to cover the solar panels, then we know that there is nowhere near the level of dust that is officially proclamed. The "red skies" are pretty much a myth, supported by using the L2 infrared filter as "red" in the official versions of the images. If you use L4 as reality dictates, the skies turn out a bland white, gray or blue.
RJ Emery wrote:
In the two years the rovers have explored the planet, did it ever 'rain' or 'snow' upon them, and of what would that precipitation consist?
While there is little evidence of precipitation per se (except for a dusting of frost or snow during the winter months on occasion), there have been numerous incidents for Opportunity that NASA refers to as "Martian car wash". In my opinion this is when the eruption of geyser spray washes off the dust. We have no images of this for the following reasons: first, the eruptions would most likely occur early in the morning, perhaps before sunrise when air pressure is lowest and therefore the differential pressure from underground would be highest. Second, without a light source on the rovers, we cannot get images of the process as it happens, so no images are taken at the times when the eruptions would be most likely to occur.
Dust devils have sometimes been hypothesized as the cleaning agent, but again, no such event has been imaged, and if you think about it, a dust devil is just as likely to deposit the dust, not remove it.
So technically, precipitation does occur on Mars, but it is probably nearly all ice crystals or snow. However, fog does sometimes form and this has been imaged. A condensate forming on the solar panels could in fact be sufficient to wash them clean, in a manner similar to dew forming on your car. However, we would expect to see some evidence of droplet patterns on the panels when imaged by the microscopic imager. This has so far not been seen.
Could it rain on Mars? Perhaps, but not much. I think it is far more likely that the evidence of erosion that we presently see is from geyser activity. Have a look at this set of microscopic images showing a badly eroded rover track.
http://marsrovers.jpl.nasa.gov/gallery/ ... _m510.html
The markings are identical to those left by raindrops driven into soil, and the erosion shows spherules on pedestals, identical to those formed in a rain on Earth. Again, I doubt that rain did this, and feel it is far more likely to be from groundwater erupting in a spray. This explains the erosion as well as the solar panels being cleaned. Compare the eroded images to these fresh rover tracks:
http://marsrovers.jpl.nasa.gov/gallery/ ... _m023.html
The conclusion is that the weather on Mars is as dependent on water as it is on Earth. Water can carry the latent heat of evaporation and through the change of state provide the thermal energy and expansion and contraction that is needed. That is of course powered by the Sun.
