UV imaging of Mars and the possibility fluorescence on Mars.

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I was reading some of the Cassini reports on the observations of the Saturn system and was interested to see the reports on the Cassini UVIS instrument. The wavelengths investigated with the UVIS go down to
100 nm and lower, i.e., within the ultraviolet range. This is about 1/5th the lowest wavelength used for Mars orbiters' optical and infrared observations. The latest Mars orbiter, MRO, has a resolution
of about 25 cm in the visible range. If the same sized mirror had also been used to make UV measurements for MRO we might have been able to get a resolution of 5cm, about 2 inches, using the smaller wavelengths. These would be gray-scale images since these are below the visual wavelengths, equivalently, above the visual frequencies, but would be useful in that they would be able to show smaller features than possible using the optical wavelengths for the same sized mirror.

Additionally, using UV wavelengths together with those in the visible range would allow greater specificity in mineral identifications because of the differences in reflectance spectroscopy in the visible
and UV range. Though Mars does have an appreciable atmosphere compared to the icy satellites of Saturn studied by the Cassini UVIS, it has no ozone layer that would impede the observation of surface features in
the UV.
The mirror used for the UVIS on Cassini only has an aperture size of 22 mm x 30 mm, equivalent in size to a circular one of only 3 cm diameter. The mirror on Cassini used for visible light imaging though is much larger than this. Anyone know if there a physical/technical reason why the same mirror could not be used on Cassini for both
visible and UV observations with the light signal from the same mirror just being split off to the different instrument for each wavelength range? The Hubble shows you can do this, thus using the same large mirror for both UV and optical observations.
And here's an article on a mirror of the same size as the one on MRO, .5 m, that has sufficient surface smoothness for UV observations as well as optical:

Optical Design & Engineering.
Sounding-rocket telescope uses new technology ultra-lightweight
David Content, Scott Antonille, Douglas Rabin, and Thomas Wallace
"Two NASA-funded programs will image the Sun and then an exoplanet: both at high angular resolution and both using the same mirror."
25 July 2006, SPIE Newsroom.

The two missions mentioned in this article had been planned to launch in 2007 but as far as I know haven't been launched yet. Since these missions are only for sounding rockets, one or more of these could be used for a Mars mission if returned safely for a soft landing by the sounding rocket.

So I was trying to find some examples where UV observations in concert with visual light observations could improve mineral identifications. Here's one interesting case where UV observations alone were able to make mineral identifications:

Oxygen-Rich Moon Minerals May Help Astronauts Breathe.
Brian Handwerk
for National Geographic News
October 19, 2005
http://news.nationalgeographic.com/news ... xygen.html

This was about detection of oxides on the lunar surface by Hubble at UV wavelengths. I presume the detection at least from Hubble wasn't possible in the optical wavelengths, either because of the better resolution in the UV or the spectra are more definitive in the UV. In either case this might also be possible on Mars. Oxides of various
types have already been found on Mars but as on the Moon UV observations may detect oxides of different types or be able to see
them in smaller deposits.

Another interesting fact I found out in reading about UV mineralogy is fluorescence of some key minerals in regards to Mars. An important one is calcite. Calcite is important in regards to Mars because it is
a kind of carbonate. The presence of carbonate in large deposits is an
important question about Mars because it would confirm large amounts of water in the past. (There has been within the last year a tentative identification of carbonate that still needs to be confirmed.)

So I thought calcite fluorescence might be detected using the UV observations of Mars. However, important to remember about
fluorescence is that it does not have to radiate in the same wavelength range. In fact it usually doesn't. So even though exposure to UV, quite common on Mars, could cause light emissions from some minerals, these would most likely be instead in visible wavelengths.
So the question occurs to me: is it possible that when you are looking at a spectrum of a sample or region on Mars, that what you take to be regular reflection spectra really has a contribution from the optical emissions due to fluorescence?

Take a look of some of the minerals in fluorescence here:


Minerals in fluorescence have an usual, unnatural look to them. I'm reminded then of the "unnatural" look to the lander and orbital color images released by NASA in attempts to produce "true color" images of

Spirit finally finds bedrock -- and new evidence of water.
http://www.news.cornell.edu/releases/ro ... 19-04.html

True, these and other "true-color" images released by NASA don't have the "day-glo" look of the examples shown of minerals with
fluorescence, but it is possible that fluorescence emissions contribute to the feeling you get from these Mars images that the colors are somehow "off".

Bob Clark
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