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Crism Trr3 Processing...


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I apologise if this is an extremely obvious question, but I'm very new to using CRISM data.


I am using CAT v6.6 to process the frt00004af7_07_if164l_trr3.img of Hale crater while following the CRISM walkthrough tutorial of Nili Fossae from crism.jhuapl.edu/CRISM.../CRISM_Workshop_090322_Nili_Fossae_Walkthrough_FPS.pdf.


Is it normal after completing atmospheric correction using the optimum wavelength volcano scan method, to still have an obvious CO2 absorption feature from 1.9-2.1 µm? If I then take a spectral ratio s1/s2, no recognisable spectral signature is apparent - as in the tutorial.




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It's not an obvious question. It's been forwarded to the CRISM team member who developed the latest version of the ‘volcano scan’ atmospheric correction. A reply should be posted within a day or two.


Any update on this please?

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An absorption-like artifact on the order of a few percent of the continuum level is typical after applying the volcano scan as currently implemented in CAT.


We've developed a method for correcting that artifact that will appear soon in CAT 6.8.


The volcano scan derives an atmospheric transmission spectrum from the base/summit ratio of images over Olympus Mons, scales it, and divides it out of the scene you're correcting to try to remove the atmospheric CO2 signature. However, the shape of the co2 absorption spectrum varies with altitude, so the volcano scan ratio of a low/high altitude spectrum isn't shaped quite like the observed spectrum at any altitude. That error in the transmission spectrum causes the imperfect correction that leaves the feature you're noticing.


The new method applies the usual volcano scan, and then scales and adds in a "patch" derived from the volcano scan data. In testing so far it helps most of the time. It only seems to hurt when there's CO2 ice in the scene; that throws off the scaling and corrupts the ice spectrum (it's still there, but the shape is messed up).


More details with the CAT 6.8 release. That release should come out in June.

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I think the size of my absorption feature should be acceptable then, thanks very much for the explanation FrankM.

I'll look foward to testing out the new CAT release in June.

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