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Need help for sulfate detection using CRISM data


Deepali
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Hello,

I am a PhD scholar exploring water associated morphology and mineralogy of the Martian surface. I am working with L band of CRISM for detection of sulfates in my area. As Leask et al. (2018) have brought forward concerns with possible artifacts at 1.9 and 2.1 micron, I am having difficulty with confirming sulfates in my area. 

I have analysed radiance data for the same (as suggested by them). But my study area is dust covered so it has a lot of noise and spikes. I have tried ratioing radiance data but CO2 bands still persist. It does show absorption at those wavelength (similar to ratioed I/F) but CO2 absorption at 2.0 micron are big enough to destroy whole band shape.

As of now, I am not sure if they are actual or spurious absorptions. It will be grateful if anyone can help me in confirming the results or suggest any method for resolving this issue.

Thank you. 

 

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Hi: I am a CRISM Team Member and PDS Geosciences Node Manager. So, you generated an IOF from radiance data. Then you did the "volcano scan" atmospheric correction, both using CAT? And you still have lots of noise in the 2 micrometer region, artifacts from trying to remove the CO2 triplet? Let me know and we can then go from there.

Ray Arvidson

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Hi Ray,

Thank you so much for replying. Hope you are doing fine.

For the other minerals I processed CRISM images in CAT following the steps advised in workshop ppts (ATP corrections, georeferencing, summar parameter calculation).

For sulfates, as you must be aware, an issue regarding artifacts at 2.1micron has been raised. So I used radiance data for them specifically. And no correction was done on them. But in the end, I ratioed the radiance data (for representation purpose basically) but still I got triple absorption of CO2. That problem was observed in 1-2 datasets only. The feature was nullified in other CRISM images upon ratioing.

Since it has been suggested that a step during atmospheric correction may be responsible for thus artifact, I did not carry out volcano scan correction in radiance data. Are you suggesting I should perhaps try that?

Beacuse of the spikes at that wavelength, it has become very difficult for me to correctly identify if the absorptions are real or not. I am willing to share the images of ratioed radiance if that is helpful.  

Thank you again for your time.

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First, do not give up on use of the IOF archive products. The artifacts mentioned in the Leask paper are associated with sharp albedo boundaries.

Second, you need to turn the radiance into IOF files in CAT by using the "Radiance to IoF" function. Then you can do volcano scans and other processing steps. This will just produce noisier results than using the archived IOF files.

I suggest do your work on archive and radiance to IoF files and compare results.

Ray Arvidson

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Hi Ray,

I have used IOF products for my work but for sulfates I have been told to confirm the absorption features by looking into radiance data also (in which I am facing this problem). 

Anyhow, so even for radiance data we have to do IOF conversion before analysis?

Thanks.

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For the radiance data you must convert to IOF using the CAT tool before doing anything else. You need IOF products, i.e., radiance divided by the solar radiance for each band where the solar radiance used in the CAT conversion is for the heliocentric distance for Mars at the time of the observation.

Ray

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