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Ausiàs R

About the data type of the CRISM I/F images

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Posted (edited)

Good afternoon.

I'm an undergraduate student of geomatics and remote sensing, and I was trying to use the CRISM images to classify minerals and chemical compounds on the surface of Mars.

I collected some information about all the products to use, also I followed the workflow proposed here and I installed CAT on ENVI. I was trying to use a an image of the Jezero Crater data product: frt00005c5e_07_if166j_mtr3, which is projected, atmosferically and geometrically corrected. The data is on I/F which as is explained here, on the FAQs means:

Q: What does I/F stand for?

A: CRISM is a reflectance spectrometer, and I/F is how reflectance is represented algebraically: I is the energy (actually radiance) observed by the CRISM instrument, and F is the energy (actually solar irradiance) incident at the top of the Martian atmosphere. I/F is a ratio of energies (radiance/irradiance), with some additional scaling so the ratio is unitless.

Acording to my knowledge radiance/irradiance=reflectance (unitless), maybe I'm wrong.

The case is that I tried to classify the minerals on the image of the Jezero Crater using this library from Viviano et al using the "CRISM RATIOED I/F CORRECTED" spectra but I get wrong or absurd results.

I noticed that the spectra of my images is clearly different from the spectra that I find on the Viviano library ratioed i/f. My Spectra always follow this pattern, and never reaches a reflectance of 0.5.

jezero.png.3105e59ce7bb0eb6c0d26f1a72407c3c.png

 

Nevertheless on the Viviano library I found spectra like this one of epidote:

epidote.png.6f0515a9feb9aa135490bce1d91047d8.png

 

Why the spectra of my image is so different? Is my data really on reflectance? How can I get real reflectances?

 

 

Edited by Ausiàs R
error

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Hi There:

I am the manager of the PDS Geosciences Node and also a CRISM Science Team Member. The I/F data you are using come from the spectral radiance for each band and pixel location divided by the solar spectral irradiance divided by pi. The is a ratio of spectral radiances and the pi converts solar spectral irradiance to spectral radiance. The solar value used is specific for the heliocentric distance when the scene was acquired, i.e., Mars distance relative to he sun.  Your I/F spectra, put through the volcano scan correction using CAT, look good. Here is what you are missing. The next step typically is to then divide the volcano scan corrected I/F spectra by a spectrum from the same corrected data for an area that looks like it is spectrally neutral. This removes any instrument artifacts and tends to de-emphasize the ubiquitous basaltic sands and regolith and thus emphasizes what might be unique in terms of mineral signatures for the area of interest. It kind is like in the lab when you use reflectance spectrometers you ratio your data relative to some standard surface. Remember there are no pure outcrops of given minerals on Mars, rather wind mixes things a lot. I hope this note helps. Please ask more questions if needed.

Ray Arvidson

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Posted (edited)

Hi Ray,

First of all I want to thank for your reply, it is really apreciated.

You said that:

On 3/3/2020 at 4:37 PM, Ray Arvidson said:

The next step typically is to then divide the volcano scan corrected I/F spectra by a spectrum from the same corrected data for an area that looks like it is spectrally neutral.

I understand that the area that looks like it and is spectrally neutral means an area that exhibes a constant reflectivity or transmissivity over the electromagnetic spectrum. But, where can I find this kind of surface/area?

Is the data that I need on the ddr files of the image? I saw that there are 14 bands on the ddr images. On this document (pages 76-77) it is explained what those bands represents. I'm not pretty sure which band I shoud use, or if any.

Once again, thanks for your answer,

Ausiàs.

Edited by Ausiàs R
Grammar

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You need to find an area in the corrected spectral data cube that looks spectrally neutral and divide your spectrum of interest by the spectrum that looks spectrally neutral. No DDR involved.

Ray

 

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