CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)
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FAQ’s- Ask a Scientist

Q: What journals are best to find background research about features on Mars?

A: I would say the most frequently published-to journals for planetary science in general are the Journal of Geophysical Research (JGR) - Planets, Geophysical Research Letters (GRL), Science, Nature, Geology, and Icarus.

However, you'll find a lot of information in conference abstracts as well, especially in terms of the most up-to-date topics scientists are working on. Popular conferences for Mars research are the annual Lunar and Planetary Science Conference (LPSC), American Geophysical Union (AGU) Fall and Spring meetings, Geological Society of America (GSA) annual meetings, and International Conferences on Mars that are held every 2-3 years.

I believe all of these conference abstracts or proceedings can be found online, however most of the journal publications require a subscription, either online or hard copy - your school or local library might have them.

As an alternative to searching all of these websites and journals separately, I would suggest using a the NASA ADS search tool: http://adsabs.harvard.edu/physics_service.html. It's not flashy but is very thorough at finding all publications and abstracts.


Q: I have noticed that there are black spots in many craters in the northern latitudes that are not present in lower latitudes. I know that black streaks on the terrain often indicate that dust has been blown away, but these spots do not appear to be the same. I was thinking that this could be some sort of basaltic sediment, but does anyone know what these really are? Thanks for the help!

A: There are many craters in the northern plains of Mars (and Mars in general) that have dark spots or streaks associated with them. In dusty areas you can see obvious wind features. Depending on the wind pattern, you might see dust deposition or erosion on the crater floor, but more noticeably you'll see streaks that extend downwind from the crater on the surrounding plains. Dunes are common on crater floors, and that would seem to suggest that deposition dominates over erosion - but both processes are likely occurring. It's highly likely that as erosion occurs, the smaller, lighter particles (dust) are being carried away by the wind whereas more dense particles (e.g., basaltic sand) are too heavy for the wind to carry and accumulate in the crater floor. Ice at high latitudes may complicate this since it tends to cement small grains together and inhibit wind erosion.

So, for your project I would suggest looking at some high resolution CTX, THEMIS, or HiRISE visible data to get an idea of the surface morphology - are there dunes present or does the surface look stripped clean? Sometimes you can see dunes in CRISM data, too.


Q: What is the difference between bound water signal and monohydrated minerals signal?

A: The term “bound water” refers to one or more actual molecules of water (H2O) being included in the crystal structure of a mineral. For instance, gypsum is CaSO4•2H2O.  Without the two molecules of water, it becomes a different mineral entirely (anhydrite). Minerals with “bound water” typically have an absorption near 1.9 micrometers that is detectable by CRISM (thus the BD1900 summary parameter).

“Monohydrated” refers to a mineral with only one molecule of bound water, whereas polyhydrated refers to more than one water molecule.  These terms are typically used in reference to sulfates on Mars; often it is difficult to positively distinguish a particular sulfate species, but you can at least tell if it is a monohydrated sulfate or polyhydrated sulfate based on the position of the 1.9 micrometer absorption. In monohydrated sulfates the absorption is shifted closer to 2.1 micrometers.


Q: Is there an angle dependence on how much signal we will get back? So if the rock face has an angle - can we see different amounts of reflection which would be interpreted as 'less of that mineral' improperly. How does the angle of the terrain affect the CRISM detections?

A: Yes, local topography can definitely affect the amount of signal reflected back to the instrument. Typically you encounter strong illumination effects when you’re looking at cliff or crater walls, or other high-standing features that create shadows. Remember that CRISM detects reflected sunlight – so if a surface is in shadow, there’s not any reflected sunlight for CRISM to see. However, a surface does not have to be completely in shadow for the signal to be diminished.

The three angles that govern reflectance spectroscopy are incidence angle (angle between the sun or illumination source and the surface nadir), emission angle (angle between the detector and the surface nadir), and phase angle (angle between incident and emergent rays). It’s too much for me to go into here, but you can read more about this and radiative transfer in general in any remote sensing textbook.


Q: a) Is there a way to detect pyrites (iron sulfides) with CRISM? b) How about carbonates?

A: a) Technically, yes, but in practice, no. Pyrite and other iron sulfides contain iron and thus have absorptions in the VNIR spectral range, but they are not diagnostic and it is very difficult to distinguish these features from those of other iron-bearing minerals, like hematite or goethite, or just poorly crystalline iron-oxide in your typical martian dust spectrum. It is also very unlikely that you will have an outcrop of pyrite large enough for its signal to dominate a CRISM pixel.

b) It is possible to detect carbonates in CRISM spectra of Mars and it has been done by members of the science team. However, it is difficult to do for several reasons and, as a result, we do not currently have a browse product useful for finding them.


Q: Is there information about the prevailing winds around Juventae Chasm or other locations near that area, (Center Latitude -4.4149, Center Longitude -62.292395)?

A: This is out of my area of expertise, but because Juventae is very near the equator I would expect the dominant wind direction to be from the east - just like in the tropics on Earth. My guess is somewhat substantiated by the dune fields that have accumulated in the western part of the chasma - the winds would have to drop their sediment load as they are forced up and over the cliff wall. However, there are almost certainly local variations in the prevailing winds due to the extreme topographic changes here. To understand the wind patterns in more detail, you'd need to run a 3-dimensional general circulation model (GCM) - you might try searching this term in the literature to find more information. Hope this helps!


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.


Q: Is there a list or library that shows what minerals correspond with certain wavelengths?

A: Yes, using Quickmap you can extract a Lambert albedo spectrum of the global multispectral map tile data at a given spot (probe) on the surface. Lambert albedo is a slightly different way of expressing reflectance from I/F, and in this case is atmospherically corrected so you shouldn't have to worry too much about atmospheric gas absorptions. Wavelength is shown on the x-axis in units of nanometers, but there is not a 1:1 correlation between wavelength and mineral absorption. Generally you have to examine at least several regions of a particular spectrum and compare the absorptions to library (or reference) spectra of well-characterized mineral samples. There is a CRISM spectral library at the PDS (http://pds-geosciences.wustl.edu/missions/mro/spectral_library.htm) that you can download and use to compare to spectra extracted from Quickmap or other tools.

 
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