Chemical Analysis of Mars from Orbit? But how?!

When I found out that water had been found on Mars my first response was to flail and shout with excitement. But once I had calmed down I started to think; how does one actually go about analysing the surface of another planet without actually being on the surface yourself? It was then that I found out NASA have managed to do all of this using a satellite currently orbiting the red planet at an altitude of 300 km (186 miles)! That’s some pretty impressive tech right there (and I imagine the specs are a well-kept trade secret). The satellite itself is known as the Mars Reconnaissance Orbiter (MRO), and it’s equipped with an analytical tool known as CRISM, or the “Compact Reconnaissance Imaging Spectrometer for Mars” if you’re feeling excessive. This device can detect and measure the wavelengths and intensity of both visible and infrared light that has been reflected or scattered from the martian surface; a technique known as “Reflectance Spectroscopy”.

Reflectance Spectroscopy functions on the principle that when light comes into contact with a material, the chemical bonding and molecular structure will cause some of this light to be absorbed. The exact wavelengths absorbed will vary depending on the type of bonding and the elements involved, and the remaining light will either be scattered or reflected depending on the macro-scale properties of the material, such as shape and size. On Mars, most of these materials seem to be grains of some sort and the potentially complex shape of such a structure can cause the light to be scattered in all sorts of directions. However, some of this light will reach the MRO, and CRISM can then detect and measure which wavelengths have been absorbed based on a decrease in intensity. How they found a way to do all of this FROM ORBIT still mystifies me, but I imagine NASA prefers it that way. This whole process then gives an output known as an “absorption spectrum”, an example of which is shown in Figure 1.

Figure 1: An example of an absorption spectrum showing wavelength (x-axis) and reflectance (y-axis). Source: CRISM website: Link:

So! What have they actually found on Mars using this technique? Well, they appear to have detected “Aqueous Minerals”, which are chemical structures that form in the presence of water by chemical reaction with the surrounding rock. The exact mineral that will form is determined by many factors, including the temperature, pH, and salt content (salinity) of the environment, as well as the composition of the parent rock. Given that this process takes an extremely long time to occur naturally, it can show where water has been present long enough to cause such a reaction, and can give an excellent indication of what the martian surface was like in the past. For example, chloride and sulfate minerals generally indicate very saline water, as well as suggesting that it was more acidic, whereas phyllosilicates and carbonates suggest less salinity and a more neutral pH. What I find most exciting is that this data can suggest where to begin looking for fossilized evidence of ancient life (if it existed at all). If the past water appears to not be too acidic and the elements for life are present, then it is certainly a possibility.

It seems that Mars just keeps getting more exiting with each new discovery, and all we can do now is wait for next announcement to be made. Here’s hoping it’s evidence of life! Also, speaking of life on Mars, everyone should go see The Martian movie in cinemas now, it’s f**king brilliant!


  • The CRISM Website. Link:
  • USGS Spectroscopy Lab – About Reflectance Spectroscopy. Link:
  • PBS Newshour. “Mars has flowing rivers of briny water, NASA satellite reveals”. Link:
  • NASA Mars Reconnaissance Orbiter Website. Link:

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