Alga Crater rim – comparing processing techniques

This image taken by the Colour and Stereo Surface Imaging System (CaSSIS) onboard the ExoMars Trace Gas Orbiter shows part of Alga Crater rim. The image was acquired on 28 January 2021.

Alga Crater is located in the southern highlands of Mars and is well-known for its colour diversity.

The top image is a crude red-green-blue (RGB) representation of the scene. But because CaSSIS doesn’t have the same red, green, and blue filters as a smartphone camera and is sensitive to infrared light invisible to humans, the shade of red is different from what you would see with your eyes, appearing more orange than yellow.

Below is the same scene shown with CaSSIS standard processing to accentuate the colour diversity of the martian surface. This is the so-called ‘NPB’ product, which puts the near-infrared filter (NIR, N) covering the longest wavelengths in the red channel of the display, the panchromatic filter (PAN, P) in the green channel, and the blue filter (BLU, B) in the blue channel. In addition, each filter image is ‘stretched’, meaning that the image is re-scaled between the minimum and maximum brightness within each colour before combining them to produce the resulting image.

While the top image does show some colour changes, they really stand out in the lower image. Bluer tones are more dust covered. The greenish colours are typically mafic minerals such as magnesium and/or iron-rich silicates, although spectroscopy is required to provide the most accurate identification.

The image is centred at 333.3ºE / 24.5ºS. North is roughly to the left.

Read more about how CaSSIS reveals the colour diversity of Mars here