Bright and dark streaks covering the slopes of the Olympus Mons aureole, as seen by the Colour and Stereo Surface Imaging System (CaSSIS) onboard the European Space Agency’s ExoMars Trace Gas Orbiter.
These enigmatic features come and go spontaneously, some last for years while others quickly fade. They change colour and brightness and show up during certain seasons on opposite hemispheres of the Red Planet.
Scientists first saw these enigmatic streaks extending for hundreds of metres down sloped terrain in images from the Viking orbiters in the 1970s. How they form, where and when has fueled scientific debate ever since.
Some researchers have interpreted these streaks as flows of salty water, or brine, that could remain liquid long enough to form them. This hypothesis suggests rare habitable zones might exist on this otherwise desert world where temperatures rarely rise above freezing.
However, a new study led by planetary scientists at the University of Bern and Brown University challenges the water-based explanation. Their paper in Nature Communications argues that these slope streaks result from dry processes involving wind and dust activity.
To bring out these features, the contrast in these CaSSIS images is stretched – the image is re-scaled between the minimum and maximum brightness within each colour before combining them to produce the published image.
“Streaks on Martian Slopes are Dry,” by Valentin Bickel and Adomas Valantinas, was published in Nature Communications on 19 May 2025.
The image covers an area of approximately 50 square kms and was captured on 3 October 2024. Mars location: 26.5°N, 223.8°E. CaSSIS image MY37_030618_155_3.
2/4 Streaks on Mars imaged by CaSSIS
Published: May 19, 2025
Credit: ESA/ExoMars TGO/CaSSIS @esa.int
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