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.

As if someone has been sweeping the surface of Mars with a broom, the origin of odd, streaked slopes has intrigued scientists for decades.

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.

Researchers turned to a machine learning algorithm to scan and catalogue slope streaks in over 86 000 satellite images from NASA's Mars Reconnaissance Orbiter (MRO).

Scientists combined several decades’ worth of orbital data and the neural power of deep learning to produce a global map with almost 500 000 streak features across Mars. The new study created the largest database yet of these features on Mars.

The team also turned to other cameras orbiting Mars, such as the CaSSIS imager on ESA’s Trace Gas Orbiter and MRO’s HiRISE, to collect more colour information in high resolution, as well as to monitor how the streaks evolved over time.

The correlations over hundreds of thousands of cases helped the team shed new light on a decades-old debate. With no evidence of water, scientists concluded that dry processes – rather than liquid flow – drive the appearance of streaked slopes on Mars.

The study found that these winding features most likely form when layers of fine dust suddenly slide off steep terrain. Multiple triggers could unleash this process, such as rocks falling, small meteoroid impacts or wind gusts causing shockwaves and shaking loose dust.

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.

ESA’s ExoMars Trace Gas Orbiter continues to image Mars from orbit to understand its ancient past and potential habitability. The spacecraft returns spectacular images and provides the best inventory of atmospheric gases and mapping the planet’s surface for water-rich locations.

Understanding the history of water on Mars and whether it once allowed life to flourish is at the heart of ESA’s ExoMars missions.

“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.