Gullies evidence of water on Mars? Microgravity experiments say perhaps not

23 December 2011

Gullies on the surface of planet Mars are often interpreted as evidence that water must have been present at its surface at some point in time. However, microgravity research by researchers from the University of Amsterdam, Utrecht University and Delft University of Technology indicate that gullies on Mars may have formed without the help of water.

Sebastiaan de Vet during the parabolic flight campaign

Gullies on the surface of planet Mars are often interpreted as evidence that water must have been present at its surface at some point in time. However, microgravity research by researchers from the University of Amsterdam, Utrecht University and Delft University of Technology indicate that gullies on Mars may have formed without the help of water.

If you form a heap of a granular material such as sugar, gravel or sand, a cone is formed. The angle of the walls of the cone is determined by the material used and is known as the static angle of repose. If you add more material to make the angle steeper, the result will be an avalanche, with sugar/sand/gravel grains tumbling down until the cone is back at the static angle of repose again. On Earth, the static angle of repose for smooth grains is ca. 20 degrees, while coarse, angular material is stable up to angle of ca. 40 degrees.

When studying the angles of gully walls on the planet Mars, researchers found them to be too shallow to have been formed by the free flowing of loose material. Instead they argued that water must have been present to act as a lubricant responsible for causing landslides at angles below the static angle of repose. Or even to directly deposit the material in question, much like rivers on Earth deposit sediment.

However, the above only holds true if the static angle of repose of material on Mars is the same as it is on Earth. Research by Maarten Kleinhans of Utrecht University together with Sebastiaan de Vet of the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the UvA and colleagues from Delft University of Technology, shows that this is most likely not the case. The key is the fact that Mars has only 38% of the gravity of planet Earth.

The research team used a parabolic flight campaign to mimic Mars’s lower gravity and test the effect on angles of repose of different materials. As the plane followed its roller coaster style path, slowly rotating cylinders containing different materials experienced one tenth of Earth's gravity, Martian gravity and the Earth's normal pull.

The results show a considerable decrease in the static angle of repose, indicating that weaker gravity such as on Mars reduces internal friction of avalanching material. This would explain the formation of gullies on Mars at too shallow an angle for Earth gravity. It also challenges the reigning paradigm that the static angle of repose is a property that only depends on the nature of the material considered and not on external factors.

The work was published in Journal of Geophysical Research in November 2011.

Published by  IBED