HiRISE has discovered an unusual landscape in this image of Daedalia Planum, high on the slopes of Tharsis at an elevation of 5,455 meters (17,900 feet). This terrain looks as if it was raked into furrows like a Japanese dry garden. Bright boulders are arranged into long curving rows separated by 5 to 10 meters (16 to 33 ft), producing a pattern that resembles a fingerprint.

Scientists have not previously seen these patterns elsewhere on Mars. They are particularly conspicuous here because of the contrast between the bright boulders and the darker soil. Some process has evidently sorted and segregated the larger boulders from the finer soil. Exactly what that process is presents a perplexing puzzle.

Mixtures of small and large grains can be sorted and separated by “grain size convection” that occurs when grains are allowed to move around relative to one another. Grain size convection can result from vibration or from thermal cycling due to temperature fluctuations or
from repeated freezing and thawing of ice. This action produces pingos and frost heaves on Earth, features that are thought to require liquid groundwater buried beneath a frozen surface.

Unique periglacial processes might be expected in Daedalia Planum because of the location’s high altitude. Vibration produces the familiar “Brazil nut effect” that forces larger grains to the surface when a mixture of grains is shaken. Seismic shaking of Daedalia Planum could take place if the interior of Tharsis is still active and rumbling. How these “Zen gardens” form on Mars remains a mystery, for now.

Written by: Paul Geissler  (7 October 2024)