Drainage pits in cohesionless materials: implications for surface of Phobos

Area of Science:

• Planetary Science
• Geology
• Geophysics

Background:

• Phobos exhibits surface features like grooves and pits, with some converging towards the Stickney crater.
• Previous hypotheses suggested these features are secondary craters, but their morphology and geometry point to underlying fractures.

Purpose of the Study:

• To investigate the formation mechanism of pits and grooves on Phobos.
• To determine if regolith drainage into fractures or gas venting is the primary cause.
• To use experimental modeling to estimate Phobos's regolith thickness.

Main Methods:

• Experimental modeling using two rigid plates simulating Phobos's fractured surface, covered with simulated regolith.
• Simulating fracture extension by pulling plates apart to observe regolith drainage.
• Analyzing the resulting patterns of pits and grooves for similarities to Phobos's surface features.

Main Results:

• Experimental drainage patterns closely mimic Phobos's grooves and pits, characterized by linearity, uniform spacing, and progression from pits to grooves.
• Gas-venting simulations produced irregular chains with raised rims, unlike Phobos's features.
• Average pit spacing in experiments correlated directly with regolith thickness, independent of the angle of repose.

Conclusions:

• Phobos's grooves and pits are indicative of regolith drainage into open fractures.
• The experimental findings provide a method for estimating regolith thickness on Phobos.
• Analysis suggests large areas of Phobos possess a uniform regolith thickness of approximately 300 meters.