The Medusae Fossae Formation is one of the most enigmatic geological units on Mars. It is a massive deposit of fine-grained, easily eroded material stretching thousands of kilometres near the Martian equator - one of the largest deposits of its kind in the solar system. Despite decades of study, its origin remains uncertain.
What it looks like
Wind erosion has carved the Medusae Fossae into yardangs - streamlined ridges aligned with the prevailing wind direction, shaped like inverted boat hulls. The yardangs of the Medusae Fossae are among the largest known in the solar system. The HiRISE image shows these formations at a resolution where individual ridge widths and the troughs between them are clearly measurable. The surface looks machined rather than natural.
What it might be
Several competing hypotheses have been proposed for the Medusae Fossae Formation's origin:
Volcanic ash - the formation may be a massive deposit of pyroclastic material from the nearby Tharsis volcanic province. This is currently the leading hypothesis. The volume of material is consistent with multiple enormous volcanic eruptions.
Polar ice - an alternative hypothesis proposes that the Medusae Fossae represents ancient polar ice deposits that migrated equatorward during periods when Mars's axial tilt was different from its current value.
Impact ejecta - a third hypothesis suggests the deposit is the fallout from a very large impact event.
The uncertainty persists partly because the deposit is so easily eroded that its original structure is difficult to recover. SHARAD radar data from the Mars Reconnaissance Orbiter suggests the deposit has a low density consistent with either porous volcanic ash or ice-bearing material.
If the volcanic ash hypothesis is correct, the Medusae Fossae represents the largest known volcanic deposit in the solar system - a finding with significant implications for the history of Martian climate, since the associated eruptions would have released enormous quantities of greenhouse gases.
The Axisophy print
The HiRISE image captures a section of Medusae Fossae yardang fields in exceptional detail. The parallel ridges and troughs create a strongly directional composition - one of the most graphically distinctive images in the Radiance collection.
