Dark, Pitted, and Possibly Alien: The Skull Hill Rock That Has Scientists Stumped

NASA’s Mars rover is on the trail of a geological mystery in the Jezero crater. While exploring an area known as Witch Hazel Hill, the rover encountered an intriguing dark rock named “Skull Hill” at a boundary between two different rock types.

Although its composition initially sparked excitement as a possible meteorite, new data suggests it may instead be a volcanic rock carried from elsewhere.

Exploring Witch Hazel Hill: A Boundary of Contrasts

Last week, NASA’s Mars 2020 rover continued its descent along the lower section of Witch Hazel Hill, located on the rim of Jezero Crater. During its journey, the rover paused at a site known as Port Anson, where it encountered a striking boundary, visible from orbit, between lighter and darker rock layers.

This type of boundary, where two distinct rock formations meet, is known as a geologic contact. Alongside this contact, the rover has also discovered several intriguing rocks known as float, fragments that likely originated elsewhere and were later transported to their current location.

Mysterious Float Rocks at Port Anson

One of these float rocks, pictured above, has been named Skull Hill. Captured by the rover’s Mastcam-Z instrument, Skull Hill stands out sharply against the surrounding light-toned terrain due to its dark color and angular shape. The rock features several small pits, and if you look closely, you may even notice spherical particles, called spherules, scattered in the surrounding soil, or regolith. For more details on these features, see Alex Jones’ recent blog post: Shocking Spherules.

Is Skull Hill a Meteorite?

Skull Hill’s dark color is reminiscent of meteorites found in Gale crater by the Curiosity rover. Chemical composition is an important factor in identifying a meteorite, and Gale’s meteorites contain significant amounts of iron and nickel. However, recent analysis of SuperCam data from nearby similar rocks suggests a composition inconsistent with a meteorite origin.

Igneous Origins? A Closer Look at Composition

Alternatively, ‘Skull Hill’ could be an igneous rock eroded from a nearby outcrop or ejected from an impact crater. On Earth and Mars, iron and magnesium are some of the main contributors to igneous rocks, which form from the cooling of magma or lava. These rocks can include dark-colored minerals such as olivine, pyroxene, amphibole, and biotite. Luckily for us, the rover has instruments that can measure the chemical composition of rocks on Mars. Understanding the composition of these darker-toned floats will help the team to interpret the origin of this unique rock!

Written by Margaret Deahn, Ph.D. Student at Purdue University

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