Origins Uncertain: ‘Skull Hill’ Rock – NASA Science

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

Last week, NASA’s Mars 2020 rover continued its journey down lower ‘Witch Hazel Hill’ on the Jezero crater rim. The rover stopped along a boundary visible from orbit dividing light and dark rock outcrop (also known as a contact) at a site the team has called ‘Port Anson’. In addition to this contact, the rover has encountered a variety of neat rocks that may have originated from elsewhere and transported to their current location, also known as float.

Pictured above is an observation named ‘Skull Hill’ taken by the rover’s Mastcam-Z instrument. This float rock uniquely contrasts the surrounding light-toned outcrop with its dark tone and angular surface, and it features a few pits in the rock. If you look closely, you might even spot spherules within the surrounding regolith! See Alex Jones’ recent blog post for more information on these neat features: https://science.nasa.gov/blog/shocking-spherules/. The pits on Skull Hill may have formed via the erosion of clasts from the rock or scouring by wind. We’ve found a few of these dark-toned floats in the Port Anson region, and the team is working to better understand where these rocks came from and how they got here.

Skull Hill’s dark color is reminiscent of meteorites found in Gale crater by the Curiosity rover: https://www.jpl.nasa.gov/news/curiosity-mars-rover-checks-odd-looking-iron-meteorite/. 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. 

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!