One of these things is not like the others. Time to sing that famous “Sesame Street” song to yourself as you gaze on a recent NASA Perseverance rover image sent back from Mars. The photo shows a rocky beige and brown patch of ground with a dark mostly smooth rock sitting in the middle of the frame. It’s the latest rock to present a science puzzle for Mars researchers to solve.
Perseverance is currently exploring the rim of the Jezero Crater in an area called Witch Hazel Hill. The rover took some time in April to study a spot where lighter and darker rock outcrops meet. That site is named Port Anson. That’s where the unusual rock came into focus.
You might notice a theme to the names, which are real locations in Canada on Earth. The theme continues with the name given to the dark rock: Skull Hill. Skull Hill is a “float” rock—a rock that came from somewhere else. That’s why it doesn’t fit in with its surroundings.
While most of Skull Hill looks smooth, it has some notable pits. “The pits on Skull Hill may have formed via the erosion of clasts from the rock or scouring by wind,” wrote Purdue University doctoral student Margaret Deahn in a rover mission update on April 17. Clasts are rock fragments or chunks. Clasts may have been embedded in the dark rock and then wore away, leaving pits behind. “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,” Deahn said.
Rovers Rock Out On Mars
NASA rovers have been exploring the red planet since the late 1990s when the Mars Pathfinder mission carried the Sojourner rover to the surface. The robotic explorers keep spotting new and noteworthy rocks. The Curiosity rover delivered a surprise in mid-2024 with the first-ever discovery of pure sulfur crystals on Mars.
Not to be outdone by its rover sibling, Perseverance found a “zebra rock” unlike any other. The striped rock is also likely a float rock like Skull Hill. When a rover finds an obviously out-of-place rock, it kicks off an effort to spot similar ones in other locations. Scientists are interested in the origin of these oddball rocks.
Fans of Mars rocks like to play spot-the-meteorite. Meteorites typically stand out from their surroundings. They’re usually dark with surfaces that show both smooth and pitted features. Skull Hill might have a meteorite-like appearance, but it likely didn’t come from space. A Perseverance analysis of similar rocks nearby “suggests a composition inconsistent with a meteorite origin.”
If Skull Hill isn’t a meteorite, that still leaves some big questions: What is it and where did it come from? The rover’s investigations suggest it “could be an igneous rock eroded from a nearby outcrop or ejected from an impact crater.” Igneous rocks often have volcanic origins. Mars has a fascinating history of volcanic activity. The tallest known mountain in the solar system is the dormant Olympus Mons volcano on Mars.
Perseverance’s science skills will be put to the test as it continues to explore the crater rim. It’s a relatively new region for the rover, so there are plenty of geological puzzle pieces left for scientists to discover and put into place.