Written by Andrew Shumway, Postdoctoral Researcher at the University of Washington
It is not common for a rover to spot nearly perfect spheres in the soil beneath its wheels. Over two decades ago, the Opportunity rover famously discovered spherules made of hematite (nicknamed “blueberries”) near its landing site in Meridiani Planum. More recently, the Perseverance rover has similarly encountered spherules embedded in bedrock and loosely scattered throughout the region informally called “Witch Hazel Hill.” In a previous blog post, we described Perseverance’s investigations of a spherule-bearing outcrop at the “Hare Bay” abrasion patch, where the team later collected a core. With the “Bell Island” sample added to the rover’s collection, the science team next decided to take a closer look at loose spherules in the area, which appear to have eroded out of the nearby bedrock.
On Sol 1555, while the United States was celebrating the Fourth of July with hotdogs and fireworks, Perseverance was hard at work studying spherule-rich regolith at the target “Rowsell Hill” using the proximity instruments on its robotic arm. SHERLOC’s Autofocus and Context Imager and WATSON camera both captured high resolution pictures of the target (shown above), while PIXL measured the elemental makeup of the spherules and surrounding grains.
Despite their superficial similarity to Opportunity’s “blueberries”, the spherules at “Rowsell Hill” have a very different composition and likely origin. In Meridiani Planum, the spherules were composed of the mineral hematite and were interpreted to have formed in groundwater-saturated sediments in Mars’ distant past. By comparison, the spherules in “Rowsell Hill” have a basaltic composition and likely formed during a meteoroid impact or volcanic eruption. When a meteoroid crashes into the surface of Mars, it can melt rock and send molten droplets spraying into the air. Those droplets can then rapidly cool, solidifying into spherules that rain down on the surrounding area. Alternatively, the spherules may have formed from molten lava during a volcanic eruption.
With these new data in hand, the Perseverance science team continues to search for answers about where these spherules came from. If they formed during an ancient impact, they may be able to tell us about the composition of the meteoroid and the importance of impact cratering in early Mars’s history. If they instead formed during a volcanic eruption, they could preserve clues about past volcanism in the region around Jezero crater. Either way, these spherules are a remnant of an energetic and dynamic period in Mars’ history!
Learn more about Perseverance’s science instruments
For more Perseverance blog posts, visit Mars 2020 Mission Updates
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