The hits just keep on coming from the Mars Perseverance rover. It’s exploring Jezero Crater on the Red Planet, looking for evidence of microbial life in the planet’s ancient (or even recent) past. Recently it spotted a very strange-looking rock with black and white stripes. Its appearance and location sparked a lot of questions. Perseverance team members have named it “Freya Castle.”
From the image, this chunk looks remarkably similar to metamorphic rocks on Earth. The most familiar are gneiss, marble, and schist (to name a few). According to Jeffrey Kargel of the Planetary Science Institute, who speculated on what Freya Castle could be, it resembles a very high-grade type of rock similar to what we find here at home. “It looks like and plausibly is, a metamorphic rock containing feldspar or other white-ish minerals arranged in something called boudinage,” he said. “That word stems from French, relating to a chain-link sausage-like structure. In the case of rocks, it forms when you have layered material, usually sedimentary rocks, where the layers are compressed from above under conditions of high heat and pressure. Much of the rock responds plastically squishing down and spreading out.”
Kargel, who is not associated with the Perseverance mission, pointed out that the conditions under which Freya Castle formed on Mars would be similar to Earth’s. “Those conditions have been common on Earth, and erosion then eventually exposes the rocks at the surface. If this is an indigenous Mars rock, it likely would have undergone metamorphism in the lower crust, and then an impact blasted it out, and the rock landed where the rover could examine it,” he said. Other transport possibilities include a deposit by fluid delivery, which makes sense since water has flooded the crater in the past.
What Kind of Rock Is It?
So, what’s the story with this rock? Based on the image, it looks pretty out of context with much of the dust and sedimentary material in the crater. That makes it worth reviewing the region in a bit more detail. An impact some 3.8 billion years ago carved out Jezero Crater. It lies on the western edge of a large impact basin called Isidis Planitia. A large impact created that basin during an impact about 3.9 billion years ago. At some point in the distant past, water filled Jezero at least twice. There’s a river delta as well as flow channels exiting the crater.
Where there’s water, there’s sediment, which hardens into sedimentary rock. ( The main types of rock are igneous (volcanic or intrusive in origin), sedimentary (deposited by wind or water), and metamorphic.) Not surprisingly, the Perseverance rover continues to find sedimentary deposits and layers at Jezero. The delta is clay-rich, and the crater contains other materials known to be in contact with water. However, some of the rocks in Jezero are also igneous. That means they were created by volcanic activity and somehow brought to the crater.
That brings us back to Freya Castle, which looks metamorphic at first glance. Such rocks have experienced some kind of heat, pressure, or other kinds of geologic stress. That process changed them from one type to another. It also altered the texture of the original rock and the mineral composition.
Creating a Metamorphic Rock
On Earth, metamorphics are a large part of our planet’s crust. They can form deep under the surface where temperatures and pressures are high. Tectonic activity also forces metamorphism. So do impact events. Both heat and compress the surrounding rock. An impact also “excavates” rocks out from deep beneath the surface and tosses them across the surface. Volcanism could be another culprit, sending hot magma into cracks and openings of existing rocks and “morphing” them. Metamorphism can also be the product of the action of hot, mineral-rich fluid injected into other rocks by hydrothermal activity. If that happened, the fluids could have found their way into the layers. The result would be deposits of “intrusive” minerals, resulting in a layered look.
On Mars, all these processes also occurred throughout history. A good analysis of the rock’s minerals could give more details about the mineral makeup of Freya Castle. That would settle the question of what kind of rock it is. Such studies could also give some insight into conditions on Mars at the time it formed.
One thing to keep in mind is that Perseverance looked at Freya Castle with its Left MASTCam-Z camera. A closer study of the rock’s mineralogy and chemistry using its onboard spectrometer could reveal far more information about Freya Castle’s origins. Planetary scientists raised questions about whether the Perseverance team might send the rover back to do a mineralogical study. For now, however, the MASTcam imagery has prompted much speculation.
“If the rock turns out to be metamorphic and from the lower crust of Mars, it might be a very rare opportunity to examine a rock from an extremely ancient period–perhaps a former sedimentary rock that formed when Mars was extremely young, formed as the Martian crust was just developing,” said Kargel. “It might possibly bear evidence of the oldest hydrosphere known on Mars, or anywhere in the Solar System.”
How Did It Get There?
Regardless of its makeup, planetary scientists now need to determine how this unusual rock got to Jezero Crater in the first place. Since the region has been inundated at least twice in Mars’s long history, the most likely interpretation is that it formed elsewhere and was likely blasted out from below the surface during an impact. Then, it got carried along by water. There’s evidence in the Perseverance image of slight rounding of the protruding edges to support the idea of fluid transport. Materials in a flood can get eroded as they tumbled and bounced along in the water. One scientist at PSI suggested that the rounding shows the rock got carried across at least a few kilometers.
At least one “outlier” suggestion is that maybe the rock has an Earth origin. An ancient impact on our planet could have sent Freya Castle out to Mars, where it landed as an Earth meteorite. That’s not a likely origin, however, since the dynamics of getting Earth meteorites out from Earth to Mars are complex.
Freya Castle’s existence at Jezero Crater points out the historical forces that shaped the planet. In particular, it’s a clue toward understanding the complex sequence of events that brought this rock to its current resting place in Jezero Crater. It takes time to analyze those events and the rock itself, which is likely what the Perseverance team is doing as the mission itself continues its trek across the Martian landscape.
Note: Special thanks to researchers at the Planetary Science Institute for discussing specific aspects of metamorphic rock formation with the author.