As the Mars rover Perseverance crested the top of Witch Hazel Hill, its operators back on Earth expected amazing things. This area on the western rim of the Jezero crater, along an ancient river delta that Perseverance has been exploring since it landed in 2021, is thought to contain some of the oldest rocks on the planet’s surface. The light-toned, layered materials promise a record of a wetter time, possibly one that hosted life.
The team did not expect what they found on March 11: a dark rock resembling a clutch of frog’s eggs. Dubbed St. Paul’s Bay, the rock looks nothing like its neighbors. Where it came from and how it formed are a mystery.
It’s just the latest weird rock discovered in nearly 30 years of rovers rolling across the Martian surface. In the past year alone, the Curiosity and Perseverance rovers have found several rocks unlike anything seen on Mars before.
Some of these rocks carry lessons about the history of water on Mars and its potential for supporting life. One even contains tantalizing hints that microbes really were active on Mars once — hints that so far have stood up to scrutiny.
“After decades, we continue to be surprised by the things we discover on this wonderfully complex planet,” says planetary scientist Abigail Fraeman of NASA’s Jet Propulsion Lab in Pasadena, Calif.
As calls to send humans to the Red Planet intensify, a thorough search for Martian life grows more important. Once humans arrive, it may be impossible to tell whether biological signs were Martian to begin with or brought from Earth.
“The perspective we get from our rovers — and helicopters! — on the surface is so important for our exploration,” Fraeman says.
Here’s a tour of some of the weirdest rocks ever seen on Mars, all spotted in the last year.
The sulfur in the stone
In late May 2024, the Curiosity rover drove over a rock and cracked it open, revealing yellow crystals of pure sulfur inside.
While scientists have seen sulfur on Mars before, it was always bound up with other elements to make salts like sulfate. Curiosity’s instruments showed that this rock contained pure, elemental sulfur.
“This has never been found on Mars before, ever,” Fraeman says.
On Earth, elemental sulfur forms in various environments, including hot springs, cold springs, hydrothermal deposits, volcanoes and their associated vents. In some settings, like the Arctic, elemental sulfur’s formation is related to microbial activity.
“That’s not the default assumption here, but it’s interesting,” Fraeman says.
History urges caution on the microbial front. Previous claims that Mars rocks contain signs of life, such as the ALH84001 meteorite in the 1990s, sparked excitement before it was shown that the purported fossils formed through geological processes instead.
To help narrow down how the yellow crystals formed, scientists at Los Alamos National Laboratory in New Mexico will compare the on-site chemistry analysis of the Martian sulfur to samples from Hawaii, the Chilean Atacama desert and a hydrothermal volcanic region in New Mexico called Valles Caldera.
How the leopard got its spots
In the Jezero crater, the Perseverance rover has been searching for signs of past life. In July 2024, it appeared to finally find it.
The excitement came from a rock named Cheyava Falls, which is speckled with black, blue and green spots researchers dubbed “poppy seeds” along with slightly larger bull’s-eye features called “leopard spots.” The poppy seeds and the outer rims of the leopard spots are rich in iron phosphate molecules.
On Earth, similar spots form in the presence of microbes. The chemical reactions that create the iron phosphate rings provide energy for microbial growth.
“These properties mark poppy seeds and leopard spots as potential biosignatures,” wrote planetary scientist Joel Hurowitz of Stony Brook University in New York and colleagues in an abstract for the Lunar and Planetary Science Conference, held in The Woodlands, Texas, in March.
NASA had originally announced the finding in a news release on July 25. Since then, scientists have been testing various scenarios to explain how the poppy seeds and leopard spots could have formed. Geobiologist Michael Tice of Texas A&M University in College Station and colleagues presented their analysis of those scenarios at the LPSC meeting.
“Although other potential abiotic mechanisms should be investigated, this analysis suggests that biological metabolisms are the most likely processes currently proposed for formation of these unique features,” the group wrote in their conference abstract.
Perseverance drilled a sample from the rock and stored it for a future mission to return to Earth. That mission’s funding is currently in limbo.
A zebra can’t change its stripes
A few months later, Perseverance found another rock reminiscent of wildlife: one with stark black-and-white stripes.
“This rock is informally named ‘Freya Castle,’ but the team affectionately calls it ‘zebra rock’ for obvious reasons,” says planetary scientist Katie Stack Morgan of the Jet Propulsion Lab. “Between the leopard spots at Cheyava Falls and our new zebra rock, we’re starting to fill up our rock menagerie.”
The rock was spotted in low-resolution navigation camera images as the rover traversed some pebbly terrain. Engineers snapped a closer photo with the rover’s main camera but drove Perseverance away before the images downloaded to Earth.
That means scientists don’t have any data on the rock’s composition, Stack Morgan says. But from the closeup image, a reasonable guess is that the white parts of the rock are feldspar, a common mineral in both Earth’s and Mars’ crust. The black stripes could be a pyroxene, often formed in volcanic lava, or amphibole, a kind of silicate-bearing mineral.
Nothing like this has been seen on Mars before, but on Earth, the zebra stripe pattern occurs in volcanic rocks formed in particular kinds of magma chambers. One thing is clear, Stack Morgan says: Freya Castle is so different from the underlying bedrock that it probably was not born where it sits now.
“We don’t know where it came from or how it found its way into the crater,” she says, but it might have come from ancient bedrock exposed within the rim of the Jezero crater. Perseverance is currently driving along that rim and may find the zebra rock’s source there.
Suddenly spherules
St. Paul’s Bay — the “frog’s eggs rock” spotted on March 11 — is reminiscent of other odd finds.
Shortly after landing in 2004, the Opportunity rover discovered hematite spherules dubbed “blueberries.” And Fraeman says that St. Paul’s Bay reminds her of another Opportunity finding: a sphere-studded rock with a different composition that the team called “newberries.”
It’s not clear if the spheres in either of those rocks are related to the ones in St. Paul’s Bay. Those newfound spheres could have formed from groundwater circulating through the rock’s pores in Mars’s past (a possible origin story for the blueberries as well). They could also have originated from rapidly cooling molten droplets following a volcanic eruption or from rock that was vaporized after a meteorite impact and subsequently recondensed.
Like the zebra rock, St. Paul’s Bay might be a geological interloper. Figuring out where it came from will be important to determining how it formed.