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NASA's Curiosity spotted iron-nickel meteorite
nicknamed "Cacao" on 28 Jan 2023. (NASA/JPL-Caltech/MSSS)
MSL Curiosity is going about its business exploring
Mars. The high-tech rover is currently exploring the sulphate-bearing unit on
Mt. Sharp, the central peak in Mars' Gale Crater. Serendipity placed a metal
meteorite in its path.
The meteorite is made mostly of nickel and iron, and
it has a name: Cacao. (Chocolate comes from cacao.) Cacao isn't very large;
it's only about 30 cm (1 ft.) across.
Curiosity has come across several meteorites since
landing in Gale Crater in August 2012.
Cacao stands out visually from its surroundings.
While the Martian surface is red from oxides, the meteorite is dark grey and
metallic-looking. It's also smooth and rounded, obvious signs that it passed
through an atmosphere.
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Cacao meteorite and its surrounds on Mars.
(NASA/JPL-Caltech/MSSS)
The image is a composite of six individual images
taken with the rover's Mastcam. Curiosity captured the images on 27 Jan 2023,
the 3,724th Martian day, or sol, of the mission. The colors in the image have
been corrected to match the lighting conditions as seen with human eyes on
Earth.
The grooves and pits are called regmaglypts. They're
particularly interesting on iron meteorites. They formed when Cacao was
travelling through the atmosphere.
Even though Mars' atmosphere is much thinner than
Earth's, it still creates enough friction to heat the meteorite's surface. The
regmaglypts are likely created by vortices of hot gas that melted the rock as
it travelled through the atmosphere.
The meteorite may have been on Mars' surface for a
long time, but nobody knows for sure.
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Curiosity found the iron-nickel meteorite
"Cacao" on 27 January 2023. (NASA/JPL-Caltech/MSSS)
This isn't the first meteorite rovers have found on
Mars. In 2016, MSL Curiosity found another metal meteorite about the size of a
golf ball named "Egg Rock." It examined that one with its ChemCam
instrument to determine its composition.
The grid pattern of five small white dots shows
where the instrument's laser struck the rock.
Iron-nickel meteorites are the rarest type of
meteorites, making up about six percent of witnessed falls. But because of
their tell-tale visual appearance, they're over-represented in collections.
That's because they're more likely to survive passage through an atmosphere and
are more resistant to weathering, even on Mars.
Most iron-nickel meteorites come from the cores of
shattered planetesimals that formed in the early Solar System. Those objects
were large enough to differentiate when they were molten. They formed a core of
dense iron and nickel, much like Earth did.
But life as a planetesimal was risky, and many of
them were shattered into asteroids. That's Cacao's likely history.
That's what makes meteorites, and especially metal
ones, so scientifically interesting. They can date back billions of years to
the beginning of the Solar System.
On Earth, meteorites like Cacao were humanity's
first source of iron. Long before smelting, people collected these meteorites
when they could and made knives and other implements out of them. King Tut was
buried with a dagger made of meteoric iron, and the Inuit people in the Arctic
and in Greenland also used meteoric iron.
They repeatedly visited one particularly large iron
meteorite called the Cape York meteorite. They hammered off chunks of iron to
shape into harpoon tips and started their own iron age without knowing anything
about smelting. They even traded iron with other groups of people.
But only our robot explorers will ever set eyes on
Cacao.
Cacao is only an interesting oddity to MSL
Curiosity. Curiosity's job is to study Gale Crater, Mt. Sharp, and features
like the sulphur-bearing unit. The unit is rich in salty minerals that formed
in the presence of water.
By researching the area, Curiosity is shedding light
on Mars' ancient history, and how it dried up to become the desiccated
wasteland it is now.
Finding Cacao is just a bonus.
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