The conventional wisdom says that Mars is made entirely of dark lava rocks called basalt. Basalts are also common on Earth, where they make up the Hawaiian islands and the ocean floor. But most of Earth's continents are made of granite, which has been notably absent on Mars. Not long ago, the Curiosity rover spotted some sand grains that looked like they could have eroded from a granite, but no one knew where the sand grains could have eroded from.
Now, scientists say they have found evidence for granite (or granite-like) rocks in several places around the Martian surface. They looked specifically at areas scrubbed free of Mars's ubiquitous dust, including the flanks of Mars's only dust-free volcano, in Syrtis Major.
"If Syrtis is representative," says James Wray, an astronomer at the Georgia Institute of Technology, then while the vast majority of Mars is indeed basaltic, "a percent or two or three" is made of these granite-like rocks, plus another 10 percent or so with an intermediate composition.
Chemically speaking, it's easy to make a basalt magma. Just heat up the inside of a planet, and the first thing that melts will be (more or less) basalt. Making a granite magma is much more complicated.
Imagine you fill a bucket with ocean water at the beach, then come home and put it in the freezer for half an hour. When you pull it out, you'll have an ice-water slushie. The ice will be pure H2O, leaving the salt and other impurities concentrated in the liquid. If you pour off the liquid into another container and then put that into the freezer, the same process will happen again. As you keep repeating the process, the liquid will become more and more salty. Eventually, even the salt will crystallize out, leaving you with a thick paste of trace minerals.
This process happens in magma chambers, too, deep underground. Different minerals have different melting points, so if basaltic magma hangs around long enough, it starts to cool down – like sea water in your freezer – and it starts to freeze. The first minerals to freeze out are iron-rich, leaving behind an iron-poor magma. If the liquid rock keeps cooling, more and more iron will crystallize out, leaving behind a sludgy liquid that has a high percentage of silica, calcium, sodium, and other pale minerals. Given enough time, even this will solidify into rock, forming granite. The dark, iron-rich minerals are known as "mafic," while the usually light-colored minerals left over after the iron has crystallized out are known collectively as "felsic" minerals, and are considered "highly evolved" since they require several rounds of crystallization to form.
Scientists have seen evidence of "evolved" magmas in the past, but none with this little iron. If Dr. Wray and his colleagues are right, this suggests a much more nuanced view of volcanism on Mars, with magmas lasting for tens or even hundreds of thousands of years in the crust before erupting to the surface.
The chemical signature that Wray's team found on Mars seems to be high in felsic minerals like feldspar, but that's actually very hard to detect directly. But they definitely aren't finding anything mafic, says Josef Dufek, a planetary geologist at Georgia Tech, and a coauthor on the paper.
"I think that's the more important result, because that says it's probably a feldspar plus-or-minus quartz rock. It probably has quartz, you just can't detect it very well." Finding quartz is essentially impossible with the instruments currently orbiting Mars, so it can only be inferred from the absence of mafic minerals.
But if finding quartz is impossible, finding feldspar was merely improbable.
"If you've ever seen feldspar, it's very light, often white, and really, really hard to detect," explains Briony Horgon, a planetary science professor at Purdue University who reviewed Wray's paper for the journal Nature Geoscience. "The fact that they were able to see distinct signatures of this mineral means that there's a ton of it there, maybe 80 to 90 percent feldspar in these areas."
Identifying rocks through spectral signatures is like piecing together a scene that has been photographed through Venetian blinds. And in the case of felsic minerals on Mars, it's like you're looking for children, but the blinds only show things from 4 feet to 6 feet tall. You'd see faces and chests of adults, plus slices of trees and tall cars, but children, birds, and airplanes would be nearly impossible to spot.
That's the challenge of spectroscopy. In designing a mission, scientists must choose in advance which wavelengths of light to look at, while knowing that different wavelengths reveal different slices of the scene on Mars.
"We're using visible and near-infrared spectroscopy, which is sensitive to iron," says Wray.
Felsic minerals don't typically have any iron at all, so finding them is as unlikely as spotting a child through that slit in the blinds. You'd need to hope for an adult carrying a child, or maybe a kid on a ladder. In the case of the rocks on Mars, scientists needed to find feldspars in which iron atoms had taken the place of calcium atoms. It doesn't happen often – just like most toddlers would rather walk than be carried – but sometimes you get lucky, says Wray, and you can smell the faint whiff of a "little bit of iron" in what should be an iron-free rock.
"The only way we can see that 'little bit' is if you don't have any truly iron-rich minerals around," says Wray. "When I say 'don't have any,' I mean a few percent at most, that could be in these rocks."
In addition to the problem of the limited wavelengths, you also have to deal with the thick layer of iron-rich dust blanketing nearly everything on Mars. That's why these scientists targeted their search at Syrtis Major, scrubbed clean by active sand dunes, and certain crater floors that have been scoured by wind.
Their findings suggest that felsic minerals may be more common on Mars than ever believed.
"If you have an impact crater that blasted through the dust on the summit of Olympus Mons, you'd be most likely to hit basalt, if it looks anything like Syrtis Major, but there would be these little patches of more highly evolved compositions," says Wray.