Geologists have now found the most compelling evidence of granites on Mars – something which prompts more complex theories about the geology and tectonic activity on the Red Planet.

Granites and basalts

Basalt and Granite. Credits: Rice University.

Granites are igneous rocks, pretty common on the surface of Earth. It is often called a ‘felsic’ (white rock) – because it is very rich in so-called white minerals, such as quartz or feldspar. It is contrasted with mafic rocks (for example basalt), which are relatively richer in magnesium and iron. Now, large amounts of feldspar have been found in a Martian volcano. Interestingly enough, minerals commonly found in basalts are completely absent from that area; considering how basalts are almost ubiquitous on Mars, this initially came as a shock, but now, geologists have come up with a theory to explain this.

Granite, or its eruptive equivalent, rhyolite, is often found on Earth in tectonically active regions such as subduction zones. However, since Mars isn’t tectonically active, there are no subduction zones there, so there has to be a different cause. The team studying the case concluded that prolonged magmatic activity on Mars can also produce these granitic compositions on very large scales.

“We’re providing the most compelling evidence to date that Mars has granitic rocks,” said James Wray, an assistant professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology and the study’s lead author.

Red Planet geology

A ‘spectral window’ into the Martian geology – bright magenta outcrops have a distinctive feldspar-rich composition. (Credit: NASA/JPL/JHUAPL/MSSS)

For many years, the geology of Mars has been considered to be very simplistic, consisting of mostly one single type of rock: basalt – a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface. The dark rock can also be found on Earth in many volcanically active areas, such as Hawaii or Iceland for example.

But earlier this year, the Mars Curiosity started to cast some doubt on those beliefs, when it reported finding soils with a composition similar to granite. No one really knew what to make of this discovery, but since it appeared to be very localized, it was just considered a local anomaly. However, this new research analyzed things at a much larger scale, using remote sensing techniques with infrared spectroscopy to survey a large volcano on Mars that was active for billions of years. The volcano is perfect for this type of study, because it is dust free (a true rarity on Mars) – some of the fastest-moving sand dunes on Mars sweep away any would-be dust particles on this volcano.

Much to the delight of researchers, the limitations of the remote sensing technology were an advantage in this case:

“Using the kind of infrared spectroscopic technique we were using, you shouldn’t really be able to detect feldspar minerals, unless there’s really, really a lot of feldspar and very little of the dark minerals that you get in basalt,” Wray said.

Separating the white and the black

So we have an island of white feldspar amidst an ocean of black basalt – how did it form?

When you have magma in the subsurface, it cools off very, very slowly. In a tectonically inactive planet like Mars, this process can be very stable. While the magma slowly cools off, low density melt separates from high density crystals, and if the conditions are just right, this process can take place for billions of years, leading to the creation of granitic rocks, as computer simulations showed.

“We think some of the volcanoes on Mars were sporadically active for billions of years,” Wray said. “It seems plausible that in a volcano you could get enough iterations of that reprocessing that you could form something like granite.”

While we are trying to figure out the existence (or lack of it) of life on Mars, this is another wake-up call, showing just how little we understand about the geologic processes on the Red Planet – which ultimately govern the appearance of life. Anyway, the geology of Mars just got a lot more interesting.

Journal Reference:

1. James J. Wray, Sarah T. Hansen, Josef Dufek, Gregg A. Swayze, Scott L. Murchie, Frank P. Seelos, John R. Skok, Rossman P. Irwin, Mark S. Ghiorso. Prolonged magmatic activity on Mars inferred from the detection of felsic rocks. Nature Geoscience, 2013; DOI: 10.1038/NGEO1994