Martian Geology More Diverse Than Thought
November 18, 2013

Martian Geology More Diverse Than Previously Thought

Brett Smith for - Your Universe Online

For years, scientists have thought of Mars as being made up of just one kind of rock – a very simplistic planet compared to the diverse geology of Earth.

However, a new study in the journal Nature Geoscience suggests there may be granite on Mars and puts forward a theory for how it could have formed there.

"We're providing the most compelling evidence to date that Mars has granitic rocks," said study author James Wray, an assistant professor of earth and atmospheric sciences at the Georgia Institute of Technology.

The study is based on the large deposit of feldspar, a mineral found in granite, recently found in an inactive Martian volcano. The location of the feldspar suggests protracted magmatic activity under the Martian surface could produce large-scale granite deposits, the study team said.

The Martian surface is mostly covered with basalt, the dark-colored rock commonly found throughout Hawaii. However, in the area around the Martian feldspar deposit, minerals rich in iron and magnesium and common in basalts are almost completely absent.

Scientists were puzzled earlier in the year when the Mars Curiosity rover made an unexpected discovery: soils with a composition similar to granite. The new study supports this initial discovery through the remote infrared surveying of a large volcano on Mars that is thought to have been active for billions of years. Thanks to nearby sand dunes being swept up and blasting away at the volcano, it is dust-free, making it ideal for the study.

"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.

The location of the feldspar and absence of dark minerals inside the volcano explain how granite could form on Mars. According to the study researchers, cooling subsurface magma causes low density melt to separate from high-density crystals during a process called fractionation. Granite is formed when this cycle is repeated over and over, which could happen inside a volcano that is active over billions of years.

"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."

The geological process is referred to as igneous distillation. In the case of the Martian volcano, the distillation progressively lowered the density of the rock, giving it the physical properties of granite, the scientists said.

"These compositions are roughly similar to those comprising the plutons at Yosemite or erupting magmas at Mount St. Helens, and are dramatically different than the basalts that dominate the rest of the planet," said Josef Dufek, an associate professor of earth and atmospheric sciences at Georgia Tech.

Another study published by Nature Geoscience by a different team offered another explanation of the feldspar deposit on Mars. That team, from the European Southern Observatory and the University of Paris, said the volcanic rocks could be anorthosite, which is common on the moon. Wray said because the feldspar is located inside of the volcano, it makes a stronger argument for granite.

Taken together, both studies indicate Mars is probably more geologically interesting than previously thought.

"We talk about water on Mars all the time, but the history of volcanism on Mars is another thing that we'd like to try to understand," Wray said. "What kinds of rocks have been forming over the planet's history? We thought that it was a pretty easy answer, but we're now joining the emerging chorus saying things may be a little bit more diverse on Mars, as they are on Earth."