In August 2012, the Mars Science Laboratory’s rover Curiosity landed at the base of Gale crater, a 5-kilometer-high mountain that formed when a meteor hit Mars billions of years ago. Using its 2-meter-long arm to drill into the planet’s surface, Curiosity scooped up and analyzed rock and soil samples, including some light-colored, crystal-studded rocks surprisingly similar to the ancient granitic rock that forms much of Earth’s continental crust.
The discovery made waves in the science community because it suggested that Mars might be the only known planet besides Earth possibly to have a continental crust. Mars traditionally is thought to be covered in denser, darker igneous rock similar to Earth’s oceanic crust, which is formed as volcanic magma sourced from Earth’s mantle cools.
Now, however, research by Udry et al. contradicts that hypothesis. Instead of seeping up between tectonic plates, the team argues, the rocks could have formed through a process similar to one on Earth: intraplate, or “hot spot,” volcanism, found in places like Hawaii, Iceland, and the Canary Islands. In hot spot volcanism, magma does not need to find the boundaries or cracks between tectonic plates to rise to the surface. Instead, it merely pushes up and breaks through weaker, thinner areas of crust.
To make their case, the researchers used a computational tool called MELTS, which can simulate the conditions required for magma to form different igneous rocks. They started with six different potential types of magma based on data from the ancient “Black Beauty” meteorite—a chunk of polished Martian rock found in the Moroccan desert in 2011—and five rocks from the Spirit rover mission nicknamed Fastball, Backstay, Esperanza, Home Plate_June Emerson, and Champagne.
They modeled how the magmas would have crystallized as they cooled, starting with the lowest temperature at which the rocks would be liquid and decreasing by 10°C intervals until they reached 600°C (or a higher temperature if the rocks had already solidified). The team found that conditions similar to those in hot spot volcanism readily transformed the magma into rocks with chemical and mineralogical compositions similar to those found in Gale crater: pale, light minerals containing lots of silica, similar to the granitic rocks that form Earth’s continental crust. The study suggests that no continental plates were needed to form the tantalizingly Earthlike rocks—just garden-variety Martian volcanoes, produced by intraplate magmatism. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2018JE005602, 2018)
—Emily Underwood, Freelance Writer