Like many planets within our solar system, Mercury has had very few studies completed in comparison with Earth, the main reason being how difficult it is to get there.
Being the closest planet to the Sun, only one probe, Messenger, has traveled to Mercury in 2015. At the time, it was concluded that Mercury’s crust was 35 kilometers thick, similar to Earth’s 40-kilometer continental crust. Though University of Arizona Lunar and Planetary Laboratory Associate Staff Scientist Michael Sori believes that it is closer to 26 kilometers thick.
His study and research paper titled “A thin, dense crust for Mercury” will be published on Tuesday, May 1.
Sori, using data collected from NASA’s messenger program in the 1970s, calculated the density of Mercury’s crust. Using the data, the density was calculated using a formula developed by Isamu Matsuyama, a professor in the Lunar and Planetary Laboratory, and University of California Berkeley scientist Douglas Hemingway.
Sori’s theory that the crust of Mercury was developed primarily by volcanism is supported by the estimates of the formula. By studying the crustal formation, scientists will be able to better understand how Mercury, a very unique planet, was formed.
Compared to other terrestrial planets sizes, Mercury is known to have the largest core. It is speculated to comprise of almost 60 percent of Mercury’s mass, compared to Earth, whose core only takes up 15 percent of its mass.
This begs the question as to why Mercury’s core is so large. “Maybe it formed closer to a normal planet and maybe a lot of the crust and mantle got stripped away by giant impacts,” Sori said. “Another idea is that maybe, when you’re forming so close to the sun, the solar winds blow away a lot of the rock and you get a large core size very early on. There’s not an answer that everyone agrees to yet.”
While Sori’s research is only the beginning, it has already revealed what comprises Mercury’s crust, which is primarily made of aluminum.
The studying of Mercury’s crust raises questions about its composition and, more importantly, its mantle.
The crust of the planet is essentially the cooled and lighter rock. When the planets were cooling, they underwent differentiation, in which heavier elements sank to the center of the planet, forming the core, and the lighter elements rose up, eventually making up the mantle and the crust.
Before Sori’s studies, research had estimated that 11 percent of Mercury’s mantle formed its crust, based on the thickness. In comparison, the moon is similar in size and is believed to have only seven percent cool rock to form its crust. “The two bodies formed their crusts in very different ways, so it wasn’t necessarily alarming that they didn’t have the exact same percentage of rocks in their crust,” Sori said.
The moon formed when less dense and light minerals floated to the surface and formed its crust. With these numbers, however, people were confused as to why the moon produced fewer rocks in comparison to Mercury. Sori’s research shows that Mercury and the moon are much more similar and there is no anomaly. This data, knowing Mercury’s crustal density and depth, allowed Sori to understand the isostasy of Mercury.
The isostasy then helps Sori understand the more specific details about Mercury’s crust, because isostasy helps us understand how the crust sits upon the mantle and how hills and valleys form without falling to form smooth planes.
There are two main types isostasy: Pratt and Airy. Both focus on balancing the masses of equally-sized slices of the planet.
If the mass in one slice is much greater than the mass in a slice next to it, the planet’s mantle will ooze, shifting the crust on top of it until the masses of every slice are equal.
Sori disproved that Mercury was dominated by Pratt isostasy by comparing the topography of Mercury with element composition. With these studies increasing and another journey to Mercury in 2025, scientists hope to learn more about the closest planet to the sun.