Surprises from Mars: Red Planet has Larger Core and Less Dense Crust than Expected
NASA InSight team, including UMD researchers, builds a new picture of Martian interior structure based on direct observations
NASA’s InSight mission landed a probe on Mars in November 2018 with instrumentation to gather the first direct measurements of ground vibrations, known as seismic waves, from the red planet. An international team of researchers, including several from the University of Maryland’s Department of Geology, conducted the first comprehensive review of the data. They determined the red planet has a larger molten core and a less dense crust than previously estimated. The results were published in three papers in the journal Science on July 23, 2021.
“These studies on Mars are special because, for the first time, we get to reconstruct the interior structure of Mars based on direct observation of geophysical activity,” said Doyeon Kim, a postdoctoral associate in UMD’s Department of Geology and a co-author of the three papers. “It’s an amazing moment kind of analogous to Earth science of the early 20th century when some of the pioneering scientists discovered major boundaries, such as the core and mantle, and put together a holistic view on how the Earth interior looks.”
Predictions about the interior of Mars have come from observations from orbiting spacecraft, which have provided extensive information about the surface of the planet, but only rudimentary data on what lies below.
“Previous studies of the Martian interior were based on geodetic data, which is indirect data on surface shape and magnetic fields gathered from orbiting the planet,” said Quancheng Huang (Ph.D. ’20, geology), another co-author of two of the papers from UMD who is now a postdoctoral fellow at New Mexico State University. “But these studies are based on recorded seismic waves traveling through the planet, so they provide the first direct evidence of the Martian interior structure.”
Seismic waves can be caused by movement of the winds on the surface, meteorite impacts, and faults or rock fractures within a planet. On Earth, these movements are called earthquakes, on Mars they’re called Marsquakes. The majority of earthquakes are driven by plate tectonics; however, on Mars where there are no plate tectonics, the faults are caused by cracking from the slight shrinking of the once-hot planet or younger volcanic eruptions. The seismic waves caused by Marsquakes echo and bounce around as they travel through different layers of rock and molten metal. Based on how the different waves propagate through the crust, mantle and core, scientists can identify the different geologic layers within the planet and make estimates about the composition of structures in the planet’s interior.
The InSight mission seismometer detected 733 distinct Marsquakes. Analyses of 35 of those quakes, which registered magnitudes between 3.0 and 4.0, were reported in the three research papers. The research showed that Mars is differentiated much like Earth, with a thin outer crust of brittle rock above a mantle of softer rock and a core that is at least partially molten.
“Prior to landing, we estimated that the radius of the core was probably between 1,450 and 1,850 kilometers, based on estimates of moment of inertia, lack of strong magnetic field and geodetic clues,” said Angela Marusiak (Ph.D. ’20, geology), a UMD co-author of one of the research papers who isnow a postdoctoral scholar at NASA’s Jet Propulsion Laboratory. “But the seismic data suggests it’s more like 1,830 kilometers, give or take 40 kilometers, which means it's on the larger end of our estimate. So, it’s going to be a little bit less dense and it's going to have more light elements like sulfur, carbon, oxygen or hydrogen than some of the other predictions.”
The unexpected size of the Martian core also indicates that the mantle structure may be simpler than it is on Earth, where an upper and lower mantle are separated by a transition in which increasing pressure and temperature cause minerals to rearrange and become denser. The larger core implies that pressures needed for this transition may not be reached in Mars’ mantle.
The research also found that Mars’ crust is much less dense, and therefore has a different composition than previously estimated. The composition of the crust can reveal important information about the history of the planet through its formation and as it cooled. Understanding the structure of the crust and the interior structure of Mars is important because it provides another example in addition to Earth for scientists to study the history and evolution of planets.
“These results are not only exciting for what they show us about Mars, they are a resounding validation of the tools and technology,” said Vedran Lekic, an associate professor of geology at UMD and a co-author of all three papers. “Even though we only have a single seismic station on Mars, our findings demonstrate that we can effectively leverage seismic techniques and analysis developed for studying Earth to study other planets, even when there is limited data available, which is a typical case for planetary missions.”
The researchers emphasize that this is just the first direct study of the Martian interior. They expect further analysis of the current data to reveal more about Mars’ interior structure. And data continues to arrive from Mars. The InSight mission was expected to last two Earth years, but the instrumentation continues to gather data.
“We are now in the fortunate position to have equipment and support from NASA that will allow us to continue the mission for as long as Mars lets us,” said UMD Associate Professor of Geology Nicholas Schmerr, who is a NASA InSight participating scientist co-investigator and the lead investigator for the UMD scientists on the mission. “We are still examining data for more details about the interior, including if there is any layering in the mantle indicative of a transition zone like on Earth. We’re learning more about where seismicity is originating from in Mars and figuring out how Mars fits into the spectrum of objects in the solar system.”
UMD postdoctoral associates in geology Ross Maguire and former UMD postdoctoral associate Foivos Karakostas (now a postdoctoral fellow at the Istituto Nazionale di Geofisica e Vulcanologia in Bologna) were also co-authors on these research papers.
The UMD portion of this work was funded in part by NASA (Award No. 80NSSC18K1628) and the Packard Foundation. This article does not necessarily reflect the views of these organizations.
The research papers, Seismic detection of the Martian core, Upper mantle structure of Mars from InSight seismic data andThickness and structure of the Martian crust from InSight seismic data, were published in the journal Science on July 23, 2021.
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