In May of 2018, NASA’s Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) landed on the Martian surface. This mission is the first of its kind, as all previous orbiters, landers, and rovers focused on studying the surface and atmosphere of Mars. In contrast, InSight was tasked with characterizing Mars’ interior structure and measuring the core, mantle, and crust by reading its seismic activity (aka. “marsquakes”).
The purpose of this is to learn more about the geological evolution of Mars since it formed 4.5 billion years ago, which will also provide insight into the formation of Earth. According to three recently published papers, the data obtained by InSight has led to new analyses on the depth and composition of Mars’ crust, mantle and confirmed the theory that the planet’s inner core is molten.
The three studies, which were published in the July 23rd issue of Science, were led by Brigitte Knapmeyer-Endrun of the Bensberg Observatory at the University of Cologne; Amir Khan, a researcher with the Physics Institute at the University of Zürich; and Simon Stähler, a researcher with the Institute of Geophysics at ETH Zurich. These papers addressed the new findings made thickness and structure of the Martian crust, the upper mantle structure, and the molten core (respectively).
Clouds drift over the dome-covered seismometer, known as SEIS, belonging to NASA’s InSight lander, on Mars. Credit: NASA/JPL-Caltech
As Bruce Banerdt, InSight’s principal investigator at NASA’s Jet Propulsion Laboratory (JPL), expressed in a recent NASA JPL press release: “When we first started putting together the concept of the mission more than a decade ago, the information in these papers is what we hoped to get at the end. This represents the culmination of all the work and worry over the past decade.”
The data that led to all three papers came from InSight’s seismometer, known as the Seismic Experiment for Interior Structure (SEIS). On Mars, seismic activity is largely the result of impacts on the surface, which causes sound waves to travel through the mantle and core to the other side of the planet. The ultrasensitive SIES was designed to let scientists hear these soundwaves, which vary in terms of speed and shape based on the materials they pass through.
These variations have given seismologists a way to study the inner structure of Mars and learn more about how all rocky planets – including Earth, Venus, and Mercury. In all cases, the rocky planets formed from the protoplanetary disc, which was made up of dust and meteoric material leftover from the formation of the Sun. As this material coalesced, it became a giant ball of molten silicate minerals, metals, and other elements.
Over the course of tens of millions of years, the planet cooled and differentiated into three distinct layers – the crust, mantle, and core – with the lighter silicate elements settling near the top and heavier elements (like iron and nickel) settling in the core. Measuring the depth, size, and structure of these three layers has always been a central part of InSight’s mission and the purpose for which SEIS was designed.
NASA’s InSight lander detected a marsquake, represented here as a seismogram, on July 25, 2019, the 235th Martian day, or sol, of its mission. Seismologists study the wiggles in seismograms to confirm whether they’re really seeing a quake or noise caused by wind. Credit: NASA/JPL-Caltech
Since the SEIS was first placed on the Martian surface, it has recorded 733 distinct marsquakes, 35 of which (all between magnitudes of 3.0 and 4.0) provided the data for all three papers. From this data, Knapmeyer-Endrun and her colleagues determined that the Martian crust is thinner than expected and may have two or three sub-layers to it. If there are two sub-layers, the crust extends 20 km (12 miles) beneath the surface, or 37 km (23 mi) if there are three.
Meanwhile, Khan and his colleagues determined that the mantle extends 1,560 km (969 mi) below the surface, while Stähler and colleagues found that the core is liquid and has a radius of about 1830 km (1,137 mi). This study is a once-in-a-lifetime chance,” said Stähler. “It took scientists hundreds of years to measure Earth’s core; after the Apollo missions, it took them 40 years to measure the Moon’s core. InSight took just two years to measure Mars’ core.”
One surprising find was that all of the most significant marsquakes detected by InSight appear to have come from one area: Cerberus Fossae, a region that is volcanically active enough that geophysicists think that lava may have flowed there within the past few eons. This is based partly on images taken by orbiting spacecraft that have spotted boulder tracks and other landslide features that appear to have been caused by marsquakes.
Another surprise was that none of these quakes were detected coming from the more prominent volcanic regions like Tharsis, where the three of the largest volcanoes on Mars are located (collectively known as Tharsis Montes). However, many quakes (large and small) may be occurring that InSight can’t detect because of shadow zones, which are caused by the core refracting seismic waves away from certain areas.
The two largest quakes detected by NASA’s InSight appear to have originated in a region of Mars called Cerberus Fossae. Credits: NASA/JPL-Caltech/Univ. of Arizona.
In the meantime, InSight’s seismometer is detecting new marsquakes every day, and the mission team hopes to detect a marsquake that is larger than a 4.0. “We’d still love to see the big one,” said JPL’s Mark Panning, co-lead author of the paper on the crust. “We have to do lots of careful processing to pull the things we want from this data. Having a bigger event would make all of this easier.”
These findings are the first of many to come from InSight’s seismic data, which are helping scientists to refine their models of Mars and its formation. They will also provide valuable information on how Mars lost its magnetosphere roughly 4.2 billion years ago, which was followed by the slow depletion of its atmosphere by solar wind over the course of several hundred million years.
That process caused Mars to transition from a warmer, wetter planet that could have supported microbial life on its surface to the icy and arid planet that it is today. Knowing how and why that transition occurred will also shed light on how terrestrial planets remain habitable (or fail to do so) over the course of their evolution. That knowledge will also assist astrobiologists looking to characterize extrasolar planets and place constraints on their potential habitability.
The research findings were also the subject of a live-streamed discussion that appeared on NASA Television on Friday, July 23rd (which you can watch below), as well as the NASA app, the agency’s website, and the JPL’s YouTube and Facebook channels. The panel included Mike Panning from NASA Jet Propulsion Laboratory, Amir Khan, and Sabine Stanely of John Hopkins University.