InSight maps the composition and bulky core of Mars for the first time
We are closer than ever to understanding the makeup of Mars thanks to the first observations of seismic activity on the planet made by the InSight lander. The NASA-led project, which landed on the surface of the Red Planet in November 2018 with the aim of probing beneath the Martian surface, observed several “earthquakes” that reveal details of its crust, mantle and surface. core.
The main findings of InSight, which are detailed in three articles published today in the journal Science, represent the first time that scientists have been able to produce a detailed image of the interior of a planet other than Earth.
“We seek to understand the processes that govern the evolution and formation of planets, to discover the factors that led to the unique evolution of Earth,” says Amir Khan, ETH Zurich and the University of Zurich, whose l The team used seismic waves reflected directly and at the surface to reveal the structure of Mars’ mantle. “In this regard, the InSight mission fills a gap in the scientific exploration of the solar system by carrying out an in situ survey on a planet other than our own. “
The results of the current NASA mission – with full title “Interior exploration using seismic surveys, geodesy and heat transport”– could reveal key information about the formation and evolution of the red planet, while helping us understand the main differences between our planet and Mars.
“A big question we would like to understand is why Earth is the only planet with liquid oceans, plate tectonics, and abundant life? Khan adds. “Mars is currently on the edge of the solar system’s habitable zone and may have been more hospitable early in its history. While we don’t yet know the answers to these questions, we do know that they are on Mars, most likely indoors.
InSight first detected the presence of earthquakes from its position in Elysium Planitia near the equator of the Red Planet in 2019 and has since detected more than 300 events, or more than 2 per day, dating back many years. ‘between them at their source.
What’s really impressive is what researchers can do with these earthquakes, using them as a diagnostic tool to “see” deep inside the planet.
“By studying the signals of earthquakes, we measured the thickness of the crust and the structure of the mantle, as well as the size of the Martian core,” explains Simon Stähler, seismologist researcher at ETH Zurich. ZME Science. “It replicates what was done on Earth between 1900 and 1940 using signals from earthquakes.”
From the crust of Mars …
Observations made by InSight have allowed researchers to assess the structure of Mars’ crust, allowing them to determine its thickness and other properties in absolute numbers for the first time. The only values we previously had for the Martian crust were relative values which showed differences in thickness from area to area.
“As part of the overall interior structure of Mars, we determined the thickness and structure of the Martian crust,” said Brigitte Knapmeyer-Endrun, a geophysicist at the Institute of Geology at the University of Cologne. . ZME Science. “Previous estimates could only rely on orbital data – gravity and topography – which can accurately describe relative variations in crustal thickness, but not absolute values. These estimates also showed great variability.
With data collected regarding the thickness of the crust over the InSight landing zone, new seismic measurements, and data collected from previous missions, the team was able to map the thickness across the entire Martian crust by finding an average thickness between 24 and 72 km.
Knapmeyer-Endrun explains that the data she and her team collected with InSight’s Seismic Experiment for Interior Structure (SEIS), specifically the Very Wide Band Seismometer (VBB) – an instrument so sensitive it can record motion at the atomic scale – and information from ETH Zurich’s Marsquake Service (MQS), suggests the Red Planet’s crust is thinner than models have so far predicted.
“We end up with two possible crust thicknesses at the landing site – between 39 and 20 km – but both mean that the crust is thinner than some previous estimates and also less dense than what was postulated on the basis orbital measurements of the surface. “
Knapmeyer-Endrun goes on to explain that InSight data also reveals the structure of the Martian crust as multi-layered with at least two interfaces that mark a change in composition. In addition to this, the team cannot rule out the presence of a third crustal layer before the mantle.
“The crust has a distinct stratification, with a surface layer about 10 km thick that has rather low velocities, which implies that it is probably made up of rather porous fractured rocks, which is not unexpected in due to the repeated impacts of meteorites “, explains the geophysicist. adding that we are seeing something similar on the Moon, but the effect is more extreme due to the much finer atmosphere of this smaller body.
Knapmeyer-Endrun is pleasantly surprised at the amount of information InSight was able to collect with a single seismometer. 4.0 from a single seismometer, ”she explains. “On Earth, we wouldn’t even be able to detect these earthquakes at a comparable distance. We typically use dozens or even hundreds of seismometers for similar studies. “
And the earthquake data collected by InSight has not only been found to be essential in assessing the thickness and composition of the Earth’s crust, it has also enabled scientists to probe deeper, into the very heart of Mars.
… To the Martian mantle and core
Using seismic waves reflected directly and at the surface from eight low-frequency earthquakes, Khan and his team probed deeper below the surface of Mars to study the planet’s mantle. They discovered the possible presence of a thick lithosphere 500 km below the Martian surface with an underlying layer at low speed, similar to that found on Earth. The study by Khan and his co-author reveals that the crustal layer of Mars is likely to be enriched with radioactive elements. These elements heat this region with this warming reducing the heat in the lower layers.
It is these lower regions that Stähler and his colleagues studied using weak seismic signals reflected from the boundary between the Martian mantle and the planet’s core. What the team found is that the core of the Red Planet is actually larger than expected, with a radius of around 1,840 km instead of previous estimates of 1,600 km. This means that the nucleus begins roughly halfway between the surface of the planet and its center.
From the new information, we can also determine the density of the nucleus and extrapolate its composition.
“We now know for sure the size of the nucleus and it is significantly larger than had been thought for a long time,” says Stähler. “Because we found that the core is quite big, so we know it’s not very dense. This means that Mars must have accumulated a substantial amount of light and volatile elements such as sulfur, carbon, oxygen and hydrogen.
This ratio of lighter elements is greater than that found in Earth’s denser core, and it could give us important clues about the differences in the formation of these neighboring worlds.
“Somehow these light elements had to get into the core. This may mean that the formation of Mars occurred faster than that of Earth, ”says Stähler. “These observations fueled speculation that Mars could represent a failed planetary embryo that describes the chemical characteristics of the solar nebula located in Mars’ orbit.”
As Knapmeyer-Endrun did, Stähler is amazed at InSight’s success in collecting seismic data, highlighting the role luck has played in the mission so far.
“We were able to observe seismic wave reflections coming from the core – like an echo – from relatively small earthquakes. And the earthquakes were just the right distance from the lander. If we had landed somewhere else, it would not have worked, ”explains the seismologist. “And the landing site was only selected because it was flat and there were no rocks, so it was really lucky.”
Stähler says he and his team will now attempt to use the seismic waves that have passed through the core of Mars to determine whether the planet’s core has an inner core of solid iron like Earth, or whether it is entirely liquid. Just one of the lingering questions Knapmeyer-Endrun says InSight will use earthquakes to tackle for years to come.
“There are still many open questions that we would like to address with seismology. For example, what geological / tectonic characteristics are the observed earthquakes related to? How deep are olivine phase transitions in the mantle? And is there a solid inner core, like on Earth, or is the entire core of Mars liquid? Said the geophysicist.
And based on the track record, smart money is on InSight to answer these questions and more. “In just 2 years of recording data on Mars, this unique seismometer was able to tell us things about the crust, mantle and core of Mars that we have been speculating on for decades.”