By Will Dunham
WASHINGTON – NasaThe InSight robotic lander, in the twilight of its service after nearly four years on Mars, reveals new details about Earth’s closest planetary neighbor based on data from two large meteor impacts that shed light on the structure of the Martian crust.
Researchers said on Thursday that the hover lander detected for the first time seismic waves moving along the Martian surface, as opposed to deeper in the planet’s body, caused by two space rocks hitting Mars in September and December last year. The reverberation of these waves along the surface gave clues to the crust, the planet’s outermost layer, over a wide geographic expanse in the northern hemisphere.
These were the largest impacts detected by InSight since it arrived on Mars in November 2018. Its science mission – originally scheduled to last two years but later extended for two more until December – is expected to come to an end as its solar panels are now covered in an accumulation of dust, robbing InSight of power.
Seismic waves generated by earthquakes and surface impacts vary in speed and shape as they pass through different materials inside a planet. Data collected by InSight’s seismometric instrument has helped scientists decipher the internal structure of Mars, including the size of its liquid metal core, the nature of its mantle and the thickness of its crust.
InSight has also established that Mars is seismically active by detecting more than 1,000 marsquakes, the largest of which occurred in May this year with a magnitude of 5.0 – medium in size compared to earthquakes on Earth.
“InSight was a success beyond my expectations,” said planetary geophysicist Bruce Banerdt of Nasa‘s Jet Propulsion Laboratory, principal investigator of the InSight mission and co-author of the new research published in the journal Science. “We are probably in the final phase of the mission.”
The three-legged InSight – short for Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport – sits on a vast, relatively flat plain called Elysium Planitia, just north of the Martian equator.
Until now, InSight had obtained data on the structure of the Martian crust – which consists mostly of fine-grained volcanic basalt rock – only in the area below its actual landing site.
Earlier findings indicated that the crust at the landing site was composed of relatively soft materials, less dense rocks. This was not the case for the other regions covered by the new data, where the crust appears denser.
There are two main types of seismic waves. Surface waves travel along a planet’s surface like ripples on water. Body waves pass through the inner layers of a planet.
“As a result of our surface wave analysis, we now understand that the crust of Mars north of the equatorial dichotomy – a visible feature of topographic variation on Mars that divides the southern highlands and the northern lowlands – has a relatively uniform structure,” said seismologist Doyeon Kim from the Institute of Geophysics of ETH Zurich, lead author of the research.
The data covered depths between 3 and 19 miles (5-30 km) below the surface.
The earth’s crust is broken up into huge plates that move inexorably over a rocky inner layer called the mantle in a process known as plate tectonics. The crust in some places under the Earth’s oceans is constantly being recycled. Mars has no plate tectonics.
“Mars’ crust is ancient and possibly similar to Earth’s crust in its early stage,” Kim said.
The December 2021 impact detected by InSight left a crater with a diameter of about 490 feet (150 meters). The September 2021 crater was about 130 meters (425 feet) wide.
“Two very large meteor impacts occurred in 90 days,” said planetary scientist and study co-author Simon Staehler of ETH Zurich Seismology and Geodynamics Group. “It’s important in terms of the hazard perspective for potential future exploration of the planet and we need to understand if this was a once-in-a-lifetime opportunity or if it happens more often on Mars.”
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