When two massive meteorites smashed into Mars late last year, the shockwaves reverberated around the red planet — giving scientists new insights into its internal structure.
Drawing on data from two Nasa missions, the first analysis of the meteorites’ impact, which left the largest fresh craters ever seen in the solar system, was published in the journal Science on Thursday.
The Mars Reconnaissance Orbiter observed the craters from space, while the InSight lander recorded the resulting seismic waves on the surface thousands of kilometres away.
“The larger impact made a crater about 150 metres across — one and a half times the size of London’s Trafalgar Square — and a blast zone around 35km across, which would cover most of the area inside the M25,” said Ben Fernando, an Oxford university geophysicist on the InSight team.
Images from the orbiter showed that the 200-tonne meteorite, which hit Mars at a latitude of 35 degrees north, pushed out chunks of water ice that had lain beneath the dry surface. “I think we were a little surprised to find ice that close to the equator,” Fernando said.
The implication is that Mars might hold more water frozen underground than scientists had realised, he added, which would be good news if humans ever manage to establish outposts on the red planet.
The impact of the two meteorites sent seismic waves racing around the surface of Mars to the InSight lander, which was 7,500km away from the first hit and 3,500km from the second. These were the first surface waves recorded by the probe’s seismometer — the only type that can provide reliable information about the planet’s outer crust.
Since landing on Mars in November 2018, InSight has recorded 1,200 “Marsquakes” generated within the red planet, which provided data about the inner core and mantle beneath the probe but little about the crust.
Preliminary analysis of the surface waves generated by the meteorites surprised the international research teams as it revealed the planet’s crust as a whole was denser and more uniform than previously supposed.
“The structure of the crust under the lander is probably not representative of the general structure of the Martian crust,” said Doyeon Kim, a geophysicist at ETH Zurich and lead author of one of the Science papers.
Domenico Giardini, ETH professor of seismology and geodynamics, said the findings shed light on a mystery going back to the early years of modern astronomy, known as the Mars dichotomy. This is the contrast between the planet’s southern hemisphere, which is predominantly mountainous and rugged, while the north is dominated by flat and dusty volcanic lowlands.
“We don’t yet have a generally accepted explanation for the dichotomy because we’ve never been able to see the planet’s deep structure,” said Giardini, “but now we’re beginning to uncover this.” The initial results suggest that the north and the south are actually more alike in their geology than scientists had realised.
The Insight lander is losing power as dust builds up on its solar panels and Nasa expects its mission to end within a few months. But scientists will continue to analyse data accumulated by its instruments for years to come.