by Jonathan Amos
Chunks of water-ice (white marks)
litter the surroundings of the crater
Space probes have witnessed a big impact crater being formed on Mars
- the largest in the Solar System ever caught in the act of
A van-sized object dug out a 150m-wide bowl on the Red Planet,
hurling debris up to 35km (19 miles) away.
In more familiar
terms, that's a crater roughly one-and-a-half times the size of
London's Trafalgar Square.
And its blast zone would fit neatly in the area inside the UK
capital's orbital motorway, the M25.
Scientists detected the
event using the seismometer on the US space agency's
The probe picked up the
ground vibrations. Confirmation came from follow-up imagery acquired
by Nasa's Mars Reconnaissance Orbiter (MRO).
This satellite routinely
pictures the planet and could produce the before-and-after proof of
a major surface disturbance, corresponding to the exact timing and
in the expected direction and distance (3,500km) from InSight.
"This is the biggest
new crater we've ever seen," said Dr Ingrid Daubar from
"It's about 500ft
wide, or about two city blocks across, and even though
meteorites are hitting the planet all the time, this crater is
more than 10 times larger than the typical new craters we see
forming on Mars."
"We thought a crater
this size might form somewhere on the planet once every few
decades, maybe once a generation, so it was very exciting to be
able to witness this event."
The impact was about 3,500km
InSight's location on Mars
The post-impact observation shows huge chunks of buried water-ice
have been excavated and thrown around the edges of the crater.
Buried water-ice has never before been seen so close to Mars'
Such deposits would be an important resource for future human
missions to the planet.
"That ice can be
converted into water, oxygen or hydrogen. That could be really
useful," said Dr Lori Glaze, Nasa's director of planetary
Using its French/UK-built
seismometer instrument, Nasa's InSight lander has detected
more than 1,300 quakes on Mars since its arrival in November 2018.
But the Magnitude 4
tremor resulting from this particular event, which occurred on 24
December, 2021, immediately piqued the interest of mission
scientists because it contained a component of so-called "surface
The vast majority of quakes picked up by InSight have produced the
traditional primary and secondary waves associated with rock
movements deep within the planet.
These newly detected ripples were travelling in the uppermost
portion of Mars, through its crust.
Before and after: The 24 December, 2021,
impact threw debris for more than 30km
"This is the
first-time seismic surface waves have been observed on a planet
other than Earth.
Not even the Apollo
missions to the Moon managed it," said Doyeon Kim from
ETH Zurich's Institute of Geophysics and a lead author on the
scholarly reports appearing in
Science Magazine this week.
The recognition of
surface waves also enabled the researchers to identify a second
This one, on 18
September, 2021, occurred roughly 7,500km from InSight. It was a
slightly smaller event and produced a cluster of craters, the
largest of which was 130m in diameter.
Scientists think both impacts can provide fresh knowledge about
Whereas the deeply
sourced quakes tell them about the structure and composition of the
planet's mantle and core, the surface waves reveal new details about
the overlying crust.
Researchers can tell that in between the InSight lander and
the impact sites, the crust has a very uniform structure and high
This contrasts with
the previously reported three layers of low-density crust
directly below InSight.
This realization may have
something to say, too, about the famous Mars dichotomy - the
observation that the Northern Hemisphere is low and relatively flat,
whereas the Southern Hemisphere of the planet is high and
Researchers have wondered whether that's because the crust in these
regions is composed of different materials.
But the new surface wave
data and its suggestion of widespread uniformity in the crust would
imply this theory is probably not the best explanation.
Mars' red dust
settled on InSight's solar panels,
reducing their efficiency
Dr Ben Fernando from Oxford University is an InSight mission
observations in the transition zone between the North and the
South have been really valuable because clearly the crust
evolved in very different ways in those regions of the planet,"
he told BBC News.
"How and why they developed the way they did is still an open
question, but I think these impact events have probably provided
more understanding on this topic than anything else we've done
so far on the mission."
There are many craters on
Mars, the consequence of billions of years of bombardment from rocks
drifting through space.
Some are true giants.
The Hellas Basin is
an impact structure over 2,000km in diameter.
But the 2021 impacts are
significant because scientists have the instrument data recording
the moment of their creation.
"Something like [the
24 December impactor] hits Earth a few times every decade, but
burns up safely in the atmosphere or drops a few meteorites.
We were amazingly
lucky to catch this one while InSight was listening," commented
Prof Gareth Collins from Imperial College London.
The InSight mission is
close to ending.
Dust is settling on its
solar panels, reducing their efficiency.
"In the next short
amount of time, perhaps somewhere between four and eight weeks
as best we can can predict, we expect the lander to not have
enough power to operate any longer," the mission's principal
investigator Dr Bruce Banerdt told reporters.