On Christmas Eve last year, a meteorite slammed into the surface of Mars, gouging out a large crater and sending a seismic wave rippling across the planet.
It was the moment scientists had been waiting for since NASA's InSight spacecraft started taking the planet's pulse three years ago.
"I think most of us on the team suspected it was a joke at first," said Bruce Banerdt, head of the InSight mission.
"Especially this late in the mission, it is really fun to still be getting completely new results."
But this event was particularly important because it was so big, said Katarina Miljkovic of Curtin University.
Flying overhead, NASA's Mars Reconnaissance Orbiter (MRO) traced the impact back to a site nearly 3,500 kilometres away, the team reports today in the journal Science.
Scattered around the 150-metre-wide impact crater were blocks of water ice excavated by the force of the crashing meteorite.
"It's the largest, fresh impact crater detected in the past 16 years," said Dr Miljkovic, one of the report's authors.
And while craters with ice have been seen on Mars before, this one is much closer to the equator than others.
The MRO also traced the source of another seismic wave, detected on September 18 last year, to a 130-metre-wide impact crater about 7,455 kilometres away from InSight.
Surface waves circle the planet
InSight has recorded more than 1,300 quakes and rumbles coming from deep within Mars, centred around an area near the equator known as Cerberus Fossae.
The two large impact events, which occurred much further to the north, shed new light on the internal structure of the planet.
Not only did these impacts produce waves that travelled through the deeper layers ― known as body waves ― they produced waves that travelled horizontally around the planet through the crust and mantle.
These two events marked the first time surface waves, like those produced in large quakes on Earth, had ever been unambiguously detected on another planet.
"We've been waiting for those meteorite strikes on Mars so we can use those as a seismic source ... to make deciphering the interior structure and the squiggly lines on the seismometer a lot easier," Dr Miljkovic said.
Pinpointing the location of impact events helps scientists understand how fast waves travel and what they're travelling through, so they can fine-tune their models of the planet's interior.
The strong impacts allowed scientists to sample a new part of the Martian underground architecture, said Hrvoje Tkalčić, a seismologist at the Australian National University.
"They provide these additional constraints that we didn't have before based on the Marsquakes that were recorded," said Professor Tkalčić, who has developed a new method of measuring seismic activity.
"They are in a completely different part of Mars."
Unpacking the mysterious Mars dichotomy
The impact events may help scientists get to the bottom of a long-standing mystery known as the Mars dichotomy.
While the northern half of the planet is thinner and dominated by volcanic lowlands, the thicker highlands of the southern hemisphere are pockmarked with craters.
Scientists speculate the difference in crust depth between the two hemispheres may have been caused by some kind of giant impact in the past, or differences in heating deep below the surface.
Until now, our understanding of the Martian crust had been limited to measurements from underneath the InSight spacecraft, said Doyeon Kim, a geophysicist at ETH Zurich's Institute of Geophysics.
"The impact events provide the first glance of what lies beneath the crust away from the [InSight] lander," Dr Kim said.
And it is very different, according to an analysis of the seismic waves in a second study published in Science, led by Dr Kim.
While the crust beneath the InSight spacecraft is a loose jumble of layers, the area to the north ― where the meteorites hit ― appears to be denser and more uniform.
The density of the rock is inferred by the speed of waves travelling through them — seismic waves travel faster through volcanic rock than broken layers of rock, like those under Insight.
"[The high velocity recorded] is consistent with the ... surface waves traversing one of the largest volcanic provinces in the northern hemisphere," Dr Kim said.
And because the waves circle the entire planet, they also provide information about the southern hemisphere.
The wave velocity detected in the south suggests the composition of the deeper layers of the crust in both hemispheres is similar.
A still active planet
Unravelling the mysteries of Mars can help us understand how our own planet formed, Dr Miljkovic said.
"The more we know about Mars, the more we know about the solar system and the terrestrial planets," she said.
Evidence is mounting that the Red Planet may still be volcanically active within Cerberus Fossae region.
A new analysis of all the Mars quakes detected by InSight, published in Nature Astronomy, indicates that magma may still flow deep within the planet.
Meanwhile, new research by Professor Tkalčić and his colleague Sheng Wang, also published today in Nature Astronomy, has confirmed measurements of the planet's core.
Their findings indicate the core has a radius of 1,812 kilometres, give or take 20km.
Instead of using a single seismometer to detect waves coming in, his method uses the location of the impact events and Mars quakes themselves as a network of sensors to scan the planet.
"Not only can we determine the existence of a core, but we can also determine how large it is," Professor Tkalčić said.
It's almost over for this little red robot
Much more data from impacts and Marsquakes are needed to understand this enigmatic planet, but InSight's days are numbered.
Already low on battery, it has been caught up in a planetary dust storm.
"The dust is very thick on our solar panels, and a huge storm on Mars has kicked a lot of dust into the atmosphere, partially blocking the Sun on our solar panels," Dr Banerdt said.
"We will probably be winding down relatively soon."
