On December 24, 2021, NASA’s InSight lander recorded a magnitude 4 marsquake with a distinct signature rippling across the surface of Mars. In a pair of papers in the journal Science, planetary researchers now show that this event, and another detected on September 18, 2021, were caused by a meteoroid impact and use the surface waves produced by the collisions to untangle the structure of the Martian crust. What’s more, the December 24, 2021 meteoroid excavated boulder-size chunks of ice buried closer to the Martian equator than ever found before.
Boulder-size blocks of water ice can be seen around the rim of an impact crater in the Amazonis Planitia regionon Mars, as viewed by the High-Resolution Imaging Science Experiment (HiRISE) aboard NASA’s Mars Reconnaissance Orbiter. Image credit: NASA / JPL-Caltech / University of Arizona.
The InSight scientists determined the December 24, 2021 quake resulted from a meteoroid impact when they looked at before-and-after images from NASA’s Mars Reconnaissance Orbiter (MRO) and spotted a new, yawning crater.
“The image of the impact was unlike any I had seen before, with the massive crater, the exposed ice, and the dramatic blast zone preserved in the Martian dust,” said Dr. Liliya Posiolova, a leader of the Orbital Science and Operations Group at Malin Space Science Systems (MSSS) and the lead author of the first Science paper.
“I couldn’t help but imagine what it must have been like to witness the impact, the atmospheric blast, and debris ejected miles downrange.”
The meteoroid is estimated to have spanned 5 to 12 m (16-39 feet) — small enough that it would have burned up in Earth’s atmosphere, but not in Mars’ thin atmosphere.
The impact was in a Martian region called Amazonis Planitia. It created a crater about 150 m (492 feet) across and 21 m (70 feet) deep.
Some of the ejected material thrown by the impact flew as far as 37 km (23 miles) away.
With images and seismic data documenting the event, this is believed to be one of the largest craters ever witnessed forming any place in the Solar System.
“It’s unprecedented to find a fresh impact of this size,” said Dr. Ingrid Daubar, a researcher at Brown University and a co-author of the first paper.
“It’s an exciting moment in geologic history, and we got to witness it.”
In the second Science paper, the scientists used surface waves from the two meteorite impacts on Mars to study the structure of the Martian crust.
“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 study lead author Dr. Doyeon Kim, a researcher in the Institute of Geophysics at ETH Zürich and the Department of Geology at the University of Maryland.
The authors analyzed the velocity of surface waves coming from the two impacts. This allowed them to exploit the relationship between surface wave velocity, frequency and depth to estimate the average properties of the crust 4.8 to 30 km (3-18.6 miles) below the surface of Mars.
On average, the Martian crust between InSight’s seismometer and the two meteorite impact sites did not vary strongly with depth and had faster seismic velocity than what was previously observed directly below the lander.
The faster velocities suggest either compositional differences or reduced porosity in areas traversed by the surface waves.
“The composition of the crust will determine some of the density, but so will factors like porosity; if you have a lot of holes in the crust, it can also decrease the density of the material,” said Dr. Nicholas Schmerr, a researcher in the Department of Geology at the University of Maryland and a co-author of the second paper.
“A volcano, with all its intrusions and magma coming up through the crust beneath it, would have also altered the crust density and composition in that region.”
“As we look further north on Mars, there’s probably some subsurface ice in the crust below the impact site, which is less porous and very different from what we see under the InSight lander.”
The findings may also provide answers to a centuries-old mystery: the crustal dichotomy of Mars.
“Mars has a very unique feature, which is the very sharp contrast between its Northern and Southern hemispheres,” said Dr. Vedran Lekic, a researcher in the Department of Geology at the University of Maryland and a co-author of the second paper.
“The southern part is really old, has high topography and is very heavily cratered.”
“Meanwhile, the northern region is volcanic, very low-lying and has comparatively few craters.”
“The surface waves we detected helped us learn more about the northern lowlands, which we’ve only been able to speculate about before.”
One popular theory behind this is that the crusts in the northern lowlands and southern highlands are composed of different materials.
However, the scientists found that their initial results appear to disprove this idea, even suggesting the crust structures may be surprisingly similar at certain depths.
“We hope that our research will continue to help researchers investigate similar mysteries and form better models of Mars as exploration continues,” they said.