New research shows that Mars has faced a constant rain of meteors during the last 600 million years. This finding contradicts previous research showing that the impact rate has varied, with prominent activity spikes. Why would anyone care how often meteors rained down on Mars, a planet that’s been dead for billions of years?
Because whatever Mars was subjected to, Earth was also likely subjected to.
Who wouldn’t want to know our planet’s history?
“On Earth, the erosion of plate tectonics erases the history of our planet,” said Dr. Anthony Lagain, a research fellow at Curtin University’s School of Earth and Planetary Sciences. “Studying planetary bodies of our Solar System that still conserve their early geological history, such as Mars, helps us to understand the evolution of our planet,” Dr. Lagain said.
Dr. Lagain is the lead author of a new paper published in Earth and Planetary Science Letters. The paper’s title is “Has the impact flux of small and large asteroids varied through time on Mars, the Earth and the Moon?“
Some previous research into meteorite impacts on the inner Solar System planets showed spikes in cratering over the last 2 billion years. Scientists think the cause of impact intensity was collisions between objects in the asteroid belt. Since Mars is so close to the belt, it’s an excellent place to investigate the issue.
The team of researchers behind this new study used a previously-developed crater detection algorithm to analyze the formation of more than 500 large craters on Mars. “The crater detection algorithm provides us with a thorough understanding of the formation of impact craters, including their size and quantity, and the timing and frequency of the asteroid collisions that made them.”
“When big bodies smash into each other, they break into pieces or debris, which is thought to have an effect on the creation of impact craters,” Dr. Lagain said. “Our study shows it is unlikely that debris resulted in any changes to the formation of impact craters on planetary surfaces.”
The researchers examined 521 large impact craters on Mars greater than 20 km (12.5 miles) in diameter. Then they showed that 49 of those craters correspond to the entire crater population of this size over the last 600 million years. They found that the impact rate is coupled for craters larger than 5 m (16.5 ft) diameter and greater than 1 km (0.6 miles) in diameter, and the two don’t vary over the last 600 million years.
This image from the study shows the locations of all craters larger than 20 km in diameter on Hesperian and Noachian terrain, purple and blue, respectively. These 521 craters are all within a +-35 degree equatorial band. Image Credit: Lagain et al. 2022.Co-author and leader of the team that created the algorithm, Professor Gretchen Benedix, said the algorithm could also work on other planetary surfaces, including the Moon, with some adaptations. “The formation of thousands of lunar craters can now be dated automatically, and their formation frequency analyzed at a higher resolution to investigate their evolution,” Professor Benedix said.
In previous research, the Moon shows evidence of a spike in impacts, but the authors of this paper say it’s a result of uncertain calibration methods. Some methods involved measuring the abundance of rocks in lunar impact ejecta. Some other calibration methods show no spikes and are consistent with a more steady rate of impacts.
In their paper, the authors write, “We conclude to a coupling of the crater production rate between kilometre-size craters (?100 m asteroids) and down to ?100 m in diameter (?5 m asteroids) in the inner Solar System.” They say that their findings are consistent with how asteroid collision debris moves into the orbital path of planets.
“This is consistent with the traditional model for delivering asteroids to planet-crossing orbits: the Yarkovsky effect slowly pushes the large debris from asteroid breakups towards orbital resonances while smaller debris is ground through collisional cascades. This suggests that the long-term impact flux of asteroids > 5 m is most likely constant over the last 600 million years and that the influence of past asteroid breakups in the cratering rate for D > 100 m is limited or nonexistent.”
This image shows an example of the crater data in the study. It shows crater counts on an ejecta blanket of a 40 km impact crater. (a-left) Ejecta blanket mapping (outlined in blue) and automatically detected craters (in green). Red circles correspond to impact craters larger than 1 km in diameter. (b-right) Close-up of a part of the ejecta blanket showing detected craters over the CTX (Camera on the MRO) Global mosaic. The diameters of the green circles are enlarged by 15% with respect to the measured rim-to-rim diameter for better visualization of the crater rims. Image Credit: Lagain et al. 2022.According to Dr. Lagain, counting impact craters is the only way to piece geological history together. Crater counts are the only way to determine when features like canyons, rivers, and volcanoes formed on Mars. These counts also help predict when future impacts might occur and how powerful they might be.
Victoria crater might be the most famous Martian crater. It was one of the HiRISE camera’s HiPODs in 2006 and was also featured in National Geographic. It’s an 800m diameter crater within the study area. Image Credit: By NASA/JPL/University of Arizona – http://photojournal.jpl.nasa.gov/catalog/PIA08813, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4211043Earth’s impact history will never be as well-known as Mars’ and the Moon’s impact history. Billions of years of geological activity have erased most of it. We know of a few big ones like the Chicxulub impact that led to the Cretaceous–Paleogene extinction event, where the dinosaurs met their demise. We know of a few others, like the Karla crater in Russia and the Vredefort crater in South Africa. But there’s no reason to believe Earth didn’t endure the same steady rain of meteors that Mars and the Moon did.
There’s some evidence on Earth for a spike in impact craters about 470 mya during the Ordovician period. But by cross-referencing this with the Martian impact results and rewinding the clock on Earth’s tectonic plate movement, Dr. Lagain and his co-authors show that the Ordovician spike is a preservation bias.
“The proximity of Mars to the main belt, as well as the Earth-Moon distance, both exclude the possibility that one of these three bodies experienced a cratering spike in their geological history whether the others did not,” the authors write in their conclusion. “The absence of such signal in the lunar and martian cratering record raises questions about the qualitative increase observed on the Earth.”
“Although a cluster of Ordovician impact craters does exist on Earth,” the authors say, “a sharp increase in the absolute impact flux at that time due to an underlying punctuated phenomenon (i.e. an asteroid breakup) is questionable.”
This image from the study shows the configuration of continental landmasses approximately 450 mya. Present-day coastlines are in grey. Image Credit: Lagain et al. 2022.According to the paper, the environmental conditions on Earth preserved more of those craters, making it look like there was a spike in impact. “The environmental conditions marking the Ordovician period have thus most likely led to better preservation of impact craters formed in the tropical region followed by a rapid burial by sedimentary layers, that were later exhumed during glacial periods, where continental masses were located at higher latitudes.”
