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Crater Counts

April 25, 2022

Our Solar System is about 4.6 billion years old, and it was a violent place in its early history at the time of planet formation. The Earth-Moon system was formed about 4.5 billion years ago when a Mars-sized planetoid, named Theia, collided with the proto-Earth to split it into the Earth and Moon. Shortly thereafter, in a geologic sense, the Earth and Moon were pummeled by many asteroid impacts about 3.8-4.1 billion years ago in the Late Heavy Bombardment. Evidence of this has been erased on Earth by environmental factors, but cratering from this is still evident on the Moon.

An artist's conception of an impact of planets

An artist's conception of an impact similar to the one that resulted in the formation of the Earth-Moon system. A Mars-sized planetoid, Theia, collided with the proto-Earth about 4.5 billion years ago to create the Earth-Moon system.

I've always believed that our large and solitary Moon was a major factor in the abiogenesis of life on Earth through its tidal forces. Humanity's interest in lunar eclipses likely accelerated the development of astronomy, and science in general.

(NASA/JPL-Caltech image via Wikimedia Commons. Click for larger image.)

Although some people were still clinging to the idea of an Earth-like Mars as imagined in Percival Lowell's 1908 book, Mars As the Abode of Life,[1], the first close-up images of the surface of Mars by the Mariner 4 spacecraft revealed a cratered surface much like that of the Moon.

Photo of Mars taken by Mariner 4, July 14, 1965

Image of Mars taken by Mariner 4, July 14, 1965.

Sending a spacecraft from one planet to another involves an initial close aiming, followed by some mid-course corrections. To do this, you need to know the orientation of your spacecraft.

For Mariner 4, this involved a star tracker that located the Sun and the star, Canopus, the second-brightest star in the night sky after Sirius.

For Mariner 4, Canopus was lost many times, a result of small dust particles that reflected sunlight and drifted through the sensor field of view.

(NASA/JPL image via Wikimedia Commons. Click for larger image.)

The question of whether meteor impacts have presently plateaued is the subject of research by an international team of planetary scientists in a recent open access article in Earth and Planetary Science Letters.[2-3] Team members are from Curtin University (Perth, Australia), the University of California - Santa Cruz (Santa Cruz, California), the University of Toulouse (Toulouse, France), the Université Félix Houphouët-Boigny (Cocody, Côum;te d'Ivoire), the Planetary Science Institute (Tucson, Arizona), the Western Australian Museum (Perth, Australia), and the CSIRO Pawsey Supercomputing Centre (Perth, Australia). Their study has revealed that the impact flux of meteors in the inner Solar System has been constant over the last 600 million years.[2]

For lack of evidence, planetary scientists have assumed that the impact flux of meteors has been constant over the last three billion years; that is, from about a billion years after the Late Heavy Bombardment to the present.[2] The data, however, suggest the presence of cratering spikes over the last two billion years caused by asteroid break-ups.[2-3] Since Mars is close to the main asteroid belt, it's a good place to verify whether such impactor spikes have happened.[2]

The research team did their study using a semi-automatic dating of 521 impact craters on Mars.[2] The crater detection algorithm was developed at Curtin University, and it automatically counted the visible impact craters from a high-resolution image.[3] They also did a statistical assessment of the cratering rate on Earth and on the Moon and compared these results with Mars. Earth posed a problem in analysis in which the impact location of craters was adjusted by a plate tectonic reconstruction.[2] This analysis of terrestrial craters showed that a cluster of craters, formed during the Ordovician period, about 470 Ma ago and indicative of a spike, existed because of a preservation bias.[2]

Man in the Moon from Le Voyage dans la lune

Man in the Moon from Le Voyage dans la lune (A Trip to the Moon), a 1902 silent film directed by Georges Méliès (1861–1938).

Crater face is a derogatory term for someone with very bad acne, and it was a common idiom when I was in high school in the 1960s and medicinal facial treatments were uncommon.

Wikimedia Commons image.

The Hesperian geologic time period on Mars about 3.7 billion years ago, was one of widespread volcanic activity that created huge outflow channels across the surface, thereby obliterating much evidence of the Late Heavy Bombardment.[2] This left just 6,554 Martian craters for analysis.[2] Some were discarded for being devoid of an ejecta blanket, and the remaining craters were then manually checked to preserve craters of 20 kilometer diameter exhibiting at least about fifty small craters superposed on its ejecta blanket.[2] The selection was limited to latitudes between 35°N and 35°S, and this yielded the final set of 521 impact craters.[2]

The images of the 521 craters were obtained using the THEMIS diurnal infrared mosaic images, which give good contrast between ejecta blankets and their surrounding terrain.[2] The main impact structure of each crater was removed by deleting a circular area with radius equal to 1.2 the crater radius from the centroid of the main crater.[2] A correlation was found between the impact flux of meteors greater than 5 meters and large asteroids (> 1 km), and this correlation does not vary over the last 600 million years.[2] The result suggests that the impact flux of asteroids greater than five meters in size was constant over the last 600 million years, and the influence of past asteroid break-ups on the cratering rate for craters larger than 100 meters is small, or non-existent.[2]

Computer-assisted crater counting

Crater counts on the ejecta blanket of a 40 kilometer impact crater. On the left, the ejecta blanket is outlined in blue, and the automatically detected craters are shown in green. Th red circles highlight impact craters larger than a kilometer in diameter. At the right, a close-up of a part of the ejecta blanket showing detected craters. (Fig. 2 of ref. 2, licensed under a Creative Commons license.[2] Click for larger image.)

As Anthony Lagain, a research fellow at Curtin University explains,
"On Earth, the erosion of plate tectonics erases the history of our planet. 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... 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... Our study shows it is unlikely that debris resulted in any changes to the formation of impact craters on planetary surfaces."[3]

Study co-author, leader of the team that created the crater analysis algorithm, and a professor at Curtin University, Gretchen Benedix, said the algorithm could also be adapted to work on other planetary surfaces, including the Moon.[3]


  1. Percival Lowell, "Mars as the abode of life," The Macmillan Company (New York:1908), PDF file at the US Census Bureau. Page image scans are available also at the US Library of Congress. It seems strange to me that many people are still trying to monetize this 1908 book, as an Internet search will reveal.
  2. Anthony Lagain, Mikhail Kreslavsky, David Baratoux, Yebo Liu, Hadrien Devillepoix, Philip Bland, Gretchen K. Benedix, Luc S. Doucet and Konstantinos Servis, "Has the impact flux of small and large asteroids varied through time on Mars, the Earth and the Moon?" Earth and Planetary Science Letters, vol. 579 (February 1, 2022), Article no. 117362, https://doi.org/10.1016/j.epsl.2021.117362 This is an open access publication with a PDF file available at the same link.
  3. Consistent asteroid showers rock previous thinking on Mars craters, Curtin University Press Release, January 21, 2022.

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