A new NASA Lunar Science Institute study suggested an asteroid, approximately 170 kilometers in diameter, striking surface of the Moon at nearly 40,000 kilometers per hour, produced the largest and oldest basin on the Moon and arguably the oldest surviving impact crater in the solar system. The impact crater is so large that it is not known by a single name, but rather for the distance it stretches from the south pole of the Moon to another impact site called the Aitken crater near the Moon’s equator. The immense South Pole-Aitken impact basin is 2,500 kilometers in diameter.

NLSI international partners in the United Kingdom, Ross Potter and Gareth Collins, working with the LPI-JSC Center for Lunar Science and Exploration in Houston, used computers to model the impact processes and then compared those models to observations made by several spacecraft missions: gravity anomalies that reflect the thickness of the crust in and around the basin and reflectance spectra that provide information about the chemical composition of material in and around the basin.

The impact was nearly a thousand times more explosive than the dinosaur-ravaging Chicxulub impact on Earth 65 million years ago. The energy of the impact was so large that it destroyed a significant fraction of the lunar crust and melted a huge region of the lunar interior, producing an instantaneous sea of molten rock that flooded the southern hemisphere of the lunar farside. Eventually the molten pool cooled and solidified, but it nonetheless permanently altered the shape of the Moon, creating a 13 kilometer deep basin that still persists on the lunar farside.

A full report of the work was written by R. W. K. Potter, G. S. Collins, W. S. Kiefer, P. J. McGovern, and D. A. Kring and appears in journal Icarus (volume 220, issue 2, pages 730-743).

CLSE’s best-fit SPA basin-forming impact; an impactor 170 km in diameter impacting at 10 km/s into the steepest LTG (50 K/km). The left panel shows material, the right panel shows pressure. The basin process is complete within ∼3 h of initial impact. The pressure scale is 0–15 GPa.

CLSE’s best-fit SPA basin-forming impact; an impactor 170 km in diameter impacting at 10 km/s into the steepest LTG (50 K/km). The left panel shows material, the right panel shows temperature. The temperature scale is 300–2800 K. The high temperatures along the central axis of the target are likely to be a consequence of the axisymmetric nature of the hydrocode.

A SPA-scale simulation for an impactor 200 km in diameter impacting at 10 km/s into the shallowest LTG (10 K/km). The left panel shows material, the right panel shows temperature. Compared to the best-fit impact, this impact (with a larger impactor and cooler LTG) excavates and places a great volume of mantle over crustal material producing a noticeable thickened annulus of crustal material. The temperature scale is 300–2800 K. The high temperatures along the central axis of the target are likely to be a consequence of the axisymmetric nature of the hydrocode.

Posted by: Soderman/NLSI Staff
Source: David Kring/NLSI Team

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