Riddles about "missing" craters solved on Eros

Shocks triggered by collisions remove traces of impact

Eros © NASA / Johns Hopkins University Applied Physics Lab
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It is the largest flying rock near the earth: the asteroid Eros. Now scientists have discovered why Eros has so few smaller craters, despite the ongoing bombardment from space.

When the probe of the Near Earth Asteroid Rendezvous (NEAR) mission circled the asteroid between February 2000 and February 2001, its photographs revealed a celestial body covered with a layer of rock, boulders, and dust embedded with numerous large rocks. In some places, the regolith even seemed to have flowed down into scree avalanches.

But what was more interesting was what the probe did not detect: there were hardly any smaller craters that, according to the scientists, would have had to cover the surface. Instead of the expected approximately 400 craters with diameters around 20 meters, the probe found just 40. "Either the craters were 'erased' by something or there were much fewer smaller asteroids than we had supposed, " explains James E. Richardson, planetary scientist at the University of Arizona in the current issue of the journal Science.

Collisions trigger vibrations

Using computer models and calculations, Richardson came to an explanation: Subsoil shocks could have wiped out around 90 percent of impact craters smaller than one hundred meters in diameter. Such seismic vibrations, according to the researcher, occur when the asteroid collides with space debris.

"Eros is only 33 kilometers long and about 13 kilometers wide. So it has a very small volume and a very low gravity, "says Richardson. "When a one to two meter object hits or an even larger eros, the impact triggers global vibrations. Our analyzes show that these vibrations easily destabilize the regolith on the surface. "Display

Due to the low gravity on the asteroid, the destabilized scree and dust layer does not crash down the slopes like a rockfall, but creeps and sometimes even spins up. Over time, according to Richardson's hypothesis, smaller impact craters are filled up

Broken inside

The results of the modeling also confirm the scientists' assumptions regarding the inner structure of the asteroid. "The NEAR mission showed that Eros is likely to be a broken monolith, a body that was once a compact piece of material, but then broken down by major impacts and held together only by its gravity, " explains Richardson. "We see evidence of this in a series of trenches and fins that extend across the surface of the asteroid, both regionally and globally."

Larger impacts could have broken Eros to its core, but many smaller impacts only shatter it on the surface. This gradient of large fractures deep inside and smaller fractures near the surface is analogous to similar formations in the moon's crust.

However, the researcher's findings may also have implications for future missions to asteroids. For when it comes to mining raw materials on the celestial body or bringing it from its earth-threatening orbit by means of explosive charges, the knowledge of the inner structure is of crucial importance.

(University Of Arizona, Dec 14, 2004 - NPO)