New lunar gravity maps point out a stark and unexpected difference between the sizes of ancient asteroids that slammed into the moon billions of years ago and the sizes of the rocky bodies that occupy the in the Main Asteroid Belt today.
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Scientists had expected to find buried evidence of several large, potentially "planet killing" asteroid impacts on the moon, as predicted by several computer models. But data collected by NASA's now-defunct, gravity-mapping GRAIL satellites show only one, previously known, behemoth crater on the moon, the 1,550-mile wide Aitken basin on the lunar south pole, according to a paper published this week Science Advances, a new journal.
Crater size provides an important measure of what was happening in the early days of the solar system. During the so-called Late Heavy Bombardment, which scientists believe occurred about 4 billion years ago -- just a few hundred million years after the solar system's formation -- Earth and the newly formed inner planets were targets in a celestial shooting gallery.
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Computer models suggest the blitz was triggered by Jupiter and the giant planets shifting orbits, causing a barrage of rocky bodies and comets to be gravitationally slingshot inward. But impact basins uncovered by GRAIL don't fit neatly into those models, astronomer and geophysicist Gregory Neumann, with NASA's Goddard Space Flight Center in Greenbelt, Maryland, told Discovery News.
"We have more of the medium-size impactors that formed the ‘face' of the moon, Imbrium (a vast lava-filled crater), and so forth, and we don't find evidence for larger impactors other than Aitken," Neumann said. "The implications are enormous."
"We have been writing this paper since 1996, trying to get an inventory of lunar impact basins from gravity. We haven't had the gravity field that we've needed to do the job until now," added GRAIL lead scientist Maria Zuber, with the Massachusetts Institute of Technology.
Overall, the number of impact basins uncovered by GRAIL was in the ballpark of what most researchers expected, Zuber told Discovery News.
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Now that definitive number, along with the crater sizes, can be used in computer models to glean a better understanding of what was happening not just on the moon, but on Earth and the other inner planets as well.
"Earth was getting bombarded at the same time as the moon was and so if the moon is missing big impacts, so was the Earth," Zuber said.
"This relates to the number of really large-scale impacts at the time when life was starting to develop. We don't know how many times life developed and got wiped out from the extreme environments associated with early impacts, but if what we see about the moon holds up -- and I think it will because this is good data -- then it allows us to have a better understanding how many large impacts the Earth could have had during a similar time," she said.