why is the moon covered in craters
From When the Soviet probe Luna 3 sent back the first shots of the dark side of the Moon, they showed that it was noticeably more pockmarked by craters than the near side. The nearside crust, by contrast, had more large, shallow basins. More than 50 years after those images first baffled researchers, a study published today in
Science explains the observations. Some theories suggest that the large basins on the near side were caused by impacts from asteroids bigger than those that caused the craters on the far side. But the latest study suggests that the observed basins do not accurately reflect the size of the initial impact, because as asteroids battered the lunar surface in the early history of the Solar System, the Moon s warmer and softer nearside crust melted like butter, producing giant lava flows that filled the impact craters and transformed them into basins. To improve on previous estimates of the size and distribution of basins, the team behind the study used data from NASA s mission (GRAIL), two satellites that since 2011 have been orbiting the Moon and mapping subtle variations in the strength of its gravitational field. Basins are characterized by thinner crust, says first author Katarina Miljkovi, a planetary scientist at the Paris Institute of Earth Physics. The team used GRAIL s gravity maps to find such thin crust and measure the true size of the basins. We didn t have to look at topography nearly at all, just at the crust thickness, says Miljkovi. The researchers found that although both sides of the Moon had the same total number of impact craters, the near side had eight basins larger than 320 kilometres in diameter, whereas the far side had only one.
Hot hit The asteroid bombardment should have battered both sides equally, Miljkovi points out. The asymmetry could have arisen from comparatively small objects punching above their weight on the near side, producing basins more easily than on the far side. Simulations showed that if the largest dark area on the near side the plain of volcanic rock known as Oceanus Procellarum was hundreds of degrees hotter than crust on the far side, impacts there would produce basins up to twice as large as impacts from similar-sized bodies on the far side (see video below). Harder and softer landing: These simulations show that the crash of an asteroid onto the surface on the young Moon would have created a larger crater in a region where the crust s temperature is heightened by radioactivity (right) than it would in a region at normal temperature (left). And indeed, around 4 billion years ago, or 500 million years after the Moon formed, the near side could have been warmer than the far side. Researchers looking at the near side have detected the presence of radioactive isotopes; their decay would have heated up the rock, explains study co-author Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology in Cambridge and principal investigator of GRAIL. The findings fit well with the observations, but there is no consensus as to what caused the startling asymmetry in isotope content between the near side and the far side, says Jeffrey Taylor, a lunar scientist at the University of Hawaii in Honolulu.
One leading theory posits that material rich in radioactive elements rose in a gigantic volcanic plume and formed a magma basin; another that it came from around 1,000 kilometres in diameter. William Bottke, a lunar scientist at the Southwest Research Institute in Boulder, Colorado, says that the work could lead researchers to revise just how dramatic asteroid bombardments were in the early Solar System. This can be used to more accurately derive what the small-body populations were like four billion years ago. This article is reproduced with permission from the magazine. The article was on November 7, 2013. Answer 1: This is an excellent question! The answer to this question includes important clues about how the Earth, moon, and other rocky planets were formed and about how Earth and the moon are geologically different. Impact craters form when meteorites crash into the surface of a planet. You have already made the good observation that the moon has a lot of these craters. Earth and other planets in our solar system formed about 4. 6 billion years ago (thatБs a long time). Early in the history of our solar system, asteroid and meteorite impacts were much more common. Our planet and other planets were struck by meteorites thousands of time more frequently than today. That is because back then there was a lot more БstuffБ (meteors and planetoids) flying around in our solar system.
Over time, those chunks of rock were БcaughtБ by the gravity of big planets. Every time a planet БcatchesБ a meteorite, it gets a crater. This is the same for the moon. In fact, our moon formed when an object the size of a small planet crashed into Earth and tore off a chunk. That chunk was caught by the EarthБs gravity and now orbits it as the moon. So, if Earth was catching meteorites just as often as the moon, why are there many more craters on the moon? The answer isБ PLATE TECTONICS!!! Plate tectonics is movement and interaction of БplatesБ of crust on the surface of our Earth. Plate tectonics is responsible for volcanoes, earthquakes, giant mountain ranges, and the recycling of crust back into the mantle. These processes shape and reshape the surface of Earth very rapidly (geologically speaking, over millions of years). Things like subduction (when crust sinks into the mantle) and erosion (wearing away of rocks at the surface) have erased many of the impact craters on earth. Only very young craters can still be seen. For example, meteor crater in Arizona is about 50,000 years old (thatБs really young compared to our planet which is 100,000 times older than this crater). The moon is tectonically БdeadБ. Plate tectonics no longer happen on the moon, so there is nothing to erase all of the craters. I ended up talking more about the Earth than the moon, but that is because plate tectonics is the big reason that so many impact craters are preserved on the moon.
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