South Pole-Aitken Basin
Published on Feb 5, 2008 at 3:23 pm.
4 Comments.
Filed under moon.
The Caloris Basin on Mercury that I wrote about a few days ago is pretty spectacular. It is huge. But, Hellas Planitia, which I wrote about yesterday, is bigger. But, bigger still is the South Pole-Aitken Basin on our own Moon. This monstrous impact basin is 2500 kilometers across, and up to 13 kilometers deep. It is quite possibly the largest impact crater in the entire Solar System. In fact, it just doesn’t seem right to call something that big a “crater.” It is a horrific scar on the surface of the Moon. And, as with the other huge impact features, the South Pole-Aitken Basin has quite unique features all its own.
Now, you might wonder why so few people have heard of such a huge impact feature on the Moon: our nearest celestial neighbor. After all, you don’t need to be an accomplished astronomer to observe the Moon. It is a favorite target for almost everyone when they get their first small telescope. The Moon can be studied in binoculars. The Astronomical League even offers a certificate for amateur astronomers finding about a hundred features on the Moon. That can be accomplished with the smallest astronomical telescope. Even with the naked eye, you can look at the Moon and see dark splotches on its surface. These are giant lava filled impact basins. So, why is something so vastly bigger than any of these so seldom heard of?
This basin is seldom discussed outside of the astronomical community because it is hard to see and study. In fact, the South Pole-Aitken basin is much tougher to study than most impact structures on Mars. Only Mercury’s Caloris Basin rivals it in difficulty of study, and that is because Mercury is so close to the Sun that it is a difficult target for both telescopes and space missions. In fact, we probably know less about the South Pole-Aitken Basin than any of the other large impact features on the Moon. The basin has historically had so little study that it doesn’t really have a name like most of the other basins on the Moon. The name South Pole-Aitken Basin refers to the fact that the basin extends from near the Moon’s south pole to near the crater named Aitken. But, what makes this lunar basin so hard to study?
One of the main reasons that the South Pole-Aitken basin is so hard to study is its location. It is mostly on the far side of the Moon. Since the Moon always keeps one face towards the Earth, we never get a direct view of the basin. Part of the rim is visible as a range of mountains on the far southern portion of the Moon. But, you might ask, what about all of the missions to the Moon? Didn’t they see the basin? Well, yes, they did. But, the basin is in the far southern hemisphere, and most of the missions were confined to near the Moon’s equator. So, the basin was not truly well studied. Hints that the basin existed date all the way back to the days of some of the earliest lunar missions, but the extent of the basin was not really known until studies during the Apollo program. Even then, though, it was viewed at an angle, so there was nowhere near as much data as one might hope for. The real bonanza of data, though, started with the Clementine Project. Clementine was a military project designed to test new technology. As part of the test, it was given the task of mapping the Moon. The image at the top of the post is a topographic representation of the Moon centered on the South Pole-Aitken Basin.
The South Pole-Aitken Basin is unique among lunar basins. Many of the largest impacts were enough to fracture the crust of the Moon, permitting magma from deep within the Moon to work its way to the surface, producing dark colored basalt rocks. These lava flows fill many of the largest basins, giving them dark and fairly smooth surfaces. These are the lunar maria, or seas. But, the South Pole-Aitken Basin lacks these lava flows. Also, this basin does not seem to have the usual mass anomalies that the other basins have. Another interesting feature is that there is nothing unusual on the opposite side of the Moon from the basin. The next largest basin, the Imbrium Basin, is less than half the diameter of the South Pole-Aitken Basin. Yes, the Imbrium Basin is filled by Mare Imbrium. Furthermore, directly opposite Imbrium Basin is a region of chaotic terrain near Van de Graaff crater. This terrain is like a much smaller and less disturbed version of Mercury’s Hilly and Lineated Terrain. The Moon is smaller than Mercury. Though it lacks a large iron core to focus the seismic energy, an impact slamming directly into the Moon to produce the South Pole-Aiken Basin should create massive seismic disturbances. The Imbrium impact created a series of fractures running around the Moon from the impact site. Yet, the much larger South Pole-Aiken Basin did not. Imbrium fractured the crust of the Moon, permitting basaltic lava flows. The impact creating the South Pole-Aitken Basin was on a much thicker portion of the Moon’s crust, but it still should have been more than powerful enough to punch through the Moon’s crust, but it seems not to have done so. These findings lead some to speculate that perhaps it was a glancing impact that created the South Pole-Aitken Basin rather than a direct hit. The glancing impact “scooped” material out to form the basin rather than simply blasting a giant hole. If so, then whatever hit the Moon to form this basin must have been sizable, indeed. It would have to take a body that was at least a significant fraction of the Moon’s size to accomplish this feat. So, where did that body go? Is it still flying around the Solar System, or did it impact Earth. Cratering studies of the basin indicate that it formed early in the Moon’s history. So, if the body that hit the Moon kept going to impact Earth, then all traces of that impact would have been lost through tectonic activity on Earth’s surface.
From the mid 1970s until the mid 1990s, the Moon was not well studied. There has now been a flurry of spacecraft to visit the Moon in the last few years, and now we have started the International Lunar Decade, which I wrote about a few months ago. Perhaps with the flurry of new activity in studying the Moon, we may have an answer to the mysteries surrounding the Moon’s South Pole-Aitken Basin, the largest known impact structure in the Solar System.
-Astroprof
Image Credit: Naval Research Lab, Clementine Project
dad2059 on February 6, 2008 at 8:10 am: 1
Any chance that the Chinese are going to study the area or the Indians next summer?
Also there used to be a rumor that ice might be buried in that crater, any word on that?
Could the glancing blow be from a comet?
Astroprof on February 6, 2008 at 10:22 am: 2
I am not exactly sure off the top of my head what areas the Chinese and Indians will be studying, but obviously the area will be studied quite extensively in this latest burst of lunar exploration.
The ice in the area got far more press attention than it deserved. There may be some ice deposits in the polar craters, but not giant frozen lakes of ice. Most likely it is layers of frost mixed with dust. At best, it is a dust and ice mixture only meters deep. And, that is probably optimistic.
While some comets can be pretty big, this sort of basin seems a bit large to be the result of a comet with a glancing blow. I am thinking perhaps something left over from the formation of the Moon would be more likely. But, there is really no telling at this point. We need more data, and even that might not tell us what we want to know.
Keplert on February 7, 2008 at 3:45 am: 3
In terms of plasma physics one might think the moon was hit by an enormous electrical (dis)charge: no impact body needed to leave a big, round scar. This would probably imply revision of planet formation if discharges this big ever could occur in the neighbourhood of our sun.
Today universe « Electronicsandcomputers’s Weblog on February 7, 2008 at 3:49 am: 4
[…] And finally, Astroprof looks at the Moon’s southern pole. A nice place to visit? […]