Caloris Basin
Published on Feb 1, 2008 at 1:52 pm.
6 Comments.
Filed under Mercury, planets.
Caloris Basin (officially, Caloris Planitia) is a giant impact feature on the planet Mercury. I find it hard to use the word “crater” for something this large. It is about the size of the state of Texas! The structure was first discovered by the Mariner 10 spacecraft in the 1970s. When Mariner 10 flew past, only part of the Caloris Basin was visible. Most of it was still on the night side of the planet. From what Mariner 10 could see, though, it was obvious that this was something very big and very unusual. While there are some massive impact features in the Solar System that resemble the Caloris Basin in many ways, Caloris Planitia on Mercury appears to have features unlike anything else that we’ve seen in the Solar System. The name Caloris means “heat,” and it refers to the fact that this basin is located at a spot on Mercury where the Sun is very nearly directly overhead at Mercury’s closest distance to the Sun every two orbits.
The image above was taken by Mariner 10 over 30 years ago. The Caloris basin is seen on the left side of the image (most of the left side of the image, in fact!). This was a truly monstrous impact. The crust of the planet was pushed up into a ring of mountains surrounding the impact basin. These mountains are in many cases over 2 kilometers high. There are also secondary rings of mountains likely created by shock waves from the impact. Radiating outward from the basin are systems of valleys, hills, and other craters. The valleys are likely a combination of crustal fractures created by the impact and scours created by ejecta hurled outward by the force of the blast. Secondary craters surround the Caloris Basin from chunks of the planet tossed outward by the blast. Some of these secondary craters themselves are in excess of 20 kilometer diameter! There are also fissures and scarps around the basin, with blocky terrain between them and the basin itself. This may be due to part of the crust sliding towards the impact basin as the planet readjusted itself to such a massive impact. The interior of the Caloris Basin is filled with mostly smooth plains that are surely volcanic in nature as lava filled the basin after whatever huge body that caused it slammed into Mercury. There are also concentric ring arcs of valleys inside the basin. The valleys become generally deeper and wider as you go into the basin from the surrounding rim mountain ranges. The valleys may be the result of some sort of faulting process as the surface of the planet relaxed in the basin area. All this was seen from the partial images of the basin sent back by Mariner 10.
Now, however, on January 14, 2008, the MESSENGER spacecraft flew past Mercury for the first time, taking far higher resolution photographs of Mercury than have every been seen before. MESSENGER and Mariner 10 are the only two spacecraft that have ever flown past Mercury. This time, though, a different portion of Mercury was illuminated by the Sun, including all of the Caloris Basin. The following image is a composite of Mariner 10 images of the basin (on the right side) and MESSENGER images (the center and left sides). The MESSENGER images show far more detail, and they let us see that the basin is rather larger and more complex than had been thought. (Note: You can click on these images to see a larger view.)
The yellow dashed line shows the Mariner 10 estimate for the dimension of the impact basin based on incomplete data. The dashed blue line shows the new estimate. This means that the Caloris Basin is now believed to be about 1550 kilometers (just under 1000 miles) across. This image shows several striking features. The rim mountains and fissures are still there, but not as easily seen in the MESSENGER image. That is because the sun was high in the sky when MESSENGER photographed the basin and thus did not cast many shadows. When Mariner 10 photographed the basin, the sun was not even shining on most of the basin, and where it was shining it was low in the sky, casting very long shadows. Those shadows created more contrast, making features easier to see. But, you do still see the concentric valleys. Interestingly, they seem to only go inward so far, and they seem far less prominent in the inner portions of the basin.
The floor of the basin is mostly smooth. This is believed to be due to solidification of lava on the floor after impact. This same feature is true of the maria (seas) on the Moon. They are smooth because volcanic activity following the formation of the impact basins filled them with lava flows. As with the Moon, we see that since the time of the formation of these largest impact basins, the rate of impacts has been far less. Most of the impacts, in fact, occurred in the earliest years of both bodies’ existence, supporting the idea of a period of intense bombardment early in the Solar System. Also, on both bodies, many of the largest impacts happened near the end of the period of intense bombardment, perhaps. The crater record seems to show the possibility of two bombardment periods early in the history of the Solar System. But, unlike the lunar seas, the floor of Caloris Basin seems brighter than the surrounding plains. On the Moon, the seas are formed from lava that originated deep in the crust or upper mantle of the Moon. This sort of lava produces basaltic rocks. Basalts are typically dark colored rocks. So, the lunar seas look darker than the surrounding parts of the Moon. But, Mercury’s Caloris Basin seems to be a lighter color. This is still a mystery that needs to be worked out. Perhaps the impact was so massive that it brought to the surface material from much deeper in the planet than the material from the lower crust or upper mantle. The chemical composition of the rocks may be unlike any normally found on the surface of a planet, and thus be rocks of a different composition than we would normally expect to find.
But one feature that looks really strange is a set of mostly radial fissures just a little left of the center of the Caloris Basin (not the crater with the bright rays, but to the right of that crater). Below I have a closeup of this part of the image. There is a crater near the middle of the pattern of fissures, it probably has nothing to do with them. It was just a coincidence that a large meteorite hit the area. The fissures show indication that they were there first, since ejecta from the impact covers some of the fissures.
I don’t really know what to make of these valleys. It looks to me that perhaps there may have been an upwelling near the middle of the basin, and that may have produced fissures leading to this formation. But, that is pure speculation. I do not have any altimeter data (if there is any) of the area. Such an upwelling, though, might not be totally unexpected in a massive impact structure of this sort. Further analysis from planetary geologists, though, will likely lead to an even better idea of what may have caused this sort of feature.
At images and data are processed, we will no doubt be hearing far more about Mercury as time passes. Already, I have enough material to significantly alter my lecture on Mercury in my planetary astronomy class later this semester.
-Astroprof
(Images courtesy NASA, JPL, Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
Darnell Clayton on February 1, 2008 at 7:06 pm: 1
I have a hard time imagining craters that large!
Is that the largest crater in our solar system?
Astroprof on February 1, 2008 at 9:38 pm: 2
Amazingly, Caloris Basin is not the largest impact crater in the Solar System. Notably, the famous Hellas Planitia on Mars is a giant impact basin 7km deep (also with 2km rim mountains) and a diameter of about 2300km.
Astroprof’s Page » Mercury’s Weird Terrain on February 3, 2008 at 12:03 am: 3
[…] Two days ago, I wrote about Mercury’s Caloris Basin, a massive impact site on the planet. Mariner 10 only say part of the Caloris Basin. However, by good fortune, Mariner 10 did see the antipodal (exactly opposite) side of Mercury from the Caloris Basin. And, what Mariner 10 saw was quite a surprise to the imaging team. They gave the nickname of “Weird Terrain” to this part of Mercury, because that is just what it looked like: weird. This portion of Mercury was filled with blocky hills and valleys. But, “weird” just isn’t a very scientific sounding name, so it is officially called the Hilly and Lineated Terrain. Sometimes, it is called the Chaotic Terrain, because it looks pretty chaotic. Below is a wide angle view of the area, and a closeup of the most chaotic portion of the area, almost directly opposite the Caloris Basin. […]
Astroprof’s Page » Hellas Planitia on February 4, 2008 at 3:35 pm: 4
[…] A few days ago, I wrote about Mercury’s Caloris Basin. It is mind boggling huge. Naturally, as Darnell Clayton of Colony Worlds asked, it is natural to wonder if this is the largest impact basin in the Solar System. As it turns out, there are larger impact structures. The largest may by our own Moon’s South Pole-Aitken Basin. I will write about that next. Instead, I wanted to focus this posting on Mars’ Hellas Planitia (or Hellas Basin). […]
Astroprof’s Page » South Pole-Aitken Basin on February 5, 2008 at 3:33 pm: 5
[…] 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. […]
Carnival of Space 40 at Orbiting Frog on February 7, 2008 at 9:26 am: 6
[…] Astroprof has two stories on Mercury: one on the Caloris Basin and one on weird terrain. […]