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.
The Hilly and Lineated Terrain covers perhaps 360,000 square kilometers of Mercury’s surface and consists of massifs filled with hills ranging, in some cases, nearly two kilometers high and several kilometers across. The hills have an appearance of almost rectangular blocks. Many ancient craters in the area are ruined by whatever event created the chaotic nature of this part of Mercury’s surface. Other craters, apparently younger, were created subsequent to the formation of the massifs, and they are largely intact, suggesting that the event was singular in nature. Some of the larger impact basins are filled with smooth plains of likely volcanic origin. Comparing crater densities of the craters that are younger than the terrain, planetary scientists have dated the formation of this chaotic terrain to about the same time as the formation of the Caloris Basin.
That gives us a clue as to what may have created the Hilly and Lineated Terrain. An impact as large as the event that formed the Caloris Basin has devastating consequences. Debris from the impact through material over much of one hemisphere of the planet. But, the devastation doesn’t stop there. Mercury has a huge core — the largest of any planet in the Solar System. Computer models suggest that this core can have an effect of focusing seismic waves at a spot opposite to the impact site. The seismic disturbance from such a large impact boggles the mind in its intensity, even on the far side of the planet.
Computer simulations suggest that the ground on the far side of the planet may be displaced upward from hundreds of meters up to perhaps a kilometer within minutes. This would break the terrain into large chunks, giving such a chaotic appearance. If this is, indeed, how this feature formed, it shows that the impact that created Caloris Basin had nearly a global effect on Mercury. This also shows, though, just how tough a planet can be. Mercury was hit with a very large body, and it survived (though scarred). Studies of the Caloris Basin and the Hilly and Lineated Terrain may tell us a lot about the interior of Mercury. The impact itself likely brought material to the surface of the planet from deeper layers than we see on the surfaces of other worlds. That, together with the disturbance to the far side of the planet, may tell us a lot about the interior of Mercury, one of the most poorly understood planetary interiors.
However, as you recall from my earlier posting about the Caloris Basin, that massive impact structure was fully illuminated by the Sun as MESSENGER passed by last month. But, remember, the Hilly and Lineated Terrain is opposite the Caloris Basin. So, if the Caloris Basin is in full sunlight, then the Hilly and Lineated Terrain is in darkness. When Mariner 10 flew past Mercury in the 1970s, less than half of the Caloris Basin was sunlit. That meant that much of the Hilly and Lineated Terrain was illuminated. But, this time, we saw all of the Caloris Basin. We’ll have to wait for a later flyby from MESSENGER in order to see the other half of Mercury. Personally, I’ll be waiting in anticipation.
(Images courtesy NASA, JPL; Diagram by Astroprof)