Hellas Planitia
Published on Feb 4, 2008 at 3:35 pm.
1 Comment.
Filed under Mars, planets.
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).
Measuring nearly 2300 kilometers across, the Hellas Basin is one of the largest impact features in the Solar System. The floor of the basin is almost 7 kilometers below what would likely have been the original terrain level, and its rims extend upward for 2 kilometers. That means that it is 9 kilometers (almost 6 miles) from the floor of the basin to its rim!
Hellas Planitia is so deep that the atmospheric pressure at its bottom reaches nearly 1160 Pascals. That is comfortably above the triple point of water, meaning that if the temperature were high enough, then liquid water could exist at the floor of the basin. However, at Hellas Planitia’s southern latitude, the temperature is believed to seldom, if ever, get that warm. Still, it suggests that perhaps that is a good place to look for any liquid water, if any might be left on Mars.
Hellas Planitia is to large that it can be seen from Earth as a bright oval region in the southern hemisphere of Mars. The name “Hellas” makes reference to Greece, and the name was bestowed onto the bright region by the famous astronomer Giovanni Schiaparelli.
The floor of the basin shows few craters. This really stands out compared with the heavily cratered highlands of the southern hemisphere. There are also apparently volcanic flow features on the floor of the basin similar to some of the “wrinkles” and ridges seen on the lunar seas. This suggests significant volcanic modification of the basin floor. It would be very interesting to see if the rocks in the floor of the crater were dark colored basalts like those of the lunar seas, or more light colored rocks that seem to form Caloris Basin on Mercury. At any rate, it would be very tough to tell from orbiting spacecraft. Mars does have an atmosphere, and the ubiquitous dust of Mars no doubt has covered the surface layers of the rock with a significant layer of dust. That gives it the lighter colored appearance.
Such a large impact, though, has far reaching consequences. Massive amounts of ejecta from the impact stretches across Mars to a distance of at least 4000 kilometers from the basin’s center. In places, the ejecta may be up to 2 kilometers thick. If all of that ejecta were piled up on Earth, it would cover the entire North American continent to a depth of well over 1 kilometer. No doubt, some of the ejecta from such an impact actually rained down across the entire Martian surface.
It is interesting to note that the Hellas impact basin is on the other side of Mars from the Tharsis region of Mars, with its monstrous volcanoes (including Olympus Mons, the largest volcano in the Solar System). Several people have suggested a link between the two, but that appears unlikely. For one thing, Hellas Planitia is not exactly opposite the Tharsis rise. Also, the volcanoes of Tharsis are likely hot spot volcanoes, not something triggered by seismic activity. So, it is likely just coincidence that the two features are on separate sides of Mars. However, Tharsis and Hellas both contribute heavily to the features on Mars, and both are likely major factors in the evolution of the climate and topology of the planet.
-Astroprof
Images courtesy NASA, JPL, STScI
Rick Laviolette on May 30, 2008 at 6:32 pm: 1
The Hellas Basin and its opposite side of Tharsis Montes give me the impression that the “bullet” hit and caused: 1. 4 Km drop in surface height 2. The Tharsis Montes Bulge on the opposite side of the Planet 3. The Tectonic crack called Valles Marinaris and the gigantic Olympus Mons Volcano as well as 2 others. So my impression is of a planet that took a bullet and died. All of its water either streamed into space or froze into the ground. Great article.