Caloris Basin

The Caloris Basin is one of the largest impact craters in this Solar System an dominates the Mercurian landscape. The basin is 1,550 km across and is surrounded by mountains that are 2 km high. The basin was most likely formed when an asteroid impacted the surface. The impact may have created shockwaves that traveled through the planet that caused a hilly region at the basin’s antipode that is known as the Weird Terrain or Chaotic Terrain.

The Caloris Basin was first discovered by the Mariner 10 spacecraft in 1974. Despite sending images of the crater back to Earth, Mariner 10 filmed it at the terminator line of the light and dark sides of the planet and was not able to image the entire crater. The full size of the crater was unknown until more information was relayed by the MESSENGER spacecraft in 2008.

Scientists estimate that the crater was formed approximately 4 billion years ago during the Late Heavy Bombardment period that swept through the Solar System. Scientists estimate that it would have taken an asteroid 100 km in diameter to create an impact crater of Caloris’s size.

The floor of the crater is filled by lava plains. Some scientists speculate that the impact opened vents that allowed lava to flow into the basin from the planet’s core. Ejecta extends 1,000 km in all directions and concentric rings surround the crater. The gravity of Mars kept the ejecta field relatively small. If such an impact had occurred on Earth, the ejected material would have covered a smaller area because of the higher gravity on this planet. At the center of the basin is the Pantheon Fossae, which is a region characterized by numerous radial troughs that appear to be extensional faults(fault that vertically thins and horizontally extends portions of the crust). There is a 40 km crater at the center of that pattern. There is no accepted explanation for the feature at this time. The basin is also thought to be the source of large amounts of sodium and potassium gasses being released to replenish the tenuous atmosphere of the planet.

Scientists have long speculated that Mercury had an much more substantial atmosphere in the early history of the Solar System. One theory that explains the end of this atmosphere is an huge impact with global repercussions. While no evidence has been found, it is possible that the theorized impact created the Caloris Basin.

We have written many stories about Mercury here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed.

Want more information on Mercury? Here’s a link to NASA’s MESSENGER Misson Page, and here’s NASA’s Solar System Exploration Guide to Mercury.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

References:
NASA Solar System Exploration
Wikipedia
NASA StarChild