Peak Ring Craters

Craters are very common across the solar system, with a large number of planetary bodies being liberally covered by them. I always feel as though I’m cheating when I use a crater on the blog, as it is definitely one of the harder landform types to match to a specific solar system body. Nonetheless this particular structure stands out as particularly impressive. This is the Aksakov Crater on Mercury, and is a very striking example of a peak ring crater. This image was captured by the messenger spacecraft in 2015, so credit goes to their team. The concentric and eccentric circles seem in this impact basin are due to two unrelated processes. Firstly the large crater is a peak ring crater, and the concentric circles are the result of the forces released during its formation. The other basins, off centre from the peak ring structure are the result of a later separate impact events, and it is just coincidence that they hit in roughly the same part of mercury. 

As we’ve discussed in previous crater focused blogs, large impact structures can exhibit a large amount of variety compared to their smaller cousins.  In the case of a smaller impact the ground around the crash site will soon settle into the familiar bowl shape of a simple crater. However large impacts produce a wider range of morphologies as the material which has been displaced by the impact returns to equilibrium. 

Complex craters occur when a large meteorite impacts the surface; this hypervelocity impact imparts a vast amount of energy to the surface into which it impacts. Shock waves are sent out from the impact site in all directions and have a devastating effect on the surroundings.  The initial “contact and compression” phase sends waves of compression rippling through the ground around the impact, and through the impacting meteorite itself. This leads to the “excavation phase” where this compressed material is forcibly ejected from the area, being scattered around the vicinity. A lot of this displaced material will be completely ejected from the area where the impact occurred and this leaves behind a large cavity. This “ejecta” can often be seen blanketing the terrain in the vicinity of the impact site. 

The result of the excavation phase is a bowl shaped crater. the centre, where the impact occurred has been forced down by the force of the impact. This displaces material all around it, forcing the crater rim to become raised relative to the original ground surface. Combined with the material that has been ejected this leaves behind a large depression called the “transient cavity”. 

It is referred to as transient because it is very unlikely to stay unmodified for very long. The final phase of crater formation is the “modification and collapse phase”. The initial shocks of the impact are over, and gravity begins to act to return the crater to an “equilibrium state”. The energy imparted by the impact has moved material very rapidly against gravity, and the result is that a lot of the craters structure is unstable. Sections of the rim will often collapse, creating shallower slopes which are more stable. The centre of a large crater will also often rebound, forming a central peak. If the crater is large enough then a complex interaction develops between the forces driving the rebound, the collapse of the resulting central peak, and the collapse of the unstable rim material. the result is a ring of central peaks like that seen at Aksakov crater.

Studying craters can be challenging, as we don’t often witness them happening. Fortunately Earth gets hit by relatively few meteorites which would be large enough to form a peak ring crater. The only peak ring impact structure we have found on Earth is the Chicxulub Crater, in Mexico’s Yucatán Peninsula. This crater is believed to result from a massive impact event which likely caused the mass extinction at the end of the Cretaceous period. Consequently we are not too keen to have any other impactors of a similar size hit the Earth any time soon. Complex computer models are thus required to work out how these forces interact, and determine what processes resulted in the complex crater morphologies we see throughout the solar system.

Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington https://photojournal.jpl.nasa.gov/catalog/PIA19212