The Caloris impact By Antonio Solazzi Based on the article: Stratigraphy of the Caloris Basin, Mercury by McCauley et al

General Information The Caloris basin is approximately 1500m in diameter. It consists of 3-6 rings of mountains, a large impact basin and associated terrain features. Also of note are secondary impact craters caused by debris from the main impact. Until recently, only part of the impact basin had been imaged by Mariner 10.

New information

Features of the Basin The Caloris impact and associated effects generated many different types of terrain features on Mercury which can be divided into four groups: Caloris Montes Formations Nervo Formations Odin Formations Van Eyck Formations

Caloris Montes Formations This type of terrain is most commonly found on the outer edges of the Caloris basin. The outer rim of the basin is made up almost entirely of this type of terrain. This terrain consists of “a jumbled array of smooth-surfaced but highly segmented rectilinear mountain massifs that rise several kilometers above the surrounding terrain.” Individual section are usually 30 to 50 km long.

An example of Caloris Montes formations The large arrow highlights the Montes formation. The Montes formation makes a visible outer edge to the basin. Thought to be made up of ejecta from Caloris combined with uplifted and fractured pre-basin bedrock.

Nervo Formations Nervo formations consist of “rolling to locally hummocky plains that lie in inter- massif depressions.” This type of terrain is generally found inside the Caloris basin. It is sometimes referred to as inter-montane plains.

An example of Nervo formations These formations are thought to be caused by fallback ejecta.

Odin Formations These formations are closely spaced, smooth hills that are about 1km across. They generally occur within 600m to 800m of the Caloris basin. They are though to be made up of blocky, high angle, late-arriving eject from the Caloris impact that has been partially covered.

An example of an Odin formation

Van Eyck Formations This terrain is made up of radial ridges and grooves along with locally swirly terrain and is extensively embayed by smooth plains. This terrain is made up of “ballistically emplaced ejecta mixed with prebasin bedrock.” It also includes several areas of craters that are interpreted as being Caloris secondaries formed by far-flung ejecta.

An example of Van Eyck Formations

Relative dating of craters using Caloris In order to relatively date the Caloris impact a crater record must be established. One way of doing this is using a 5 point scale where C 1 are the oldest craters and C 5 the newest. Using this system the Caloris impact is relatively dated to the period to the late C 3 time period.

C 1 and C 2 craters

C 3 and C 4 craters

C 5 Crater

Hilly and lineated terrain On the other side of Mercury from the Caloris basin terrain exists that is very different from other terrain on Mercury. This terrain consists of hills 5km to 10km wide and 2km deep along with valleys up to 15km wide and 120km long. This terrain is thought to have been caused by the shock wave from the Caloris impact propagating through the core of Mercury.

Hilly and lineated terrain

The spider

Conclusions The Caloris impact was a major event that left marks over a large section of Mercury’s surface. Some of the features of the Caloris impact can be used to relatively date different craters and features on the planet.