Impact! by Gordon Dennis

In my November 2020 Blog we considered colliding galaxies; we saw that the number density of stars in the Galaxy was so small (just one star per 2.63 cubic parsecs) that collisions between stars are very rare events. 
Let’s look at a much smaller volume of space - the solar system.  Here, number densities are much higher – there are eight major planets, thousands of asteroids and an unknown number of comets. Collisions are much more frequent, although less frequent now than in earlier epochs.  Anyone who’s observed the Moon through binoculars or a telescope knows that the Moon’s surface has many craters.  Craters are the result of impacts between massive bodies in evolving planetary systems. This is believed to be a fundamental process in planetary formation. 
The Barringer crater in Arizona (Figure 1) is the most perfectly preserved impact structure on Earth.  The reasons this crater is so perfectly preserved include the very dry Arizona climate and the fact that the impact event happened very recently in astronomical terms – about 50,000 years ago. 
The crater is approximately 1.2km wide and 170m deep and was formed by the impact of a nickel-iron meteorite just 50m in diameter.

How could such a small object create a hole so much larger?  The answer lies in the enormous kinetic energy of the impact. Kinetic energy scales linearly with mass and exponentially (specifically a square law) with velocity:

Typically, an impacting asteroid will have a velocity between 15 -30 km per second.  The kinetic energy of the Barringer impact is estimated to have caused a blast equivalent to the detonation of a 10-12 megaton bomb. The main cause of damage after impact would have been due to the atmospheric shock wave.  Two km from the impact site, the shock wave would have arrived approximately 6 seconds after impact.  The peak overpressure would have been around 95.1 psi (normal air pressure is 14.7 psi).  The maximum wind velocity would have been an astonishing 1360 mph (approximately Mach 1.8) and the sound Intensity 117 dB (i.e. threshold of pain).
That’s quite a score sheet.  But, as Table 1 shows, the Barringer event was actually a relatively small event in solar system terms.

 

1. Precise crater morphology is unknown since most of the crater itself has been eroded.
2. The 40km crater in the Minch basin is the largest known in the UK (at least it’s in the UK at the moment) and has only recently been identified (Katz, 2019).  It has yet to be added to the Earth Impact Database
3. Initial depth estimated 40km before rebound occurred.
4. Impactor diameter estimates vary between 11 to 81 km.
5. A group from Kobe University in Japan has provisionally identified a 7800km diameter crater on Ganymede, the largest planetary moon in the solar system. If this is confirmed, it will be the largest vestigial impact crater discovered in the solar system so far. (Hirata et al 2020)
   

 
Simple and complex impact structures

The Barringer crater is an example of a simple impact crater, having a bowl shape with a covering of shattered rock and mineral fragments.  On Earth, simple craters are generally less than 4 km in diameter (Ball, Kelley and Peiser, 2007).
Larger impactors produce complex impact craters.  Large-diameter craters develop not only a central peak, but often one or more peak rings (French, B 1998) and also concentric ring structures.  Many examples of this are seen on the Moon, such as the crater Tycho (Figure 2 and Figure 3)

Why are impact craters circular?

One might conclude that if the impactor arrived exactly at 90° to the impact site, the crater would be circular.  Otherwise it might be more oblate in shape.  In fact, nearly all impact craters we observe are more or less circular, as shown by the examples in Figure 4 and Figure 5 below.

The basic mechanism of impact crater formation is an explosion rather than a ‘skid mark’.  Earthquake or volcanic events can be quite geographically widespread, and particularly in the case of volcanic activity, take place over relatively long timescales.  Impact events are concentrated at a single point on a planetary surface.  The release of enormous amounts of kinetic energy takes place in the case of a small crater in a fraction of a second; and even in the case of a larger impactor in just a few minutes over tens or hundreds of kilometres  (French, B 1998). 

Counting impact craters

On planetary surfaces, the more craters there are, the older the terrain is believed to be.  This is the case of the heavily crated regions of Mercury (Figure 6).
However, there are other considerations as well.  On Mars, the surface has experienced erosion as well as burial of craters (Figure 7). a surface covered with many small craters on Mars is often one that is more resistant to erosion, and not necessarily older.

Observation of impacts

There have been quite a few impacts observed on Earth and elsewhere in the Solar system. 
A small meteorite impacted Mars’ surface sometime between September 2016 and February 2019 – the uncertainty being because the MRO can’t be everywhere at once.  The impactor is estimated to have been about 1.5m in diameter and the resulting crater to be 15 to 16 meters in diameter (Figure 8).

Comet Shoemaker–Levy 9 was a comet that broke apart into 21 main fragments in July 1992 and collided with Jupiter in July 1994.  This was the first time a cometary impact with a Solar system planet had been observed.  As Jupiter is a gas giant, no crater was formed as such. However, the vast impact scars caused by the explosive entry of the comet were very evident (Figure 9).

The Chelyabinsk meteor was a small asteroid about 17 meters in diameter that struck Earth's atmosphere at an  estimated 18km/second  over the city of Chelyabinsk, Russia, on Feb. 15, 2013.  The incident was captured on dashcam footage and the luminosity of the object was comparable to the solar luminosity.  The atmospheric pressure shock wave caused major damage over a very wide area and over 1200 people were injured.  
The largest meteorite fall recorded (NB ‘recorded’, not ‘happened’) in the UK occurred in the Leicestershire village of Barwell on the evening of Christmas Eve 1965. Several villagers did what any English person would do: they reported the matter to the Police, who duly took several fragments into custody.  Subsequently, many fragments were found around the local area; the largest weighed over 7.7 kg so it was very lucky nobody was hurt. 
Among those to visit Barwell not long after the event was Patrick Moore (then, plain Mr. Moore, later Sir Patrick).  He found a fragment of the meteorite and offered it to the local museum.  He later said, “They told me ‘we have plenty of it so you can keep it for display as long as you make sure it comes to us in your will’”.
There is a wonderful story about a Barwell resident whose car was damaged in the incident and he tried to claim off his insurance.  His insurers helpfully told him it was an Act of God and therefore they were not liable to pay for the damage. So, he went along to the local church and said since it was an Act of God maybe they could pay, but they didn’t do so. 

References 
Katz, B (2019). An Ancient Asteroid Crater May Be Hiding Off Scotland’s Coast https://www.smithsonianmag.com/smart-news/ancient-asteroid-crater-may-be-hiding-scotlands-coast-180972393/  Accessed January 6th 2021.
Matson, J (2010). Meteorite That Fell in 1969 Still Revealing Secrets of the Early Solar System.  https://www.scientificamerican.com/article/murchison-meteorite/  Accessed January 6th 2021.
Earth Impact Database (EID) 
Ball, A; Kelley, S; Peiser, B (2007). Near-Earth objects and the impact hazard.  ISBN 978 0 7492 1887 4
French B. M. (1998) Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. LPI Contribution No. 954, Lunar and Planetary Institute, Houston. 120 pp.
Hirata, N; Ohtsuki, K Keiji; Suetsugu, R  (2020). A Huge ring-like structure on the surface of Jupiter’s moon Ganymede may have been caused by a violent impact   https://www.kobe-u.ac.jp/research_at_kobe_en/NEWS/news/2020_08_05_01.html   Accessed January 6th 2021.