Crater Crisis

by Max Harvey

(image: Tony Hicks)

When I was looking up at the moon on Sunday night through my telescope I posed the question as to why it is that there are so many craters. Upon the surface appears a complex arrangement of ever increasing detail. Craters within craters within craters. But how come Earth has not come to see the same pattern ingrained upon its skin - why is it that the ground we walk upon is not littered with crevices as a result of catastrophic collision events? The answer, it turns out, lies not with the moon's ability to attract impacts from other objects but rather with Earth’s ability to cover its scars up.

A process with which many will be familiar is the concept of plate tectonics. This process that is responsible for the separation of pangea into the now distinct continents is also responsible for the systematic renewal of the crust that covers the surface of the Earth. Through plate subduction the process of plate tectonics destroys past evidence of high velocity impacts and replaces the remains with fresh new crust, formed at constructive boundaries.

In addition to this, the elements such as wind and rain, erode and weather the sites of impact, removing any trace that they existed in the first place. It is rare for impacts to happen anyways as objects often burn up when entering the atmosphere. 

Whilst I could focus on any one of these factors, I will choose to look at the process of tectonics in greatest detail as it poses the most interesting explanation. Rather on focussing on why the Earth has tectonics, it is more useful to consider why the moon doesn’t.

The moon was formed in a collision with a Mars-sized planet just over 4.5 billion years ago. In its infancy, the newly formed moon was a flowing sea of molten rock which was made of the same constituents as the surface of the Earth (because it was actually the surface of the Earth - formed from displaced matter as a result of the collison).

When we think about how convection currents occur on Earth, it is due to the heating effect of the core which causes plumes of mantel magma to rise, consequently driving tectonic plate movement. What can be inferred from this fact is that in order for convection currents to occur, there must be some form of heat source at the centre of the body which lasts for a significant length of time. Unfortunately for the moon, it is unable to maintain a high enough temperature for convection currents to occur in a liquid mantel for 3 reasons:

Firstly, the moon loses energy much more quickly than the Earth. This is due to the square-cube law which represents the relationship between increasing volumes and surface areas. The surface:volume ratio of the moon is much greater than that of the Earth and consequently the moon loses energy at a much faster rate through emission of electromagnetic radiation.

Secondly, whilst the Earth is still losing energy, it began with a much greater starting amount of energy as a result of its accretion. This means that the moon already started with less energy than the Earth and additionally lost energy at a much faster rate.

The third and final reason is that the moon is volatile depleted meaning that it has significantly lower amounts of Potassium - 40 in its composition. This is important as potassium-40 is a radioactive isotope which decays via electron capture into argon-40. This process releases energy and so keeps the temperature of the Earth high enough to support a liquid mantle and also convection currents within this mantle. The moon, with its severe depletion of K-40, was unable to maintain the high temperature required and consequently cooled to a level where the molten seas solidified and have remained ever since as a canvas to document the moon's interactions with foreign projectiles.

One day it is assumed that the Earth will follow suit, and its interior will cool to a point where tectonic processes can no longer occur. But, until then these phenomena will continue to patch up the damages inflicted by our planet's battle with outer space, whilst the moon continues bearing her scars.