Scientists the NASA Ames Research Center conducted an experiment in the late 1960s to simulate the evolution of the lunar surface. Without atmosphere and notable geological activity, the only processes that alter the surface are erosion and sedimentation, both caused by impacts: new craters and ejecta continuously reshape the landscape. So researchers started with a smooth surface of quartz sand, shot different sized projectiles into it and took photographs which was later combined into a film. The experiment was named Mare Exemplum (Sea of Examples).

The experiment provided some very important observations. Firstly, large impact craters, at first very characteristic of the landscape, are continuously fading away. This happens both through the constant bombardment of smaller particles that smooth away the sharp edges and disrupt ejecta blankets and through other large impacts that reshape the surroundings instantly. Secondly, after a transient period at the beginning, the number of craters saturates: it reaches a maximum value and only fluctuates around it. There is simply no place left for newer craters to from, only if older ones are eradicated in the process. The consequence is that if the surface is old enough (e.g. didn't reformed through volcanism or other processes), the craters we see have a limited age, and the ones we observe only represent a tiny fraction of all the impacts that occurred during the lifetime of the heavenly body.

Two similar-sized craters with different ages on the Moon. The right one is young, with a deeper pit and surrounded with ejected boulders. The left one is older and looks more subdued: the sharp features and boulders have been smoothed ad broken down by the cosmic sand blasting. By the time the crater on the right reaches the state of the old one, the latter may not be there any more!

However there are some notable differences from natural processes too. The size of the projectiles was limited to six different values - instead of a continuous size distribution - and every smaller group was ten times more abundant than the larger. The lack of even smaller particles explains the sometimes funny, grainy sort of texture of the surface. Also, the size of the projectiles were small - at that scale the simulation represented the cratering that affects the lunar regolith over 1 to 10 years. As we go to higher scales (larger impacts, longer period of time), other effects pop up: the surface will melt from the energy of the impact, creating more complex craters - central peaks, multiple walls, shallower basins. Nevertheless, the main findings of this experiment about the erosion and surface age were very important, just at the time when the exploration of the Moon entered a new age.

László Molnár

Image soures:

1) Remote Sensing Tutorial

2) LROC / NASA / GSFC / Arizona State University

Video source: This film was originally produced in a 16 mm format. A copy of the film was kindly provided by Friedrich Hörz and digitized by the LPI-JSC Center for Lunar Science and Exploration in 2011.