A Cosmic Beating
The Moon is battered, scarred, and pummelled from thousands of impacts over billions of years. The Moon though, like the Earth took most of its beating in the early days of the solar system, and yes Earth took just as much punishment as the Moon did. But Earth has weather, erosion, and is geologically very active, covered in forests, deserts, oceans and everything in between. All this activity has done a good job of concealing, and wearing down the craters. Also the Earth has an atmosphere that takes care of the smaller objects, burning them up before they even reach the ground. The Moon though has no atmosphere, meaning it is exposed to every incoming projectile from space.
The Moon is pristine with no wind, erosion, or present geological activity. This means that lunar features are mainly ‘frozen’ in time with nothing to disturb them, apart from maybe the occasional new impact, or darkening action by the Sun on lunar dust. So craters, ravines, lava flows, etc look as fresh as when they were first made. In fact boulders have been photographed on the lunar surface that have rolled down a crater wall or slope, leaving a trail behind them in the lunar dust that looks like it was made yesterday. In reality astronomers and scientists know they rolled down that slope tens of millions of years ago, and with little to disturb them they generally stay like that.
The Crater Impact
When a comet or asteroid hits the Moon they can be travelling at many thousands of miles per hour, so the resulting collision is kind of violent to say the least. When a sufficiently large object hits it creates extreme heat, actually melting the rock and compressing it into the surface, the melted rock can then ‘rebound’ producing a central peak which then cools and is often as high as the crater walls themselves.
A bowl shaped depression is dug out of the lunar surface and all the rock, dust, and lunar soil gets thrown upwards and outwards. No surprise there then I hear you say, but this can create some really striking effects. Lunar material gets launched just a few metres, or sometimes hundreds of miles from the original impact, to land back down to the surface as ejecta blanket, or crater rays. Younger lunar craters usually have bright crater rays, because this fresh material has been excavated from beneath the surface and hasn’t had the effect of Sun weathering. The action of sunlight will darken lunar dust over time, giving an idea of the age of craters from the tone of the ejecta.
Here are some of the best images of crater rays from impacts large and small, as photographed from the Nasa Lunar Reconnaissance Orbiter, CLICK TO ENLARGE…