Fresh White Craters, the brightest things on the Moon
Fresh white impact craters are the most recent impacts on the Moon. Anything less than a billion years old (which means it is from the current Copernican era), is considered young in lunar terms. Some may be very young indeed. There are very few large craters with the bright ray systems associated with fresh craters. Tycho is thought to be the youngest large crater unless a group of 12th century astronomers were right and this accolade should go to the far side crater Giordano Bruno.
They are important because their ejecta blankets are as fresh as they were on the day of the impact and have not been disturbed by micrometeorites. Many of these craters have extensive ray systems and in some cases ejecta was flung out for hundreds of kilometres (in Tycho’s case 1500 km stretching to the Apollo 17 landing site in the Taurus-Littrow region.) Landing sites for robotic or manned missions can, therefore, be chosen to take advantage of this to maximise the different types of rock available to analyse. You don’t have to go to Tycho to see what Tycho is made of. Analysis of the Tycho ray samples brought back by the Apollo 17 crew show Tycho to be around 100 million years old.
The rays of a fresh crater can be spectacular to view through binoculars or a telescope when the sun is overhead with respect to the crater so full Moon is the best time to observe them. The rays look white not because the rocks excavated are bright white in colour but because their newly exposed and broken surfaces are clean and shiny and have a relatively high albedo in comparison to the mature, darker mare material they lie on top of which has been battered and dulled by micrometeorite impacts.
So why is Moon Zoo interested in them? There are several reasons.
- By counting the number of fresh impact craters the team can calculate the current impact rate of the Earth-Moon system which is of interest for assessing the risk of asteroid and meteoroid impacts.
- Also small fresh, impact craters of of just a few kilometres in diameter are the most likely locations from which lunar meteorites found on Earth have been ejected and pinpointing the source of these meteorites is the subject of much research.
- And because fresh craters are undisturbed their crater walls, interior features and secondary craters can be studied in detail.
Forum member Tom128 developed an interest in freshly formed craters and started a forum thread to collect “Great Fresh Whites.” Here are some of the early finds:
And there is more information in the Fresh White Crater Reference Resource here.
Jules is the volunteer Moderator of the Moon Zoo Forum