Author Topic: Fresh White Crater Reference Resource  (Read 4378 times)

Geoff

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Fresh White Crater Reference Resource
« on: July 16, 2010, 06:44:08 pm »
This topic explains what fresh while craters are and gives some examples of craters that are fresh and white and some that are not. The topic is a combination of the Image of the Week: Fresh White Craters - the brighest things on the Moon and IreneAnt's additional comments in the Great Fresh Whites thread.

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 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:


ID: AMZ20003g7 (DJ_59)
ID: AMZ20004r5 (Tom128)
AMZ1001dn8 (Geoff)
ID: AMZ1000j38(Aliko)

You can read more about craters here and watch a cool animation here.

And some additional notes from IreneAnt.

Fresh white craters, are important because they represent the freshest, youngest craters. When a crater first forms, it's ejecta and rays are made up of broken little pieces with flat, smooth surfaces that reflect sunlight very well. This is what makes the ejecta and rays look white. Over time, micrometeorites batter these smooth surfaces, pitting them, melting parts into blobby glass, and forming microscopic particles of pure iron. The result of this "space weathering" is that the surfaces no longer reflect light so well and the brightness of the ejecta and rays fades away.
  • So, when looking for fresh white craters, we want to see bright white ejecta and possibly rays around the crater rim.
Sometimes, the ejecta and rays are made up of material that is naturally lighter than the general surface stuff, such as when a crater punches through the bottom of a mare and excavates the underlying highland material. In such cases, the bright ejecta and rays will never fade away. Copernicus crater is one of these types of craters.

The walls of craters are very steep and so material on the walls often slides down slope. This can keep the walls from accumulating space weathered material, so they continue to look fresh and bright. These types of craters are not considered fresh white craters.

Examples of Fresh White Craters

Examples that are NOT Fresh White Craters


« Last Edit: July 16, 2010, 07:40:01 pm by jules »