what are the dark bits?

[rockystar]

I was at one of my local astronomy club's open observing sessions a couple of weeks ago; this was a question I got asked, and I didn't really have a good answer. I know it's going to be something to do with the areas not rejecting reflecting the same amount of light, but what is the cause of the different shades when looking at the moon?

is it that mountains reflect more light and valleys/craters reflect less? Or is all to do with shadows? Or maybe composition?

I also heard recently that the visible colour of the moon is like charcoal, but we see it as white because of the contrast against the black sky - is this true? It sounds like a good fact to tell people given the opportunity.

[VilleM]

Dark bits are less reflective: http://www.universetoday.com/19626/color-of-the-moon/

-V

[ollypenrice]

Yes, some parts are less reflective but there is also a shadowing effect. Here, in the morning, a tree may cast a shadow to its west. In the evening the same tree will cast a shadow to its east. The higher the sun the shorter these shadows become. The same effects arise on the moon from the sunlight's angle. It can be interesting to observe some effects which arise very much from reflectivity and compare them with shadowing effects. The rays which we see around full moon emanating from some craters are the result of reflectivity. Lacking relief they cast no shadows when the light is coming from the side so they seem to disappear.

Olly

[rockystar]

thanks for the link, some interesting stuff that I can relay.

I'm assuming then that the lower reflectivity is down to the composition of the rock?

[John]

3 hours ago, rockystar said:

thanks for the link, some interesting stuff that I can relay.

I'm assuming then that the lower reflectivity is down to the composition of the rock?

From VilleM's link:

"....That gray color you see comes from the surface of the Moon which is mostly oxygen, silicon, magnesium, iron, calcium and aluminum. The lighter color rocks are usually plagioclase feldspar, while the darker rocks are pyroxene. Most of the rocks that you can see are volcanic, and were extruded from the inside of the Moon during volcanic eruptions. Some rare rocks called olivine are actually green.

The dark regions you see on the Moon are called lunar maria, and they were formed by ancient volcanic eruptions. They’re less reflective than the lunar highlands, and so they appear darker to the eye. The maria cover about 16% of the lunar surface, mostly on the side we can see from Earth. Astronomers think the lunar maria were formed about 3-3.5 billion years ago, when the Moon was much more volcanically active.."

[rockystar]

2 hours ago, John said:

They’re less reflective than the lunar highlands, and so they appear darker to the eye.

I read this sentence and went "but why" I didn't associate it with the previous paragraph, so the make up of the rocks just didn't flew flow into this sentence. Just the way i read and interpreted it.

Thanks for highlighting

 

[CraigT82]

The highlands are mainly composed of a light coloured, silicate rich rock called Anorthosite, whilst the dark coloured Maria are composed of free flowing basalts, low in silicates but rich in iron and magnesium (mafic).  It's the lack of silicate minerals in the basaltic lavas that make them dark coloured.