Moon Craters?

[abumuhannadh]

Hi,

i have always wondered, why one side of moon have so much more craters then the other. anyone has the answer?

[brianb]

Mostly because the dark areas have been "resurfaced" by basalt lavas which flowed out over the existing surface (which we presume was cratered) after the majority of the craters had formed.

Tidal locking of the moon's rotation to its orbit may also have had an effect, the leading side will of course be struck more than the trailing side. We can't assume that the side which is leading today was the same as was leading when the major bombardment which created most of the craters occurred, later impacts may have caused some rotation.

[abumuhannadh]

thanks brianb.

so that means any crater on the sea of lava must be younger than most of the craters around it.

how old might be the youngest big crater (several miles at least) on the moon?

[brianb]

how old might be the youngest big crater (several miles at least) on the moon?

We don't know ... having minerological samples is the only reasonably reliable way of getting dates, bit in general the craters with bright rims & ray systems are more likely to be "new" because of the tendency of infalling micrometeorites to be dark. Copernicus is thought to be about one billion years old - one quarter the age of most of the craters of similar size. Very bright craters (Aristarchus, Proclus) are younger than that but probably unlikely to be younger than e.g. the Chicxulub impact which is associated with the KT boundary (dinosaur extinction etc)

There are a few craters in the 10 - 20 metre class which have been created by impact of spacecraft hardware and I believe the Lunar Recconaisance Orbiter has found one or two small (5 metre class) craters which appear to have formed by "natural" means since high resolution photography was last carried out by Lunar Orbiter & Apollo programmes in the 1960s & 1970s.

[abumuhannadh]

... I believe the Lunar Recconaisance Orbiter has found one or two small (5 metre class) craters which appear to have formed by "natural" means since high resolution photography was last carried out by Lunar Orbiter & Apollo programmes in the 1960s & 1970s.

does that mean small meteorites fell on the moon after the Apollo program? i guess that shouldn't be surprising eh? since earth is also hit by small meteorites all the time?

[brianb]

does that mean small meteorites fell on the moon after the Apollo program?

Yes. And small lumps of rock - even very friable ones - which don't make it through Earth's atmosphere can & do hit the Moon's surface. Increasingly large ones are of course increasingly rare but, given enough time, will happen.

[abumuhannadh]

as earth has a gravity 6x that of moon, does that make earth 6x more the target of a meteoroid. oh is it solely dependent on the direction of travel of the meteoroid?

[abumuhannadh]

sorry about these questions, i heard that there isn't any thing called 'stupid question' in SGL

[brianb]

as earth has a gravity 6x that of moon, does that make earth 6x more the target of a meteoroid.

Rather more than that ... the "gravity well" is 6 times deeper but has a bigger area too. Also an object will acquire more velocity falling into Earth's gravity well than it does into the Moon's, so an object that happens to hit the Earth will have a lot more energy than it would if it happened to hit the Moon instead. Good job we have an atmosphere to protect us from most of the smaller impactors, though it will have very little effect when the Big One eventually comes our way.

[toml42]

as earth has a gravity 6x that of moon, does that make earth 6x more the target of a meteoroid. oh is it solely dependent on the direction of travel of the meteoroid?

The short answer is: it depends.

The majority of asteroids are moving fast enough that their trajectory is not significantly altered by the Earths gravity.

So the main factor is cross sectional area - earth has roughly 13 times the cross sectional area of the moon.

for slower asteroids, it's slightly more complicated (the many body problem is ugly), but i would imagine mass would be the deciding factor. you could probably approximate it using the 'Hill spheres' of the Earth and the Moon. (The hill sphere being the sphere where the gravitational influence of the body is greater than the suns influence)

it works out for objects at the same distance from the sun to be proportional to the cube root of the mass, which means the earth at 83 times the mass of the moon has a capture radius 4.4 times larger, meaning its capture cross section is nearly 20 times higher. of course, it's probably quite a bit higher than that because these spheres are intersecting but it might do for an approximation.