It’s been a short but productive life for the two satellites that made up the NASA’s latest successful moon mission. Named Ebb and Flow both were each about the size of a washing machine and have been flying together round the moon since the January 1 this year, making up the Gravity Recovery and Interior Laboratory (GRAIL).
Now, after a stupendously successful end to their science work, they have been crashed into the moon.
Flying in tandem (with Flow forever chasing Ebb) they have been in a very tight polar orbit, as near circular as the NASA flight engineers could manoeuvre them into. This high precision in their tandem orbit allowed NASA to see when they moved slightly towards or away from each other. This small change would then be because of gravity changes over the lunar surface.
Detecting these gravity changes is essentially about measuring the distribution of mass about the moon or planet. If you sync a detailed gravity map up with one of topography, you can understand the tie-in between say, a large mountain giving a bit more mass and hence a bit more gravity.
This gets really interesting when the gravity field departs from what is expected from the terrain. This then reveals what is going on beneath the surface, providing a sort of CAT scan of the inner moon.
But on the whole it would seem that the moon, unlike a lot of other bodies in the solar system, “wears its gravity on its sleeve” with the highs and lows of gravity being as expected from the terrain.
When you look at the variation of the moon’s gravity you can see that some of the craters are low in the gravity field … which you would expect for them being a big hole in the ground.
But you’ll also see that some of them are red, indicating they are higher in gravity. These are the older craters that have been filled with dense rock that has flowed from under the surface of the moon very early in its history.
One of the chief findings that Ebb and Flow have given us was that the density of the moon’s highland crust (the brighter stuff you see when you look up at it) is quite a bit lower than was thought before. Using this new information, models of how the moon formed can be brought up to date.
That this new lowering of the highland crust density only serves to support the idea that the moon was born from a violent impact involving the early Earth.
Once they entered their orbit Ebb and Flow were always locked into a course of doom, inevitably to end in a new impact crater of their making. Much lobbying of the NASA mission control led to today’s controlled impact, preserving the moons heritage sites and also providing the engineers with much loved data.
It was a glorious – if sudden – ending to a mission with big impacts … now quite literally on the moon.