2012年12月9日星期日

Grail satellites show Moon's violent history
















Ebb and Flow - together known as the Grail mission - have mapped the subtle variations in gravity across the surface of the lunar body.
They show the Moon's crust to be a mass of pulverised rock - the remains of countless impacts.
Scientists say the beating was far more extensive than previously thought.
And this observation, they add, has relevance for the study of the Earth's ancient past.
It too would have been pummelled in the first billion years of its existence by the left-over debris from the construction of the planets.
And when the gravity measurements are combined with topographical information from another of Nasa's lunar satellites showing the surface highs and lows, it becomes possible to separate out that signal related just to the Moon's internal structure and composition.
The resolution of Grail's maps far exceeds anything previously achieved - a thousand to a hundred-thousand times' improvement.
This will be a boon to researchers as they study not just the general evolution of the body but how individual features on its surface formed - from the largest impact features like the ringed basins, right down to craters just 20-30km across.
One standout observation is that the Moon's crust - its topmost layer - ranges in thickness from 34km to 43km. These numbers are about 10-20km less than previously proposed.

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