New calculations that indicate how the Moon鈥檚 surface and interior react to the gravitational pull of the Earth and the Sun have produced further evidence that molten 鈥渟lush鈥� exists beneath the lunar surface.
A team led by James Williams at NASA鈥檚 Jet Propulsion Laboratory in California calculated how the gravitational pull of the Earth and the Sun affects the Moon鈥檚 crust. They used measurements from Earth that show how of the Moon鈥檚 surface changes over time.
Using formulas developed by the British mathematician Augustus Love, the researchers calculated that the way the Moon responds to these gravitational forces is partly due to a soft portion beneath its surface.
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鈥淗idden secrets鈥�
鈥淪ince we can鈥檛 go inside the Moon, we have to use indirect methods to learn its hidden secrets,鈥� says Williams. 鈥淚n this case we were able to use the tidal distortion of the Moon.鈥�
The first evidence of a soft region near the Moon鈥檚 core was found using seismological equipment placed at different places on the surface during the Apollo missions. These found that moonquakes lost their energy when they traveled further than 1,000 kilometers below the Moon鈥檚 surface. Since 1977, when these measurements ended, there has been no further evidence.
Robert Massey, an astronomer at Greenwich Observatory in the UK, says that the new results could provide fresh insight into the way a planet鈥檚 interior cools. He adds that similar calculations could be applied to ongoing experiments to refine measurements of the distance between the Earth and the Moon.
Bouncing lasers
鈥淚t would be interesting to see whether it is possible to refine these calculations,鈥� Massey says.
The rate at which the Moon鈥檚 surface moves up and down is currently measured to within a few centimeters by bouncing laser pulses off reflectors left on the surface of the Moon by US and Russian missions. A new experiment announced by Tom Murhpy of Washington University in January 2002 should improve these measurements by collecting more laser measurements and removing some of the uncertainty produced by the distortion of the Earth鈥檚 atmosphere.
William鈥檚 team鈥檚 research will be presented at the Lunar and Planetary Science conference, in March 2002.