THE man in the moon is humming a tune, but thankfully the noise won’t prevent sensors on future moon missions from peeking at the lunar interior.
A steady barrage of small meteorite impacts should cause the moon to “ring”, but no seismometers sent to the moon to date have been sensitive enough to hear it. So Philippe Lognonné at the Paris Institute of Earth Physics in France and colleagues decided to work out how loud the ring is.
The team estimated the meteorite population in the solar neighbourhood, and calculated the likely seismic signals that would be created by a range of meteorite sizes and velocities as they strike the moon. Their calculations revealed space rocks with masses ranging from a gram to a kilogram do indeed create a hum, but it is subtle. Earth’s hum – created by pounding waves – can be more than 1000 times louder (Journal of Geophysical Research, in press).
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The moon-hum’s quietness means future lunar seismometers should be able to use seismic imaging to peek deep within the moon without the hum creating problematic background noise.
“The moon-hum’s quietness means lunar seismometers should be able to peek deep within the moon”
The network of seismometers left by the Apollo missions has been shut down since 1977, so Lognonné hopes more sensitive instruments will be sent to the moon soon. Imaging the interior could help reveal more about the origin of shallow moonquakes – occasionally powerful tremors whose source has yet to be identified. Future seismometers could also reach deeper than the Apollo network to measure the size of the moon’s core. “The area within 500 kilometres of the centre of the moon is completely unknown to seismology,” Lognonné says.
The first instrument may be a seismometer proposed for Japan’s Selene-2 moon mission, which aims to send a lander to the surface, perhaps as early as 2015.
