A steady barrage of small meteorite impacts on the moon should cause it to "ring", but no seismometers sent there have been sensitive enough to hear it. So Philippe Lognonné at the Institute of Earth Physics of Paris and colleagues decided to work out how loud the ring is.
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The team estimated the meteoroid population in the solar neighbourhood, and calculated the likely seismic signals the space rocks would create if they struck the moon at a range of sizes and velocities.
To determine how the vibrations from these impacts would be seen by seismometers, the team used data taken by Apollo seismometers four decades ago. These measured the vibrations created by the landings of lunar modules and spent rocket stages.
Since the precise locations and timing of these landings were known, they could be used to gauge how long it would take vibrations caused by meteorite impacts to travel through the moon, and how much the signals might dim.
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 – is more than 1,000 times louder.
"This shows that all planets may hum, those with and those without atmosphere," says Lognonné.
It's oh so quiet
The moon-hum's quietness means future lunar seismometers should be able to peek deep within the moon without the hum creating problematic background noise, says Lognonné.
Instead seismometers can focus on measuring waves created by moonquakes, tremors created by a variety of sources, including the tidal tug of the Earth. Because seismic waves are sensitive to the type, arrangement and density of rocks they pass through, studying the quakes can reveal more about the moon's interior.
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. These could 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 complete unknown to seismology," Lognonné says.
"I think [the study] is a great idea," says Clive Neal of the University of Notre Dame in Indiana, who was not associated with the research. "Estimating the actual background noise is critical for designing the next generation of seismometers to go to the moon."
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.
Journal reference: Journal of Geophysical Research