Observations of the Moon's Interior

 The Moon: A possible interior of the moon from outside to inside: Crust, Mantle, Core.

The Moon became the second celestial body that humans set foot on when Neil Armstrong and Buzz Aldrin landed there in 1969 and remains the only body that we have been on besides Earth. Due to the costs and limited number of trips to the Moon, we do not have a very extensive knowledge of the interior of the Moon. However, we were able to place four devices on the Moon from 1969 to 1972 that measure vibrations from Moonquakes to observe the interior of the Moon. Moonquakes are the equivalent of Earthquakes, except that they tend to release thousands of times less energy. The deep (700+ km below surface) Moonquakes observed in this paper are caused by the gravitational pull from the Earth causing stresses and fracturing inside of the Moon. There are other sources of Moonquakes, but they are not relevant to these deep Moonquake studies. These Moonquakes can be more easily felt because the Moon is colder and more brittle, which makes it more rigid, so it will vibrate more easily. This is similar to the comparison between hitting a metal bell with a hammer and hitting a pillow with a hammer. The bell will vibrate much more and much longer. The vibrations that the devices are detecting from the Moonquakes are actually Primary (P) Waves and Secondary (S) Waves because Primary Waves move faster and get detected first. Primary Waves vibrate in a motion towards and away from you (compressional), like the sound of someone's voice being projected towards you. Secondary Waves move in a direction that is at a right angle to the P-waves and they can also not pass through a liquid. The data from these stations was analyzed in the 1970s and was reanalyzed to find new results using new methods and technology in 2001 to make different observations. The results found that there appears to be a dislocation similar to the motion of the S-waves (shearing motion), which shows the source of these deep Moonquakes is internal, as opposed to meteorites. The results also show that there seems to be a general direction that the waves propagate, which could imply some sort of preferred orientation in the deep fractures of the Moon.

 

Conversions: This figure shows the S-p and P-s conversions at the lithosphere-asthenosphere boundary and also shows how multiples influence P-s conversions by reflection off of the surface and Crust-Mantle Boundary (Moho).

This data was analyzed using a method different from the method that was used when the data was originally gathered.  The method that was originally employed was a method used where the waves observed are waves that began as P waves and got converted to S waves before they reach the device.  The new method used was one where the wave began as an S wave and was converted to a P wave before it reaches the device.  This method is used because a wave can be converted from P to S or S to P when it passes through a structure, such as a fracture.  New layers, where the physical properties of the interior change, can also cause the conversion before the wave reaches the device that detects vibrations at the surface.  The older method is more effective for observing shallow structures in the moon because if you try to observe deep structures using the P to S method, it will be covered up with multiples.  Multiples create an effect of there being a lot of incoherent noise before the wave you are trying to observe arrives, similar to the effect of trying to pick one specific persons voice out in the middle of a crowd.  Using this alternate method instead allows for you to observe the first wave before the noise gets there or listen to the person you want to hear before everyone else starts talking.  This allows for observations of deep features and can allow for different interpretations about the sub-surface structure of the Moon. 

The first image is from http://www.daviddarling.info/images/Moon_interior.jpg

The information from this paper is from:

Vinnik, L., Chenet, H., Gagnepain-Beyneix, J. & Lognonne, P.  First seismic receiver functions on the Moon. Geophysical Research Letters, volume 28(15). pp. 3031-3034, August 1, 2001.