Geosciences column special: Planetary science, part 2

13 Jan

This month we have a special edition of our Geosciences column with two pieces on planetary science written by external contributors. Whereas the first piece, published yesterday, focused on Martian water, this second article examines the internal structure of the Moon.

If you’d like to contribute to GeoLog, please contact EGU’s Media and Commmunications Officer, Bárbara T. Ferreira at media@egu.eu.


Moon not made of cheese!

A Science paper published last year re-examines previously obtained lunar seismograms to provide evidence that the moon’s core, like that of Earth, has a partly liquid exterior and a solid interior.

Although its surface is barren, the moon's internal configuration is multilayered and resembles Earth's (Source: Wikipedia)

It is likely that, as generations of star-crossed lovers gazed towards the round face of the moon in the night sky, they could not help but wonder what it was made of. “Green cheese” (then referring to freshly made, or immature), as John Heywood proposed in 1546, was probably as good a guess as any. When telescopes allowed a closer view it came with a big disappointment for cheese lovers: all this time, humankind had been staring at a rocky sphere that, furthermore, appeared passive and sterile.

But things are not always as they seem. By re-evaluating data obtained decades ago by the Apollo missions, Renee Weber and her team of planetary scientists at NASA provide, for the first time, a detailed picture of the moon’s interior. Moreover, they show that the moon is remarkably similar to Earth. It comprises a small inner core enclosed by a slightly larger fluid outer core, both of which are surrounded by an even larger partially molten zone, a solid mantle, and finally, a crust.

The Science study relates the polarization of shock waves created by seismic events to the likely internal configuration of the moon. The waves, recorded in the early 1970s, were digitally stacked and filtered, enabling the researchers to better pinpoint the precise point where the shock event took place, determining its velocity and direction. By identifying waves which may have been directed towards the centre of the moon from each area of seismic activity, or cluster zone, and reflected back from the lunar core, the results provide indirect information on the boundaries separating each of the moon’s layers.

Seismic events create different types of waves. Longitudinal, or primary (p-), waves are fast and weak, progressing in a vertical motion, superficially resembling the locomotion of a caterpillar. Shear, or secondary (s-), waves are slower but stronger, and their movement is horizontal, as in the movement of a snake. When p- and s- waves travel in the same direction, they are orientated perpendicular to each other and are polarized.

The waves also differ according to the medium in which they are moving. In fluid, shear waves are attenuated, gradually losing their energy (which is why on Earth we cannot measure direct shear waves from quakes occurring on the other side of the planet, as they would have to pass through its outer core comprising mostly molten iron). This is how Weber and her colleagues were able to indirectly investigate the density of each of the moon’s internal layers.

The recent study would be impossible without data obtained from the Apollo missions, the last of which left the moon in 1972. They left behind enough strategically placed seismic detectors to form a triangle, with edges of over 1,000 km in length, and thus distant enough from each other to pinpoint the location of underground tremors, previously not known to have existed. The lunar Passive Seismic Experiments (PSE), as they were called, continuously recorded five years of data and sent them back to Earth, identifying over 12,000 seismic events, including likely meteorite impacts and moonquakes. These data were reinforced by later research showing that most deep moonquakes occurred repeatedly in particular source regions, located around 1,000 km below the surface, and were associated with constant tidal pressure changes that the moon experiences as it rotates around Earth and the sun.

Plans to place additional seismometers on the moon have thus far been postponed or scrapped and, therefore, the PSE catalogue has remained the only data source for moonquakes. However, as lovers may today still gaze at the moon, at least they can be certain it is not made of cheese.

By Till  F. Sonnemann, researcher at the University of Sydney 

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