Friday, 8 March 2013

Finding Dikes on the Moon

Earth dike, Moon data
NASA's GRAIL mission finds volcanic dikes on the Moon. The image on the right is of a dike on Earth. The data on the left shows how these kinds of features show up in the lunar data.
Image credit: Left: NASA/JPL-Caltech/CSM, Right: Photo Copyright © Louis Maher
For a while now, I have wanted to write about dikes on the Moon. Dikes are a proper geological term for molten rock, or magma, that comes up through vertical cracks in the rock and freezes there, leaving behind large sheets of volcanic rock literally standing in the cracks. No-one has really seen these kinds of things on the Moon, though they are very common on the Earth and Venus. But, recent studies of the lunar gravity field have revealed a large number of these structures on the Moon.

In September 2011, NASA launched the Gravity Recovery and Interior Laboratory (known as GRAIL) mission. This small cost mission used two washing-machine sized spacecraft to measure the gravity of the Moon. The two GRAIL spacecraft, named Ebb and Flow by 4th grade students from Montana, orbited the Moon for a little less than a year, measuring the very small changes in the distance between them. These changes resulted when one of the spacecraft flew over a region of different mass, corresponding to a change in gravitational attraction that altered in the spacecraft's orbit. For example, whenever the leading spacecraft flew over a mountain, the extra mass of the mountain would produce a larger gravitational attraction on the spacecraft, causing it to speed up, and so moving it further away from its companion. 

But, gravity data does more than measure topography (the highs and lows of a planetary exterior). It also has the potential to tell us what's below the surface.  We can use information from other instruments that also measure topography to subtract the gravity effects of mountains and valleys from the data. What is left is information from deeper in the crust.

Using these techniques, the GRAIL mission compiled Bouguer (pronounced boo-gay) gravity maps of the Moon. Bougeur is the name given to the gravity data after you have removed the topography effects. On a global scale, the Bouguer gravity maps give a very good estimate of the thickness of the lunar crust.

The images below compare GRAIL Bouguer gravity data with Lunar Reconnaissance Orbiter Camera (LROC) data of the surface. High Bouguer numbers (red) correspond to thinner crust, and low numbers (blue) correspond to thicker crust. In general, it can be seen that many, but not all, of the large basins (the big dark circular regions) have had their crust greatly thinned by the impacts that formed them. In other cases, basins that are barely noticeable in the LROC data (like the large South Pole-Aitken basin on the southern far side) show up spectacularly in the Bouguer gravity maps.

GRAIL Bouguer Gravity Map
Global Bouguer gravity map from the GRAIL mission, showing the eastern limb (left), far side (centre), and near side (right) of the Moon. Comparison with the global LROC image data below shows that many areas of very thin crust (red zones) correspond to large impact basins.
Image credit: NASA/JPL/GSFC/MIT

LROC global image mosaic
Global Lunar Reconnaissance Orbiter Camera mosaic of the eastern limb (left), far side (centre), and near side (right) of the Moon. Many of the large impact basins are filled with dark volcanic lavas and so appear dark in these images. 
Image credit: NASA/GSFC/Arizona State University (compiled by I. Antonenko)

But these are global maps that show average data over relatively large regions. Work published just last month in the journal Science, by Jeffrey Andrews-Hanna and his colleagues, shows that things are much more complex at the regional scale. Dr. Andrews-Hanna and his team found long linear gravity anomalies spanning hundreds of km in length, but relatively thin in width. These features are too small to represent differences in crustal thickness. Another mechanism must account for the difference in gravity. The most likely explanation is density, where denser materials have a greater gravitational attraction than less dense materials. 

So, here is where we get to the dikes. The gravity anomalies identified by Dr. Andrews-Hanna and his colleagues suggest higher densities than their surroundings. So, they propose that these long thin anomalies are volcanic dikes, where materials with higher densities, pushed up through the lower density rocks that make up the crust of the Moon.

Gravity vs Topography data for a lunar dike
A 500 km long gravity anomaly, located on the lunar far side, is interpreted to be a volcanic dike. The feature is clearly seen in gravity data (left) but not in topography data (right), suggesting that this feature is older than all the crater that are seen in the topography data.
Image credit: NASA/JPL-Caltech/CSM

The really interesting thing about these dikes is that they are old. Dr. Andrews-Hanna argues that since these dikes can't be seen in topography data, the many impact craters and large basins that cover the surface of the Moon must have formed after these dikes were emplaced. It is estimated that these dikes were formed during the first billion years of the Moon's history.

Also, the distribution and orientation of the dikes suggests the crust of the Moon went through a period extensional stress during this early time, where the crust was pushed out as the material just below the crust heated up and expanded. The resulting stretching caused the crust to fracture, allowing the expanding material from below to squeeze into the cracks. 

If you would like to learn more about this topic, NASA has a very well written press release on these first results of the GRAIL mission. Also, Emily Lakdawalla of The Planetary Society has an excellent blog post on the GRAIL mission's first results, with the best primer on gravity data I have ever read.

Andrews-Hanna et al. (2013),  Ancient Igneous Intrusions and Early Expansion of the Moon Revealed by GRAIL Gravity Gradiometry, Science, 8 February 2013, Vol. 339, p. 675-678, DOI: 10.1126/science.1231753.

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