Why the Far Side has no "Man in the Moon"

Lunar Reconnaissance Orbiter Wide Angle Camera mosaic of the near side (left) and far side (right) of the Moon. Mare Moscoviense is the largest dark lava deposit on the far side of the Moon, where only a few small lava patches can be seen.
Image Credit: NASA/GSFC/Arizona State University

Looking up at the night sky, anyone can see that the near side of the Moon has many gigantic dark splotches. In many cultures, these splotches make up the "Man in the Moon". In others they form a rabbit. So, when the Soviet probe Luna 3 took the first pictures of the far side of the Moon in 1959, we were surprised to see no giant splotches, no Man (or rabbit) in the Moon. Scientists now know that those dark splotches represent large basins that formed when asteroids impacted the young Moon, and which were then filled with massive amounts of volcanic lava. But the question of why there are so few large basins and almost no lava deposits on the far side is still a source of some mystery.

Generally speaking, the distribution of impact basis should be fairly similar on the two hemispheres of the Moon, with both the near side and far side having the same amounts of large, medium, and small impact features.  The size of an impact structure is generally determined by measuring its diameter using the basin's edges, which are called rims. The problem for near side basins is that they are filled with lava, which can often hide important clues for determining where exactly the rim is located, making it hard to measure the basin's size. Also, during the final stages of the impact process, the basin sides collapse inwards due to gravity. For very large basins, this can result in multiple concentric ring structures, where it is not clear which, if any, of these ring structures represents the true basin diameter. So, the question is, are there more large impact basins on the near side because we have incorrectly measured their size?

To answer this question, scientists are using data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission. Launched in September 2011, the GRAIL mission consisted of two satellites, Ebb and Flow, which were named in a contest (school children from Montana submitted the winning entry). The two satellites measured the gravity of the Moon by tracking the changing speeds and distances between them.  When the GRAIL mission ended in December 2012, the two satellites were purposefully crashed into the Moon. But, during their year-long operating mission, the satellites collected a great deal of data, which scientists are still evaluating today.

Global near side (left) and far side (right) map showing the thickness of the Moon's crust, derived from gravity data obtained by NASA's GRAIL mission. Basins identified in this study are outlined by black circles.
Image Credit:  NASA/JPL-Caltech/S. Miljkovic

One interesting product that has been derived from the GRAIL data is a global map showing the thickness of the Moon's crust. Working together with colleagues from around the globe, Dr. Katarina Miljkovic from the Institut de Physique du Globe de Paris, France is using this crustal thickness map to study the sizes of lunar impact features.  Instead of measuring the diameter of rims to determine a basin's size, Dr. Miljkovic is using the diameter of thinned crust. During an impact, a great deal of material is excavated from the target surface, which then rebounds, allowing the mantle to push up under the crust. Together, this causes the crust to be much thinner under impact basins than in other parts of the crust. And this thinned crust can be used to represent a basin's size. The beauty of this technique is that it doesn't suffer from the same problems as the basin rim measurements. Because gravity measurements see into the interior of the crust, surface lava flows and ring structures are not an issue. The drawback of the crustal thickness technique is that it is generally limited to larger basins since the gravity signature of smaller basins can be difficult to resolve from other features.

Using this crustal thickness technique, Dr. Miljkovic and her colleagues measured all the lunar basins larger than 200 km in diameter.  The results of this work were published just last week in the journal Science, where the researchers reported that basins on the near side are truly larger than those on the far side. The problem is that the total number of craters on the two sides is the same, with just the distribution of their sizes being different. The probability of this occurring randomly is estimated to be less than 2% if both sides of the Moon were subjected to the same population of impacting asteroids. So, what could have happened to put all the big basins on the near side, leaving the far side devoid of large impacts?

There is currently no satisfactory way to get all the large asteroids to target the near side, while the smaller asteroids veer to the far side. For this reason, Dr. Miljkovic and colleagues propose a different reason for the basin size dichotomy. They suggest that the basins on the near side were made by exactly the same kinds of asteroids as those on the far side, but that these asteroids literally made a bigger impact on the near side because it was much warmer than the far side.

 If you recall, the dark splotches on the near side of the Moon also represent the presence of volcanic lava.  Most of the lava is disproportionately located on the near side of the Moon, with 99% of the lava-covered surfaces being found on the near side. This volcanism dichotomy is thought to be a result of two things: 1) a high concentration of radiogenic, heat-producing elements on the near side and 2) a thinner crust on the near side.  Early in the Moon's history, radiogenic elements would have decayed, producing radiogenic heat (though why these elements were concentrated on the near side is still not clear). The resulting heat would have melted parts of the lunar mantle, creating magma which was able to erupt onto the surface because of the thinner crust on the near side.  So, the near side of the young Moon would have been much warmer than the far side, which had a thicker crust and fewer radiogenic elements. These different temperature states on the two hemispheres would have persisted for the whole time the large impact basins were being formed.

Simplified diagram showing how the temperature of the crust affects impact basin formation. Warm crusts experience more rebound during basin formation, decreasing the amount of gravitational collapse. Cold crusts experience less rebound, allowing more gravitational collapse to occur.
Image Credit: Irene Antonenko

To study how the temperature and thickness of the crust affects impact basin sizes, Dr. Miljkovic and her coworkers ran computer simulations. Using exactly the same impactors, they studied how the size of the resulting basin differed for impacts into a warm thin crust (simulating the lunar near side) and those into a cold thick crust (simulating the lunar far side). They found that the crustal temperature had an effect during the last stages of basin formation, when a temporary "transient" basin cavity rebounds and collapses due to gravity. Impacts into a warm crust experienced more rebound, because warm material moves more easily.  The increased rebound means there is less difference in height between the rim and centre of the basin, so less material collapses into the interior due to gravity. This means that outer parts of the basin are not thickened as much by rim material collapsing inward, resulting in a larger diameter of thinned crust. In a cold crust, less rebound means that more rim material collapses into the interior, thickening the outer parts of the basin, making the diameter of thinned crust smaller. Dr. Miljkovic estimates that, for exactly the same impactor, basins in the warm crust can appear to be as much as 2 times larger than their counterparts in the cold crust.

Using this result, the researchers "corrected" the sizes of the near side impact basins, to reflect what they would have been if they had impacted into the cold, thick far side crust. After this correction, the distribution of basin sizes on the near and far sides of the Moon is more comparable, confirming that they were both bombarded by the same population of asteroids. 

Dr. Maria Zuber from the Massachusetts Institute of Technology in Cambridge is the principal investigator of the GRAIL mission. She sums up these findings very well, saying "GRAIL data indicate that both the near side and the far side of the moon were bombarded by similarly large impactors, but they reacted to them much differently.” So now we know why the near side looks so different from the far side. Early in the Moon's history, the near side was much warmer than the far side. This allowed very large basins to form, making huge bowls into which the volcanic lava flowed, so creating the big dark splotches we see today as the "Man in the Moon."

But only on the near side.

Image Credit: Irene Antonenko

Sources:
NASA's GRAIL Mission Puts a New Face on the Moon, NASA News Release, Nov 7, 2013.

Miljkovic et al., 2013, Assymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties, Science, 342, p724-726, DOI: 10.1126/science.1243224