THE USM SOUTHWORTH PLANETARIUM

207-780-4249     www.usm.maine.edu/planet

70 Falmouth Street     Portland, Maine  04103

43.6667° N                   70.2667° W 
Altitude:   10 feet below sea level

Founded January 1970

Julian date:  2458645.5

THE DAILY ASTRONOMER
Tuesday, June 11, 2019

Mysterious Moon Matter

Something strange has been detected on the moon.  Or, more precisely, something strange has been detected  under the moon's surface.  It is a mysterious mass that is likely metallic, located under the moon's biggest crater and, according to a report published in the April 2019 Geophysical Research Letters, could be five times larger than Hawaii's biggest island, Hawai'i.   

The problem is:  scientists aren't sure why such an immense metallic collection is embedded in the moon.       Its recent detection has certainly puzzled lunar scientists who are even now developing theories to explain this mysterious moon matter.     One substantial clue is it location, within the South Pole-Aitken Basin, the largest crater on the moon.  This low topography region is so large it covers one quarter of the moon's surface.    It is possible that the enormous asteroid that formed that crater about 4 billion years ago was highly metallic and after the impact, it deposited vast quantities of metal well below the lunar surface.  We recall that during this early period the moon was still largely molten and any large impacting  body could have interred such material deep under the lunar terrain.

A false color image of the moon.  These colors indicate differences in topography.   Regions of low topography are shaded blue, whereas regions of higher topography are indicated in shades of reds and yellows.     The dashed white circle shows the location of the "mass anomaly" within the South Pole -Aitken basin, the location of the moon's largest crater.    (Image: NASA/Goddard Space Flight Center.)

Data from NASA's Gravity Recovery and Interior Laboratory (GRAIL)  and the Lunar Reconnaissance Orbiter helped scientists pinpoint the mass's location.   They detected surprisingly strong gravitational anomalies around the South Pole-Aitken Basin.    An anomaly is a region where the surface gravity deviates significantly from the norm.  Let's imagine that the moon were uniformly dense and perfectly spherical.   One would be unable to detect any anomaly along such a "perfect" moon because each point along the surface would be at the same distance from the center and the mass of the material below it would be equal to all the others location points along the perfect moon surface.    The actual moon is neither uniformly dense nor perfectly spherical.*    For this reason, the moon exhibits  various gravitational anomalies.    The anomaly around the South Pole-Aitken Basin is so significant that it indicates the presence of a highly dense material, most likely metallic.  

Don't let any of us smug astronomy types persuade you to believe that the solar system is completely mystified.     Not only are astronomers still finding hitherto unknown worlds around the Sun; they are still  not even close to truly understanding the structures and compositions of those worlds.    Many mysteries remain....

*The moon is slightly oblate, meaning that its equatorial radius is larger than it polar radius.  (Equatorial radius:  1738.1 km;  Polar radius:  1736 km)  Also, the side of the moon facing Earth is larger than that away from Earth.