Honey, I Shrunk the Moon!

Today at a press conference at NASA HQ in Washington, DC, scientists from the Lunar Reconnaissance Orbiter Camera (LROC) team announced their discovery of lobate thrust fault scarps on the surface of the Moon that indicate that not only has the Moon's radius shrunk by 100 meters, but it has done so in geologically recent times.  High-resolution images of these scarps show that they cross-cut small impact craters, which would normally be removed over hundred-million-year timescales by thermal cycling of the soil by the moon's month-long day, solid body tides from its gravitational interaction with the Earth, and micrometeorite impact gardening.

What would cause such shrinking?  When the moon was formed out of the impact of a large planetoid into the infant Earth, it initially was molten.  Over time the crust and core solidified, but its mantle still contained a large amount of molten silicate magma.  Magma from this "magma ocean" on occasion reached the Moon's surface, filling many of its large impact basins, creating the dark mare basalt regions.  Over time, the Moon's mantle cooled and solidified as a result of reduced heating from radioactive elements, conduction through the Moon's crust, and the removal of hot magma through volcanism.  Now as you may remember, for most materials, the phase change from a liquid to a solid causes a change in the density of the material.  For silicate magma, the density increases, so for a given amount amount of magma, the volume will decrease when it solidifies.  When this occurs on a planetary scale, such as a the Moon or Mercury, as the interior of the planet cools and solidifies, the planet or moon shrinks.

When this shrinking occurs, the crust of the planet must accommodate it.  After all, when the volume decreases, the surface area also decreases.  This causes compression within the crust which can be taken up by compacting loose surface material or reducing pore space, or through thrust faulting.  Thrust faults are low-angle faults that allow the surface above the fault to be pushed up on top of the rock beneath it. The surface expression of thrust fault on the Moon or Mercury is a arcuate scarp.  Examples of these scarps are shown above from the Moon (shown on the left side of the side-by-side image above from LROC data) and from Mercury (shown on the right).  The lunar scarps are smaller than their Mercurian cousins, rising only 100 meters above the surrounding terrain at most, with the majority only rising a few tens of meters.  On Mercury, like Beagle Rupes shown above cutting across the oblong Sveinsdóttir crater, these scarps are much than those found on the Moon, often rising a kilometer or above the surrounding terrain.  Their height, length, and estimated horizontal displacement suggest that Mercury may have contracted by several kilometers.  MESSENGER images will be used to date these scarps to determine when this shrinkage occurred just as the LROC group has done.

The situation is a bit different for icy bodies.  Unlike silicates, when water solidifies its density decreases and volume increases when it freezes.  So the radius of icy bodies increases rather than shrinks like rocky worlds like the Moon and Mercury.  The equivalent feature to the lobate scarps of the rocky bodies formed from this expansion are the narrow extensional fractures that criss-cross ancient worlds like Tethys, Dione, and Rhea.  The image at left, taken last Friday by Cassini, shows a pair of narrow fractures that cut across Penelope, an ancient impact basin on Saturn's moon Tethys. These fractures suggest that these worlds have expanded as a result of the solidification of water in their interior more recently than the heavy bombardment that produced the plethora of impact crater visible in scenes like the one at right.  Like the work being performed at the Moon and Mercury, finding and mapping these fractures and measuring the amount of displacement they accommodated will tells us how much these moons have expanded (and thus how much of their interiors remained molten until comparatively recently) and approximately when, based on superposition relationships with impact craters.

Now this wouldn't be an Io-centric blog without pointing out that the lobate thrust fault scarps seen on the Moon and Mercury are puny when compared to equivalent features on Io.  Now, obviously, unlike those two geologically dead(ish) worlds, Io is much more active and its mantle still has a large melt fraction.  Its volcanoes dredge up enough material every year that it could cover the entire surface of Io in a layer one centimeter thick (though the amount of resurface varies greatly from place-to-place).  These centimeters add up, and after a million years, the surface of Io is covered in approximately 10 kilometers (6.2 miles) of cooled lava, sulfur, and sulfur dioxide (though some of this material is locally recycled, so 10 km maybe an upper limit).  This has the effect, for the present-day surface, of shrinking the radius of Io by 10 kilometers in one million years.  As a result of global subsidence, thrust faults drive up massive "lobate scarps", also recognized as the mountains of Io.  Two clear examples of this are shown at left, Gish Bar Mons and Euboea Montes.  Both mountains reach 8-10 kilometers above Io's surrounding plains.  Of course, this may be an over-simplified model for how Io's mountains form, so I would recommend reading my article on various formation models for more information on the details.

This study of lunar lobate scarps highlights the need for global high-resolution studies, whether we are talking about the Moon, Mars, Mercury, or Io, as LROC images allowed researchers to assess the global population of these cliffs. Once their true distribution was determined, they could then come up with a proper model of how they formed. This provided an important glimpse to the Moon's recent geologic activity, after the formation of the mare basalts.

Link: NASA's LRO Reveals 'Incredible Shrinking Moon' [www.nasa.gov]