On the road: Planetary Sciences in Reno, day 3

Posted by Rich Talcott
on Thursday, October 18, 2012

Michael Carroll receives the 2012 Jonathan Eberhart Planetary Sciences Journalism Award from Division of Planetary Sciences Chair Rosaly Lopes for his August 2011 Astronomy article, "Storm warning." // Rick Fienberg, (c) 2012 AAS.
The 44th annual meeting of the Division for Planetary Sciences (DPS) of the American Astronomical Society continued on Wednesday in Reno, Nevada (my final full day at the conference). Among the many talks were ones discussing the latest activities of the Curiosity rover on Mars, which recently scooped up some martian soil and delivered it to the chemical analysis instruments onboard, and the ongoing discoveries of the MESSENGER spacecraft after 1.5 years in orbit around Mercury. Today also saw DPS present its 2012 prizes. Near the top of the list was the Jonathan Eberhart Planetary Sciences Journalism Award, which went to Astronomy author Michael Carroll for his article “Storm Warning” in the August 2011 issue.

Among the top scientific results presented today were two that particularly caught my fancy. The first involved new models of the Moon’s formation. Most astronomers think the Moon was born when a Mars-sized protoplanet smashed into the still-forming Earth. Much of the debris cloud formed a disk around our planet, where it soon coalesced into the Moon. Previous models concluded that most of the disk material came from the impactor; however, the Moon’s composition bears an uncanny similarity to Earth’s.

Sarah Stewart of Harvard University in Cambridge, Massachusetts, reported on her team’s model, which still involves a giant impact but alters the initial conditions. In this scenario, the young Earth rotated once every two to three hours, and the rapid spin caused a large equatorial bulge. Under these conditions, most of the debris disk that formed after the impact would have come from Earth. The model then predicts that a resonance between Earth’s orbit around the Sun and the Moon’s orbit around Earth slowed our planet’s period of rotation to five hours. In the 4.5 billion years since, tidal interactions between the Moon and Earth have slowed the period to what it is now.

Robin Canup of the Southwest Research Institute in Boulder, Colorado, also tweaked the initial conditions of the impact model and came up with a Moon that matches Earth’s composition. In this version, both the impactor and the proto-Earth have masses about four to five times that of Mars.

My other favorite story also involves moons, but these orbit Saturn. Erik Asphaug of the University of California, Santa Cruz, presented a new model that can explain why the ringed planet has one giant moon, Titan, and half a dozen midsize icy satellites. Compare these to Jupiter’s family of moons, which comprises four giants that possess more than 99.99 percent of the system’s mass.

Asphaug suggests that Saturn started with a satellite system similar to Jupiter’s. But something destabilized the saturnian system, perhaps during the final stages of their formation or when the giant planets migrated to new positions a few hundred million years later. The disruption caused the moons to collide with one another. Titan grew big as it acquired most of the mass from each colliding body. But the mergers also liberated material that came mostly from the outer layers of the smaller colliding moons. Gravity then caused this icy material to clump into objects with sizes and compositions that resemble Saturn’s midsize moons. The jovian satellites never underwent this process because Jupiter’s much larger mass kept its system stable.

The model nicely explains the diversity of Saturn’s moons. Of course, it’s just a hypothesis now. But ongoing Cassini spacecraft observations of the moons can test its predictions and, perhaps, solve the riddle of the ringed planet’s worlds.

Related blogs:
On the road: Planetary sciences in Reno, day 2

On the road: Planetary sciences in Reno, day 1

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