September 18, 2013
Europa Surface Cracks Associated With One-Time Tilted Axis
[ Watch the Video: Jupiter Moon's Cracks Due To Tilted Axis ]
Lee Rannals for redOrbit.com - Your Universe Online
New evidence shows that Jupiter's moon Europa most likely was once spinning around on a tilted axis.
NASA scientists analyzed distinctive cracks lining the surface of Europa and found that the moon used to have a small tilt in its spin axis. This information could influence calculations of how much of the moon's history is recorded in its frozen shell, how much heat is generated by tides in its ocean, and how long the ocean has been liquid.
"One of the mysteries of Europa is why the orientations of the long, straight cracks called lineaments have changed over time. It turns out that a small tilt, or obliquity, in the spin axis, sometime in the past, can explain a lot of what we see," said Alyssa Rhoden, a postdoctoral fellow with Oak Ridge Associated Universities who is working at NASA's Goddard Space Flight Center in Greenbelt, Marylandt.
Europa features crisscrossing cracks across its icy surface, which serve as a record of the stresses caused by massive tides in the moon's ocean. These tides occur because the moon travels around Jupiter in a slightly oval-shaped orbit. As Europa closes in on Jupiter, it gets stretched out like a rubber band, with the ocean height at the long end rising nearly 100 feet. When Europa moves farther away, it relaxes back into the shape of a ball.
The moon's ice layer stretches and flexes to accommodate changes in orbit, but when the stresses become too great, the ice cracks. Scientists have been puzzled as to why Europa's icy layer cracked in different directions over time, even though the same side of Europa always faces Jupiter. The new evidence, reported in a published paper in the journal Icarus, helps explain these changes in the cracks.
"Galileo produced many paradigm shifts in our understanding of Europa, one of which was the phenomena of out-of-sync rotation," said Claudia Alexander of NASA's Jet Propulsion Laboratory in Pasadena, California and project manager when the Galileo mission ended.
Researchers compared the pattern cracks near Europa's equator to predictions based on three different explanations. The first set of predictions was based on the rotation of the ice shell, while the second set assumed that Europa was spinning on a tilted axis, making the orientation of the pole change over time. This effect looks like what happens when a spinning top begins to slow down and wobble. A third explanation would be that the cracks were laid out in random directions.
"By extracting new information from the Galileo data, this work refines and improves our understanding of the very unusual geology of Europa," said Greenberg.
The ancient tilt could also affect the estimates of the age of Europa's ocean. Scientists believe tidal forces generate the heat that keeps Europa's ocean liquid, but a tilt in the spin axis could suggest that more heat is generated by tidal forces, which would keep the ocean liquid longer.
"One of the fascinating open questions is how active Europa still is. If researchers pin down Europa's current spin axis, then our findings would allow us to assess whether the clues we are finding on the moon's surface are consistent with the present-day conditions," said Rhoden.