The meandering
poles of Jupiter's moon Europa etched tell-tale scars across the satellite's
icy surface, a new study finds.
Europa's
ice-heavy poles shifted almost 90 degrees, from near the current equator to
their current north-south alignment, which caused the moon's spin axis to
change as well.
"A spinning
body is most stable with its mass farthest from its spin axis," said Isamu Matsuyama,
planetary scientist at the Carnegie Institution who participated in the study.
"On Europa, variations in the thickness of its outer shell caused a mass
imbalance, so the rotation axis reoriented to a new stable state."
Stresses
from the changing spin axis caused fractures that stretch more than a third of
the way across Europa's surface.
Matsuyama
and other researchers mapped arc-shaped scars extending more than 310 miles
(500 km) across the Europa, using images from NASA's Voyager, Galileo, and —
more recently — the Pluto-bound New Horizons spacecraft that zipped past
Jupiter last year. The Jovian moon
has a radius of just over 930 miles (1,500 km), making it slightly smaller than
Earth's moon.
The
research is detailed in the May 15 issue of the journal Nature.
Europa
joins Earth, Mars, and Saturn's icy moon Enceladus as planetary bodies that
have experienced "true polar wander" where their spin axis shifted, Matsuyama said.
The new
finding adds more weight to the notion that Europa's icy crust slides over a
liquid, subsurface ocean which could harbor conditions for life. A similar
possibility exists on Saturn's
moon Titan, leading NASA and Europe to consider future
deep space missions to one or both places.
"The
large reorientation on Europa required to explain the circular depressions
implies that its outer ice shell is decoupled from the core by a liquid layer,"
Matsuyama said. "Therefore, our study
provides an independent test for the presence of an interior liquid layer."
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