Arc-shaped troughs (black and white arrows) extend 100s of kilometers on the surface of Jupiter's moon Europa. These enigmatic features are likely fractures resulting from a shift in Europa's spin axis. Vertical scale bar (right) is 100 km. Credit: P. Schenk/NASA/LPI
Curved features on Jupiter鈥檚 moon Europa may indicate that its poles have wandered by almost 90掳, report scientists from the Carnegie Institution, Lunar and Planetary Institute, and University of California, Santa Cruz in the 15 May issue of Nature. Such an extreme shift suggests the existence of an internal liquid ocean beneath the icy crust, which could help build the case for Europa as possible habitat for extraterrestrial life.
The research team, which included Isamu Matsuyama of the Carnegie Institution鈥檚 Department of Terrestrial Magnetism, used images from the Voyager, Galileo, and New Horizons spacecraft to map several large arc-shaped depressions that extend more than 500 kilometers across Europa鈥檚 surface. With a radius of about 1500 kilometers, Europa is slightly smaller than the Earth鈥檚 moon.
By comparing the pattern of the depressions with fractures that would result from stresses caused by a shift in Europa鈥檚 rotational axis, the researchers determined that the axis had shifted by approximately 80掳. The previous axis of rotation is now located about 10掳 from the present equator.
The drastic shift in Europa鈥檚 rotational axis was likely a result of the build-up of thick ice at the poles. 鈥淎 spinning body is most stable with its mass farthest from its spin axis,鈥� says Matsuyama. 鈥淥n Europa, variations in the thickness of its outer shell caused a mass imbalance, so the rotation axis reoriented to a new stable state.鈥�
Such a change is called 鈥渢rue polar wander鈥� as opposed to apparent polar wander caused by plate tectonics. There is evidence for true polar wander on Earth, and also on Mars and on Saturn鈥檚 moon Enceladus. 鈥淥ur study adds Europa to this list,鈥� says Matsuyama. 鈥淚t suggests that planetary bodies might be more prone to reorientation than we thought.鈥�
The study also has implications for liquid water inside Europa. Scientists have hypothesized that Europa has an extensive subsurface ocean based on spacecraft photos that revealed its fractured, icy surface. The ocean beneath the crust would be kept liquid by heat generated by tidal forces from Jupiter鈥檚 gravity. The presence of heat and water may make life possible, even though the subsurface ocean is cut off from solar energy.
鈥淭he 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,鈥� says Matsuyama. 鈥淭herefore, our study provides an independent test for the presence of an interior liquid layer.鈥�
Source: Carnegie Institution
