In 2005, NASA gathered information that suggested that a vast sea of water underneath the frozen surface of Saturn’s moon, Enceladus, spewed water vapor into the atmosphere. Images captured by the Imaging Science Sub-system (ISS) camera of NASA’s Cassini spacecraft featured parallel, linear ridges on the surface of Enceladus’ southern region.
These eruptions of water called “tiger stripes” are unique to the Enceladus. These observations led scientists to believe that they had potentially discovered a new, large source of liquid water that was possibly fueling these eruptions. Nine years later, data collected by Cassini in 2005 have been fully analyzed to confirm that a large subsurface ocean can exist underneath the Enceladus’ surface.
Scientists worldwide collectively analyzed gravity measurements taken by Cassini to determine the internal structure of the Enceladus. The Cassini flew past the moon three times between April 2010 and May 2012 while collecting extremely precise measurements of Cassini’s position, which were constantly transmitted to NASA via microwave carrier signals. This data can be used to construe the gravitational tug of the Enceladus on Cassini. By surveying the effect of the Enceladus on Cassini’s microwave signals as it moves past the moon, scientists learned about the Enceladus’ gravitational field and from this, the distribution of mass within the moon.
This is not a new method used to learn about the internal structure of distant objects and is currently the most accurate geophysical method of determining internal structures without seismometers on the object’s surfaces. In the case of Cassini’s measurements, a negative mass anomaly was discovered at the exact location of the tiger stripes. This anomaly indicates that there is a lower density at the tiger stripes than if the surface were a part of a uniform spherical body.
Previously, scientists confirmed the presence of a depression at the Enceladus’ south pole, but the magnitude of the negative mass anomaly was smaller than it would be for a depression. The presence of a subsurface ocean would provide one possible explanation for the small negative mass anomaly and eruptions of water at the moon’s south pole.
Although scientists cannot confirm whether the tiger stripes are spewing water or another liquid, the possibility has not yet been ruled out. If the liquid is water, then the Enceladus will add to a growing list of known bodies of water outside of Earth, such as Jupiter’s moon Europa.
