Geysers erupting on Enceladus, one of Saturn’s smaller moons, raise the possibility that there may be life present in a subsurface ocean. While Enceladus is covered in a thick layer of ice, there are long cracks, so-called “tiger stripe” cracks, present on the surface. These cracks are thought to be caused by the gravitational effect of Saturn and Dione, another of the planet’s moons.
NASA’s Cassini spacecraft first began to stream data back from Enceladus in 2005. At that time it revealed an icy surface marred by stripes that included geyser eruptions. Scientists were surprised that such a small moon would be so active. It was initially thought that the vapor and ice eruptions signaled the presence of a subsurface ocean.
Hypothesizing such a conclusion is one thing, proving it is quite another. Between 2010 and 2012, scientists instructed Cassini to perform three flybys. During each flyby very precise measurements were taken of the spacecraft’s trajectory using Cassini’s microwave carrier signal with NASA’s Deep Space Network. Gravitational tugs on the spacecraft by the moon alter the path of the craft very slightly. By measuring such deflection on the frequency of Cassini’s signal, scientists were able to understand Enceladus’s gravitational field, which, in turn, revealed details about the moon’s distribution of mass.
The captured data identified a negative mass depression near its southern polar region. In this case, the absence of density on the surface is compensated by the presence of greater density than ice below. Water is the only plausible explanation.
This process has taken approximately 10 years, during which 101 geysers were charted near the southern pole. As the moon orbits Saturn, it does so in an elliptical orbit that contributes to surface fractures that make geyser eruptions possible. Also of vital importance is that the geysers originate from deep below the surface, giving further credence to the existence of a large body of water below.
There is so much fluid emitted by the geysers that they contribute to one of the planet’s rings. On a pass through that ring, tests showed that the vapor was salty in nature.
Any biological activity within the ocean below the surface should be detectable in the geysers’ plumes. Knowing that life on earth began in its salty oceans, there is excitement among scientists that this Saturnian moon could possibly exhibit signs of life. Unfortunately, the instruments aboard Cassini are not capable of making such a determination. Another expedition with the proper equipment is necessary to verify that hypothesis. All of these findings are published in the online edition of the Astronomical Journal.
Oceans below icy surfaces are not new or unusual. It has long been known that Jupiter’s moon Europa has a large subsurface ocean. What makes the Enceladus discovery so exciting is that, through the plumes, scientists are able to get a glimpse of what goes on below. Water has long been surmised as the necessary ingredient to spawn life.
Orbital resonance occurs when two orbiting bodies exert gravitational influence on one another. Enceladus is currently in a 2:1 orbital resonance with the moon Dione. It completes two orbits for every one Dione completes. This orbital ratio not only maintains Enceladus’ orbital stability, but also provides some of the heating energy for Enceladus’ geological activity.
All of this data has made Saturn’s moon Enceladus and its geyser eruptions a scientific marvel. In the astronomical community nothing is more exciting than a distant object exhibiting the possibility of life.
By Hans Benes