Worlds that might have an underground ocean
Sen—The Solar System has many diverse and interesting worlds beyond the eight planets, with moons, dwarf planets, asteroids and comets. We continue to learn more about these bodies from current missions, such as NASA's Dawn and New Horizons probes, and the European Space Agency's Rosetta spacecraft. Future missions are being planned to further our understanding of these diverse worlds.
In recent years, we've seen an explosion of information concerning icy moons. The evidence is mounting that there is liquid water underneath at least some of their surfaces, which could be a possible indicator of life-friendly environments.
Life requires more than water to exist, to be sure; a source of energy is also required, and a stable environment. But for now, scientists will have to whet their appetite using observations from spacecraft flying overhead and telescopes on Earth's surface. Here are a selection of icy moons in our Solar System that could harbour underground oceans.
Europa (moon of Jupiter)
While Europa came under scrutiny during NASA's Galileo mission in the 1990s and 2000s, more recent observations lead credence to the idea that Europa has an ocean under its cracked, icy surface.
In 2013, spectroscopic observations released from the Keck telescope in Hawaii suggested a mineral called epsomite (a mangesium sulfate salt). The team suggested this would have been created when a mineral inside the theoretical ocean was oxidized.
Later that year, results from the Hubble Space Telescope showed a plume of water erupting from Europa's surface. Follow-up observations of the moon and examinations of the Galileo data, however, have found no other plume events.
Europa imaged by NASA's Galileo spacecraft. Image credit: NASA/JPL-Caltech/SETI Institute
Callisto (moon of Jupiter)
So far, Galileo observations of Callisto in the 1990s provide the strongest evidence of an ocean at the moon. Callisto's magnetic field changes with Jupiter's 10-hour rotation, with electrical currents produced inside the moon.
Because ice is considered a poor conductor of electricity, scientists in 1998 suggested that Callisto could have a salty ocean underneath. This would be enough to generate the currents, scientists suggested at the time.
Callisto imaged by NASA's Galileo spacecraft. Image credit: NASA/JPL/ DLR(German Aerospace Center)
Ganymede (moon of Jupiter)
While Ganymede was observed a few times by NASA's Galileo spacecraft in the 1990s and 2000s, it was only for a few hours at a time. Longer observations with the Hubble Space Telescope, released in 2015, showed that Ganymede likely has an underground ocean that is affecting how auroras appear in the moon's atmosphere.
Ganymede is the only known moon with a magnetic field, and in addition it is embedded in the intense magnetic field of Jupiter. Both affect the appearance of auroras. Scientists discovered these auroras are rocking back and forth between the fields less than predicted. A possible explanation would be the existence of a salty ocean that creates a secondary magnetic field, suppressing the rocking.
Ganymede imaged by NASA's Galileo spacecraft. Image credit: NASA/JPL-Caltech
Titan (moon of Saturn)
Titan is considered a strong location for life considering that it has a liquid cycle and also is replete with substances that are reminiscent of the possible chemistry of Earth before our own planet had life. The Cassini spacecraft has spotted numerous seas and lakes on the moon, but of liquid methane rather than water. But could there also been a subsurface ocean?
In 2012 scientists said the tides observed on Titan suggest there would be an ocean underneath its surface. Saturn's pull on the moon creates tides that are 30 feet (10 meters) tall, about 10 times higher than would be expected if Titan had a rocky interior.
Liquid water could also better explain how methane sticks around in Titan's atmosphere, even though it should dissipate in a relatively short time. The data suggests methane could be generated from Titan's interior and be making its way to the atmosphere.
Titan with the small moon Tethys in the background. Both were imaged by the NASA-ESA Cassini spacecraft. Image credit: NASA/JPL-Caltech
Enceladus (moon of Saturn)
While the Cassini spacecraft has repeatedly seen geysers spewing from Enceladus, it wasn't until 2014 that data released from the spacecraft revealed more evidence of a possible ocean.
Cassini looked at gravity variations in the moon that suggest a huge ocean (possibly regional) underneath a thick layer of ice. Specifically, during flybys the spacecraft detected a gravity anomaly at the south pole that was smaller than models of the depression there predicted, implying that there would be water (which is thicker than ice) lying underneath.
As for the plumes, Cassini observations suggest they are replete with salty water and organic molecules, the latter of which are considered the basic buildings blocks of life. The theoretical ocean is heated through gravitational interactions with Saturn.
Enceladus viewed from the NASA-ESA Cassini spacecraft. Image credit: NASA/JPL-Caltech
Pluto and Charon (moon of Pluto)
While Charon is quite distant from the Sun, the largest of Pluto's five moons could have an underground ocean. That's because in the past, the moon had an orbit that was more oval than circular, according to a 2014 study from NASA's Goddard Space Flight Center. After Charon was formed, likely because a huge impact ejected material off Pluto's surface that coalesced into the moon, its eccentric orbit had a tug-of-war with Pluto that could have heated up Charon's insides.
The ocean, however, is theoretical. Also, it could now be a frozen wasteland because Charon's orbit is more circular and tidal heating would not be as extreme. Scientists will get a better look at the moon when the New Horizons spacecraft flies by Pluto's system in Jul. 2015, which will provide more information about its history and formation.
As for Pluto, a 2006 model of its interior suggests that the dwarf planet could also possess an ocean. If there is liquid methane there, it would create a layer of methane clathrates (ice) that could influence how much ice is above it, the study says.
Artist's conception of Pluto (center) and its moon Charon as viewed from another moon in the system. Image credit: NASA, ESA and G. Bacon (STScI)
Ceres (dwarf planet)
While not technically an icy moon, Ceres is a unique member of the asteroid belt. It is so big that it is round and has been classified as a "dwarf planet", similar to Pluto. As NASA's Dawn spacecraft orbits the world to learn more about its mysteries, whether Ceres possesses an ocean will be one of them.
So far, long-range observations of Ceres show that its mass is likely made up of 30% water. When Ceres was young and still feeling the energy of its formation, it is possible that the water was sloshing around in the form of an underground ocean. Today, NASA cautions, it is most likely a frozen icy mantle.
That said, the Herschel Space Observatory did spot water vapor coming from Ceres, although the origin is a mystery. Nevertheless, NASA says Ceres could have astrobiological potential, just like the icy moons of our Solar System.
Ceres viewed from NASA's Dawn spacecraft. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
This list may only be a partial one, as we are discovering new things about the Solar System all the time. It may be that oceans are quite common in worlds in the outer Solar System. Further observations and study by robotic missions will help us understand not only how our Solar System formed but whether there is microbial life on—or perhaps under—some of these diverse worlds.