Last year, I put together a post describing volcanoes as heat engines, vehicles for transferring higher temperature materials to locations of lower temperatures. The post went on to take a look at widespread volcanic activity on planets, moons, asteroids and comets in the solar system. You can find it here: https://volcanohotspot.wordpress.com/2016/06/27/volcanoes-as-heat-engines/
We will take a look at geysers discovered and investigated on one of Saturn’s moons, Enceladus by the Cassini probe some 15 years ago.
The Cassini – Huygens probe was launched in October 1997 as a joint NASA – ESA exploration mission. It was inserted into a Saturn orbit in June 2004 following three inner system gravity assist fly-bys and a single gravity assist from Jupiter. The Huygens probe landed on Saturn’s largest moon, Titan January 2005. Orbits were constructed to allow multiple fly-bys and investigation of the most interesting of 60+ moons and rings of the Saturn system.
The exploration mission was extended several times and eventually ended in 2017 with a re-entry into Saturn’s atmosphere. The timing of mission’s end was based on the desire to not have an out of control spacecraft landing on one of the moons.
Mission planning while at Saturn was iterative, meaning that as new surprises were discovered (and there were surprises big and small nearly continuously), fly-bys and close investigations could then be scheduled to take a closer look and gather additional data.
One of the interesting bodies was Saturn’s moon Enceladus, which upon arrival in the system, was only thought to be an exceptionally bright moon, the most reflective body in the entire solar system. An early look with an onboard instrument called a magnetometer found a disturbance in its magnetic field centered on the south pole of the moon. There was also a faint, diffuse ring around Saturn at the distance of Enceladus’ orbit.
This discovery changed the mission completely, adding 20 additional close fly-bys to an original three planned close fly-bys. With the first fly-by, a series of geysers near the south pole of the moon were discovered. Many of the additional fly-bys went through the plume of the geysers, sampling the output. The geysers were imaged from multiple viewpoints. We found that the plumes were a mix of salty water ice with some organic materials and water vapor.
In the solar system, salty water in any quantity combined with heat creates and environment where life is possible. Other than Enceladus, only Jupiter’s moon Europa and Earth have those sorts of known oceans. Over the years, planetary oceans have been suggested for Titan, Ganymede and Calisto. Other than the idea that they exist, very little else is known about those oceans.
The discovery of a planetary ocean on a tiny moon only just over 500 km in diameter is a profound event. It raises all sorts of questions starting with how do you generate sufficient heat to keep an ocean in a liquid state inside a very small moon over the course of 4.5 billion years?
With the discovery of active geysers, Enceladus joins Io, Triton, and Earth as the only bodies outside active comets with observed eruptions. Some would also add Venus to that list.
Enceladus is the second major moon out from Saturn and the ring system. It is 504 km in diameter and completes an orbit every 1.37 days. Due to its close proximity to Saturn, it is tidally locked like our moon, with the same face always pointing toward Saturn. This means that like our moon, its day is the same length as its orbit.
This orbit is in a 1:2 resonance with that of Dione (second largest Saturn moon), two moons farther out. It primarily the resonance with Dione working on Enceladus’ elliptical orbit that provides tidal forces thought to keep the oceans liquid over geologic time.
I have not found anything yet suggesting resonances with Tethys or Mimas driving tidal heating, though a roughly 3:4 resonance with neighboring Tethys (third largest Saturn moon) and a roughly 2:3 resonance with neighboring Mimas (396 km in diameter) should have some measurable impact.
There are five types of terrain on Enceladus. The oldest is cratered, with crater diameters no larger than 35 km. The lack of larger diameter craters is yet another observation that needs some sort of explanation. A younger portion is relatively crater free. This may have been resurfaced with a massive outflow from the subsurface ocean. Think of this as the functional equivalent of a flood basalt with liquid water acting as basalts do here on the Earth or moon. Additional terrain types are still younger and include fissures, plains and corrugated terrain. The South Polar Terrain where the geysers are located is covered with snow from the ice particles ejected by the glaciers. It also has house-sized boulders.
The smooth plains are thought to have been resurfaced in the last few hundred million years, and if so, means the moon is still geologically active on a large scale. Volcanoes that erupt water are referred to as cryovolcanoes.
There is no detectable atmosphere (too small) and no magnetic field.
The moon itself has an icy crust 30 – 40 km thick on top of a regional or global ocean as deep as 10 km. Under that ocean is an interior with some mix of rocky and icy materials. Hot spots at the bottom of the ocean near the South Pole are suggested as a way to keep the geysers active.
There are perhaps 101 geysers active on the south polar region of Enceladus (South Polar Terrain – SPT). They are located along four rifts in the surface first called “Tiger Stripes” and more formally sulci (subparallel furrows and ridges). Activity varies with Enceladus’ position in its orbit, as tidal flexing of its elliptical orbit by interactions mainly with neighboring moon Dione tends to open the rifts a bit more when Enceladus is at its farthest from Saturn. Overall activity has decreased by some 30% during the 15 years they were observed.
The Tiger Stripes were given names from cities and countries referred to in The Arabian Nights – Alexandria Sulcus, Baghdad Sulcus, Cairo Sulcus and Damascus Sulcus. The geysers were confirmed visually by fly-by photographs, as hot spots via thermal imaging during fly-bys, and as the location of a water vapor plume. The rifts are typically 130 km long, 2 km wide, and 500 m deep. They are perhaps 35 km apart. Individual hot spots are perhaps 10 m across.
Generally, in the solar system, the younger the surface, the fewer impact craters you see. The southern polar region of Enceladus has few visible impact craters, meaning that it is very young, perhaps 4 – 0.5 Ma at the oldest.
Maximum activity and maximum tidal stress along the Tiger Stripes are not uniformly distributed across the SPT. They are concentrated in the Saturn-facing hemisphere and are thought to be related to the evolution of tidal deformation across time. Perhaps a third of the geysers cycle on and off during the 6.5 years observed. But they do not do so as predicted by any of the tidal – eccentricity models.
The mechanism of tidal modulation of geyser activity is so far unknown, though there are several theories. Perhaps the activity is controlled by strike – slip motion of the rifts. Perhaps it is driven by eccentricity tides with a time delay of 5 hours. The best suggestion are eccentricity tides plus an observed physical libration of the moon. This requires a subsurface ocean to allow the observed movement of the icy crust. It is also testable via close observation over time. Analysis of jet activity has shown jet activity is high in portions of the Sulci with the highest shear and normal stresses.
The erupted water was initially thought to be due to stresses on cracks beneath the surface of the rifts. But this explanation does not explain the salt, ammonia and organic content of the geysers. An alternate and perhaps better explanation is that water-filled cracks propagate through the weakened ice of a Tiger Stripe fracture zone (rift) and extends through tens of kilometers of ductile ice. This provides pathways for liquid and vapor to reach the surface.
Once scientists realized there were actual geysers spouting actual vapor and salt water, the argument about where that liquid water was coming from began. Initial speculation was a regional ocean under the SPT. This explanation works, as the smaller the ocean, the less energy is required to keep it liquid. The ice cap above the ocean is thought to be 30 – 40 km thick above a south polar ocean perhaps 10 km thick.
The problem with this theory is the question why there is an ocean at one location and nowhere else. The other problem is explaining the libration of the ice cap observed as the surface of the moon. Updated explanations suggest a global ocean, thicker underneath the SPT.
There is not a systematic variation in plume height, though there are large variations in plume brightness. This may be the result of supersonic flow through the geysers, but so far the details of eruption process are not yet understood. Individual geysers do not appear to shut on and off in relation to tidal flexing. Rather their total output appears to change over the course of each orbit (1.38 days).
The plumes themselves have two components – micro water particles and vapor. Total mass flux of vapor is around 200 kg / sec. Total thermal emission is around 5 GW. Modeled temperatures on the hot spots approach 200 K, which is some 130 K warmer than the average surface temperature of Enceladus. Impurities such as salt and ammonia can decrease the freezing temperature of water. Both have been detected in the plumes. Water salinity is close to that of earth oceans.
One of the other discoveries at Saturn was confirmation that Enceladus was the source of material for the E Ring, discovered in 1967 and confirmed by Pioneer 11 in 1979. It is centered on Enceladus and composed of microscopic icy or dusty particles. It is also dynamically unstable, with a lifetime no longer than a million years. It was created by and is constantly replenished by eruptions from the geysers. The E Ring is diffuse and located some 3 – 8 Saturn radii. Neighboring moons and four Trojan moons are coated with deposits of ice from the ring, making them very bright and smoothing out their features.
The big question in all of this is how such a small icy moon like Enceladus generates sufficient heat to keep water in a liquid state? For rocky and icy planetary bodies, there are several options, especially if that body is located close to the sun. A sufficiently large body like earth or Venus can generate enough internal heat by radioactive decay to keep water in a liquid state. Being relatively close to the sun is also a huge plus.
Too close, and all the original water is separated into H2 and O2 by the action of solar radiation. The H2 is driven away and lost, as it all the water. You end up like Venus, with what is essentially a planetary ocean of CO2 at its surface.
Too far away from the sun, or too small, like Mars, and the water ends up as ice.
The moon systems of the large planets are neither close enough to the sun nor individually large enough to keep water in a liquid state due to internal radioactive decay. There is another mechanism in play here, the tidal flexing of the moons themselves by gravitational interaction with their orbits, those of their neighbors, and tides raised by these interactions and the primary. We see this with the four Galilean moons of Jupiter, all of which have oceans of some sort.
These orbits are also elliptical, introducing additional tidal forces on the moons.
The Galilean moons are also large, meaning they have a reasonable proportion of silicic rock and in turn some residual radioactive decay in their interiors. The largest, Ganymede is larger than the planet Mercury. The smallest, Europa is a bit smaller than the Earth’s moon. But for them the largest energy input is the daily tidal flexing of their structure. When you do this to solids, you mechanically heat those solids up.
All four Galilean moons have some sort of oceans beneath the surface. Io, the closest to Jupiter is the most volcanically active body in the solar system. It erupts sulfur and silicates and is thought to have a magma ocean under the solid crust. The next moon is Europa. It has a planetary ocean perhaps 100 km deep capped with a few tens of kilometers of ice. The largest moon is Ganymede. It is thought to be home to one to several layers of oceans. The largest ocean is also thought to 100 km thick topped with nearly 100 km of ice. Final moon is Calisto. It is farthest out and subject to the least tidal flexing. It is also the coldest and thought to have an ocean some distance below the surface.
Step out to Saturn and we have another family of icy moons. Like Jupiter, Saturn also has over 60 moons. Unlike Jupiter, it does not have four large moons. Instead, it has a single giant moon, Titan, and a family of seven other major moons that range in size from just under 400 km (Mimas) to over 5,100 km (Titan). While there are tidal forces, they are not sufficient to drive the spectacular volcanic activity we see on Io.
There are resonances and tidal forcing of elliptical orbits in the system of Saturn’s moons. Enceladus is in a 2:1 resonance with Saturn’s second largest moon, Tethys. This interaction is thought to be sufficient to tidally flex Enceladus’ interior sufficiently to maintain an ocean over geologic time.
The problem is that there may not be enough energy transferred via this interaction to keep the ocean liquid and driving both geyser and resurfacing activity since its formation 4.5 billion years ago. Additionally, the system of inner moons and rings may not be dynamically stable over geologic time.
Clearly, there is a lot we don’t know and even more that we can only speculate about today.
One of the interesting results from the Cassini mission’s multiple fly-bys and gravity assists is the suggestion that the system of Saturn’s primary moons is not stable since the formation of the solar system some 4.5 billion years ago.
A paper published in 2016 suggests tidal interactions between the eight major moons are such that the current configuration of moons may be formed as recent as 65 Ma ago. This includes the ring system and shepherding moons. Before that time, you would have a series of collisions with the current system of moons and rings being formed from that debris. https://arstechnica.com/science/2016/03/saturns-inner-moons-may-have-formed-only-recently-from-a-giant-ring/
While most scientists believe that there is enough internal heat generated by tidal interactions and forcing of elliptical orbits to keep Enceladus’ ocean liquid over geologic time, there is not universal agreement. The alternate view is there is simply not enough heat generated that way to maintain a liquid ocean, meaning there must be another heat source. A related question is why geysers on Enceladus and not on other Saturn similar moons? What makes this moon different?
If the alternate view is correct and there is not a good explanation for the necessary quantity of excess heat driving the geysers of Enceladus over time, we are in an interesting predicament. Heat from a recently-formed body would be enough to power an ocean at least for a while. At this point, it is a mystery, and a most interesting one.
We have a very small, icy body circling Saturn with active geysers. These geysers appear to be erupting material from a planetary ocean under a cap of ice. They are driven by heat from the interior of the moon. They are ejecting salty water with organic compounds into space around the moon. There is some dispute about the idea that tidal flexing by neighboring moons and interacting elliptical orbits provides sufficient energy to keep this ocean in a liquid state over the lifetime of the solar system. With that in mind, there is a second possible explanation based on the notion that the system of inner moons is dynamically unstable over geologic time. But at this point, we do not have sufficient information to determine which idea is most accurate.
The discovery of warm, salty water with organics is a very big deal in the search for life in other places than this planet. We know that life here on Earth does quite well in the dark, cold depths of the oceans on this planet around the warmth and chemicals provided by volcanically-heated black smokers. It may turn out that it will be easier to find life in the oceans of icy moons than on other terrestrial planets.