Icy Moons and Habitable Zones by Gordon Dennis

Not that long ago, the phenomenon of cryovolcanism was a regarded as a theoretical possibility on some bodies in the outer part of the solar system.  On Earth, volcanoes consist of an outpouring of molten rock, mainly silicates.  Cryovolcanism is defined as the extrusion of liquids and vapours of materials, mainly water.  It was until recently only a matter of scientific speculation and no such thing had been observed.
At the distance at which the gas giant planets orbit the Sun, surface temperatures on planetary moons are very low.  In the case of Enceladus, the sixth moon of Saturn (see Figure 1), the warmest surface temperature is around just 70K.  At that temperature, water is frozen solid and as hard as granite.  
Enceladus was discovered in 1789, one of many discoveries made by the German-born English astronomer  William Herschel  (1738 –  1822). The moon is very small with a diameter of approximately 500km and little more was known about Enceladus until the late twentieth and early 21st centuries.

The first images of this diminutive moon that showed any surface features were taken by Voyager 1 and Voyager 2.  These were not high resolution and taken at long range (so nothing like as detailed as shown in Figure 1). 
In 2005, the NASA Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface in much greater detail.  We can see that there is very little cratering, suggesting a very young surface.  The very high albedo (loosely speaking, reflectivity) of Enceladus’ surface is due to it being composed of water ice.  The near-linear surface features located particularly near the poles  (Tobie, 2015; Ray et al, 2020) were named “tiger stripes”.  The crucial images taken by Cassini revealed water-rich plumes venting from the south polar region as shown in Figure 2 below.

The important point to remember here is that Figure 2 is NOT an artist’s impression.  This is an actual observation of active and on-going cryovolcanism taking place right now. 
Approximately 93% of the plume material falls back to Enceladus’  The escape velocity  at the surface of Enceladus is only ~240 ms-1, so the rest escapes the moons gravitational attraction, and supplies most of the material making up Saturn's E ring.  Saturn’s atmosphere does have water clouds, but the water can’t reach as high in the planet's atmosphere as the water from Enceladus showering down.  This makes Enceladus the only moon in the solar system known to influence the chemistry of its parent planet.
Evidence from the observed libration of Enceladus plus chemical analysis of the observed plumes using a mass spectrometer instrument aboard the spacecraft  provides evidence with a high degree of certainty that the moon has a global sub surface ocean largely (though not entirely)  of water (Witze, 2014).  Mixed in with water in the plumes are silicates, indicating interaction between the ocean and its underlying rock floor (Choukroun et al, 2021); Tobie, 2015).  This is assumed to be caused by hydrothermal venting in the depths of Enceladus’ oceans, which also happens in Earth’s oceans.  They are made possible by tectonic processes on Earth, giving the strong implication of similar processes occurring on Enceladus.
A hydrothermal vent known as a black smoker near the Galapagos Islands is shown in Figure 3.

Somewhat astonishingly, around these geothermal vents live primitive life forms generally called extremophiles, since they exists under pressures and temperatures which would kill most species.  These extremophiles do not rely on any energy from sunlight and instead receive energy from chemical processes associated with the vent.  Given that over 500 such sites are known on Earth, plus the chemical analysis of Enceladus’ plumes, there is a possibility that similar primitive life forms might exist in Enceladus’ sub-surface ocean.

Habitable zones in planetary systems

As far as we know, all living species need water, including extremophiles mentioned in the previous section which may exist on Enceladus.  The definition of a stars habitable zone has been the range of distances where liquid water can exist on a planetary surface.  Too close to the star, the water will boil off; too far out, the water will freeze.  This concept is shown in Figure 4.
This all sounds very simple, but as with many things, the situation is actually more complex.  For one thing, the fact that a planet orbits the star within that stars habitable zone does not immediately indicate that the planet is inhabited.  Consider for example the case of the Sun.  Within the Sun’s habitable zone there are two planetary-sized objects.  One, which we call Earth, is full of a vast number of living species.  The other, which we call the Moon, is not fare distant from Earth, and within the Suns habitable zone, but is devoid of all life.

The next aspect to consider is the type of star.  Red dwarfs are by far the most common type of star in the Milky Way.  They have surface temperatures of ~2,000K, and so, referring once again to Figure 4, those stars habitable zones will be closer in to the star than as in the case of the Sun, whose surface temperature is just below 6,000K.
One might reasonably assume that in such a system a planet would be as likely as the Earth to harbour life, albeit the planet would need to be orbiting closer in.  For example, the TRAPPIST-1 system in Aquarius, discovered in 2015, has at least 3 earth-sized planets in its habitable zone – see Figure 5.

However, red dwarves are known to produce very intense flares of radiation.  For example, studies by NASA’s Chandra X-ray observatory concluded that about 25% of the time, Barnard's Star, located approximately 6 light years from the solar system, unleashes scorching flares, which may damage the atmospheres of planets closely orbiting it. So once again, while the habitable zones around stars such as Barnard’s or TRAPPIST-1 may be long term theoretically conducive to life, in the short term, life may not even get started due to the intense radiation environment.
So let's get back to icy moons.  As well as Enceladus, evidence of cryovolcanism has been observed on Neptune's moon Triton and is suspected on Jupiter's moon Europa.  And yet, should any of these moons harbour life in their sub-surface oceans, their parent planets - the gas giants Jupiter and Saturn, with the ice-giant Neptune - are well outside the Suns habitable zone.  All of these planets would be well off to the right of the diagram in Figure 5.
From the foregoing, we can see that the “conventional” view of the habitable zone can be somewhat  misleading. We might assume false positives in the case of red dwarf star systems.  Equally we might neglect the cases of life existing in orbits outside of what is conventionally thought of as the habitable zone, such as the possibility of life in the sub-surface ocean of Enceladus.

Conclusions

1. Saturn’s moon Enceladus is a prime target  for life detection.  Plumes containing water, CO2 and sodium chloride, NaCl as well as silicates originating from the ocean floor have been positively identified by the Cassini mission data.
   
2. Sample-return missions to Enceladus have already been proposed (Mackenzie et al, 2021; Neveu et al, 2021)

References

Choukroun, M et al (2021). Sampling Plume Deposits on Enceladus’ Surface to Explore Ocean Materials and Search for Traces of Life or Biosignatures.  The Planetary Science Journal,2:100(7pp), 2021 June.  https://iopscience.iop.org/article/10.3847/PSJ/abf2c5/pdf    Accessed June 6 2021.
Mackenzie, S et al (2021). The Enceladus Orbilander Mission Concept: Balancing Return and Resources in the Search for Life.  The Planetary Science Journal, 2:77(18pp), 2021 April.  https://iopscience.iop.org/article/10.3847/PSJ/abe4da/pdf  Accessed June 10, 2021
Neveu, M et al (2021). Returning Samples from Enceladus for Life Detection.
Ray, C et al (2020). Oxidation processes diversify the metabolic menu on Enceladus.  https://arxiv.org/pdf/2012.08582.pdf  Accessed June 11 2020
Tobie, G (2015).  Enceladus’ hot springs https://www.nature.com/articles/519162a.pdf Accessed June 10 2021
Witze, a (2014). Icy Enceladus hides a watery ocean.  Nature News. https://www.nature.com/articles/nature.2014.14985.pdf   Accessed June 10 2021