Life in the outer solar system
1) No to Gas Giants
We’ll deal with the first question quickly. No extremophile we can imagine to date would be able to deal with conditions within the atmospheres of the gas giants. This article discusses the possibility of life on Jupiter very nicely.2) Enthusiastic YES to Icy Satellites
The answer to the second question is much more promising, in fact the icy satellites of the gas giants are some of the most promising locations for life within the solar system. And with one exception that we’ll discuss later, the possibility of life on these worlds is all based on the same principal. We already discussed in the previous section that there is ample evidence for liquid water below the icy surface on a number of these satellites.Enceladus
Since the Cassini mission is providing the most recent data on Enceladus, let us start there and then turn to the Jupiter system. In the previous module, you learned Enceladus likely has a global ocean, and that the water erupting as geysers contains some hydrocarbons. The water chemistry is the result of geochemical reactions between the water and the magnesium and iron-rich rocks in Enceladus’ core. This is not so dissimilar to what we see within Earth’s oceans today. And these similarities in setting and in the chemistry that Cassini has detected to date make this world a great candidate for life. We had visited Carolyn Porco’s Captain’s Logs before and so we’ll cite her from this 2012 article :The kind of ecologies Enceladus might harbor could be like those deep within our own planet. Abundant heat and liquid water are found in Earth's subterranean volcanic rocks. Organisms in those rocks thrive on hydrogen (produced by reactions between liquid water and hot rocks) and available carbon dioxide and make methane, which gets recycled back into hydrogen. And it's all done entirely in the absence of sunlight or anything produced by sunlight.
What makes Enceladus particularly attractive as a place to search for life is that the geysers are an easy means of providing samples from the interior. No landing would be required, and we might simply sample the geysers from orbit. The title of the article cited above is: ”Is it Snowing Microbes on Enceladus?”. Multiple missions to Enceladus have been proposed (e.g. here), but at the time of writing, none have been approved.
Even though the Cassini mission has now ended, analysis of the data collected during fly-bys continues. In a 2018 Nature article entitled "Macromolecular organic compounds from the depths of Enceladus" scientist report that they found not just simple organic molecules, but very complex ones. You can read the release here, but the opening paragraph sums it up nicely:
"We are, yet again, blown away by Enceladus. Previously we'd only identified the simplest organic molecules containing a few carbon atoms, but even that was very intriguing," said SwRI's Dr. Christopher Glein, a space scientist specializing in extraterrestrial chemical oceanography. He is coauthor of a paper in Nature outlining this discovery. "Now we've found organic molecules with masses above 200 atomic mass units. That's over ten times heavier than methane. With complex organic molecules emanating from its liquid water ocean, this moon is the only body besides Earth known to simultaneously satisfy all of the basic requirements for life as we know it."
And that brings us to Europa
Europa is another icy world that we have already discussed in module 4. Since it too is thought to have a subsurface ocean it is often considered one of the prime candidates for life in the solar system. Please watch this 4 min video on Europa. It brings together some of the aspects that we have discussed in this course.
There are two missions to the icy moons of Jupiter in the planning stages, NASA's Europa Clipper (launch 2023) and ESA’s JUICE (JUpiter ICy moons Explorer, launch 2022). Neither of these missions includes a lander. As the names imply, the NASA mission will focus on Europa, while the European mission will visit three worlds with oceans, namely Ganymede, Europa, and Callisto. Both missions will in some form investigate the icy shells of the satellites. In addition to determining the surface composition, both missions will use sub-surface radar to determine the thickness of the shell. One of the big unknowns about these oceans is how deep they are. In the case of the icy satellites that really means: "How thick is the shell above the liquid ocean?".
The launch for NASA's mission is still to be determined, though it will be in the 2020’s. Note: NASA’s JUNO mission reached Jupiter on July 4, 2016 and is already returning data, but it is aimed at investigating the gas giant. But there is a very long range plan to study Europa that will eventually include a lander and then perhaps "even autonomous vehicles, called cryobots (upper right), that melt through the ice to explore the ocean below, perhaps sometime later in this century".
Planning for JUICE is more advanced; contracts were signed in Dec. 2015 and it is set for launch in 2022. After spending 7.5 years travelling through space, it will arrive at Jupiter in 2030 and then spend 3.5 years in the Jovian system. It will conduct two targeted Europa flybys, a Callisto flyby and finally a dedicated Ganymede orbital phase. The reason why JUICE will study the potential habitability of three icy moons is nicely summed up in its YELLOW book.
4.5.1.3 Habitability
JUICE will give the opportunity to study a variety of ocean worlds at the Jupiter system. The particular characteristics of Europa, Ganymede and Callisto, such as the internal structure, geological activity and global composition, determine the liquid layers context and the final fate of habitability of these environments. Comparisons among these ocean worlds have implications for astrobiology.
While Europa’s ocean is probably in contact with the rocky layer, which may be a direct source of biologically important elements, Ganymede and Callisto’s oceans are sandwiched between layers of different water ice phases. However, rise of deep silicate-rich melts has been argued to contaminate the Ganymede’s ocean. Another example of intriguing implications is concerning the effects of the radiation environment, which is less intense at the surface of Ganymede than at Europa, thus alteration of analogous materials will be different and particularly interesting to explore in the case of organics and potential biosignatures.
In short, while these icy moons share significant similarities of watery oceans, their geology may be quite different. One of the icy moons is sufficiently different that it warrants its own discussion.
