There are a handful of moons in our solar system that may be an abode for extraterrestrial life. After comparison, which of the candidates makes the strongest case?
During the course of my career as an educator, I’ve tried to create a wide variety of opportunities for my students to earn extra credit. Some of these activities have undoubtedly been more popular than others, and a few were especially well received.
About two years ago, I came up with a relatively simple task for my students to perform over a weekend near the end of the school year. Any student successfully completing the optional assignment by the following Monday would receive a handful of bonus points for that grading period.
I put this list of traditionally female names on my chalkboard:
Students were permitted to copy the list. Along with the list, students were posed with this question to answer:
What is this a list of?
Throughout the course of the day, which included six classes, a few students wanted to offer a guess or two right away. Feeling confident that no student would immediately know the answer, I allowed for a few guesses to be taken at the end of each class.
“A list of Disney characters?”
This guess was actually made several times throughout the day. Although not the answer I was looking for, I do understand why that assumption would be put forth given the first name on the list.
“A list of women you’ve dated?”
To this colorful response (which I am sure was made largely in jest), I offered back only that “I’ve actually never dated a Juliet.”
Several students in each class did submit a correct answer by the following Monday. This list is, of course, of moons (natural satellites) in our solar system. Most of the students submitting correct responses had found the answer in the way I assumed the majority would: an online search engine. However, at least one student insisted that a video game had provided him with enough information to take an educated guess. Finding out how and where the students sought an answer to my question was definitely interesting to me.
Many students were surprised to learn that our solar system includes nearly two hundred moons or “natural satellites”, including our own Moon – which is one of the largest.
I’ve recently been researching some of the moons in our solar system to a considerable extent. My goal had been to decide which would serve me best as one of the settings of a science fiction piece I’m hoping to complete this summer – Dying Up There. The only real requirement for selection was that the moon be a realistic candidate to host life (and, in this case, specifically aquatic life). This is not a complete list of all the candidates, but these three really seemed to make strong cases to me: Europa, Enceladus, and Titan. Each of these moons appear to possess considerable quantities of liquid. There are good arguments supporting the belief that the bodies of liquid on each moon may be suitable for extraterrestrial life.
Europa is a moon of Jupiter, whereas Enceladus and Titan both orbit Saturn.
Prior to my research, my mind went immediately to Europa. Europa has been a standby in the world of science fiction, especially over the last four decades, as a potential home to extraterrestrial life. However, my research nearly convinced me to stray elsewhere.
Comparable to our moon in size, Europa is probably best known for being featured prominently in the 1984 film 2010: The Year We Make Contact and the novel on which the film is based. More recent films, such as Europa Report, have kept the moon in the popular imagination. Europa is a favorite of science fiction writers (and of NASA) for good reason: there is strong evidence to support that this moon is home to an ocean 60 miles deep. By comparison, the deepest part of the Earth’s ocean is not quite seven miles from surface to bottom.
A quick look at an image of Europa will not reveal this massive ocean. Europa is, at any given time, five times farther away from the Sun than Earth. Europa’s surface is also far colder than any spot on Earth, even the most frigid regions of Antarctica. As a result, Europa’s ocean is believed to be frozen upwards of 20 miles deep. Thanks to tidal heating, a considerable part of this ocean may remain liquid. If hydrothermal vents on the seafloor are functional, as some strongly believe, this may allow for life forms to exist using chemosynthesis in a body of water with a chemical composition similar to our oceans. There is a minority view that this ice-covered ocean may even occasionally melt through to the surface, producing what is referred to as “chaos terrain”. If this theory holds true, there may be a handful of locations on Europa’s smooth surface where the ocean is just “tens of meters” below the ice. Moreover, NASA has recently suggested that some features on Europa’s surface are covered in sea salt.
There are some regrettable drawbacks of Europa serving as a setting within my current work. The first being that Europa’s familiarity through many other appearances in science fiction over the last few decades boils away some of the “unknown” quality the moon may have previously possessed. Europa has essentially become the Mars of our generation, in terms of where science fiction writers suggest some kind of advanced extraterrestrial life may be encountered. There is also the pesky fact that Europa is bathed in radiation from Jupiter, making future human exploration there especially problematic.
Also worth mention is the fact that at least a few in the scientific community suspect that Europa’s ocean is biologically dead. Despite these less optimistic feelings, NASA has proposed a wide array of exploratory missions for Europa.
Enceladus could very well be what everyone wants Europa to be: a bastion of life in our solar system beyond the Earth. This tiny moon of Saturn is very remote, nearly ten times farther away from the Sun than the Earth, and has been only a bit player in the world of science fiction since first being discovered in 1789. Take a good look at the picture to the left. Doesn’t Enceladus look like a snowball? Oddly enough, cryovolcanoes on this little moon blast water vapor into space. As a result, some of this vapor falls back to the surface as what might be described as snow.
These cryovolcanoes are largely found in the southern region of the moon, and researchers believe that an ocean exists there beneath an ice shell. This is not a Europa-sized ocean, as Enceladus is only believed to be home to an ocean comparable to some of the larger bodies of water on Earth. However, this smaller Enceladus ocean is also believed to have a generally thinner ice covering than the oceans on Europa. Recent studies suggest that the ice covering this southern ocean on Enceladus is just three miles thick. Compare that to Europa’s expected average ice thickness and the ocean on Enceladus becomes immediately easier to access and explore.
An ocean on Enceladus could be kept liquid under a layer of ice by the same processes that may be working for Europa. Tidal heating is believed to trigger considerable geological activity on this moon, creating an array of fractures across the surface. This is conducive to keeping at least some of this ocean as liquid and helps create the unique chemical mix believed to exist within that sea. There is strong evidence that suggests hydrothermal activity may be at work below the ice and that the core of Enceladus allows water to freely enter through fissures and cracks to pick up additional heat.
However, Enceladus is not necessarily ideal for the setting I need. The conditions on Enceladus are not as grand as on Europa and the sheer number of possibilities seem somewhat limited compared to the latter. More troubling is a new study that states Enceladus is only one hundred million years old. That may seem ancient, but in the grand scheme of existence that is a relatively quick tick of the clock. Most sources suggest that life on Earth took almost one billion years to emerge. Enceladus may be painfully young on a cosmic scale and any life developing in that moon’s ocean must assuredly be very primitive.
Titan, no stranger to the world of science fiction, is Saturn’s largest moon and brings an entirely different set of attributes to the table. Larger than Europa and Enceladus, Titan is the second largest moon in the solar system. Aside from the Earth, Titan is also the only other body in the solar system with surface liquid. Titan is shrouded in a thick atmosphere and only in the last two decades have researchers gained a good understanding of this moon’s surface. Unlike nearby Enceladus, Titan is believed to be ancient. However, Titan’s surface is fairly young, which suggests recent geological activity. Titan is oddly Earth-like: there is precipitation, eroded channels cutting across the surface, and large bodies of liquid present.
Of course, Titan is very, very cold. This is not liquid water collecting on the surface of Titan, but liquid methane. Possessing a much lower freezing point, methane is able to exist as a liquid on Titan (also possessing a much lower boiling point, methane appears as a gas on Earth). Enough liquid methane rains down on the surface that substantial lakes and seas have formed in a few locations on Titan. Several of these bodies of liquid have been named. The largest three are Kraken Mare, Ligeia Mare, and Punga Mare. Kraken Mare is the most impressive at over 500 feet deep and covering an area larger than Earth’s Caspian Sea. The largest bodies of liquid methane on Titan are found near the poles, where the indirect sunlight struggles to evaporate these seas.
Titan may also have liquid water under the surface, but most studies regarding such a ocean have taken a backseat to the lakes and seas on the surface. Life on Titan would be seriously hindered by the extreme cold and the moon’s atmosphere offers many additional complications.
If Titan is without life now, the future may be somewhat brighter. Literally. When the sun becomes a red giant, Titan may warm up enough to become much more hospitable to life. Unfortunately, these conditions would not last forever. The Earth would also be destroyed or jettisoned from the solar system by the same changes in the Sun. So, I don’t think too many people are looking forward to Titan’s potentially improved conditions.
Also worthy of note is that in 2005 a spacecraft from Earth landed on Titan, making the latter the most distant body landed on by a craft from the former.
Titan is incredibly interesting and really seems to have a very unique existence within our solar system. However, Titan is almost too far from where my imagination initially started. Titan’s methane lakes are undoubtedly science fiction worthy, but I really needed a definitely ice-covered sea to serve as a setting within Dying Up There.
Tentatively, I am going with Europa. Europa may not as edgy as Enceladus, but the wonder and possibilities associated with the former have created a legacy of intrigue. Enceladus makes for a solid backup plan. The very thought of jumping (or falling) into an alien sea, regardless of the temperature, is incredibly unnerving to me and that’s exactly the type of feeling I’m looking to create in my future readers.
Outside the realm of storytelling though, any of these moons make interesting candidates to host life and are worthy of exploration. This is, of course, a limited list. Which of these three seems the most likely to actually host life? Are there any other moons in our solar system worth throwing into the debate for best place to find extraterrestrial life in our solar system?