Enceladus: An Exploration Priority
By Nathalie A. Cabrol
Nathalie is a planetary scientist and Director of the SETI Institute Carl Sagan Center
As we embark on the search for life in our Solar System, starting with Mars in 2020 and with Europa a few years later, Enceladus, the tiny moon of Saturn (504 km in diameter – or 313 mi), is becoming a top-priority in the ever growing list of worlds to be explored.
If it were anywhere on Earth, Enceladus would qualify as a five-star vacation destination! It should have great skiing, with snow flurries of fine crystals that have accumulated for millions of years. But, if you think that a 100-m layer of powder might be too much to handle, you could instead visit the so-called Tiger Stripes geothermal center. The geothermal center at the moon’s south pole showcases over 100 geysers spewing water vapor, dust, and ice particles into space at up to 2,189 km/h (1,360 mph) and at a rate of 200 kilograms (440 lb) per second. Old Faithful at Yellowstone pales in comparison. Incidentally, you will still have to deal with the powder because these geysers are the reason why it is snowing on Enceladus.
Then there’s the view! Imagine a non-stop ballet of moons of all sizes and shapes filling the sky, and a line of sight on the rings of Saturn and its stormy atmosphere that occasionally lights up with aurorae! Breathtaking!
But I digress. Certainly, for future generations, the Saturn system will be one of the hottest tickets for planetary tourism. But for now, let’s focus on scientific exploration and discoveries. Before Cassini’s latest (and regrettably last) flyby of Enceladus a few days ago, we already knew that the small moon was an excellent candidate for astrobiological exploration. The latest pictures are screaming for us to get a mission concept together and put it on a launch pad. Have you seen the pictures? Literally out of this world! A stunning landscape of faults, fractures, pits, lineaments, heaves, and smoothed impact crater rims (Figure 1). In case you missed them, I’m reposting them for you here.
Figure 1: The interplay between impact tectonic and larger scale tectonic is deeply dissecting the crust of Enceladus. Here, we see an active terrain with crisscrossing fracture systems in impact basin deposits, curvilinear fractures following crater topography, subdued impact crater rims, and smooth inter crater plains.
So, what did we already know and what new things are being revealed in these images?
Gravity measurements, hydrothermal activity, geology, geochemistry – including the salty nature of the particles and obviously tectonic deformation and geomorphology - are all making a compelling case for the existence of a subsurface ocean resting 30 to 40 km (19 to 25 mi) down. This ocean appears to be global, as opposed to a collection of dispersed melt pockets. This information was inferred from the wobble in the orbit of Enceladus around Saturn. The amount of wobble also indicates that it could be a very deep ocean, maybe 25-30 km deep – everywhere… That’s about 10 times more than the average depth of Earth’s global ocean!
So what’s new? To the trained eye, last week’s pictures brought more than just further evidence that something strange, or dare I say “fishy,” is going on down below. They open incredible perspectives in terms of developing new mission concepts!
When I am not wearing my Director’s hat, I am spending a lot of time designing and field testing exploration strategies for robots of all kinds that could support future missions to Mars or Titan, or elsewhere in the Solar System. Therefore, pictures like the Cassini images of Enceladus send my neurons on a field trip.
Let’s step back for a moment and look at the terrain. What is striking is how smooth the rims of all the craters are, even subdued. The lack of impact ejecta on the inter-crater plains is also striking. Clearly, this is an active landscape. The level of tectonic activity is astonishing, ranging from small to very large scale. The relatively local tectonic systems include fracture patterns inherited from impact cratering and diffusing radially away from ground zero. Interestingly, the patterns are generally not completely circular, which might give us some indirect information about subsurface heterogeneity and/or impact direction. You will notice that such radial features are often associated with coalescing pit formations. These are typically the telltale signs of ice sublimating or caving in, and/or the sign of venting. The wide and long tectonic features (canyon-size) meandering across the landscape (Figure 2) are generated by much stronger processes, as the crust responds to stress from Saturn’s gravitational pull.
Figure 2: Deep, canyon-size, curvilinear faults cutting across the landscape through impact basins and formed as a response of the crust to stress from Saturn’s gravitational pull
Spectacular would be a fair way to describe this landscape; But to engineers, it’s rather nightmarish! Engineers have to consider the challenges associated with landing something on such a landscape, since if you don’t land safely, you don’t have a mission. Some of the existing concepts from NASA and ESA involve an orbiter and/or a lander. The lander would be equipped with an icemole (ESA) to drill its ways through the ice. Most other mission concepts are flybys that could potentially investigate both Titan and Enceladus at once. One of the most recent concepts envisions flying through the geyser plumes and bringing samples of this ejected material back to Earth.
The idea of landing any object by design or by mistake (as in an orbiter crashing) on the surface of Enceladus will have to be carefully scrutinized because of Planetary Protection implications, which adds a further layer of complexity to any mission. Personally, I would like to see imagination (grounded in a good dose of realism) take over. I love the sample return idea. It is achievable and we should do this sooner rather than later. But, I also want to understand what’s going on down below and not just remotely from orbit. I am a field geologist. I want eyes and boots on the ground – granted that they won’t be human for quite some time.
Landing big robots is expensive, complicated, and made even more complex (= more expensive) by the type of terrain we are dealing with on Enceladus, but there are ways around these challenges. One of my favorite ideas is to think about swarms of intelligent micro-robots that could use the landscape to their advantage and gain critical knowledge about the unique surface and subsurface environment of the icy moon. Intelligent micro-robots are robust, cheap, and even cheaper by the dozen! Losing some is not losing a mission. I truly believe that Enceladus provides a fabulous opportunity to become a testing ground for a brand new type of astrobiology exploration. The same concept could be later used on other worlds for robotic missions, and for human missions, where micro-robot swarms could be sent as precursor explorers of special regions under remote human supervision.
Remember, whatever is being expelled in the geyser plumes comes from the subsurface ocean…and it is snowing down and piling up at the surface. I want to know what it is made of. Just saying.