Of all the possible life-beyond-Earth questions hanging fire, few are quite so intriguing as those surrounding the now famous plumes of the moon Enceladus: what telltale molecules are in the constantly escaping jets of water vapor, and what dynamics inside the moon are pushing them out?
Seldom, if ever before, have scientists been given such an opportunity to investigate the insides of a potentially habitable celestial body from the outside.
The Cassini mission to Saturn made its closest to the surface (and last) plume fly-through a year ago, taking measurements that the team initially said they would report on within a few weeks.
That was later updated by NASA to include this guidance: Given the important astrobiology implications of these observations, the scientists caution that it will be several months before they are ready to present their detailed findings.
The reference to “important astrobiology implications” certainly could cover some incremental advance, but it does seem to at least hint of something more.
I recently contacted the Jet Propulsion Lab for an update on the fly-through results and learned that a paper has been submitted to the journal Nature and that it will hopefully be accepted and made public in the not-too-distant future.
All this sounds most interesting but not because of any secret finding of life — as some might infer from that official language. Cassini does not have the capacity to make such a detection, and there is no indication at this point that identifiable byproducts of life are present in the plumes.
What is intriguing is that the fly-through was only 30 miles above the moon’s surface — the closest pass through a plume ever by Cassini — and so presumably its instruments produced some new and significant findings.
The scientists writing the paper could not, of course, discuss their findings before publication. But Jonathan Lunine, a Cornell University planetary scientist and physicist on the Cassini mission with a longtime and deep interest in Enceladus, was comfortable discussing what is known about the moon and what Cassini (and future missions) still have to explain.
And thanks to that briefing, it became apparent that whatever new findings are coming, they will not make or break the case for the moon as a habitable place. Rather, they will essentially add to a strong case that has already been made.
“I think the evidence shows that Enceladus is the most promising target (for finding life beyond Earth) in the solar system,” Lunine told me.
Any new findings from the October 28, 2015 fly-through would certainly be useful in terms of understanding the habitability of the moon, but he said that the logical question to ask now takes the story much further. “Is the moon inhabited? That’s what I want to know now.”
That Lunine is such an enthusiastic supporter of a habitable Enceladus is not surprising: He is concept principal investigator for the Enceladus Life Finder, probably the most advanced of several proposed missions looking for NASA support.
What is surprising, at least to those who have not followed Enceladus developments with the intensity they seem to deserve, is how much is already known about the moon and its potential for supporting life.
I’ll lay out Lunine’s case, but first a little background:
Enceladus is one of the 53 (or more) moons of Saturn, and is roughly the width of Colorado– about 310 miles in diameter. It is one of Saturn’s major inner moons, is covered in ice and as a result reflects a lot of light and is one of the brightest objects in the sky. But it didn’t attract much scientific attention until 2005 when those water vapor and dust plumes were detected shooting out from its south pole by Cassini.
Further study strongly suggests that Enceladus has a global liquid ocean between its rocky core and its icy surface, and the plumes, or geysers, consist of water pushed out through cracks in that surface. The ocean is small in comparison to that of Jupiter’s watery moon Europa — the first is roughly the volume of Lake Superior while the latter has more water than all the oceans of the Earth put together — but as Lunine put it, “bacterium could do just fine in a Lake Superior-sized ocean.”
The history of Enceladus and its ocean are little understood in comparison with Europa, which Lunine said has probably had a stable ocean under its ice cover for billions of years. But unlike Europa, Enceladus has that singular advantage of constantly spitting out its insides for us to study and gradually understand. (Yes, researchers using the Hubble Space Telescope have detected what they concluded could be some water vapor plumes on Europa, too, but that finding is not confirmed.)
The equatorial surface of Enceladus is a beyond frigid -340 degrees Fahrenheit, but the temperatures around the southern polar fractures are a still cold but much warmer -100 to -130 degrees Fahrenheit. What’s important is the huge difference in temperatures — in the range of 200 degrees Fahrenheit.
The presumed sources of the heat are friction caused by gravitational forces from Saturn, and scalding heat from the core that enters the water through hydrothermal vents.
So, what is known about the geysers being pushed out of Encedadus, and about the dynamics causing the phenomenon?
Already published papers report that the water vapor, which can extend out three times the diameter of the moon, is salty, filled with fine dust particles, and contains molecules including carbon dioxide, methane, molecular nitrogen, propane, acetylene, formaldehyde and traces of ammonia. While none of these compounds are a biosignature per se, many are associated with life.
Recent analysis of some of the dust particles concluded that they were from the floor of the Enceladus ocean, and based on their characteristics appear to have been formed by the interaction of water and rock. The most logical site for this kind of interaction is at hydrothermal vents, where heat from the core makes its way up into the water. Some have argued that life on Earth may well have started at potentially similar hydrothermal vents on early Earth.
One of the primary goals of that final close fly-through was to collect data that would allow the Cassini scientists to measure how much hydrothermal activity is occurring within Enceladus.
If substantial amounts can be detected, that increases the chances for the existence around of vents of simple forms of life. Measurements for hydrothermal activity depend on the detection of methane (which has already occurred) and of molecular hydrogen (which scientists were looking for in that final fly-through.) Measurements for molecular hydrogen can be difficult to make, which might explain some of the time lag.
At the low altitude, the team also expected to be more sensitive to the possible presence of heavier and more massive molecules — including organics — that would not be observed during previous, higher-altitude passes through the plume.
One potentially complicating issue is that measurements of the pH of the water has come back with quite high alkaline levels. If that is limited to areas around hydrothermal vents then it isn’t a problem for life, Lunine said. But if it was far more widespread, it could be.
So these are some of the results we are now awaiting. But to Lunine (and others on and off the Cassini team) the case for habitability and a possible home for life on Enceladus has already been made.
“What we already know is that the ocean has the general characteristics of habitability. Obviously, it has liquid water and so there’s an energy source keeping it from all freezing. It appears to have varied thickness but is still global. It’s salty and has organic molecules, as well as those small grains of silica. The simplest model for why they exist is that water is cycling through quite warm rock at the base of the ocean, dissolving silica and delivering it to the ocean.
“Put this and more together and you have a signal, a big red arrow pointing to this moon saying it may well support life, and needs to be explored more and soon.”
But even if the upcoming Enceladus paper adds significantly to the habitable moon story, another mission to study the plumes may be long in coming. Limited resources are the major reason why but so too is the congressionally-mandated mission to Europa, a target not dissimilar to Enceladus.
Texas congressman John Culberson has pushed long and hard for the Europa mission (or missions), arguing that the Jovian moon offers our best chance of finding extraterrestrial life in the solar system. That huge and stable ocean is such a tempting target that the miles of ice encasing it are not seen as an deal-breaking obstacle. (“Thick-icers” and “thin-icers” are in constant debate about how deep that ice might go.)
That Europa is promising in terms of astrobiology is a conclusion that many scientists agree with, and NASA seems eager to cooperate. But it is nonetheless quite unusual to have Congress require NASA to mount a specific and costly mission and to set a timetable for doing it — as Congress did for Europa in 2015.
The congressional requirement follows years of waiting for a Europa mission. The Galileo mission to Jupiter produced convincing information starting in 1998 that the moon had a large ocean under its ice surface, but almost two decades have gone by without an Europa-specific mission.
Lunine, and others, are pressing to make sure that doesn’t happen with Enceladus. Last year he proposed a NASA Discovery mission to the moon that wasn’t selected, and has ideas for other sorts of NASA efforts.
“It was a sixteen or seventeen year odyssey to get a mission planned for Europa, and we just hope that doesn’t happen with Enceladus,” he told me. “We could be testing for bio-activity there and really, where else would that make so much sense?”
(Update: The most recent selection of NASA solar system missions did not include any to Enceladus, alas. Breakthrough Initiative founder and billionaire Yuri Milner has proposed a private mission to the moon.)
Many Worlds 