Variability of Enceladus’ Plume

by Paul Gilster on August 1, 2013

I’ve come to believe that building the system-wide infrastructure we’ll eventually need for interstellar flight will depend on satisfying two imperatives: Planetary defense and astrobiology. The first demands the ability to move payloads quickly to distant targets, allowing plenty of time to change the trajectory of problematic objects. The second is all about science and answering key questions about our place in the cosmos. I know few people who think life doesn’t exist elsewhere, but if it really is out there, finding and cataloguing it will involve human crews operating deep in the Solar System.

After all, we’re learning how widespread internal oceans may be, and it’s possible that even places as remote as Triton may have spawned some kind of organisms. And while we’ve had our eye on Europa’s hidden ocean for some time now, Enceladus is a recent entrant into the astrobiology sweepstakes with its jets of water ice and organic particles emmanating from the so-called ‘tiger stripes’ at the south pole. The Cassini orbiter discovered the jets back in 2005, and we’ve been learning more ever since. Now Cassini’s VIMS instrument (Visual and Infared Mapping Spectrometer) has given us data showing that the plumes vary predictably over time.

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Image: Effects of the gravitational pull of Saturn on the amount of spray coming from jets at the active moon Enceladus. Enceladus has the most spray when it is farthest away from Saturn in its orbit (inset image on the left) and the least spray when it is closest to Saturn (inset image on the right). This analysis is the first clear finding that shows the jets at Enceladus vary in a predictable manner. The background image is a mosaic made from data obtained by Cassini’s imaging science subsystem in 2006. The inset image on the left was obtained on Oct. 1, 2011. The inset image on the right was obtained on Jan. 30, 2011. Credit: NASA/JPL-Caltech/University of Arizona/Cornell/SSI.

Cornell University’s Matt Hedman likens what’s happening to adjustable garden hose nozzles. As Enceladus orbits Saturn, the ‘nozzles’ in the south pole fissures close when the moon is closest to the planet, and open again as it swings further away. Think of Enceladus being alternately squeezed and released. The new findings confirm a variability in the plumes that had been long suspected but never observed until now, one that has implications for the moon’s internal structure. Says Christophe Sotin (JPL), a co-author of the paper on this work:

“The way the jets react so responsively to changing stresses on Enceladus suggests they have their origins in a large body of liquid water. Liquid water was key to the development of life on Earth, so these discoveries whet the appetite to know whether life exists everywhere water is present.”

Yes, and with New Horizons bearing down on Pluto/Charon, we’re soon going to learn whether there is evidence for internal oceans in this interesting binary system. One day, we’re going to investigate Kuiper Belt objects like Makemake and Haumea, the latter especially interesting because of its ellipsoid shape and the probability this was caused by an ancient impact. We may learn much by comparing and contrasting Haumea with other Kuiper Belt objects as we find out whether radiogenic heating could keep internal oceans liquid even in these environments.

Enceladus itself continues to teach us that objects once thought quiet can be active and filled with potential. The famous jets being sprayed from Enceladus turn out, when examined in the infrared, to be dimmest when the moon is at its closest point to Saturn, while three to four times as bright at its most distant point. Gravitational effects like these are now visible, thanks to Cassini, but they remind us of the unseen internal action taking place as the giant planet squeezes and relaxes tiny Enceladus, an energy source that has shaped the moon and perhaps created conditions hospitable to life.

The paper is Hedman et al., “An observed correlation between plume activity and tidal stresses on Enceladus,” published online in Nature 31 July 2013 (abstract).

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{ 19 comments }

Tulse August 1, 2013 at 9:47

Enceladus seems to me to be a prime candidate for an exobiology mission, as its subsurface water seems far easier to sample than Europa’s — just do a flyby through its plume with a bucket. An analysis of plume contents should surely tell us if there is something interesting biological on/in Enceladus. There would be no need for a lander, much less drilling through kilometres of ice. It’s a farther distance for a probe, but a massively simpler mission technically.

Rob Flores August 1, 2013 at 13:52

These ice capped moons with oceans underneath stir up the exobiologists
quite a bit. But the pattern of evolution on this planet shows that more
complex organisms evolved when elements higher up on the Periodic table
were incorporated into their physiology. Do we have a good estimate as
to how abudant and varied are say the first 53 chemical elements these moons’ (stopping at Iodine the heavyest element used by animals).
These moons seem to me to be equivalent of the mid ocean basins (away from smokers). Very few nutrients would imply low energy simple microscopic organisms may only flourish there. Maybe archaic bacteria is the best these worlds can do.

Astronist August 1, 2013 at 18:17

“two imperatives: Planetary defense and astrobiology” — so you don’t think that economic growth will be a major imperative? How important is economic growth in present-day society versus planetary defence and astrobiology?

Stephen
Oxford, UK

Paul Gilster August 1, 2013 at 19:37

Stephen, I think economic growth is a given — without an economic case, building the infrastructure is that much harder. If my assumption of long-term economic growth is awry, then my two imperatives will be hard to fulfill.

Enzo August 2, 2013 at 8:33

If one just think what a proper asteroid strike could do to the human race, let alone to the so beloved economic growth, then planetary defense looks a lot higher priority than it might seem.
As for me, pushing for an exponential process in a finite world is essentially a Ponzi scheme, just with longer timescales than the usual financial variety.
Infinite growth in a finite world makes little sense and infinite growth at exponential rate make little sense very very fast.

william f collins August 2, 2013 at 10:48

Paul,
Assuming continued world-wide economic growth, interplanetary space travel /exploration will have to produce a “profitability factor(s)” in order for interplanetary expansion to occur and to continue . Where do you think such sources of profitability might be? I also suspect that a combination of human/robotic crews will be very much involved in the exploration/exploitation of interplanetary space – itself a precursor to the launching of interstellar probes. A speedier interplanetary propulsion system is key to reducing the danger factors – weightlessness, radiation exposure, psychological breakdowns, etc. – for human space crews.

Paul Gilster August 2, 2013 at 11:44

It’s a good question, and not one I have an easy answer for. I think we need to see how the early commercial activities play out. Can Planetary Resources turn a profit, and just how much of one? How about Deep Space Industries? We’ll get some ways to project a bit further once we see how these efforts pan out. And yes, I certainly agree with you that speedier interplanetary propulsion methods are called for, and I’m an advocate of nuclear thermal on that score.

coacervate August 2, 2013 at 17:38

@Tulse, agreed. I would add that Earth borne microbial spores are very lightweight, yet have a high surface charge. It is possible (likely?) that our magnetic field is strong enough to lift them to great heights at the poles. Perhaps they are blown away by the solar wind?

There are numerous ways that life could be entering space. Now we have “CubeSat”s available. Does anyone here know if a Petrie -dish type experiment is possible. Specifically I am trying to determine if we could design an aerogel or simlar material to non-destructively catch space spores should they exist. A self-contained incubator could be structured within the cube to capture, moisten and culture specimens taken in Earth orbit, Enceladus plumes or even a crash and tumble mission to Europa. Remember the “Dam Busters”? I’ll bet the Beagle II crew could finesse one right in the groove! Heh.

Low risk, high potential payoff experiment.

william collins August 2, 2013 at 18:03

Would human crews who travel to Enceladus confront intense radiation exposure from Saturn in ways similar to the projected intense exposure that may confront human crews traveling to the Jovian moon, Europa?

Astronist August 2, 2013 at 19:33

@ Enzo: yours is a point that anti-growth people (such as Tom Murphy) love to push. But I would suggest that it depends on confusing short-term and long-term perspectives.

In a finite world, unlimited exponential growth is impossible, agreed. But nobody’s suggesting *unlimited* growth. We are still a tiny civilisation in a vast, undeveloped Solar System. Our immediate prospect, if we accept it, is of continued exponential growth for a period of 1000 years or more before we start to run out of resources. Exponential growth cannot continue forever, true, but if we are to become a genuinely spacefaring civilisation then exponential space-based growth for a period of centuries to come will be necessary.

@ william f collins: for the immediate future, space adventure tourism will surely be the basis for growth of an initial LEO-based space economy. That leads naturally to Earth-Moon cycler stations, and thence to deep space. Later on, of course, when large numbers are living permanently away from Earth, the sources of profitability in space will be the same as on Earth: development of housing estates, shopping malls, industrial estates, energy, transportation, mining, software, package holidays to visit the ancestral planet.

O’Neill identified space solar power for return to Earth as the economic driver, but that’s looking weaker now with extremely large unconventional natural gas deposits being announced, and nuclear fission nowhere near full exploitation. So space solar power may be more important for supply to customers already in space or on other planets.

Stephen
Oxford, UK

william f collins August 2, 2013 at 21:35

Paul, Stephen,
Thank you for your responses. I do wonder if large numbers of folks (thousands, millions) will actually live in on or above Enceladus, Titan, Europa, or Ganymede, etc. vice NEO or on the Moon/Mars. Who knows? My speculation on the direction of interplanetary space travel is almost limitless. If we do discover native life on these moons ( and I hope that we do), perhaps the long distances and the prohibitive costs of transit /settlement might preclude the appearance of large human populations in the outer system – that might be a good thing (for the native life forms.) As I told my daughter ( who lives in one of the BRICS nation which will become an economic and political powerhouse in the near future) , I believe that my grandchildren and their children will have an opportunity to fully participate in our interplanetary future. When we will take off as a spacefaring race.

Alex Tolley August 3, 2013 at 18:17

“A self-contained incubator could be structured within the cube to capture, moisten and culture specimens taken in Earth orbit, Enceladus plumes or even a crash and tumble mission to Europa”

The problem is that even on Earth, we can only culture a small fraction of microbes that exist. It just isn’t as simple as innoculating a generic food supply. Hoping to culture “alien” microbes would probably be a crap shoot.

Enzo August 4, 2013 at 23:59

@Astronist

I agree that exponential growth is beneficial in the short term. And I wouldn’t mind if it happened somewhere else in the solar system, it’s just that I can’t see any there. I actually have a gut feeling (just that, no proof) that our lack of expansion is space is due to our preoccupation with managing the difficulties of exponential growth here.
Two things worry me most : the fact that it is an exponential process and that there’s not even a hint to a switch to a stable regime (quite the contrary, exponential growth is actively promoted left, right ad center on Earth).
People, even technically trained, are hill equipped to intuitively understand what exponential really means. They might have studied exponential functions, their derivatives, compounding interest, etc. and still not getting it.
I still remember an engineer that was adamant that any computer problem could be solved by putting more processors in parallel. You might smile at that, but how many people have really thought that cracking the AES256 encryption algorithm by brute force is not twice as hard as AES128, but more than 10E38 times harder ?
Similar is the effect on time scales. One toy example I once read was the following : imagine a pond. In it some algae doubles in size every day. After 30 days the pond is full. What day was the pond half full ? The 29th.
A slightly less toy example can be derived from this. Imagine some vital resource or it could be ecosystem capacity, whatever. Imagine the amount used doubled at every human generation. This could go on, say, for 5-6 generations or more (some 150 yrs with a generation every 25 yrs) until half of all the capacity is gone. What chances do you have to get people to understand that they are one generation away from disaster with something that has gone on for 150 yrs ad still only half used ? None whatsoever. And how much does something need to grow to double every 25 yrs ? Less than 3%/yr.
It’s a toy example, I know, its just to give an intuitive feeling for a process that we are hill equipped to intuitively understand.
If technically minded people have difficulties with reality of exponential processes, the general population hasn’t got a clue, let alone that it could be a problem.
Finally, as the population grows exponentially, so does the resistance to change. People HATE change and this growing inertia makes even less likely a paradigm switch from exponential growth to some sort of equilibrium.
A case in point is global warming : the official position from most government is that it is real, but but emissions continue to rise and so does the concentration of CO2 in the atmosphere. Whether you believe or not it’s a real problem is immaterial : it is considered a problem at government level, even at global level, still, because any real action would affect growth (and a lot of other things), very little is done.
So, I’m not optimistic : we have exponential growth (both population and consumption), actively promoted and exponential resistance to change, I can’t see it just petering out, slowly morphing itself into equilibrium any time soon.

Rob Henry August 6, 2013 at 2:17

Enzo, are you absolutely sure that the problem is others misunderstanding of the exponential curve? How can you tell that the greater problem is not your concept of what exponential growth means in an economic context? You like examples so here’s one for you.

Say we never leave Earth, we become a thousand times wealthier than we are today, but use no more energy at all. A few times more annual weight of material is transformed into synthetic objects than today, but programmers programming computers, artists painting canvasses, and waiters serving wine makes up 99.9% of GDP. Can growth continue? Of cause it can, we could become ten times richer and the energy and manufacturing sector would just shrink from 0.1% to 0.01% of GDP. Given that you like the overly simplistic, you should also note that far more of money would be spent on securing supply of energy, than on the energy itself, so NO you couldn’t easily corner the market for energy and wreck the economy.

This also addresses the problem of political charge to the global warming dilemma. If we were really have to optimize the situation because it becames life-or-death, we would want to confine growth to the first world, because growth here has already almost completely uncoupled from energy usage, and that is where all potential new solutions will be generated. Now do you see why it’s not really possible to address this issue apolitically?

Here is the real problem. Exponential growth keeps providing us with problems and opportunities that our species has never faced before. We cannot use the example of the past to demonstrate the likely security of our future. I note that you already seem to have grasped that but you seem to use it in the context or resource exhaustion, something that capitalism and the price signal is superlative at avoiding.

Also note that space colonisation will ameliorate the problem, and it is hard to see how Astronist could be wrong to suggest it.

Rob Henry August 6, 2013 at 3:51

@Coacervate. You have tried something that I have not seen attempted before. You have put radiopanspermia in planet to moon context within the Solar System. It would be much easier to follow the modern preference for lithopanspermia as the means for such transfers, but I would love to see the alternative fleshed out. That would entail addressing deceleration without aid of atmospheric breaking, as well as the usual problems with radiation exposure.

Enzo August 6, 2013 at 22:11

@Rob Henry,

“Say we never leave Earth, we become a thousand times wealthier than we are today, but use no more energy at all.”

How ? You might as well say “say we invent black magic and learn how to materialize what we need from vacuum”.
And, in any case, if the population continues to grow exponentially, so will the need for materials “transformed into synthetic objects” and the energy required to make them.
If you mean continued economic growth without population growth and without growing consumption of energy and materials, then I’m not sure, I never really thought about it because a the moment things are very very different from that.
Unlike yours, my example is based on what you can observe now : population has gone from 3.5 to 7 B in 40 years, consumption has probably grown accordingly, but, in any case exponentially. No sign of change, and strong, strong push for business as usual. Is this a competition for who creates the most simplistic model ? Because I’m loosing badly so far.

“We cannot use the example of the past to demonstrate the likely security of our future.”
My point exactly, so we shouldn’t assume that “capitalism and the price signal is superlative at avoiding” will always be there to solve any “resource exhaustion”.

“Also note that space colonisation will ameliorate the problem, and it is hard to see how Astronist could be wrong to suggest it.”

Which is *exactly* what I said about growth with “And I wouldn’t mind if it happened somewhere else in the solar system, it’s just that I can’t see any there.”

Rob Henry August 7, 2013 at 17:53

Enzo, while the bulk of our economic needs are met with a greater mass of material goods produced per person, energy and mineral requirement measures have historically been a good proxy for the size of a countries economy. This is no longer the case in richer nations (even though you insist that it can only be due to black magic). Being richer starts to become about spending a higher percent of income on services.

You might find it worthwhile to think what proportion of our wealth is tied up in real estate, and is land in Manhattan intrinsically worth that much, or does its value reside in the interconnections that it can provide in that proportion of our economy that is more abstract than material? You might also note that while avoiding resource exhaustion problems at minimal overall cost is a strong point, creating bubbles has been capitalisms weak point, thus the future may be problematic – just not through any mechanism you posit.

Rob Henry August 7, 2013 at 18:48

Oops Enzo, I didn’t notice it. You wrote “if the population continues to grow exponentially”. And, off hand, I can’t think of a single wealthy country (Israel perhaps?) whose fertility rate is above replacement, and most look in terminal decline. I note, that if trend for even further reduction in fertility is actually arrested, someone born in Italy today will see the youngest generation shrink to less than a fifth their current size. The confusion here arises because humans live longer than a single generation. Also life expectancy keeps increasing, and reasons for when that might reach saturation give differing results.

Enzo August 11, 2013 at 3:18

Unfortunately lower local fertility does not mean stable population, presumably because of migration from countries with high population growth. Even Italy grew 0.4%, USA 0.7% and Australia the same as Bangladesh, 1.2%.
World one is still above 1%/yr.

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