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New Evidence for Water on Enceladus

New measurements from Cassini, made on a flyby through the plume of Enceladus on March 12, 2008, bolster the case for liquid water in the Saturnian moon. Cassini found negatively charged water ions in the plume, and its plasma spectrometer also traced other kinds of negatively charged ions including hydrocarbons. That adds Enceladus to a fairly select list — the Earth, Titan and the comets — where negatively charged ions are known to exist in the Solar System. They’re found in Earth’s ionosphere, and also in places at the surface where liquid water is in motion, such as waterfalls.

Image: Is there liquid water beneath this surface? Portions of the tiger stripe fractures, or sulci, are visible along the terminator at lower right, surrounded by a circumpolar belt of mountains. The icy moon’s famed jets emanate from at least eight distinct source regions, which lie on or near the tiger stripes. However, in this view, the most prominent feature is Labtayt Sulci, the approximately one-kilometer (0.6 miles) deep northward-trending chasm located just above the center of the mosaic. Credit: NASA/JPL/Space Science Institute

We’ve been examining the case for water, and hence the possibility of life on Enceladus, for some time now. The latest results strengthen that prospect, according to Andrew Coates (University College London), lead author of the Icarus paper on the Cassini data:

“While it’s no surprise that there is water there, these short-lived ions are extra evidence for sub-surface water and where there’s water, carbon and energy, some of the major ingredients for life are present. The surprise for us was to look at the mass of these ions. There were several peaks in the spectrum, and when we analysed them we saw the effect of water molecules clustering together one after the other.”

A BBC story on the findings points out that Cassini has already detected sodium in the plumes of Enceladus, a signature typical of liquid water in contact with deep rock for extended periods of time. And what processes add extra electrons to water molecules? Coates believes the answer may be friction as water comes out of the jets, telling the BBC reporter it would be “like rubbing a balloon and sticking it on the ceiling.” Another possibility: The ambient plasma environment, which further Cassini passes will help us characterize more fully.

Cassini’s plasma spectrometer had previously found large negative hydrocarbon ions in flybys of Titan, with the largest being found at the lowest altitudes that Cassini flew, some 950 kilometers above the surface. Coates’ team suggested in December of 2009 that these large ions could be the source of the dense haze that screens Titan’s surface from view. This news release speculates that the large ions are the organic mix called ‘tholins’ that represent a prebiotic melange of chemicals that can be produced in gases known to be present on Titan.

The paper is Coates et al., “Negative ions in the Enceladus plume,” in press at Icarus (abstract). It’s interesting to see that this is yet another case of a scientific instrument being taken beyond its intended use to return key data. Cassini’s plasma spectrometer was designed to measure the density, flow velocity and temperature of ions and electrons around Saturn. Who among its design team could have imagined it would sample jets around a tiny moon whose evidence of subsurface water excites the interest of astrobiologists?

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Comments on this entry are closed.

  • Adam February 9, 2010, 16:11

    I’m constantly amazed by the hoops they’re making “Cassini” jump through. Future probes should be designed with an eye to increased adaptability/versatility.

  • andy February 9, 2010, 18:31

    I’m still not convinced Enceladus is a good candidate to host life – there are various models which suggest the current active state is transient and for most of the time this icy moon is frozen.

  • Gary Allen February 9, 2010, 18:48

    There’s been some hallway gossip about doing an Enceladus sample return. The exobiology arguments concerning Enceladus are similar to those previously used for Europa. However exploring Europa is almost impossible because of the ridiculous radiation environment (someone told me it’s 30,000 rads near Europa). Also getting through the thick ice crust on Europa is very hard to do. However with Enceladus, there’s a plume of water jetting out from a watery mantle. This means a spacecraft doesn’t even need to land on Enceladus to get a sample. Simply fly through the water plume and collect a sample just like Stardust did for its sample. Of course the $64,000 question is:

    Could a biologically significant sample be recognisable after impact at orbital velocity?

    It’s my understanding that the rings of Saturn were actually produced by water expelled from Enceladus. If there is biological activity in the mantle of Enceladus then there should be frozen biological traces in the rings of Saturn. Rather than a fast flyby through the water plume of Enceladus, a better mission might be a slow rendezvous to the rings of Saturn where a sample could be collected. The gotcha is delta-V. It would take a huge amount of delta-V to drop down from a hyperbolic orbit into a ring rendezvous orbit. Then another huge amount of delta-V would be needed to get away from the rings. The solution might be a combination of aerocapture and nuclear electric propulsion, i.e. aerocapture down to the rings, grab a sample and then use nuclear electric to return to Earth. Another option would be to analyse the sample in situ but that would require a sophisticated science payload. Unfortunately either option requires a multi-billion dollar spacecraft. No way that’s going to happen any time soon. I guess a hyperbolic flyby through the water plume is the only practical option.

  • Enzo February 9, 2010, 22:09

    Gary,

    I think only the e-ring is replenished/made by Enceladus :
    http://apod.nasa.gov/apod/ap070327.html

  • Gary Allen February 9, 2010, 22:42

    Enzo,

    That picture you linked is very cool. Thank you!

    So the e-ring IS the water plume from Enceladus. This means the low-cost sample return mission needs to fly through the e-ring.

    It’s still going to have too high of a relative velocity.

  • tesh February 11, 2010, 5:55

    From a very lay point of view, I don’t think that sample return from anywhere other than the innersolar system should be considered, with Mars being the primary target. Anything else is just dreaming and will ultimately lead to a “half assed” approach with many compromises along the way and will be of little scientific value. As people have pointed out, how do you even recognise biological samples. I guess you could look for enrichment or dearth of specific isotopes in hydrocardons or specific hydrocarbons – that’s even assuming it is life as we known it. The BEST result possible now or in 20-40 years from now, will still only be a maybe/maybe not result.

    Mars is doable, in a fairly robust way, within the next 40-60 years – the outer solar system is not. Get humans in situ, start to explore and analyse samples there (as well as return them), form hundreds of locations and then maybe, just maybe, we will have a answer that will mean something. Is/was there life?

    The outer solarsystem can wait till technology and desire (propulsion, long term habitation, bigger rockets, detection approaches, government commitment, etc) has caught up with our dreams.

  • kurt9 February 11, 2010, 14:11

    I don’t see the big deal about this discovery. We already know that the outer solar system has lots and lots of water and methane. The fact that one of the smaller moons of Saturn is full of water should be expected.

  • kurt9 February 11, 2010, 14:16

    Speaking of life, did anyone catch this one? Does anyone here have access to the full paper in Science Journal (without me having to pay for it)?

    http://www.abc.net.au/science/articles/2010/02/05/2811555.htm

    The details are sketchy in this article and the Science abstract is not any better. Can any of the astrobiologists here tell me if this finding means that the endosymbiosis theory (and especially the Hydrogen hypothesis) on the origin of the Eukaryote is wrong?

  • Tulse February 11, 2010, 16:14

    As I read that piece, it is looking at how the endosymbiosis occurred (or more accurately how the bacterial endosymbiont was properly integrated into eukaryotic organisms). It does not contradict the basic notion of endosymbiosis.

  • Gary Allen February 12, 2010, 14:51

    Tesh said: “Mars is doable, in a fairly robust way, within the next 40-60 years – the outer solar system is not. Get humans in situ, start to explore and analyse samples there (as well as return them), form hundreds of locations and then maybe, just maybe, we will have a answer that will mean something.”

    I strongly agree with Tesh’s comment about how to do exobiological exploration on Mars, i.e. have humans do it in-situ. There is a subtle and probably intractable problem with a robotic Mars Sample Return (MSR) mission justified by exobiology. An MSR is very hard to do and consequently very expensive. In order to acquire funding it was necessary to sell the idea that highly exotic and interesting life forms are currently living on Mars. Also, in order to keep the costs down, the sales job required that these life forms be near the Martian surface (deep-dark life that is multiple kilometers underneath the Martian surface is not interesting). So we fabricate the belief that exotic life is near the surface of Mars and we are going to deliberately bring this life back to Earth for analysis. Of course this description immediately brings up the worry of an “Andromeda Strain”. One can not justify the funding for an MSR mission AND dismiss concerns about an Andromeda Strain. So the engineers designing the sample return vehicle must come up with a design that has a tiny probability of releasing the supposed Andromeda Strain into the Earth’s environment.

    To what probability does one design the sample return vehicle?

    This is obviously a political decision. Who makes that decision? Obviously the people making that decision will be proponents of MSR and people who are against it for various reasons. The people who are against MSR know they can kill the project by requiring the probability of release to be so small that no credible technology can achieve it. There was an attempt at an MSR proposal a few years ago but the exercise was abandoned because no one could come up with a rational sample return technology.

    I’ll close by saying that we should justify Mars exploration as a precursor to human colonization (Mars colonization should be the priority of our Space Program). If there is indigenous life on Mars then the Mars colonists should have the distinction of discovering it.

  • James M. Essig February 13, 2010, 18:02

    Hi Folks;

    Great discussion!

    Regarding potential aquatic lifeforms on some of the moons within our solar systems containing aqueous environments, I would love to take part in a mission to any of these moons, provided the radiation levels are shieldable and/or are not to intense, and serve as a specimen collector. Catching an ET crab, or an ET fish, or perhaps even an ET mollusk, would be an absolute thrill.

    My hope is that biology textbooks by 2050 will have whole chapters devoted to taxonomical discussion of such ETI lifeforms, including photographs. That such could occur seems to be a very likely possibility, even before we send our first manned expeditions to other star systems.