Europa is interesting enough without throwing in a new theory about energy sources. But Robert Tyler (University of Washington) has been studying the possibilities in Europan tides, using computer simulations that offer a different way of getting energy out of this icy world. We’ve speculated that Europa experiences enough tidal flex from Jupiter to create possible energy sources for life. What Tyler is saying is that the moon may experience not just internal pressures but large waves pushing through the submerged ocean. These waves, of course, could be a way of distributing heat and dissipating tidal energies.
This being the case, the assumption that energy may come from flexing at the core, as well as pressures on the oceanic ice sheets, has to be supplanted by a different view:
“If my work is correct then the heat source for Europa’s ocean is the ocean itself rather than what’s above or below it,” Tyler says. “And we must form a new vision of the ocean habitat that involves strong ocean flow rather than the previously assumed sluggish flows.”
Causing the waves is obliquity, the axial tilt of the moon in relation to its orbital plane, which results in a tidal force not previously considered in Europan terms. Earth’s axial tilt is 23 degrees. Europa’s hasn’t been measured, but Tyler believes that even at minimum values, it should be sufficient to produce significant heating. If obliquity does cause waves in an ocean we’ve long assumed as calm, then we have another way of explaining how Europa’s ocean manages to stay liquid. After all, the surface of this world is extremely cold — minus 160 degrees Celsius — while any sources of heat produced by radioactive decay seem meager.
Obliquity could be highly significant in the overall energy picture. From the paper:
“…the minimum kinetic energy of the flow associated with this resonance (7.3 X 1018 J) is two thousand times larger than that of the flow excited by the dominant tidal forces, and dissipation of this energy seems large enough to be a primary ocean heat source.
Tyler’s work gives us another take on the energy possibilities on Europa, and by extension on other moons suspected to have oceans, such as Ganymede and Callisto. And that has to play well with astrobiologists speculating on life’s development under distant ice. The paper is Tyler, “Strong ocean tidal flow and heating on moons of the outer planets,” Nature 456 (11 December 2008), pp. 770-772 (abstract).
Hi Paul
Europa is looking more interesting all the time. The higher heat flow means a much thinner ice-crust, while all the vigorous stirring means very rapid cycling of irradiated ice (and photochemically created oxidants.) The extra energy might mean stronger biogeochemical recycling of reductants in any Europa biosphere, thus by-passing what seemed like a strong limit on any life there. Now I’m looking forward to the eventual discovery of those “macrofauna” that were mentioned in that paper a year or two ago.
Wonder if there’s analogous tidal energising of Triton? Its “zero-age” crust is strong evidence of very high heat-flows at some point in the recent past, perhaps meaning strong remnant heat and a thinnish crust over any ocean below. Will it be an ammonia/water eutectic or something more watery like Europa? No way of knowing yet.
It turns out oceans and massive atmospheres are important to consider when tidal evolution is being studied. For example, the massive atmosphere of Venus makes the 1:1 spin-orbit resonance unstable for that planet. Also the configuration of the oceans on Earth has an effect on the tidal evolution of the Moon’s orbit.
You’re reading my mind about Triton, which I was thinking about last night — it’s in that retrograde orbit, too, a very interesting place for tidal speculations!
OK I can see the T-shirt now….
SURF EUROPA
Does anybody know why Europa’s axial tilt hasn’t been measured yet? Do we know the axial tilts of other satellites in the Outer Solar System, and how exactly is the axial tilt measured?
Hi Paul
Triton has been on my mind ever since I read Baxter’s “Manifold: Space” and his (flawed) “terraforming by dropping Nereid on it” approach. Actually it fell off my “Interesting Places” list in 1989 after Voyager 2 showed it was smaller and colder than imagined, and didn’t have seas of methane or nitrogen that some expected. But all the later work on the near negligible surface age has made it very interesting indeed. There’s a heat source mystery about the place – what’s keeping the geology going? The new tidal work by Tyler might be an answer, or a deeper source like some of Louise Riofrio’s primordial Black Holes.
Pluto and Charon may also have active geysers as well, which means
that many other members of the Kuiper Belt and Oort Cloud may be
geologically active as well:
https://centauri-dreams.org/?p=1360
Does it help to have another nearby body to create tides and therefore
geysers in the outer Sol system? Perhaps all binary and trinary KBOs
should be examined for geological activity.
Remember when Earth was the only place in the Sol system that
was considered to be “alive”?
Origin of Europa and the Galilean Satellites
Authors: Robin M. Canup, William R. Ward
(Submitted on 30 Dec 2008)
Abstract: Europa is believed to have formed near the very end of Jupiter’s own accretion, within a circumplanetary disk of gas and solid particles. We review the formation of the Galilean satellites in the context of current constraints and understanding of giant planet formation, focusing on recent models of satellite growth within a circumjovian accretion disk produced during the final stages of gas inflow to Jupiter.
In such a disk, the Galilean satellites would have accreted slowly, in more than 10^5 yr, and in a low pressure, low gas density environment. Gravitational interactions between the satellites and the gas disk lead to inward orbital migration and loss of satellites to Jupiter. Such effects tend to select for a maximum satellite mass and a common total satellite system mass compared to the planet’s mass.
One implication is that multiple satellite systems may have formed and been lost during the final stages of Jupiter’s growth, with the Galilean satellites being the last generation that survived as gas inflow to Jupiter ended. We conclude by discussing open issues and implications for Europa’s conditions of formation.
Comments: 62 pages, 7 figures. To appear in “Europa”, University of Arizona Press
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0812.4995v1 [astro-ph]
Submission history
From: Robin Canup [view email]
[v1] Tue, 30 Dec 2008 01:01:52 GMT (2440kb)
http://arxiv.org/abs/0812.4995
Books and Arts
Nature 457, 384-385 (22 January 2009) | doi:10.1038/457384a;
Published online 21 January 2009
Is there life on Europa?
BOOK REVIEW by Kevin Hand
Unmasking Europa: The Search for Life on Jupiter’s Ocean Moon
by Richard Greenberg
Praxis/Springer: 2008. 278 pp. £17.50/$27.50
In the field of astrobiology, the discovery of life beyond Earth sits like a gem inside the nested Russian dolls of physics, geology, chemistry and, ultimately, biology. Efforts to understand the habitability of worlds within our Solar System began with physical and astronomical surveys, and have now moved on to the challenge of cracking open the geological secrets of key destinations such as Mars and the large, icy moons of Jupiter and Saturn.
Understanding the geological context for life is critical. Rock cycles, whether they are of silicates or ices, enable chemical cycles that can then be exploited by biological systems. Such cycles are central to life on Earth. On Mars, the demise of mantle convection may have led to the planet becoming cold and dry. Near the giant planets of the outer Solar System, and perhaps around massive extrasolar planets, rock cycles may be driven by the gravitational squeezing of icy moons due to tidal interactions.
On icy moons such as Jupiter’s Europa, the mixing of irradiated, oxidant-rich surface ice with a water ocean could maintain a chemically rich environment capable of sustaining life.
In Unmasking Europa, planetary scientist Richard Greenberg details in depth our geological understanding of the tidally tormented icy surface of Europa. Without pulling any punches, he also describes the equally tormented scientific debate that has led to the current canon.
More than a decade after the Galileo spacecraft returned magnetic-field and gravity data that strengthened the case for a subsurface, liquid-water ocean on Europa, we still do not know whether that ocean lies beneath an ice shell just a few kilometres thick or a shell with a thickness of more than ten kilometres.
From an astrobiology perspective, a thin shell could permit direct cycling of oxidant-rich ices with the ocean. A thick ice shell, however, would impede the cycling of surface material, possibly limiting the chemical energy available to any life below the surface. On this contentious debate over the ice thickness, Greenberg notes, “by itself, modelling of heat transport on Europa is too uncertain to definitively discriminate between thin conductive or thick convective ice”.
However, on the basis of a host of geological features observed in images from the Voyager and Galileo missions, many of which are reproduced in the book, Greenberg argues compellingly that only a thin shell is consistent with the observed ridges, cycloidal features and chaotic terrain of Europa, all of which can be explained through tidal dynamics.
Although Greenberg occasionally strikes an acerbic tone when describing scientific differences with those on what he calls the thick-ice bandwagon, his motivation seems noble. He fears that “the most brilliant young minds may leave science if they perceive it to reward something other than good research”.
He feels that the data point towards a thin ice shell but that political powers have marginalized this interpretation and those scientists who advocate it. When discussing his own work, Greenberg generously bestows much credit on his former students, postdocs and colleagues.
In Unmasking Europa, Greenberg succeeds in conveying a story, not of heroes and villains, but about the rise and fall of ideas and how some become accepted for reasons that perhaps go beyond empirical support. In Greenberg’s earlier work, Europa the Ocean Moon (Springer, 2005), which is of similar scope but targeted to a research audience, the political storyline is not particularly appropriate.
In his latest work, he delivers an accessible and well-laid-out popular-science treatment in which the political narrative is more pertinent, although obviously biased towards his own perspective. Greenberg uses humour to balance out the tone, as in his suggestion that the reader should buy a second copy of the book just to cut out the images and do the geological reconstructions while reading the first copy.
Tides are the recurring theme of Greenberg’s treatment – they “connect the orbits of Jupiter’s moons to the geology of Europa, creating environments potentially suitable for life”. Only one short chapter deals with the possibility of a biosphere; more detail on the known chemistry of Europa would have been welcomed. Nevertheless, his treatment of tidal dynamics is thorough.
Europa has not yet revealed a smoking gun, as have the icy plumes of Enceladus, to indicate that it is geologically active today. This has left the planetary geology community staring at the limited imagery of Europa, wondering what its surface features reveal about the interior.
Centuries ago, geologists began adopting the uniformitarian mantra of ‘the present being the key to the past’. In the ebb and flow of planetary science, with data streams punctuated by missions that are all too rare, we often find ourselves struggling to decipher the geological present, much less the past.
Unmasking Europa provides a comprehensive and engaging account of Europa’s past and present, and sets the stage for the many questions that will be answered by future missions as we continue our search for life beyond Earth.
See Editorial, page 358.
Europa and Enceladus Might Harbor Oceans With Tides, Life
by Dave Mosher
The scoop: Our eyes might be on Saturn’s icy moon Enceladus, but scientists haven’t forgotten about Jupiter’s moon Europa.
Richard Greenberg, Galileo spacecraft scientist and author of “Unmasking Europa,” chats about the potential for oceanic life off Earth.
http://dsc.discovery.com/space/im/europa-enceladus-richard-greenberg.html