[X] My Assistant

[remcook]

If anyone's around Oxford in two weeks time...

ASTOR LECTURE IN PLANETARY SCIENCE

'Exploring the Outer Planets: Extending the Boundaries of the Habitable Zone'

to be given by

Professor Andrew P. Ingersoll
Earle C. Anthony Professor of Planetary Science, California Institute
of Technology

Martin Wood Lecture Theatre, Clarendon Laboratory

4.30 p.m. Friday, 16 March 2007

Abstract: The words "habitable zone" used to mean a band around the
sun at the Earth's orbital distance, where water exists as liquid.
New observations force us to re-define this zone as an archipelago
that includes many of the icy moons of the outer solar system. The
latest entry is Enceladus, a small satellite of Saturn. Observations
by the Cassini spacecraft reveal a surprising amount of activity on
this small object - a geologically young surface, warm fissures that
emit plumes of water vapour and water ice particles, and organic
molecules coating the surface around the vents. The talk will cover
the observations, the attempts of scientists to infer the conditions
below the vents, and the possibility of sub-surface life.

This lecture is open to members of the University and the general public.

[martin peters]

...if liquid was flowing on Enceladus in any significant amount, I'd have to think we'd see fluid flow features on the surface.

The 6 GW of heat radiating from the tiger stripes might be provided by a total fluid flow rate (for all tributaries flowing toward the tiger stripes) of as little as 0.4 m^3 per second. Knowing neither the chemical system carrying the energy nor the particular energy density of the system, of course, the best we can do is look around for some representative values. The 0.4 m^3 s^-1 is for a system of liquid ozone and liquid propane. If they could be made to react completely to form H2O and CO2, 0.2 m^3 of propane and 0.2 m^3 of ozone would produce roughtly 6 GW. I am not suggesting the system of ozone and propane to be a candidate to run this process on Enceladus, but it serves as a real example of energy density. A second example might be the decomposition of ozone to O2. If this decomposition were to proceed so that all O3 was converted to O2, a fluid flow rate of 1 m^3 s^-1 would provide the needed 6 GW. Naturally, systems with less energy density would require proportionally greater fluid flow rates.

If all the tiger stripes are hot, then the total fluid flow would have to be divided up into at least four branches. Thus, even if the total flow is 10 m^3 s^-1, there would still be only 2-3 m^3 s^-1 in each. Furthermore, these flows are not necessarily in wide, shallow, easily observable flow channels, but more probably, in my view, in the bottoms of cracks or valleys. Therefore, I suggest that the likely flow rate combined with the fact that the flows might be confined into narrow channels is not inconsistent with the lack of identifiable flow features in existing images of the surface of Enceladus.

martin

[martin peters]

Several days ago I proposed a cold-interior, solar model to explain why the tiger stripes of Enceladus are hot. One component of that process was for a liquid to condense on the surface and flow to the tiger stripes. It was, however, overly restrictive of me to require a flowing liquid. What the model requires is that material move (by any process) toward the tiger stripes. Glacier-like flow of an 'ice-field,' would suffice, if the total chemical energy passing any arbitrary point amounted to 6 gigajoules per sec. The cross-section of the moving mass would have to be appropriately large, and the slower it moved, the larger the cross-section would have to be. If there were to be a material on Enceladus containing 1 gigajoule per m^-3 (chemical potential energy), then the total flow volume would have to be 6 m^3 s^-1. In that case, average flow velocities of 10, 1, 0.1 mm/s would require cross-sections of 600, 6000, and 60,000 m^2, respectively. Needless to say, nothing about this model prevents there being a combination of glacier-like (nearer to 55 deg S, for example) and stream-like (nearer to the tiger stripes, for example) flows.

martin

[ugordan]

Press release: A Hot Start Might Explain Geysers on Enceladus.

[Rob Pinnegar]

Hmmm. This "hot start" theory is almost certainly going to be tied in with the notion that Iapetus had a lot of Al-26 at the outset.

[remcook]

Another enceladus modelling paper in press at Icarus:


Tidal Heating in Enceladus • SHORT COMMUNICATION
In Press, Accepted Manuscript, Available online 19 March 2007,
Jennifer Meyer and Jack Wisdom

the abstract is short and clear:

"The heating in Enceladus in an equilibrium resonant configuration with other Saturnian satellites can be estimated independently of the physical properties of Enceladus. We find that equilibrium tidal heating cannot account for the heat that is observed to be coming from Enceladus. Equilibrium heating in possible past resonances likewise cannot explain prior resurfacing events."

[belleraphon1]

All.....

Emily Lakdawalla posted a TPS piece a while back that was most comprehensive
http://www.planetary.org/news/2007/0322_Ch...est_a_Soup.html

This weeks (04/01/07) TPS Radio piece elaborates on the South Polar Sea piece with an interview of one of the authors: http://www.planetary.org/radio/show/00000230/

This all reminds me of ..... Jules Verne..... ahhhhhhhhhhhh the Central Sea, the Saknussemn Sea...
http://jv.gilead.org.il/vt/c_earth/27.html

Craig

[Guest_AlexBlackwell_*]

A couple of new entries in the Enceladus-related literature, courtesy of Geophysical Research Letters:

Juhász, Antal; Horányi, Mihály; Morfill, Gregor E.
Signatures of Enceladus in Saturn's E ring
Geophys. Res. Lett., Vol. 34, No. 9, L09104
10.1029/2006GL029120
05 May 2007
Abstract

The second paper will be published online in GRL, I believe, on Monday, May 7, 2007. Here's the title and abstract:

Convection in Enceladus' ice shell: Conditions for initiation
By Amy C. Barr and William B. McKinnon

Abstract
Observations of Enceladus by the Cassini spacecraft indicate that this tiny Saturnian moon is geologically active, with plumes of water vapor and ice particles erupting from its southern polar region. This activity suggests that tidal dissipation has become spatially localized, perhaps due to a compositional, rheological, and/or thermal anomaly in its ice shell. Here we examine the role that solid-state convection may have played in Enceladus' prolific activity by creating a suitable rheological and thermal anomaly. We find convection can only initiate in the pure water ice I shell of a differentiated Enceladus if the ice grain size is less than 0.3 mm, which is quite small, but may be realistic if non-water-ice impurities (and/or tidal stresses) keep grains from growing. This grain-size restriction becomes more severe for lower basal ice temperatures, which implies that any ammonia present has not become strongly concentrated in a thin basal ocean (while convection occurs). For a maximally thick pure ice shell and underlying ocean, convective heat flows are ~7–11 mW m^−2 for ice grain sizes of 0.1–0.3 mm, compared with the ~100 mW m^−2 measured for Enceladus' south polar terrain from Cassini CIRS observations. Thus whereas solid-state convection may be a prerequisite for Enceladus' geological activity, the observed heat flow requires strong tidal dissipation within the convecting region, and possibly, that convection reaches the surface.