Paper: astro-ph/0510798

Date: Fri, 28 Oct 2005 10:23:28 GMT (316kb)

Title: Modeling the Jovian subnebula: II - Composition of regular satellites
ices

Authors: Olivier Mousis and Yann Alibert

Comments: 9 pages, A&A, in press
\\
We use the evolutionary turbulent model of Jupiter's subnebula described by
Alibert et al. (2005a) to constrain the composition of ices incorporated in its
regular icy satellites. We consider CO2, CO, CH4, N2, NH3, H2S, Ar, Kr, and Xe
as the major volatile species existing in the gas-phase of the solar nebula.
All these volatile species, except CO2 which crystallized as a pure condensate,
are assumed to be trapped by H2O to form hydrates or clathrate hydrates in the
solar nebula. Once condensed, these ices were incorporated into the growing
planetesimals produced in the feeding zone of proto-Jupiter. Some of these
solids then flowed from the solar nebula to the subnebula, and may have been
accreted by the forming Jovian regular satellites. We show that ices embedded
in solids entering at early epochs into the Jovian subdisk were all vaporized.
This leads us to consider two different scenarios of regular icy satellites
formation in order to estimate the composition of the ices they contain. In the
first scenario, icy satellites were accreted from planetesimals that have been
produced in Jupiter's feeding zone without further vaporization, whereas, in
the second scenario, icy satellites were accreted from planetesimals produced
in the Jovian subnebula. In this latter case, we study the evolution of carbon
and nitrogen gas-phase chemistries in the Jovian subnebula and we show that the
conversions of N2 to NH3, of CO to CO2, and of CO to CH4 were all inhibited in
the major part of the subdisk. Finally, we assess the mass abundances of the
major volatile species with respect to H2O in the interiors of the Jovian
regular icy satellites. Our results are then compatible with the detection of
CO2 on the surfaces of Callisto and Ganymede and with the presence of NH3
envisaged in subsurface oceans within Ganymede and Callisto.

\\ ( http://arXiv.org/abs/astro-ph/0510798 , 316kb)

Astrophysics, abstract
astro-ph/0504649

From: Paul Estrada [view email]

Date (v1): Thu, 28 Apr 2005 20:20:34 GMT (67kb)
Date (revised v2): Tue, 10 May 2005 23:19:41 GMT (68kb)
Date (revised v3): Tue, 13 Dec 2005 21:42:03 GMT (107kb)

A Gas-poor Planetesimal Capture Model for the Formation of Giant Planet Satellite Systems

Authors: P. R. Estrada, I. Mosqueira

Comments: 45 pages, 11 figures, 3 appendices, uses rgfmacro.tex, accepted for publication to Icarus

Assuming that an unknown mechanism (e.g., gas turbulence) removes most of the subnebula gas disk in a timescale shorter than that for satellite formation, we develop a model for the formation of regular (and possibly at least some of the irregular) satellites around giant planets in a gas-poor environment. In this model, which follows along the lines of the work of Safronov et al. (1986), heliocentric planetesimals collide within the planet's Hill sphere and generate a circumplanetary disk of prograde and retrograde satellitesimals extending as far out as $\sim R_H/2$. At first, the net angular momentum of this proto-satellite swarm is small, and collisions among satellitesimals leads to loss of mass from the outer disk, and delivers mass to the inner disk (where regular satellites form) in a timescale $\lesssim 10^5$ years. This mass loss may be offset by continued collisional capture of sufficiently small $< 1$ km interlopers resulting from the disruption of planetesimals in the feeding zone of the giant planet. As the planet's feeding zone is cleared in a timescale $\lesssim 10^5$ years, enough angular momentum may be delivered to the proto-satellite swarm to account for the angular momentum of the regular satellites of Jupiter and Saturn.(abridged)

http://arxiv.org/abs/astro-ph/0504649

Paper: astro-ph/0602033

Date: Thu, 2 Feb 2006 02:50:36 GMT (387kb)

Title: A low density of 0.8 g/cc for the Trojan binary asteroid 617 Patroclus

Authors: Franck Marchis, Daniel Hestroffer, Pascal Descamps, Jerome Berthier,
Antonin H. Bouchez, Randall D. Campbell, Jason C. Y. Chin, Marcos A. van Dam,
Scott K. Hartman, Erik M. Johansson, Robert E. Lafon, David Le Mignant, Imke
de Pater, Paul J. Stomski, Doug M. Summers, Frederic Vachier, Peter L.
Wizinovich, Michael H. Wong

Comments: 10 pages, 3 figures, 1 table

Journal-ref: Nature, 439, 565-567, 2006
\\
The Trojan population consists of two swarms of asteroids following the same
orbit as Jupiter and located at the L4 and L5 Lagrange points of the
Jupiter-Sun system (leading and following Jupiter by 60 degrees). The asteroid
617 Patroclus is the only known binary Trojan (Merline et al. 2001). The orbit
of this double system was hitherto unknown. Here we report that the components,
separated by 680 km, move around the system centre of mass, describing roughly
a circular orbit. Using the orbital parameters, combined with thermal
measurements to estimate the size of the components, we derive a very low
density of 0.8 g/cc. The components of Patroclus are therefore very porous or
composed mostly of water ice, suggesting that they could have been formed in
the outer part of the solar system.

\\ ( http://arXiv.org/abs/astro-ph/0602033 , 387kb)

Should this be moved to the KBO thread, then?

Suddenly, the Jovian Trojans are begging for a mission...Dawn 2, anyone?

From a manned SF perspective, this could be quite significant as well in the distant future. The Jovian Trojans may have the most economically accessible supply of volatiles in the whole Solar System!

A 'Dawn 2' launched on an orbit with a perihelion at earth's orbit that aphelions quite a ways past Jupiter and that had a 4 year period from crossing Jupiters orbit outbound to inbound (sorry, not smart enough to figure out the rest of the orbit parameters), would if launched at the right time, cross 60 degrees ahead of Jupiter and 60 degrees behind.

This would give you a fair chance to look over at least 2 Jovian Trojans, and maybe some mainbelt asteroids both ways. If total period of orbit is close to an integral number of years, earth gravity could send craft on to Chiron (timed correctly) for a look at that interesting piece of real estate.

No heavy radiation shielding as we don't get close to Jupiter, but we do get to pass down range through Jupiters magnetotail 2 years after first Trojan encounter.

Let's get going!

Yeah! We could call it "TRACE": TRojan Asteroid/Chiron Explorer!

Heck, maybe this is actually a job for a New Horizons 2...seems like a better platform for multiple flybys and long-endurance transit to the outer system (gotta lose the solar arrays).

A catchy acronym is the halfway point for getting a mission off the ground!

Okay... ... here are a few attempts at that, then:

BOLD/FACE: Belt Object (Lagrangian) Discoverer/Flyby Assessment of Chiron Environment
JOLT-CE: JOvian Leading Trojans-Chiron Explorer
JAR/CARFET: Jovian Asteroid Reconnaissance/Chiron After Redirected Flyby of Earth (Tentative)

L unar
I nserted
P olar
O rbiting
T est
O bservatory
V ehicle
A ssembly
N etwork

A-D Vehicles would be intended as stress analysis missions, perhaps sponsored by JAXA.

Bob Shaw

There has already been a lot of interest in a solar-powered Trojan flyby mission -- maybe even one that flies by more than one Trojan -- which could very likely be made within the Discovery cost cap. Beth Ellen Clark of Cornell was associated with one such Discovery proposal called "Andromache", although it's hard to find anything on it.

But: there is also now a proposal floating around -- neat acronym and all -- to actually have a New Frontiers-class craft orbit one or maybe even two Trojans. It's called PARIS; it would use the new concept of a low-powered but long-duration ion drive powered by a particularly large 1-kilowatt RTG (which must use the new, more efficient future RTG designs to reduce its plutonium load); and there have already been at least two abstracts on it, the most recent being at http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1922.pdf . (This new type of propulsion system is attracting increasing interest; it would apparently also allow a relatively low-cost giant planet orbiter which could do an awful lot of putt-putting around the planet's system of moons -- including orbiting one or more of them -- after the orbiter was initially braked into orbit around the planet by aerocapture.)

Some JPL documents on the overall subject:

Trojan flybys:
http://trs-new.jpl.nasa.gov/dspace/bitstre...9/1/95-0948.pdf

Trojan REP orbiter:
http://trs-new.jpl.nasa.gov/dspace/bitstre...1/1/05-2069.pdf
http://trs-new.jpl.nasa.gov/dspace/bitstre...0/1/05-2173.pdf

someone really needs to email this to our JAXA blogger friend...
and thanks Bruce for those links.

Review: Europa, the Ocean Moon

Thu, 16 Feb 2006 - Our five senses are all we have to allow our brains to interact with the world outside our bodies. Space exploration relies almost entirely on one, the sense of sight. Space probes send us images of planets, moons and other objects which we then have to decipher as best we can.

Richard Greenberg in his book Europa, The Ocean Moon uses recent images of Europa, together with our understanding of celestial mechanics and plate tectonics, to unravel this little moon's mysteries. For Europa's biggest mystery is whether it harbours life who may be looking right back at us from their own little world.

http://www.universetoday.com/am/publish/bo...on.html?1622006

Huh... Here's another review, from Mike Belton, leader of the Galileo Imaging Team.

Excerpts:

The descriptions of how Greenberg developed,
promoted, and defended his ideas is another significant part of
the book and one that I find at times to be absurd, generally
irreverent, and, in professional terms, possibly approaching
suicidal.

...

Paranoia is not a word to be used lightly, but there is much that I
am certain is delusional in this aspect of the book. We see this
even in the preface (though there similar instances
throughout the book): “I never felt welcomed by the team…
Then, when it became clear that my field was the key to
understanding what we saw at Europa and evident how
significant those discoveries were, attempts to keep me
marginalized were driven by transparent social, political, and
financial motives.” This is absurd. The book is filled with
words and phrases like: “professional gladiators,” “powerful
infighters,” “perverted definition of ‘team’,” “sycophantic,”
“hustlers,” “enforcers,” “malignity, envy, and ignorance,”
“locked out,” “pontificating,” “jealousy,” and “animosity.”
Nowhere, as far as I can see, do the concepts on which the
everyday world of our profession is based, that is, integrity,
trust, and skeptical inquiry, get a fair shake. Even the players
who are mentioned by name are stereotyped. Those who
worked with the author are uniformly brilliant and insightful;
those who question or raise opposing ideas are to one degree
or another vilified. As I noted before: this is absurd.

Because I actually bought the book (what was I thinking ), I wrote a review for my blog last year:

http://volcanopele.blogspot.com/2005/06/re...search-for.html

Your hatred for Europa is bit disturbing.

Or is it the possibility for life on Europa that you dislike!?









Werid.

Jason:

Glad to hear you're blogging again!

And Titan is *much* nicer than Europa!

Bob Shaw

Bah. Jason just prefers orange stuff...

I imagine he's spot-on about Greenberg, though -- I have yet to see a convincing explanation from him about how Europa's ice crust could be so thin right now without any visible changes or geysers being seen by Voyager and Galileo. I think the cyclical tidal heating theory of Europan history is probably right: the thickness of its crust varies over cycles of several tens of millions of years as its degree of tidal heating changes. It last became thin enough for all the surface features from its previous cycle to be totally erased roughly 30-60 million years ago, and since then it's been gradually chilling and re-thickening -- more in some regions than others, which has allowed Pappalardo-type solid-ice diapirs to form up to this time only in the thicker areas of its crust, where they have erased the Greenberg-style cracks and ridges formed during its thinner period.

I don't think anyone wants to hear my astrobiology rant again... I just prefer places that don't have the taint of astrobiology.

As for Europa, it is a world trying to be cool like Io but failed miserably, and gave up.

And Bruce, I've warmed up to Enceladus and it isn't orange. Neither is Triton for the most part.

I'm not sure I can explain why, but that is one of the funnier things I've read lately. I do get a little tired of the implication that 'if we could conclusively prove that there were no life in the solar system outside of Earth, anywhere, ever, then that proves the system is boring and we should just go home and forget about exploring it.' I think it would be fun to find extraterrestrial life, but I don't think that that's the whole ball game, or even most of the ball game. But then my space-related obsession is planetary geography, not exobiology.



I thought it was a world trying to be hot, like Io, but gave up?

My point is that astrobiology and the search for life beyond earth is not a goal. A goal (to me) is something that if you put enough effort toward achieving it, it can be achieved. Building a colony on Mars is a goal because if we put enough money and man-power into it, we can put a person on Mars and build a colony. Yes, it would be a lot of money, but it something that can be achieved. Finding life on another world is not a goal. If there is no life on Mars or in Europa's ocean, then no matter how much money you pour into achieving that goal, you can never succeed, no matter how hard you try. My other beef is that it is used to sell missions to congress and the public far more often than it should.

So I essentially agree with you, while finding life on another world would be great, we, as a community, should be careful how much we advertise astrobiology, lest it become the noose we hang our selves with.

I don't want to hijack this thread any more than I already have so I'll stop there.

true

We still have to "discover" what the rocky surface of Europa looks like. It might be Io, but smoking into an ocean instead of spraying into a vacuum.

The map of Europa's "other" surface will be interesting to see, sometime in the 23rd century when we have it.

Interesting turn of a phrase. I didn't know searching for life beyond Earth or even
just thinking about it was an abomination to science. Just like the astronomers of
the early Twentieth Century who focused on distant galaxies for "respectability".

When extraterrestrial life is found, the bandwagon will be practically crushed by
all the scientists who won't hesitate to jump on it. Just like with exoplanets,
which were ignored and sometimes outright put down as rarities by many
mainstream astronomers until they were proven in 1995.

My problems with astrobiology have nothing to do with my opinions on whether there is or is not life on other worlds in this solar system. In fact I think it is quite possible on Europa, a slightly more distant possibility on Mars, and maybe a few other places. I just have a problem with using astrobiology as a rationale for missions that will not actually search for life. I have a problem with using astrobiology to have some future missions jump to the front of the line while missions to non-astrobiologically interesting worlds, but are important for other reasons, are subjugated to the back.

All I am saying is that we shouldn't put all our eggs in the astrobiology basket. Okay, I have really steered this thread in a wrong direction. If someone wants to start a thread in Policy and Strategy, I'll join in.

John that's a rather pessimistic view of our future in planetary exploration. Certainly we'll have a spacecraft that can penetrate the ice on Europa this century.
Are you saying this because of the pressures at the bottom of this deep ocean would be immense? How much would you estimate the pressure would be given the depth of this ocean but Europa's weaker gravity?

- Bob Clark

I actually agree with you in certain cases here. As much as I want us
to find extraterrestrial life, I also think the worlds of our Sol system
certainly justify exploration and study in and of themselves, whether
they have life or none. I would not want any place left untouched
just because someone thinks it may not have organisms.

Case in point: The Viking mission. They were actually called failures
by some because the landers did not find any conclusive evidence of
Martian life. The whole point was that they went there to see if the
planet had life, not to confirm what some thought they already knew.

Both landers and orbiters made it to the Red Planet intact and returned
information and images that has only been surpassed in the last few
years. Hardly a failure just because little bugs didn't dance around in
the biology equipment saying Here We Are! And thanks in large part
to the Vikings, we have a much better idea of how and where to really
look for Martian life.

I know I am preaching to most of the choir here, but I felt it had to
be said.

JOVIAN DREAMS

- New Recipe For Oxygen On Icy Moons

http://www.spacedaily.com/reports/New_Reci..._Icy_Moons.html

Richland WA (SPX) Mar 27, 2006 - Researchers at Pacific Northwest National
Laboratory said they have uncovered the most detailed picture to date of how
oxygen could be manufactured on water-rich but frigid moons in the outer solar
system.

Outer irregular satellites of the planets and their relationship with
asteroids, comets and Kuiper Belt objects

Authors: Scott S. Sheppard (Carnegie Institution of Washington)

http://arXiv.org/abs/astro-ph/0605041

What a belated reply on my part! I lost track of this thread, obviously.

Anyway, with the water+ice depth being 100 km, Europa's seafloor isn't going to be mapped from orbiting spacecraft to any but the crudest degree (like the identification of mascons).

Supposing a craft did penetrate the ice and ocean, the chore of mapping more than a tidbit of the surface would be gargantuan. And returning results to the surface (and on to Earth) yet more difficult. I imagine a straight-shot descent from a hole to the seafloor could be done this century (although I can't prove it... and note that "century" doesn't look so long when you see that the delay between Voyager 2 and the *second* subsequent Europa mission from then (with Galileo being the next subsequent) is going to be perhaps 40 years.

So, when (if) a submarine does make the trip, an obvious, yet still dazzlingly difficult, mission scope would be to descend straight down, communicate via something left in the melt-path all the way through the crust, and to have its lateral mobility (easy in water) made irrelevant by the need to keep the craft near the antenna. I suppose it could wander a circle 10 km in radius or so, but that's still a small pinpoint of Europa's area. And -- this is painful, light would be nonexistant, so it's not like it could "fly" over the ocean floor and map away from an altitude of several km -- it would have to get very close to the floor and map with sonar (semi useful) or VERY close and map a room-sized area at a time with a flood lamp.

Now imagine how many submarines it would take to cut lots of razor-thin swaths around Europa, recording their flood-lamp-lit swaths, and beaming the results back to Earth (but through the ice: apparently impossible).

At this point, several uber-expensive solutions come to mind (a massive network of communication antennas in the ice?)... c'mon, this isn't going to happen. It almost seems like it would be easier to develop a flyby craft for exploring other stars than it would be to produce a global map of Europa's seafloor of anything like the resolution and coverage of the maps we have of, geez, even Pluto.

Does the military (or a military) have the capability of communicating
through thick ice when their submarines are, say, under the Arctic ice
pack, or is it all hush-hush?

I recall a line from the novel version of Tom Clancy's The Hunt for
Red October how whatever the military designs in secret the public
sees about twenty years later.

Naval surface to subsurface radio comms using ELF (Extremely Low Frequency) transmission was around and not very secret since the late 60's. It can\could penetrate up to few hundred feet of water and possibly ice but it's technical limitations are severe.
The US version used a carrier frequency of 76hz. Data rate was around 3 minutes per character (!). The effective transmitter antenna length needed to be somewhere in the 200km range although the physical installations used were in the 10's of km range and took advantage of local geology to be efficient enough to work.
The biggest problem with it is that it was a one way communication channel - that probably had something to do with the 200km transmitter antenna - and was only ever really used as a "bell ringer" to alert subs so they could then manoeuver to a location where two way comms was possible.
Some relevant info here - can't vouch for the site but the pdf docs look fairly official.
It was fully decomissioned a couple of years back presumably because the neutrino comms system had finally come on stream allowing high speed DTE\DTW* communications.

*Direct Through Earth\Direct Through Water. **

** This is a joke.

Radio/radar through water is heavily dependent on whether the water is fresh or not. Salt water is a fairly good conductor which means that radio waves are quickly absorbed (the decline is more or less exponential), normally the penetration depth under such circumstances are only a small fraction of the wavelength. This is the reason for ELF. Even a small fraction of a 76-Hz wave is quite a long way. Ordinary LF ("Long-wave") can be used for communicating with submarines, but only if they are close to the surface ("periscope depth").
As for the arctic pack ice I think that ordinary LF would probably pentrate to a sub that was just under it if you used a strong transmitter.

In principle things work the same for ice, but penetration is better than for water. That's why the MARSIS scientist feel fairly sure that there is no water layer under the polar ice. It would presumably be at least somewhat salty and thus show up as a more or less impenetrable layer.

As for Europa a strong radar might be able to penetrate the ice if it is not too thick, but almost certainly not an underlying ocean (which would be salty). That would require seismic measurements. i. e. set up a network of seismometers and then set of a lot of bangs, or you might perhaps use a rover/hopper with some kind of a "thumper".

tty

I'm not so sure you would need any kind of artificial seismic inputs (thumps, bangs, or impacts) to study Europa seismically. I once read an analysis of Europa's position in the Jovian tidal sequence that predicts Europan surface movement on the order of hundreds of meters per orbit. Whether the surface is a thin or a thick layer of ice over a subsurface ocean, it *must* flex to some degree under the tidal stresses.

That flexing would provide sufficient seismic energy that a set of passive seismometers ought to be able to get a lot of internal structure information, I would think...

-the other Doug

Yes, but you need a minimum of three seismometers operating simultaneously, and even then there would be a great deal of ambiguity in the data. You can get much more precise results when You know exactly when and where the energy release is.

tty

Did Galileo ever detect any surface shifts? Are comparisons between
the Voyager images of Europa and Galileo years later possible?

For triangulation, yes. But a single geophone would be a damned sight better than nothing. My guess is that it would produce something useful.

There's been a HUGE amount of discussion at science working groups over how much a single seismometer (including a short-lived one) could tell us about Europa. The consensus at this point is that it would be worthwhile -- at least to tell us the overall seismic activity background to design a better seismometer later, and maybe for additional information as well. Nicholas Makris says that measuring the speed at which seismic waves of different frequencies arrive could probably by itself tell us how thick the ice crust is.