And if you're thinking of actually stopping at Pluto and taking your dog out for a walk, you'd have to do velocity matching, and with Pluto's poor excuse for an atmosphere, you couldn't do aerocapture to bleed off your hyperbolic excess velocity. On the other hand, heliocentric velocities are low, so it might not be much delta-V.
-John Sheff
Cambridge, MA
jsheff@comcast.net

[quote name='ugordan' date='Sep 6 2006, 04:20 AM' post='66909']
This writeup by Bruce Moomaw was shamelessly copied from the Cassini-Huygens Yahoo group. I hope noone objects (John Sheff ?) posting it here as well because it contains some very interesting new bits of info.

-----
I'm not in a position to object, since I myself shamelessly copied it from the excellent Jupiter list. I'd like to credit Bruce Moomaw as author, and while we're at it, here's an earlier post Bruce made about the upcoming DPS meeting:

John Sheff
Cambridge, MA
john@jsheff.com

My initial scan of the abstracts for the upcoming meeting of the DPS (
http://adams.dm.unipi.it/~dps06/ ) indicates that -- while there are quite
a few interesting subjects covered -- the star of the show is likely to be
Titan. Specifically, the fact that the prolonged analysis of the data from
Cassini's VIMS -- which can only view the surface through the half-dozen or
so narrow spectral "windows" allowed by Titan's methane atmosphere -- is
nevertheless finally starting to reveal some fascinating (if sometimes
contradictory) information on that world's surface composition, which is the
subject of a whole series of abstracts, especially in conjunction with the
data on Titan's upper-atmospheric chemistry from Cassini's mass
spectrometer.

First, the Cassini VIMS and INMS teams, in the "Titan Surface 1" session,
combine to solidly confirm something already hinted at by Huygens' GCMS
analysis of the vapors near the surface after its landing. Three definite
spectral features are visible to the VIMS by peeping through the window
around 5 microns. One simply seems to match liquid methane and/or ethane.
Another, while as yet not nailed down, somewhat resembles that from
acetonitrile (CH3CN), suggesting that it may be some other nitrile (more on
this in a moment). But the third seems beyond doubt to be due to really
large amounts of solid benzene on Titan's surface -- something indicated by
Huygens' clear detection of benzene vapor there despite the fact that the
stuff has a boiling point as high as +80 C!

This, it turns out, meshes with the fact that Cassini's INMS, during its
Titan flybys, is detecting amounts of benzene in Titan's upper atmosphere
greater than predicted by "orders of magnitude" (along with toluene, which
is just benzene with one of its hydrogen atoms replaced by a methyl group
[CH3 ]). See also the papers by Waite and Vuitton in the "Titan Atmosphere
4" session. Moreover, the VIMS spectra are NOT showing any solid acetylene
on Titan's surface, which was expected to exist in large amounts; apparently
the upper atmospheric reactions are creating the benzene instead. Sure
enough, a poster by Imanaka ("Titan Atmosphere 2") shows that in a
ground-based simulation, solar UV does indeed produce very large amounts of
benzene and toluene out of a nitrogen-methane atmosphere. (Since benzene
rings have also combined in previous similar simulations to form PAHs --
"polycyclic aromatic hydrocarbons" -- it is, I think, possible that the
latter may be the mysterious stuff, getting steadily darker from .83 to 1.42
microns, that was revealed by Huygens' floodlight-lit near-IR spectra of
Titan's surface just before and after landing, which seems to be mixed with
water ice and tholins there and doesn't match anything yet identified in a
lab spectrum.) The new abstract adds that "All three absorption features
are primarily associated with dark materials on the surface of Titan;
benzene specifically maps in channels, 'lakes,' and boundaries of bright and
dark terranes" -- just as you'd expect if it and the nitrile are getting
washed off of higher-altitude terrains by methane rain.

As for that "nitrile" (a carbon-hydrogen-nitrogen compound) that VIMS also
seems to be seeing on Titan's surface: Cassini's INMS spectra also revealed
amounts of cyanogen (C2N2) and cyanoacetylene (HC3N) comparable to the
benzene, and Huygens also sniffed a whiff of cyanogen vapor on the surface
despite its boiling point of -20 C -- still tremendously higher than
Titan's surface temperature. (HC3N is only 1 AMU different in mass from
C2N2, and Huygens's mass spectra may thus actually have been seeing both of
them together.) Vuitton's DPS abstract also reports that the INMS saw CH5N,
but there's no sign of this in Huygens' mass spectra, nor have any of the
previous articles on cassini's upper-atmospheric mass spectra mentioned
it -- could this be a misprint? Finally, there's another odd possibility:
on sifting through my old papers I've found a November 1987 "Icarus" article
by Thomas Jones and John Lewis mentioning that, if Titan's nitrogen really
was made out of its original supply of ammonia by solar UV at its beginnings
(as is now believed), large amounts of frozen hydrazine have often been
predicted as a side product.

So far, so clear. But now we come to the intriguing puzzle: Larry Soderblom
will be delivering a talk (in the "Titan Surface 2" session) repeating
something he's been saying at conferences for a year, on which Thomas McCord
agrees with him:

"Titan's vast equatorial fields of longitudinal dunes seen in radar images
correlate with one of two dark surface units discriminated as 'brown' and
'blue' in color composites (RGB as 2.0, 1.6, 1.3 microns) of near-IR
spectral cubes. [These color names are purely symbolic; they just mean that
the 'blue' surface areas are lighter-colored in the relatively short
1.3-micron wavelength than the 'brown' regions -- Moomaw.] Earth-based
spectroscopy shows a surface consistent with dirty water ice; VIMS data show
more evidence of water ice in darker than brighter units (McCord et al.
2006). Our work shows that relative to the VIMS dark blue unit, the albedo
of the dark brown unit is lower at 1.3 microns, higher at 2.0 microns, shows
less evidence of water ice, and correlates with the radar-dark dunes. This
suggests that the dunes are dryer, higher in hydrocarbon or nitrile
composition.

"VIMS bright units show even less evidence of H2O, and are inferred to
consist of very fine tholin dust. If the rate of deposition of hydrocarbons
is ~0.1 micron/yr (Yung et al. 1984), the surface would be coated
(optically) in a few years unless cleansing processes are active. The dunes
must be mobile on this timescale to prevent accumulation of bright coatings.
Likewise, fluvial/pluvial processes every few decades must be cleaning the
dark floors of the incised channels and dark scoured plains at the Huygens
landing site. In this model Xanadu is a large inactive region where eolian,
fluvial, and pluvial activity is currently at a low ebb." (Soderblom adds
that Huygens landed in one of the "dark blue" areas which do contain a fair
amount of water ice along with tholins, which nicely matches its own near-IR
surface spectra. See http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1326.pdf
for more on this.)

In short, Soderblom and McCord stand one of our common beliefs about Titan
on its ear -- they think the light-colored regions, especially Xanadu, are
NOT water ice washed clean of accumulated tholins by methane rain. Instead,
they think that these regions are precisely the parts of Titan's water-ice
surface that have not been swept clean by either winds or methane rain for a
very long time, and are thus still coated with some accumulation of very
slowly falling unidentified tholin (light-colored in the near-IR).
Soderblom will say this again in the Cassini radar team's talk on Xanadu in
the "Titan Surface 2" session.

But what is this mystery tholin? And why isn't the falling solid benzene
powder following the same distribution pattern? And the assumption about
Xanadu has always been that it is a high-altitude water-ice region swept
clean of falling dark materials by liquid methane and/or winds -- an
assumption backed up by VIMS' altitude maps of Titanian terrain made by
measuring the amount of atmospheric methane above that part of
Titan(http://www.aas.org/publications/baas/v37n3/dps2005/271.htm ).
Moreover, if it is high-altitude, Titan's winds up there are considerably
higher-speed.
(Cassini's radar maps of it suggest that it is a previously low-lying region
that underwent a lot of rainfall and fluvial erosion, and was then gradually
raised to a higher altitude by a huge underlying mantle plume or diapir,
like Mars' Tharsis bulge.)

All this suggests strongly that Soderblom and McCord are wrong, and that
Titan's VIMS-bright areas are not regions that have accumulated some
mysterious light-colored tholin powder. But, when you look at their
spectra, it's very hard not to agree that the bright regions -- whatever
they may be -- are most definitely not bare water ice. See McCord's
comparison of the spectra of water ice and standard-type tholins with
Cassini's measurements of the albedo of the bright regions visible through
the various methane spectral windows at
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1398.pdf -- this stuff is
clearly brighter than water ice at 5 microns and MUCH brighter than water
ice at the 2-micron wavelength where water ice is quite dark, and a mixture
of known tholins doesn't change this.

So what are we looking at in Xanadu and the other VIMS-bright regions of
Titan? Could it possibly be that Titan's cryovolcanism -- which was
predicted, even before Cassini's arrival, to resurface the moon at 200 times
the extremely slow rate at which organic smog falls onto the surface, and
which now appears to be even more active than had been thought -- has been
burying the slowly falling smog throughout Titan's history and exposing most
of it to a mix of liquid water and ammonia which has chemically changed it
into entirely new types of compounds that are now mixed into Titan's frozen
surface water ice, radically changing the ice's IR spectra? Carl Sagan and
W. Reid Thompson, back in the early 1990s, mixed the predicted Titanian
tholins with liquid water and got "amino acids...nucleotide bases, PAHs and
a wonderful brew of other compounds." And Somogyi and Smith, in a poster at
the upcoming DPS meeting's "Titan Atmosphere 2" session, will report similar
reactions occurring very fast to produce a fascinating range of complex
prebiotic compounds. (To complicate things still more, McCord will declare
in the "Titan Surface 1" session -- the same one at which the VIMS team will
describe their spectral detection of benzene and a nitrile on Titan's
surface -- that his own analyses of Cassini's VIMS spectra reveal "no
spectral features above the intrinsic noise level in any of the methane
windows". There is likely to be a very interesting fight at that session.)

There will also be another interesting twist: at the "Titan Surface 1"
session, Jason Barnes et al will present a global map of Titan's surface
composition as indicated by VIMS. "The equatorial regions are a vast, dark
dunefield intersperced with brighter material including unique Xanadu; the
mid-latitudes are bright, but spectrally distinct from the equatorial
brightlands; and the south pole is again a mixture of bright and dark, with
the different dark material than that near the equator." The latter just
might be connected to still another VIMS observation to be described by
Caitlin Griffith in the Titan Atmosphere 3" session:

"We present spectra from Cassini's VIMS, that reveal the presence of a vast
tropospheric cloud at latitude 51-68 north and all longitudes observed...
Methane clouds are not expected at these latitudes, where the general
circulation is predicted to transport dry air from the stratosphere to the
troposphere, which thereby becomes subsaturated. The derived characteristics
of the cloud indicate instead that it is composed mostly of ethane. The
cloud forms as a result of stratospheric subsidence. Ethane undersaturated
above ~65 km is transported to 30-50 km altitude, where its mixing ratio
exceeds saturation by a factor of several hundred. This explanation implies
that we are seeing the edge of a massive polar ethane cloud, and the
preferential condensation, sedimentation and surface accumulation of ethane
(and other photochemical products) within 35 degrees latitude of the north
pole. The polar accumulation of ethane, perhaps as ice, may partially
explain the lack of liquid ethane oceans on Titan's surface at middle and
lower latitudes. The sedimentation of material predominately at Titan's
poles further suggests a polar surface composition and geology that differs
from those at lower latitudes." (Meanwhile Sushil Atreya, in his article
"Titan's Methane Cycle" in an upcoming issue of "Planetary and Space
Science", proposes instead that the processes in Titan's upper atmosphere
may manufacture a lot less liquid ethane than had previously been thought,
and a lot more of other solid organics instead . Maybe that benzene is
substituting for it as well as for the expected acetylene.)

Finally, there will be two talks by Wood and Mitchell in the "Titan Surface
2" session on the north-polar methane lakes found by Cassini on its latest
radar pass -- and specifically on the fact that they now look very much like
a dense accumulation of volcanic calderas, many including multiple rings
around the lake rims suggesting multi-stage collapses. "The concentration
of dozens of possible caldera volcanoes in this northern region of Titan
suggests the existence of an extensive hot spot region of heat loss. This
differs from the other 8% of Titan so far imaged by radar where volcanic
features are infrequent and relatively isolated... If volcanic and
lacustrine environments are concurrent, then geothermal systems rich in
organic materials may provide suitable conditions for the emergence of
life." And Robert Nelson, in the same session, will once again describe the
sudden change in the near-IR appearance of one patch of Titan's surface,
suggesting a cryovolcanic eruption in the last two years. Combine all these
talks with that fact that there will be a lot of others on the subject of
Titan's surface composition (whose conclusions are not yet stated in the
printed abstracts) at the Titan sessions of this DPS meeting, and it
promises to be fascinating for that reason alone, let alone the multitude of
other subjects to be covered at the meeting. I hope to attend.

Would this process also explain the albedo assymetry on Iapetus?

I don't have a problem with roundness as a criterion, but:

1) why didn't they define an upper limit as well? The phrase "is not a star ..." is insufficient. Has the word "STAR" been defined somewhere else by the IAU? They could have said the upper limit is when a body is capable of initiating a thermonuclear reaction in its core, or that its 13 Jupiters, or whatever. Just saying "is not a star" is a cop-out.

2) They defined a planet, for better or worse. They should have left it at that! It seems most people are objecting to the "double-planet" phrase; personally I'm confused as to why they felt the need to define a "pluton". There are already definitions for things like ""plutinos", or "cubewanos", or "Kuiper objects" or "extended scattered disk" that are already in use by informal convention, and it's been working fine; there has not been any controversy about that.

3) Er, need I state the obvious? ... a planet is a "celestial" object. What does "celestial" mean? In common usage it is used to distinguish something that's not earthly. So doesn't this definition exclude Earth? (We're back down to 11 planets, LOL!)

- John Sheff
Cambridge, MA

Results are in:

IAU

Right you are, Emily. On p. 155 of his book he discusses karst terrain in which - on Earth - water dissolves limestone and creates caves, arches, and sinkholes. The idea was that methane and other hydrocarbons would dissolve the water ice on Titan. "The idea was neatly self-consistent in that liquid methane would create the cave systems in which it would hide". Unfortunately, it was based on the assumption that methane dissolved water - a conclusion borne out in only one lab experiment and then retracted and discredited.

This doesn't mean there are no caves on Titan - just that they're not created by this meechanism.



- John Sheff
Cambridge, MA

Correct me if I'm wrong, but I don't think anyone had predicted that we'd find caverns on Titan (if indeed they exist! ).

So far we've been thinking of Titan, basically, as a 2-dimensional world to explore. If this interpretation is correct, Titan might have an inside as well as an outside worthy of investigation.

Yes, whoever does that is an unsung hero.

Notice the tiny black dot in the upper left of the crescent? In a similar image released last week
Ciclops such a dot was identified as Epimetheus. I wonder if that's what it is here or perhaps another satellite?

A Tale of Two Moons:

http://ciclops.org/view.php?id=1841


Three Moons Meet:

http://ciclops.org/view.php?id=1784


Staying with Epimetheus:

http://ciclops.org/view.php?id=1788


The Silent Spheres:

http://ciclops.org/view.php?id=1840


Rhea Occults Saturn:

http://ciclops.org/view.php?id=1962


Cruising with Pan:

http://ciclops.org/view.php?id=1982



I think Arthur C. Clarke will appreciate these clips!
- John Sheff

The Cassini team has composed a tribute to Paul McCartney.
Caution! The file is 147 MB in size. Right-click on it and do a "Save Target As ..." to your local machine. Even using a broadband connection, it is a very long download!
- John Sheff
Cambridge, MA


A SATURNIAN MUSICAL CELEBRATION TO HONOR PAUL MCCARTNEY

MEDIA RELATIONS OFFICE
CASSINI IMAGING CENTRAL LABORATORY FOR OPERATIONS (CICLOPS)
SPACE SCIENCE INSTITUTE, BOULDER, COLORADO
http://ciclops.org
cpcomments@ciclops.org

Preston Dyches (720) 974-5859
CICLOPS/Space Science Institute, Boulder, Colo.

Image Advisory: June 18, 2006


A SATURNIAN MUSICAL CELEBRATION TO HONOR PAUL MCCARTNEY

On the occasion of Paul McCartney's landmark 64th birthday, the Cassini
Imaging Central Laboratory for Operations (CICLOPS) is releasing today
an 8-minute movie as a birthday gift to the former Beatle. Sixty-four
of the most dramatic and spectacular images taken by NASA's Cassini
spacecraft, including one mosaic from the European-built Huygens probe
of the surface of Titan, are composed together in a cinematic voyage
through the Saturn system and put to the music of the Beatles.

"In their creation of new musical forms and directions, and in their
expansive vision of the art of popular music, the Beatles reached
heights of achievement in their brief time together that nobody has been
able to surpass", said Carolyn Porco, Cassini imaging team leader and
the producer/director of the movie. "And being a major element in that
incredible story makes Paul McCartney, in my mind, one of planet Earth's
brightest stars. It makes me very happy to be able to celebrate him in
this way. I hope he likes our movie."

The movie `Sixty-four Sights from Saturn' is available today on the home
page of http://ciclops.org. High resolution versions may be obtained
by emailing cpcomments@ciclops.org.

The Cassini imaging operations center (CICLOPS) is based at the Space
Science Institute in Boulder, Colo.

-end-

Thanks, Alex. I don't have access to the entire paper, but how can they deduce the following?

"Evidence suggests that the Neptune Trojans are more numerous than either the main asteroid belt or the Jupiter Trojans,"

That's a pretty significant statement. There are tens of thousands of asteroids in the main belt. Does this imply there are at least that many Neptune Trojans?

- John Sheff
Cambridge, MA

And, lest we forget, there's the ring that Cassini discovered shortly after SOI: S/2004 1R. I know it was announced to be 138,000 km out from Saturn's center. That would put it between Atlas and Prometheus, and between the outer edge of the A ring but interior to the F ring. I haven't heard much more about it since. Does anyone know if this is still considerd a distinct ring?

Here's another idea:
As someone here previously mentioned, the problem limiting power output from a nuclear fission reactor is operating temperature. Power is proportional to temperature, but beyond a certain temperature, the core melts.
There has been an idea advanced called the gaseous-core fission reactor, where the core is undergoing fission in a plasma state, so operating temperature is no longer a limitation. I remember reading a book by a propulsion engineer named Hunter back in the sixties (which I wish I could find again), and he worked out some of the performance aspects of such a reactor. You'd have the uranium in the form of a plasma that reaches criticalilty inside a chamber with "glass" walls, that it does not come into contact with. Instead the plasma simply reaches criticality and then gets exhausted out the back of the ship before it comes into contact with anything it can melt. But unlike in Orion, the fissioning material does not do the pushing. Instead - and here's the key point: the plasma heats fuel in a surrounding chamber by the radiative flux going through the "glass" walls. The heated fuel then exhausts out the back (along with the uranium plasma) and produces thrust.
He calculated that such a ship would be capable of speeds of 100,000 - 1,000,000 feet/second. It could do 1 AU (Earth-Sun distance) in about 11 days. At such speeds you don't need to travel along a long curving Hohmann trajectory to reach another planet, and you don't have to wait for the planets to come into position. The trajectory of the ship is esssentially a straight line from one planet to another. Such a ship may still be way too slow to travel to Alpha Centauri, but it would open up the solar system to commerce. Pretty cool.

That's very cool! Thank you. I've used Celestia before but not to the extent of simulating a flyby. Now that I know it can be done, I'm going to use it.

- John Sheff

I know how horrendous the delta-vee requirements are for landing on Mercury, but they're not, even with present technology, impossibly high. If you remember, the lander portion of Beppi-Columbo was not nixed for technological reasons - it was simply deemed too expensive for the program's budget!

I agree that manned landings, when they happen, will probably not occur within our technological horizon, i.e., this century, and only as a "mopping-up" exercise after the rest of the solar system has been thoroughly explored.

I remember as a kid reading a SF novel by Alan E. Nourse, called, I think, "Brightside Crossing" (You might want to look it up, Rem31, if you can find a copy; it might answer your questions). It may even have been written before Mercury's true rotation period was known. His characters mounted a surface-crawling manned expedition to traverse Mercury's dayside, timed to arrive at the center right when the planet was at perihelion! The expedition was mounted not for the sake of science, but for the glory, as it was "the last great challenge left in the solar system". This was science-fiction, I know, but I wonder ...

Today you have people willing to pay $60,000 and put their lives at great risk to climb Mt. Everest. They do it not for science, nor for the sake of being as high up as they can. (You can, after all, get higher in a aircraft or spacecraft!) They do it for the sheer challenge of it, "because it was there". So the fact that Mercury is, as you say, the most difficult place in the solar system to get to, may not repel people, but may be precisely what makes it an irresistable draw for some. Funny things, these humans...

As I recall, Mariner 10's discovery of a magnetic field at Mercury was something of a surprise. That, plus the more accurate determination of the planet's density (which turned out to be much higher than all but Earth's) provided by Mariner 10's measurement of the size and mass of the planet, led scientists to posit a large iron core for Mercury. That alone is worthy of study, and while Messenger and BepiColumbo orbiters will constrain the models of the planet's interior, there's nothing like a seismic network of landers to really study the planet's geology. A number of people posting here have suggested that such a network could be emplaced at the poles or at high latitudes, but I see nothing far-fetched about a low-latitude seismic network of landers. All the probes would have to do is land early in the Mercurian night, as others have suggested, and dig into the regolith a few dozen meters! (We are already developing automated drilling technology for Mars exploration.) There is no need for the spacecraft to be "cooked"; a few meters down, there is bound to be a benign temperature regime. Such landers could function for a very long time, whether powered by RTGs or some sort of suitably-hardened solar panels. (And if the latter are developed, SEP would make even the daunting task of reaching and orbiting Mercury not so far beyond present technology, I would think.) But the automated drilling is an enabling technology, no?

- John Sheff

Have you seen these stunning descent images from Huygens?

Huygens views

Enceladus just jumped to way near the top of the list of biologically interesting places. We know how to get to Mars; we some ideas of further exploration of Euiropa and Titan. Has anyone designed a mission to land on Enceladus? What would it take? How much delta vee?