Interesting Titan stuff at the upcoming DPS meeting Options

[ugordan]

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.

-----

In addition to the really fascinating- looking new information that will be
released at the upcoming DPS meeting ( http://adams.dm.unipi.it/~dps06/ )
on Titan surface composition data from Cassini's VIMS, there are several
interesting- looking abstracts on its radar data:

(1) Karl Mitchell and Charles Wood have two talks at the "Titan Surface 2"
session on Thursday on the north polar lakes found on the T16 flyby,
suggesting that many (though not all) of them are inside cryovolcanic
calderas. "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." And: "If volcanic and lacustrine environments are concurrent,
then geothermal systems rich in organic materials may provide suitable
conditions for the emergence of life."

(2) In the same session, Randoph Kirk reviews the T13 radar mapping of
Xanadu: "Morphologically, Xanadu is populated with ubiquitous, closely
spaced hills ~5 km across, which locally form chains and appear to be
dissected by numerous channels and low areas filled by radar-dark sediments.
Radarclinometry indicates typical hills are at least 500 m high, but the
results are asymmetric, strongly suggesting that the foreshortened bright
slopes are unresolved. If so, the hills are ~1000 m high with 30 degree
slopes. In either case Xanadu contrasts strongly with the rest of Titan,
where topographic features are rare and mostly <300 m high... We therefore
hypothesize that Xanadu was formed by an initial period of compressive
tectonism and cryovolcanism that triggered the erosion that sculpted the
rugged surface." Sounds less like Mars' Tharsis bulge than I previously
thought, and more like Venus' Ishtar Terra -- commonly thought to be a patch
of surface crust shoved together by a local convergence of underlying mantle
convection currents (like scum wrinkling up together above the drain on an
emptying bathtub. But that would still mean that it's higher-altitude now
than the rest of Titan's surface.

(3) According to Essam Marouf (in Wednesday's "Titan Atmosphere 4"
session), Cassini's radio occultations during its T14 flyby last May showed
clear specular radio reflections off its surface: "The echo appears
consistent with reflection from localized hydrocarbon liquid regions
embedded in mostly nonspecularly reflecting terrain." No description from
the abstract, though, of just what parts of Titan's surface were covered by
this study.

(4) Back at the "Titan Surface 2" sesssion, Jani Radebaugh summarizes the
conventional conclusions regarding Titan's huge networks of longitudinal
dunes, including the idea that they have heights of about 100 meters. But
in a "Titan Surface 3" poster the same day, Flora Paganelli presents an
alternative appraisal of Cassini's dune observations which is a real
eyebrow-raiser.

To wit: Before the T16 flyby, all of Cassini's dune-detecting radar flybys
had involved the craft flying over Titan's surface roughly parallel to the
dunes' long axes -- that is, with its SAR radar beam (which slants off to
one side of the craft) perpendicular to the dunes' long axes, so that it was
assumed that the "light" parallel streaks in the images were from that side
of each dune that tilted toward Cassini and thus reflected back a brighter
radar echo, while the "dark streaks" were the opposite sides of the dunes
that tilted away from Cassini and its radar beam. But during the T16 flyby,
for the first time it flew roughly perpendicular to the dunes' long axes --
and damned if the light and dark streaks didn't show up on the radar with
just as much clarity as during the previous dune observations. This raises
the serious possibility that the "dunes" aren't really dunes at all, with
surface slopes -- but instead "might be superposed streaks with none or
minimal topography, and that they are visible because of differential
erosion between the radar-bright rougher substrate and the radar dark of
fine-particle smooth surface deposits." In short, Titan still retains its
ability to keep totally surprising us.

(5) No less than three entirely new, and very promising, new techniques
have been developed to utilize Cassini's radar for Titan surface mapping in
ways it wasn't originally designed for -- all of them described in "Titan
Surface 3" posters. First, Richard West and Ralph Lorenz discuss the new
"High-SAR" technique in which the radar's scatterometer data is acquired and
processed in new ways to generate genuine and very useful SAR images at far
greater distances from Titan than in its standard SAR mode (albeit, of
course, at lower resolution). So far it's been used at distances ranging
from 11,000 to fully 37,000 km from Titan, with resulting resolutions
varying from 1.5 to 5 km. (The closest of these observations was a second
SAR inspection of Huygens' landing site.)

Second, Lauren Wye describes another way to utilize the radar's
scatterometer data for imaging at 15-km resolution over far bigger areas
than the High-SAR mode (and even obtaining some data on surface
characteristics that neither regular nor High SAR can achieve). Finally,
Bryan Stiles describes how very precise monitoring of Cassini's position and
attitude allow the center of its SAR swath to be used for radar altimetry,
albeit at lower resolution (50 km horizontal and 200 meters vertical) than
the nadir-pointing altimetry for which the system was originally designed.
Sine the latter type of altimetry can't be done at the same time that a
region is SAR-mapped, and since it's become crystal clear that altitude maps
of surface features are absolutely crucial to make any sense out of Titan,
this is very important indeed. But then, given the extraordinary complexity
and continuing mystery of the place, this is true of any new Titan study
technique at all.

[ngunn]

Fantastic stuff - thanks for posting it. By the time we really understand Titan we'll have learned a heck of a lot about not jumping to conclusions!

[jsheff]

[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.