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Slow Weather on Titan

With a dense atmosphere of nitrogen and methane, Titan is the only moon in our Solar System that shares Earth-like characteristics in climate. But Titan’s climate, receiving one hundred times less sunlight at ten times Earth’s distance from the Sun, operates at a much slower pace. The seasons on the distant moon last more than seven Earth years, and the motion of its clouds is slow and deliberate.

We’ve had a good look via the Cassini spacecraft at the movement of those clouds, some two hundred of them being examined between July 2004 and December 2007 in a study of global circulation patterns. Summer changes to fall at the equinox in August of this year. We’re at a time when the circulation models say clouds in the southern latitudes should have already disappeared, but it’s clear from the Cassini imagery that many clouds remained as late as 2007.

titan_circulation

Image: This infrared image of Saturn’s moon Titan shows a large burst of clouds in the moon’s south polar region. These clouds form and move much like those on Earth, but in a much slower, more lingering fashion, new results from NASA’s Cassini Spacecraft show. Titan’s southern hemisphere still shows a very active meteorology (the cloud appears in white-reddish tones) even in 2007. According to climate models, these clouds should have faded out since 2005. Credit: NASA/JPL/University of Arizona/University of Nantes.

Sebastien Rodriguez (University of Paris Diderot), who has been working with the Cassini visual and infrared mapping spectrometer team, has this to say about the phenomenon:

“Titan’s clouds don’t move with the seasons exactly as we expected. We see lots of clouds during the summer in the southern hemisphere, and this summer weather seems to last into the early fall. It looks like Indian summer on Earth, even if the mechanisms are radically different on Titan from those on Earth. Titan may then experience a warmer and wetter early autumn than forecast by the models.”

Rodriguez’ comment reminds this jazz buff of the lush Woody Herman tune (with lyrics by Johnny Mercer) called ‘Early Autumn.’ I’ve always loved the Stan Getz version of this standard, but I’m listening to Jo Stafford’s rendition as I write, enjoying the juxtaposition of a jazz classic and imagery from an exotic moon around a ringed world. One of the pleasures of digital music is being able to pop up favorites on a whim.

Cassini’s extended mission will run until the early autumn of 2010, which will offer plenty of opportunities to monitor climate change on Titan — the spacecraft makes its next flyby of the moon on June 6. So expect to learn much more about sluggish weather on Titan, including whether it’s the result of a slow rate of temperature change at the surface and in the low atmosphere. Until then, I can’t help thinking that an autumn that lingers is something Johnny Mercer would have appreciated.

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  • ljk June 23, 2009, 13:24

    Saturn satellites as seen by Cassini Mission

    Authors: A. Coradini, F. Capaccioni, P. Cerroni, G. Filacchione, G. Magni, R. Orosei, F. Tosi, D. Turrini

    (Submitted on 22 Jun 2009)

    Abstract: In this paper we will summarize some of the most important results of the Cassini mission concerning the satellites of Saturn. Given the long duration of the mission, the complexity of the payload onboard the Cassini Orbiter and the amount of data gathered on the satellites of Saturn, it would be impossible to describe all the new discoveries made, therefore we will describe only some selected, paramount examples showing how Cassini’s data confirmed and extended ground-based observations.

    In particular we will describe the achievements obtained for the satellites Phoebe, Enceladus and Titan. We will also put these examples in the perspective of the overall evolution of the system, stressing out why the selected satellites are representative of the overall evolution of the Saturn system.

    Comments: 34 pages, 10 figures, to appear on the special issue of Earth, Moon and Planets for the Elba workshop

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:0906.4064v1 [astro-ph.EP]

    Submission history

    From: Diego Turrini [view email]

    [v1] Mon, 22 Jun 2009 16:55:05 GMT (2046kb,X)

    http://arxiv.org/abs/0906.4064

  • ljk June 28, 2009, 22:22

    DNA Nucleobase Synthesis at Titan Atmosphere Analog by Soft X-rays

    Authors: S. Pilling (PUC-Rio), D. P. P. Andrade (PUC-Rio), Alvaro C. Neto (Unicamp), R. Rittner (Unicamp), A. Naves de Brito (LNLS)

    (Submitted on 19 Jun 2009)

    Abstract: Titan, the largest satellite of Saturn, has an atmosphere chiefly made up of N2 and CH4 and includes traces of many simple organic compounds. This atmosphere also partly consists of haze and aerosol particles which during the last 4.5 gigayears have been processed by electric discharges, ions, and ionizing photons, being slowly deposited over the Titan surface.

    In this work, we investigate the possible effects produced by soft X-rays (and secondary electrons) on Titan aerosol analogs in an attempt to simulate some prebiotic photochemistry. The experiments have been performed inside a high vacuum chamber coupled to the soft X-ray spectroscopy beamline at the Brazilian Synchrotron Light Source, Campinas, Brazil.

    In-situ sample analyses were performed by a Fourier transform infrared spectrometer. The infrared spectra have presented several organic molecules, including nitriles and aromatic CN compounds. After the irradiation, the brownish-orange organic residue (tholin) was analyzed ex-situ by gas chromatographic (GC/MS) and nuclear magnetic resonance (1H NMR) techniques, revealing the presence of adenine (C5H5N5), one of the constituents of the DNA molecule.

    This confirms previous results which showed that the organic chemistry on the Titan surface can be very complex and extremely rich in prebiotic compounds. Molecules like these on the early Earth have found a place to allow life (as we know) to flourish.

    Comments: To appear in Journal of Physical Chemistry A.; Number of pages: 6; Number of Figures: 5; Number of Tables: 1; Number of references:49; Full paper at this http URL

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    DOI: 10.1021/jp902824v

    Cite as: arXiv:0906.3675v1 [astro-ph.EP]

    Submission history

    From: Sergio Pilling [view email]

    [v1] Fri, 19 Jun 2009 14:12:09 GMT (1052kb)

    http://arxiv.org/abs/0906.3675

  • ljk July 1, 2009, 14:41

    Cassini/VIMS hyperspectral observations of the HUYGENS landing site on Titan

    Authors: S. Rodriguez (AIM), S. Le Mouélic (LPGN), C. Sotin (LPGN), H. Clénet (DTP), R. N. Clark, B. Buratti (JPL), R. H. Brown (LPL), T. B. Mccord, P. D. Nicholson, K. H. Baines (JPL)

    (Submitted on 30 Jun 2009 (v1), last revised 1 Jul 2009 (this version, v2))

    Abstract: Titan is one of the primary scientific objectives of the NASA ESA ASI Cassini Huygens mission. Scattering by haze particles in Titan’s atmosphere and numerous methane absorptions dramatically veil Titan’s surface in the visible range, though it can be studied more easily in some narrow infrared windows.

    The Visual and Infrared Mapping Spectrometer (VIMS) instrument onboard the Cassini spacecraft successfully imaged its surface in the atmospheric windows, taking hyperspectral images in the range 0.4 5.2 ?m. On 26 October (TA flyby) and 13 December 2004 (TB flyby), the Cassini Huygens mission flew over Titan at an altitude lower than 1200 km at closest approach.

    We report here on the analysis of VIMS images of the Huygens landing site acquired at TA and TB, with a spatial resolution ranging from 16 to14.4 km/pixel. The pure atmospheric backscattering component is corrected by using both an empirical method and a first-order theoretical model. Both approaches provide consistent results. After the removal of scattering, ratio images reveal subtle surface heterogeneities.

    A particularly contrasted structure appears in ratio images involving the 1.59 and 2.03 ?m images north of the Huygens landing site. Although pure water ice cannot be the only component exposed at Titan’s surface, this area is consistent with a local enrichment in exposed water ice and seems to be consistent with DISR/Huygens images and spectra interpretations. The images show also a morphological structure that can be interpreted as a 150 km diameter impact crater with a central peak.

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Journal reference: Planetary and Space Science 54 (2006) 1510-1523

    DOI: 10.1016/J.PSS.2006.06.016

    Cite as: arXiv:0906.5476v2 [astro-ph.EP]

    Submission history

    From: Sebastien Rodriguez [view email] [via CCSD proxy]

    [v1] Tue, 30 Jun 2009 11:10:35 GMT (1768kb)

    [v2] Wed, 1 Jul 2009 12:54:49 GMT (1768kb)

    http://arxiv.org/abs/0906.5476

  • ljk July 6, 2009, 3:03

    Global circulation as the main source of cloud activity on Titan

    Authors: Sébastien Rodriguez (AIM, LPGN), Stéphane Le Mouélic (LPGN), Pascal Rannou (GSMA, LATMOS), Gabriel Tobie (LPGN), Kevin H. Baines (JPL), Jason W. Barnes, Caitlin A. Griffith (LPL), Mathieu Hirtzig (LESIA, AOSS-PSL), Karly M. Pitman (JPL), Christophe Sotin (LPGN, JPL), Robert H. Brown (LPL), Bonnie J. Buratti (JPL), Roger N. Clark, Phil D. Nicholson

    (Submitted on 3 Jul 2009)

    Abstract: Clouds on Titan result from the condensation of methane and ethane and, as on other planets, are primarily structured by circulation of the atmosphere.

    At present, cloud activity mainly occurs in the southern (summer) hemisphere, arising near the pole and at mid-latitudes from cumulus updrafts triggered by surface heating and/or local methane sources, and at the north (winter) pole, resulting from the subsidence and condensation of ethane-rich air into the colder troposphere.

    General circulation models predict that this distribution should change with the seasons on a 15-year timescale, and that clouds should develop under certain circumstances at temperate latitudes (~40\degree) in the winter hemisphere.

    The models, however, have hitherto been poorly constrained and their long-term predictions have not yet been observationally verified.

    Here we report that the global spatial cloud coverage on Titan is in general agreement with the models, confirming that cloud activity is mainly controlled by the global circulation. The non-detection of clouds at latitude ~40\degree N and the persistence of the southern clouds while the southern summer is ending are, however, both contrary to predictions.

    This suggests that Titan’s equator-to-pole thermal contrast is overestimated in the models and that its atmosphere responds to the seasonal forcing with a greater inertia than expected.

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Journal reference: Nature 459 (2009) 678-682

    DOI: 10.1038/NATURE08014

    Cite as: arXiv:0907.0606v1 [astro-ph.EP]

    Submission history

    From: Sebastien Rodriguez [view email] [via CCSD proxy]

    [v1] Fri, 3 Jul 2009 12:01:33 GMT (993kb)

    http://arxiv.org/abs/0907.0606

  • ljk July 6, 2009, 3:05

    Impact of aerosols present in Titan’s atmosphere on the CASSINI radar experiment

    Authors: S. Rodriguez (AIM, L3ab), P. Paillou (L3AB), M. Dobrijevic (L3AB, Oasu), G. Ruffié (LPIOM), P. Coll (LISA), J. M. Bernard (LISA), P. Encrenaz (OP, Lerma)

    (Submitted on 3 Jul 2009)

    Abstract: Simulations of Titan’s atmospheric transmission and surface reflectivity have been developed in order to estimate how Titan’s atmosphere and surface properties could affect performances of the Cassini radar experiment.

    In this paper we present a selection of models for Titan’s haze, vertical rain distribution, and surface composition implemented in our simulations. We collected dielectric constant values for the Cassini radar wavelength ($\sim 2.2$ cm) for materials of interest for Titan: liquid methane, liquid mixture of methane-ethane, water ice and light hydrocarbon ices.

    Due to the lack of permittivity values for Titan’s haze particles in the microwave range, we performed dielectric constant ($\varepsilon_r$) measurements around 2.2 cm on tholins synthesized in laboratory.

    We obtained a real part of $\varepsilon_r$ in the range of 2-2.5 and a loss tangent between $10^{-3}$ and $5.10^{-2}$. By combining aerosol distribution models (with hypothetical condensation at low altitudes) to surface models, we find the following results: (1) Aerosol-only atmospheres should cause no loss and are essentially transparent for Cassini radar, as expected by former analysis. (2) However, if clouds are present, some atmospheric models generate significant attenuation that can reach $-50 dB$, well below the sensitivity threshold of the receiver.

    In such cases, a $13.78 GHz$ radar would not be able to measure echoes coming from the surface. We thus warn about possible risks of misinterpretation if a \textquotedblleft wet atmosphere\textquotedblright $ $is not taken into account. (3) Rough surface scattering leads to a typical response of $\sim -17 dB$. These results will have important implications on future Cassini radar data analysis.

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Journal reference: Icarus 164 (2003) 213-227

    DOI: 10.1016/S0019-1035(03)00125-8

    Cite as: arXiv:0907.0613v1 [astro-ph.EP]

    Submission history

    From: Sebastien Rodriguez [view email] [via CCSD proxy]

    [v1] Fri, 3 Jul 2009 12:14:38 GMT (390kb)

    http://arxiv.org/abs/0907.0613

  • ljk August 6, 2009, 10:28

    http://www.technologyreview.com/blog/arxiv/23948/

    Thursday, August 06, 2009

    How Titan Got Its Atmosphere

    The methane in Titan’s atmosphere has puzzled astronomers for decades. Now they think they know where it came from

    Methane doesn’t last long in sunlight. The Sun’s rays rapidly break it down into other organic molecules. So the discovery of methane anywhere in the Solar System causes a frisson of excitement among astronomers.

    And understandably so. The methane cannot have been there for long (otherwise it would have been broken down by sunlight). So it must have been recently released into the atmosphere. On Earth, most methane in the atmosphere is produced by the ongoing, unstoppable farting of living things.

    That’s why the recent discovery of methane in small amounts on Mars caused such excitement. Could it be that methane farting Martians could be responsible? Probably not. Many commentators ignore the fact that methane on Earth is also released by volcanoes, hydrothermal vents and in some reactions between rocks and water.

    However, the 800 pound gorilla is Saturn’s moon, Titan, which has a dense nitrogen atmosphere with a sizable fraction of methane. The question is how does this methane gets there, if it is constantly being replaced as it is broken down by sunlight.

    There are two suggested answers (ignoring the wild suggestion that some kind of farting organisms could be responsible).

    The first possibility is an ongoing reaction beneath Titan’s surface between iron or magnesium silicates, water and carbon dioxide to produce methane. This is called serpentization and occurs in various places on Earth such as in the Precambrian rocks beneath parts of Canada.

    The second is that methane ice was incorporated in Titan’s interior when the moon formed in the early Solar System and that the atmosphere is constantly refreshed by huge methane belches from below as this ice melts and escapes.

    Now a group an international group of planetary geologists say they known which. They say that recent measurements of the the ratio of hydrogen to deuterium in Titan’s methane cannot be explained by serpentization reactions. The water involved would have to have an improbably strange mix of these isotopes.

    On the other hand, primordial methane may well have had a mix of hydrogen and deuterium that is closer to what we see on Titan today. And the difference can be explained by the way photolysis prefers one isotope over the other.

    Interestingly, the team suggest a way of testing their idea. They say that another of Saturn’s moons, Enceladus, must have formed from the same primordial methane. Enceladus seems to occasionally burp this stuff into orbit around Saturn. A measurement of the isotopic ratio of this methane could settle the question, or at least strongly back the argument.

    And who could do such a measurement? Over to the team at the Saturn-orbiting spacecraft, Cassini.

    Ref: http://arxiv.org/abs/0908.0430: A Primordial Origin for the Atmospheric Methane of Saturn’s Moon Titan

  • ljk September 29, 2009, 22:22

    Monday, September 28, 2009

    P.S. on the problem with science

    I should have, of course, provided the two papers in question so you can decide for yourself. I can’t quite do that. I can give you the link to my paper, here:

    http://www.gps.caltech.edu/~mbrown/papers/ps/vimsclouds_final.pdf

    And I can even provide you with a link to their paper:

    http://www.nature.com/nature/journal/v459/n7247/full/nature08014.html

    But it’s possible that you can’t read theirs. (but wait: read the comments below; people found all of the parts of this article posted online in various locations, so you’re in luck!) Why not? Because, even after $1B of taxpayer money going to send Cassini to Titan and get these results, the copyright to the paper is now owned by Nature. And they say you’re not allowed to read it unless you subscribe or pay. If you are logged in from an academic institution, you probably will get access from their subscription. But if you’re elsewhere you are simply out of luck. Seems a bit crazy, huh?

    If you do get the two papers, be sure to check out the supplementary information in the Nature paper: that is where all of the important details (like where there are and are not clouds) lie. At first glance the two papers look more or less like they say there are clouds in the same spots. It helps that the figures are all really really small so details are hard to discern. But when you blow them up and look carefully things just don’t match up nearly as well as two papers using exactly the same data should.

    Full article here:

    http://www.mikebrownsplanets.com/2009/09/ps-on-problem-with-science.html

  • ljk October 8, 2009, 20:38

    October 6, 2009

    New Evidence of Seasonal Change on Titan

    Written by Nancy Atkinson

    New images of Titan’s surface from the Cassini spacecraft show changes which are evidence of seasonal change. Objects identified earlier as liquid hydrocarbon lakes are shrinking and disappearing over the course of one to several Earth years.

    Scientists say seasonal temperature variations causing evaporation is the most likely cause for the changes observed.

    Cassini’s Synthetic Aperture Radar (SAR) repeatedly peered through Titan’s thick atmosphere, and data show that the lakes exhibit more than an order of magnitude increase in radar return and have disappearing borders between observations, suggesting surface change. These changes cannot be explained without invoking temporal variability, scientists reported at the American Astronomical Society’s Division for Planetary Sciences meeting now under way in Fajardo, Puerto Ric

    Full article here:

    http://www.universetoday.com/2009/10/06/new-evidence-of-seasonal-change-on-titan/