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Cassini’s Exquisite Last View

The release of Cassini’s last images of Saturn and its rings is a welcome event, a capstone to the mission that has taught us so much. What we see below is a series of images that have been grafted together, 42 red, green and blue images that allow us to see a wide-angle mosaic of Cassini’s view. The images were taken by the spacecraft’s wide-angle camera on September 13, and include the moons Prometheus, Pandora, Janus, Epimetheus, Mimas and Enceladus.

Image: After more than 13 years at Saturn, and with its fate sealed, NASA’s Cassini spacecraft bid farewell to the Saturnian system by firing the shutters of its wide-angle camera and capturing this last, full mosaic of Saturn and its rings two days before the spacecraft’s dramatic plunge into the planet’s atmosphere. During the observation, a total of 80 wide-angle images were acquired in just over two hours. This view is constructed from 42 of those wide-angle shots, taken using the red, green and blue spectral filters, combined and mosaicked together to create a natural-color view. Credit: NASA/JPL-Caltech/SSI.

I was glad to see some Cassini team members reminiscing about Voyager, whose journeys opened up the outer Solar System to close view, and continue to inform us about the interstellar medium. Thus Carolyn Porco, former Voyager imaging team member and Cassini’s imaging team leader at the Space Science Institute in Boulder, Colorado:

“For 37 years, Voyager 1’s last view of Saturn has been, for me, one of the most evocative images ever taken in the exploration of the solar system. In a similar vein, this ‘Farewell to Saturn’ will forevermore serve as a reminder of the dramatic conclusion to that wondrous time humankind spent in intimate study of our Sun’s most iconic planetary system.”

Here is a brightened view, processed to bring out detail in the fainter areas of the image. The six moons mentioned above show up faintly, but the annotations should help.

Image: The ice-covered moon Enceladus — home to a global subsurface ocean that erupts into space — can be seen at the 1 o’clock position. Directly below Enceladus, just outside the F ring (the thin, farthest ring from the planet seen in this image) lies the small moon Epimetheus. Following the F ring clock-wise from Epimetheus, the next moon seen is Janus. At about the 4:30 position and outward from the F ring is Mimas. Inward of Mimas and still at about the 4:30 position is the F-ring-disrupting moon, Pandora. Moving around to the 10 o’clock position, just inside of the F ring, is the moon Prometheus. Credit: NASA/JPL-Caltech/SSI.

We’re looking toward the sunlit side of the rings from about 15 degrees above the ring plane, with Cassini at approximately 1.1 million kilometers from the planet and on its final approach.

Titan’s Polar Vortex

Although Cassini is gone, we have vast amounts of data that will continue to generate new discoveries for quite some time, as witness the latest, a paper out of the University Of Bristol that discusses the atmospheric chemistry on Saturn’s largest moon, Titan. Lead author Nick Teanby has been studying Titan’s upper atmosphere, where in the polar regions we are seeing unexpected cooling, a process that differs from what we see on Earth, Venus and Mars.

Indeed, Titan’s polar vortex seems to be extremely cold. What we see on the other worlds is that the high altitude polar atmosphere on a planet’s winter hemisphere is warmed as a result of sinking air heating as it is compressed. It was Cassini’s Composite Infrared Spectrometer (CIRS) instrument that produced the observations Teanby has used to study this anomaly on Titan.

CIRS data showed the expected polar hot-spot beginning to develop in 2009, but temperatures dropped significantly by 2012 and remained as low as 120 K until late 2015, after which the expected hot-spot returned. Teanby explains what’s happening this way:

“For the Earth, Venus, and Mars, the main atmospheric cooling mechanism is infrared radiation emitted by the trace gas CO2 and because CO2 has a long atmospheric lifetime it is well mixed at all atmospheric levels and is hardly affected by atmospheric circulation. However, on Titan, exotic photochemical reactions in the atmosphere produce hydrocarbons such as ethane and acetylene, and nitriles including hydrogen cyanide and cyanoacetylene, which provide the bulk of the cooling.”

Image: Titan’s winter polar vortex imaged by the Cassini Spacecraft’s ISS camera. The vortex is now in deep winter and can only be seen because the polar clouds within the vortex extend high above Titan’s surface into the sunlight. The vortex was extremely cold from 2012-2015, giving rise to unusual nitrile ice clouds. Credit: NASA/JPL-Caltech/Space Science Institute/Jason Major.

Hydrocarbons and nitriles appear high in the atmosphere and are strongly susceptible to vertical atmospheric circulation, meaning that over the southern winter they accumulate in great amounts over the pole, creating the cooling Teanby and team are studying. The work also draws on Cassini data from 2014, when hydrogen cyanide ice clouds were observed over the pole, a reminder not only of Titan’s intriguing chemistry but the continuing vitality of Cassini data.

The paper is Teanby et al., “The formation and evolution of Titan’s winter polar vortex,” published online by Nature Communications 21 November 2017 (full text).

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{ 4 comments… add one }
  • ljk November 28, 2017, 11:29

    http://astrobiology.com/2017/11/role-of-the-global-water-ocean-on-the-evolution-of-titans-primitive-atmosphere.html

    Role Of The Global Water Ocean On The Evolution of Titan’s Primitive Atmosphere

    Press Release – Source: astro-ph.EP

    Posted November 27, 2017 9:33 PM

    During the accretion of Titan, impact heating may have been sufficient to allow the global melting of water ice and the release of volatile compounds, mainly constituted of CO2, CH4 and NH3.

    The duration and efficiency of exchange between the primitive massive atmosphere and the global impact-induced water ocean likely play a key role in the chemical evolution of the early Titan’s atmosphere.

    To investigate the atmospheric composition of early Titan for a wide range of global (atmosphere + ocean) composition in volatiles, we first developed a gas-liquid equilibrium model of the NH3-CO2-H2O system, where the non-ideal behavior of both gas and liquid phases, and the speciation of volatiles dissolved in the aqueous phase are taken into account. We show that the relative abundance of CO2 and NH3 determine the composition of Titan’s atmosphere. For CO2/NH3 1, CO2 is the main constituent of Titan’s primitive atmosphere while the NH3 atmospheric content is dramatically decreased. We then investigate the conditions for the formation of CH4-rich clathrates hydrates at Titan’s surface that could be the main reservoir of methane for the present-day atmosphere. In absence of reliable experimental data in the CH4-CO2-NH3-H2O system, the dissolution of methane in water is included using a simplified Henry’s law approach.

    We find that if the concentration of CH4 in Titan’s building block was higher than ~0.1 mol.kg-1 and CO2/NH3 < 3, a large fraction of methane may be stored in the primordial crust, which would form at temperature below ~10{\deg}C.

    Nadejda Marounina, Olivier Grasset, Gabriel Tobie, Sabrina Carpy
    (Submitted on 24 Nov 2017)

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

    Cite as: arXiv:1711.09128 [astro-ph.EP] (or arXiv:1711.09128v1 [astro-ph.EP] for this version)

    Submission history

    From: Nadejda Marounina

    [v1] Fri, 24 Nov 2017 20:37:25 GMT (6498kb,D)

    https://arxiv.org/abs/1711.09128

  • Michael Fidler November 29, 2017, 1:40

    MISSION HERA: NASA AND ESA WILL RETURN TOGETHER ON SATURN?
    “Hera will aim to dive, protected by a heat shield, to 100 000 km / h in the upper atmosphere, then to deploy a parachute, before transmit scientific information for almost two hours! We hope to measure all atmospheric parameters of Saturn up to about 400 kilometers deep, which represents 20 bars of pressure, that is unheard of … “.

    https://www.science-et-vie.com/ciel-et-espace/mission-hera-la-nasa-et-l-esa-vont-elles-retourner-ensemble-sur-saturne-10033

    https://hera.lam.fr/

  • Digbijoy November 29, 2017, 13:21

    Is there any picture of saturn captured by Cassini just as it was approaching its atmosphere for plunging in? Like, last minute pics before its demise? It would have been fascinating to see how Saturn’s atmosphere appears, say, from 1000 km away.

  • Curious November 30, 2017, 3:15

    An amazing picture! What is the tiny object just to the left of the word “Epimetheus” in the annotated image?

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