Australian Sky & Telescope — January 01, 2018

32 AUSTRALIAN SKY & TELESCOPE January 2018

8: NASA / JPL / SPACE SCIENCE INST. / JASON MAJOR; 9 & 11: NASA / JPL / SPACE SCIENCE INST.; 10: NASA / JPL / UNIV. OF ARIZONA

/ UNIV. OF IDAHO

10.GEOMETRIC ODDITYDiscovered by Voyager

2in1981,thehuge,hexagon-shapedcloudpattern

surroundingSaturn’snorthpoleremainsanenigma.

Here’showitlookedtoCassinionNovember27,2012.

Computersimulationshaveshownthatashallow

eastward-movingjetnearthepole,likeoneofthejet

streamsinEarth’satmosphere,candevelopintoa

hexagonal pattern.

11.RHEA IN TRANSITSaturn’s two largest moons,

Titan(5,150kmacross)andRhea(1,528km),appear

tobestackedontopofeachotherinthistrue-colour

scene recorded June 16, 2011. Titan’s dense, nitrogen-

dominatedatmospheresportsadetachedhazelayer

(seenatupperright)knownastheNorthPolarHood.

12.TITAN REVEALEDCassini visited Saturn’s biggest

moon often, making 127 flybys in all. Ordinarily cloaked by

opaquehaze,thesurfaceofTitanemergesintoviewwhen

seen at infrared wavelengths. This view shows terrain

that is mostly on the moon’s Saturn-facing hemisphere.

Several places show more details than elsewhere because

the spacecraft took those images when closer to Titan

during its November 13, 2015, flyby. The scene features

the dark, dune-filled regions named Fensal (toward north)

and Aztlan (closer to equator), which together form a

crude, sideways H.

13. BACKLIT BEAUTY Saturn’s backlit rings, gauzy
    Titan, and Enceladus created this dramatic scene
    on June 10, 2006. Hazes in Titan’s thick atmosphere
    scatter sunlight to encircle the disk. The crescent of
    Enceladus encroaches slightly along its lower-right limb,
    with eruptive plumes faintly visible at its bottom. In this
    viewing geometry, the brightest parts of the rings are
    otherwise tenuous features that are rich in tiny, forward-
    scattering particles.

appearance results from a feedback loop driven

by ice’s faster evaporation from dark regions

than from bright ones. This ‘thermal runaway’

began billions of years ago, when Iapetus’s

leading hemisphere was coated with dust blasted

from the surfaces of Saturn’s even-more-distant

irregular moons, the biggest of which is Phoebe.

Cassini flew close to Phoebe during its initial

approach to Saturn and measured the moon’s

density, which is higher than is typical of

Saturn’s ice-dominated satellites. By implication,

Phoebe must have a rock-and-ice composition

more like that of Pluto, suggesting that this

moon originally orbited the Sun before being

captured by the planet’s powerful gravity.

Like Earth, Saturn has aurorae near its poles

produced by the interaction between its magnetic

field and the solar wind. The aurorae cause Saturn

to emit radio waves whose strength varies as the

10.

13.

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CASSINI’S SATURN