Wednesday, 17 April 2013

William Herschel's Exploration of Saturn

I've been continuing to read through Alexander's excellent historical overview of Saturn observations before what we think of as the modern era of planetary exploration, and I've been astounded at how quickly our knowledge expanded in those early years.  My first post covered the work of Galileo and Huygens to explain the observations of Saturn's rings, and the second post looked at Cassini's discoveries of the major icy satellites.  But now I'm interested in observations of activity and structure on the planet itself, which seemed to be largely ignored for nearly a century.

Cassini had first noted vague observation of contrasts associated with a South Equatorial Belt (SEB) in 1676 (northern winter solstice in Saturn's third year).  Ninety years later, Charles Messier's observation in March 1766 (approaching northern winter solstice in Saturn's sixth year) noted 'two darkish belts, they were indeed extremely faint, and difficult to be discerned, directed, however, in a right line parallel to the longest diameter of Saturn's ring.' (Phil. Trans., 1769, vol. 59, p459).  Those bands must have both been in the southern equatorial and temperate regions, given the ring opening angle at the time.  Ten years later in May 1776 (northern spring), Messier observed 'a belt of a fainter light on the body of Saturn.... pretty broad and almost as distinct as those of Jupiter', which was presumably the north equatorial belt (Phil. Trans., 1776, vol. 66, p543).  But a comprehensive study of features on Saturn had to wait for a new hero of planetary astronomy to come along.  

William Herschel

William Herschel
William Herschel (1738-1822) had been born in Germany, but migrated to Britain at the age of 19 to seek refuge from war with France.   He constructed over four hundred telescopes during his lifetime, and his discovery of Uranus in 1781 came as he was cataloguing multiple star systems using his 160-mm aperture, two meter long reflecting telescope from the back garden of his home in Bath, Somerset.   Herschel had first taken up astronomy as a hobby in 1773, and observed a ring plane crossing in April 1774 (spring equinox at the start of Saturn's seventh year):  "I found the planet as it were stripped of its noble ornament, and dressed in the plain simplicity of Mars...".

In 1789 at the autumnal equinox he used a 40-foot reflector to discover two additional satellites, Enceladus and Mimas, bringing the total count to seven (the names were later provided by William's son, Sir John Herschel).  Saturn had been moving across the sky rather rapidly, so Herschel commented that over 2.5 hours "I had the pleasure of finding, that the planet had visibly carried them all away from their places," confirming that these points of light were indeed associated with Saturn.  Herschel crater on Mimas is named in his honour. He measured their orbital periods and comparable brightnesses; observed a shadow of Titan transiting Saturn's disc; and made detailed assessments of the leading/trailing brightness asymmetry of Iapetus, suggesting that the regularity of the observations implied the absence of any discernable atmosphere. He confirmed that the Cassini division was a true gap in two concentric rings, and measured the angular dimensions of the ring system.  These satellite and ring observations were all reported in Phil. Trans. 1790, vol. 80, p427 and Phil. Trans., 1791, vol. 82, p1.

Saturn's Atmosphere

Herschel's observation of Saturn, and a dusky
spot in June 1780.
And now we come to my original reason for reading Alexander's book:  the first study of features on Saturn's disc.  During Saturn's northern spring and summer between 1775-1780, Herschel observed the bright equatorial zone and one or more dusty dark belts, of variable width and brightness, in the northern hemisphere.  A large dusky spot observed in 1780 seemed to move from the centre to the limb, showing for the first time that Saturn rotated on an axis, just like Jupiter.  He surmised that he was seeing atmospheric features, and that "very probably Saturn has an atmosphere of a considerable density."  

As the rings opened and closed, the belts appeared to curve and then straighten, suggesting bands around a spherical object.  At the autumnal equinox and ring plane crossing in 1789, he observed a symmetry in the belts between the two hemispheres, and made precise measurements of the equatorial and polar radii of the planet, noting the polar flattening of Saturn due to it's rapid rotation.  Despite such apparent differences, Herschel was beginning to compare Jupiter and Saturn, stating "that every conclusion on the atmosphere and rotation of the one, drawn from the appearance of its belts, will apply equally to the other."  That theme of comparative planetology of the two gas giants continues to this day, two centuries later.

Herschel's discussion of quintuple rings in the southern
hemisphere, November 1793.
In November 1793, Herschel recorded more and more features in the southern summer hemisphere - a 'quintuple belt', probably consisting of the dark SEB in Saturn's tropics and two dusky temperate belts at higher southern latitudes.  He also noted that the south polar regions were of "a pale, whitish colour; less bright than the white equatorial [zone]."  (Phil. Trans. 1793, vol. 84, p28).  He used a series of observations of any heterogeneous features in Saturn's belts between November 1793 and January 1794, selecting pairs of similar features at two different times to estimate the rotational period of Saturn.  He was guided by his earlier deduction of a ring rotation period of 10.5 hours, and over many trial solutions arrived at an estimate of 10 hours, 16 minutes and 4 seconds (Phil. Trans., 1794, vol. 84, p48), around 15-30 minutes off from the current (variable!) rotation rate being measured by the Cassini spacecraft.  Alexander's book notes how incredible this result is, given that Herschel was using very slight differences in intensity in 'ill-defined stretches of belts.'  

More than a decade later, Herschel was compelled to return to his Saturn observations, comparing new observations of Saturn's north pole in 1806 (northern spring), to south polar views from 1793-1795 (southern spring).  At both poles, he noted that they had changed from their former whiteness to a duller tone during continued exposure to the summer sunshine.  Drawing analogies from the seasonal changes to the polar caps of Mars, Herschel suggested seasonal changes in Saturn's atmosphere, creating "large, dusky looking spaces of a cloudy atmospheric appearance" at the poles (Phil. Trans., 1806, vol.96, p455).

I had never truly appreciated Herschel's contribution to the exploration of Saturn before.  Since Cassini's time the planet had been rather neglected, but a century later Herschel provided dramatic insights into the satellite system, rings and the atmosphere that have stood the test of time. His enthusiasm for Saturn exploration was real:

(Phil. Trans., 1805, vol. 95, p272).  Sounds like the start of hundreds of subsequent research proposals!

Summary of Saturn Years, Measured from Spring Equinox (Heliocentric Longitude of Zero)

Saturn Year One:  1597-1627:  Galileo discovers Saturn's 'strange appendages'.
Saturn Year Two:  1627-1656:  Several theories proposed to explain Saturn's servants.
Saturn Year Three:  1656-1685:  Huygen's solves the Saturn problem and discovers Titan, Hooke and Flamsteed observe Saturn's ring progression; Cassini discovers Iapetus, Rhea, Dione and Tethys.
Saturn Year Four:  1685-1715:  Repeated observations of the new satellites, early evidence of C ring.
Saturn Year Five:  1715-1744:  Rev. James Bradley accurately measures ring diameters and satellite orbits.
Saturn Year Six:  1744-1774:  Charles Messier's observations of equatorial belts.
Saturn Year Seven:  1774-1803:  Herschel's discoveries of Enceladus and Mimas, discoveries of atmospheric features, measurement of rotation rate.
Saturn Year Eight:  1803-1833:  Herschel compared polar colouration on Saturn.

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