Early Views of Saturn: Cassini Populates the Saturnian System

My last blog entry talked about the first two Saturnian years between Galileo's first observations of Saturn's unusual 'appendages' and acceptance of Huygen's explanation of these features as a broad, flat ring around the planet's equator.  In 1656 Huygens had also discovered Saturn's largest moon, Titan.  But the latter part of the 17th century was dominated by the observations and discoveries of one Jean Dominique Cassini (1625-1712), who later gave his name to the most sophisticated spacecraft ever sent into the outer solar system.

Jean Dominique Cassini.

Cassini was an Italian, and served as professor of astronomy at Bologna for 19 years, distinguishing himself as a great observer of the planets.  He was invited to Paris in 1669 to become the first director of the new observatory, and became a naturalised French citizen, changing his name from Giovanni Domenico to Jean Dominique.  Cassini's first Saturnian satellite was discovered in 1671, just as the planet was passing the northern autumnal equinox (i.e., ring plane crossing) for the third time since Galileo's first observations.  The discovery using a 17-foot telescope tube, was record in Phil. Trans. volume 8, p5178 - Cassini compared the motion of a new point of light, compared to Saturn and Huygen's discovery of Titan:  its distance to Saturn increased westward from October 25th to November 1st, and then decreased in distance until November 6th.  He noted that the satellite had a long orbital duration (80 days), a maximum distance from Saturn of 10.5x the diameter of the ring, and an inclination to the plane of Saturn's equator.  Cassini had made his first discovery - the icy satellite Iapetus.

The discovery of Rhea came less than a year later in 1672, reported in Phil. Trans. volume 12, p. 831.  Cassini measured an orbital period of 4.5 days, and an orbital distance of 5/3 the ring diameter.  One remark shows how difficult these observations were:  "The apparent magnitude of these planets is so little, that posterity will have cause to wonder, that their discovery was begun by a glass of 17 foot."  In the same report, Cassini made an astonishing discovery - Iapetus was not always so readily visible, and suggested that one side was less effective at reflecting sunlight than the other, and that Iapetus orbited with one face always towards Saturn.  The bright trailing hemisphere was visible when Iapetus was on the western side of Saturn, the dark leading hemisphere when on the eastern side.  So Iapetus' two-tone appearance was first observed 332 years before the spacecraft arrived!  The dark terrain is named Cassini Regio in his honour.

Cassini's diagram showing the gap in the rings and the SEB,
Phil. Trans. volume 11, 1676.

Saturn's rings continued to open up towards southern summer solstice, and Cassini's report in Phil. Trans. volume 11, in August 1676 reported two new discoveries:  the first suggestions of a South Equatorial Belt ("we have discerned on the globe of Saturn a dusky zone [zona obscura], a little farther south than the centre, similar to the zones of Jupiter"), and of a division in Saturn's ring ("also the breadth of the ring was divided into two parts [dividebatur bifarium] by a dark line, apparently eliptical but in reality circular, as if into two concentric rings").  He may have discovered further atmospheric banding in later years, but the dark line separating the A (outer) and B (inner) rings is called the Cassini Division in his honour.

Dione and Tethys were discovered in 1684, as Saturn's northern winter hemisphere was slowly approaching the spring equinox (the end of Saturn's 'third year').  These two orbited closer to Saturn than Rhea, and completed a complement of four icy satellites (in addition to Titan) that Cassini wished to name for his sponsor, Louis XIV.  Their names were actually suggested from classical mythology by John Herschel in the 19th century.

Huygens and Cassini continued to observe the Saturnian system through its fourth year (1685-1715) since Galileo's observations, and in 1714 the ring ansae were again observed to disappear at the northern spring equinox.  As the end of the 17th century was dominated by Cassini's discoveries, the end of the 18th would be dominated by another great astronomer:  William Herschel, subject of the next blog post.


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.

Early Views of Saturn: Galileo and Huygens over Two Saturn Years

I've been busy writing an article concerning the history of astronomical observations of Saturn, spurred in part by the Royal Society's release of the first 'Philosophical Transactions' from 1665 featuring, among other things, drawings of Jupiter and Saturn by the 'Ingenious Mr. Hooke'.  I've just come across a book from the 1960s by Alexander, called 'The Planet Saturn:  A History of Observation, Theory and Discovery.'  It's full of rich detail from original sources, lots of hand-drawn diagrams, and has certainly opened my eyes to how much planetary meteorology was possible even before the use of photographic film in astronomy.  Although known to the ancients as a wandering point of light among the fixed stars, and providing a character for many of our ancient mythologies, observing the details of the Saturnian system required Galileo's invention of the telescope in 1610.

Compilation from Huygen's Systema Saturnium (1659)
showing how Saturn's appearance had changed from
1610 to 1646.

Galileo's first, distorted views of Saturn's disc in July 1610 would have occurred at a heliocentric longitude of 150 degrees (i.e., the position of Saturn in its orbit as observed from the Sun, measured from a longitude of zero at the northern spring equinox, 90 at the northern summer solstice, 180 at the northern autumnal equinox and 270 at the northern winter solstice).  So Galileo was observing in late northern summer. For reference, Cassini arrived at Saturn in 2004 (heliocentric longitude of 293, just after the northern winter solstice), and today we're in northern spring (heliocentric longitude of 43 today).  It's interesting to note that Saturn has been around the Sun 13.5 times between Galileo's observations and those of the Cassini spacecraft 400 year later.  We know the story of Galileo seeing 'strange appendages' that appeared fixed in position and brightness (unlike Jupiter's moons, which moved from night to night), and describing Saturn as a 'triple planet.'  Those accompanying 'servants' vanished two years later at the autumnal equinox, when the ring opening angle had closed to zero as viewed from Earth.  "Has Saturn, perhaps, devoured his own children?", he asked?

Huygens' diagram of a ring cycle from his Systema Saturnium.

Almost half a century passed before Christiaan Huygens (1629-1695), at the age of only 26, solved 'the Saturn problem.'  Alexander's book describes Huygens' discovery of Titan, in March 1655, and how repeated observations between 1656 and 1659 (as Saturn's northern hemisphere was emerging into spring sunlight) confirmed his theory that the two ansae were indeed the broad flat ring that we now know and love.  His Systema Saturnium was published in 1659, and suggested that the rings were inclined by more than 20 degrees to the ecliptic, explaining the variations that observers had witnessed over the past 50 years.

Robert Hooke's observation of Saturn
from June 1666, published in the first
volume of the Philosophical Transactions.

Many observers began to train their telescopes on Saturn in the latter part of the 17th century, including 'the Ingenious Mr. Hooke' in 1666, as reported in the first volume of the Philosophical Transactions.  Robert Hooke's (1635-1703) observations suggested he could actually see the ring itself, crossing the planet's southern hemisphere and disappearing behind the northern hemisphere (observations at a heliocentric longitude of 118 degrees, near northern summer solstice).   Here's what the journal says (rough translation):

A Late Observation about Saturn made by the Same (Mr. Hooke)

June 26 1666 between 11 and 12 at night I observed the body of Saturn through a 60 foot telescope and found it exactly of the shape represented in the figure.  The ring appeared of a somewhat brighter light than the body, and the black lines crossing the ring and crossing the body (whether shadows or not, I dispute not) were plainly visible whence I could manifestly see, that the southernmost part of the ring was on this side of the body, and the northern part behind, or covered by the body.

[Incidentally, Hooke had been the assistant to Robert Boyle of Boyle's Law, and they conducted their experiments in an Oxford house now part of my college on the High Street, University College].

John Flamsteed (1646-1719), the first Astronomer Royal, observed Saturn in 1671 very close to the northern autumnal equinox, and found the rings to be 'very slender, and to one that thought not of them, scarce discernable' (volume 6 of Phil. Trans., heliocentric longitude of 180 degrees, northern autumnal equinox two Saturnian years after Galileo had first viewed the planet's disc).   This brings us full circle in solving the mystery of Saturn's appendages and up to the time of Jean Dominique Cassini, and his discoveries of Saturn's icy satellites and the division in the rings, subject of a later blog post.

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.

Early Exploration of Venus - The Path to Venus Express

On February 22nd, Oxford physics invited Dr. Dmitrij Titov to present a colloquium on his studies of Venus from ESA's Venus Express mission. Titov was among the original proposers of this mission (along with Oxford's Fred Taylor, so AOPP has always has a close connection to Venus science), which has been returning new insights into our sister planet, revealing a world far removed from our original ideas. Inspired by his talk, I decided to look into Venusian exploration a little more closely.

To study Venus is to study our place in the solar system. Galileo was the first to see the phases of Venus and realise that the planet was rotating around the sun inwards of the Earths orbit. Recent images of transits of Venus across the face of the sun served to show how fragile our worlds are, but if you missed the 2012 transit then you'll have to wait until 2117 until the next one. Our understanding of Venusian weather really started with the observations of Lomonosov in 1761, who saw a tiny crescent around the planet during a transit and was the first to discover the planet's atmosphere. But early 20th century observations were misleading, showing cloud top temperatures of 240 K and an atmosphere of water vapour and carbon dioxide, making Venus appear as an Earth-like world. This period in the thirties and forties was a great period in Venus research, with poets and artists placing beautiful women in tropical pools, lakes and jungles on the surface of Venus. It gave the idea that the universe must be widely inhabited, with someone on the world right next to us. But then radio observations in the 1950s saw emission temperatures of 500 K, providing a dilemma that would last until the beginning of the space age.

First Steps to Venus

Russia was attempting to send missions to Venus even before the launch of Gagarin, but the missions apparently failed. The US flyby of Mariner 2 in 1962 ended our naive views of Venus, and the first data returned to Earth revealed a hellish world with temperatures exceeding 500 K. Venera 7 achieved the first soft landing in 1970, being incredibly over-designed as all previous attempts had failed several kilometres above the surface. Even Soviet U boat engineers were enlisted to ensure the lander could survive the extreme pressures, and resulted in huge spacecraft several tonnes in mass. The flotilla of Venera spacecraft between 1970 and 1980, two sent every ballistic opportunity, provided ground truth on Venusian temperatures, winds, chemistry, plasmas and radio waves... It was a wonderful time for space research, when no one seemed to be counting the money! Venera 13 and 14 provided the first panoramas of the stony desert surface, and left images of Lenin behind to bake in the extreme heat.

Pioneer Venus consisted of two components flown in 1978, one a dedicated orbiter and one with four entry probes. The in situ experiments taught us a lot about the Venusian climate, showing how different the atmosphere is to Earth - pressures of 90 bar (equivalent to an ocean 1 km deep) and temperatures of 460 K at the surface; an atmosphere of 96% carbon dioxide, clouds of sulphuric acid and so little water vapour that it would only be 3 cm deep if condensed as a liquid on the surface. Above the clouds, in the region known as the mesosphere, temperatures appeared to increase from the equator to the poles, exactly the reverse of what we'd expect, and ultimately explained by a Hadley cell of air moving towards higher latitudes, sinking and adiabatically heating. Seemingly persistent cold collars of air were discovered at mid latitudes in both hemispheres, sitting on top of the cloud decks and encircling the polar regions.

The visible appearance of the planet is due to sunlight reflected from the thick clouds between 50-70 km altitude, with characteristic dark patterns due to some unknown ultraviolet absorber within the clouds (possibly sulphur based). We don't see the surface, but if we were to fly within the clouds they would appear as a fog or light haze consisting of tiny particles. The sulphuric acid composition had been observed by both spectroscopy and polarimetry, but the chemistry within the clouds remains an enigma.

The Pioneer Venus mission revealed a rich dynamics, including a super rotating atmosphere, moving six times faster than the equatorial velocity of the surface, a global whirlpool. Project Vega placed two balloons into these clouds, which floated for fifty hours in 1984, and scientists are left dreaming of being able to do it again. Magellan was a US radar mission to unveil the surface beneath the clouds, showing unusual corona features, circular volcanic features and rift zones. Magellan showed the surprising result that the surface of Venus is very young, geologically speaking. The crater record shows an age of only half a billion years, suggesting catastrophic resurfacing, although the mechanism remains unclear.

Beyond this flotilla of missions, there was then a large gap in our exploration of Venus. The planet had been slowly unveiled, far beyond our naive perspective of the early 20th century. After such a deluge of interplanetary visitors, the next thing to attempt was a systematic survey of our sister planet, for which Titov and colleagues developed the Venus Express mission. This spacecraft is still operating at Venus today, and some of the key results will be the topic of a future blog post.