What Cassini Discovered

Cassini has rewritten the book on Saturn, making many scientific discoveries; the first six years of study were summarized in 2010 in two papers in the journal Science.19 Two of the greatest advances were conceptual. The first was to paint a sharply etched portrait of a gas giant planet and its moons as intriguing worlds with “per­sonalities” and quirks that made them noteworthy. Voyager paved the way, but Cassini got closer to its targets and spent a larger amount of time in their neighborhood. Another realization was the plausibility of the outer Solar System for biology. Although far from the Sun’s warming rays, the largest of Saturn’s moons get internal energy from radioactive decay of their rocky material, and many of the moons get extra heat by being tidally “squeezed” by the gravity of the massive planet. If life’s minimum requirements are energy, liquid water, and organic material, those conditions may be met on half a dozen of the moons of Saturn. This animates the search for life elsewhere in the universe, since giant planets with attendant moon systems are expected to be commonplace.

Not all scientific data are equally digestible to public audiences. Spectra or measures of magnetic field strengths and charged par­ticle fluxes are abstract and esoteric without a lot of background information and context. So the general awareness of Cassini has been based on its imaging. In that respect the Imaging Science Sub­system is preeminent, and ISS team leader Carolyn Porco has been an eloquent spokesperson for why we should care about the outer planets and their moons. The portraits made of the Saturn system, best seen at a glance in a poster mosaic of sixty-four scenes from Saturn on the ciclops. org website, are an eloquent testimony to the interplay of shadows and light in the realm where there’s no air to diffuse and scatter light. The shadows of moons fall on rings, or on each other, and shallow-angled light casts deep shadows on the hills and pock-marked surfaces of the moons. The chiaroscuro is worthy of Caravaggio. In fact, there’s an unbroken lineage in the natural depiction of strong light and shadow that connects Leon­ardo and Galileo’s sketches of the Moon, Bonestell’s paintings, and the actual images from Cassini.

The science started flowing long before Cassini got to Saturn. As it passed Jupiter to get a gravity assist, the spacecraft took 26,000 pictures and studied the circulation patterns that produce counter­rotating atmospheric bands on the giant planet. It also provided evidence that Jupiter’s faint ring originates from micro-meteorite impacts on the smaller moons.30 Cassini gave a nod to Albert Ein­stein with a new test of his general theory of relativity. Gravity very slightly bends and slows down light and any other form of radiation passing a massive object. Cassini sent radio signals past the Sun to the Earth so this delay could be measured. The results agreed with the predictions of general relativity theory with a pre­cision of one part in 50,000, improving on the precision of previ­ous tests by a factor of fifty.31

In 2004, Cassini discovered three new moons of Saturn, bring­ing the total to sixty-one. They’re small, between three and four kilometers across, like free-floating mountains in space. A year later it found a slightly larger moon in one of the gaps in the rings. The moons and rings engage in a complex gravitational waltz. Some gaps in the rings are caused by a moon clearing out particles at that distance, but others are caused by a more distant moon driving a resonance. If orbits have periods that are related by the ratio of two small integers, then the outer object can influence the inner one, and in the case of an outer moon, it clears out a gap in the rings, as described in the last chapter. Since there are many moons, there are many ratios that can cause resonance. Harmonic effects like this cause much of the complexity of the ring system.32

They orbit silently in the vacuum of space, but ring and moon systems have the timbre of a beautiful musical instrument, each one unique.

Cassini’s arrival at Saturn in July 2004 involved a daring and risky maneuver. It shot the gap between the F and G rings, equiva­lent to threading the eye of a needle. The High-Gain Antenna had to be pivoted away from Earth and along the flight path to protect it from hits by small particles, and then the rockets were used for a very precise deceleration that allowed Saturn to capture the space­craft. At its closest approach it skimmed just 13,000 miles above Saturn’s cloud tops. Imagine the majestic view if humans had been along for the ride, watching the towering cloudscapes of a planet so big 760 Earths would fit inside. Cassini provided fascinating new details on the ring system.33 To a casual observer it seems miraculous that such subtle and complex patterns could arise by unguided natural processes, but the gravitational mechanisms be­hind the rings have been known for decades. Cassini measured the size distribution of ring particles more accurately than before, and its data showed how ring particles can join into loose aggregations or “piles of rubble.” And, it found instances of moons stealing par­ticles from rings as well as times when moons eject particles. New data showed that the rings are 90 percent water, so they’re most similar to a huge bumper car ride with careening chunks of ice of all sizes—from microscopic to the size of a house. Surprisingly, when seen up close the rings are tinged red, which scientists specu­late is due to rust or small organic molecules mixed in with the water ice (plate 7). To the connoisseur of gravitational dynamics, there’s a life’s work in understanding the shifting spokes, the spiral density waves, the embedded moonlets, and the features that look like waves and straw and rope.34

Much of the time thereafter has been spent planning and execut­ing flybys. An early passage near Phoebe was the only one that will be possible for this curious little moon. Phoebe is one of the out­ermost moons and only 150 miles across, but parts of its heavily cratered surface appear very bright and scientists think there’s ice under the surface. The lion’s share of the flybys have been used to explore Titan. Titan is the second largest moon in the Solar System.

Only Jupiter’s Ganymede is larger. Titan is 50 percent larger than the Earth’s Moon and nearly twice the mass. It is unprecedented in having an atmosphere thicker than Earth, made of the same pri­mary ingredient, nitrogen. Setting aside episodic lava flows on Io, it’s the only object beyond Earth where stable bodies of liquid have been seen on the surface. Cassini has used radar to penetrate the thick murk of the atmosphere and reveal surface details as small as a kilometer.35

Titan’s orange haze is naturally produced photochemical smog. Methane and ethane are mixed in with the nitrogen and they form clouds and rain, which falls on the surface.36 There’s weather here but it’s a completely alien chemical environment. Other trace in­gredients include propane, acetylene, argon, and hydrogen cyanide. Add some oxygen and you’d have the recipe for a real conflagra­tion, and it’s not anything humans would want to breathe. Hydro­carbons break up and recombine in the upper atmosphere under the action of sunlight. The methane present would be converted into more complex molecules in only 50 million years, suggesting that it must be replenished from Titan’s interior. Robert Zubrin pointed out that the base of Titan’s atmosphere is so dense and its gravity is so gentle that astronauts could potentially fly by wearing powered wings attached to their arms.37

The real prize on Titan is not the chemical haze in the atmo­sphere but the glittering liquid on the surface. In late 2009, NASA released a gorgeous picture of the northern polar region, back-lit by the Sun, with a glint of specular reflection from a body of liq – uid.38 Other images show that the liquid levels do not vary by more than 3 millimeters—there’s little surface wind on Titan. The north and south polar regions are dotted with lakes varying from a mile across to larger than the largest lake on Earth. The main ingredi­ents of the lakes are likely to be methane and ethane, with smaller contributions from ammonia and water.39 Evaporation from the lakes is not enough to supply the methane seen in the atmosphere, implying even larger reservoirs of liquid methane underground. Titan is literally swimming in organic materials. There’s hundreds of times more mass of liquid hydrocarbons on Titan than the sum of all the oil and gas reserves on Earth.