Friday July 26th, 11am
It’s our final day at sea, and the vast bulk of the Atlantic Ocean is now behind us, with anticipation for our arrival in New York City tomorrow morning. The fourth and final lecture in this Royal Astronomical Society series concerns an enormously successful robotic explorer in the distant solar system, the Cassini-Huygens mission to Saturn. Many of the topics I’ll describe have been the subject of previous posts on this blog, so I’ll refer the interested reader to those for more details. For example, I begin with a brief history of observation of the Saturn system, from the strange appendages observed by Galileo, to the explanation of the rings and the discovery of enigmatic Titan by Christiaan Huygens; Jean Dominique Cassini’s discoveries of a division in the rings and a multitude of icy satellites; and William Herschel’s in depth observations of Saturn’s changing appearance. Saturn’s first human-made satellite was named for Cassini; and Titan’s first entry probe was named for Huygens, honouring their pioneering studies of the ringed planet.
The Cassini mission was born of the early 1980s, as one of the first fully international robotic space missions, a collaboration between NASA and the European Space Agency. Indeed, when budgets were being slashed it was the international aspect of this mission that probably saved it. It launched in 1997; spent seven years in the frigid depths of space and finally arrived in orbit around Saturn in 2004 just after the northern winter solstice. It has been working out there on its lonely orbit for almost a decade now – following completion of its four-year primary mission; it’s lifetime was extended and extended and we now hope it will survive through until 2017, northern summer solstice. That is, the spacecraft will have observed all four seasons at Saturn, from winter to spring, summer and autumn, providing arguably our most comprehensive picture of a giant planet ever obtained. Ignoring the wonderful science from this mission for just one moment, the Cassini mission is a triumph of engineering – a spacecraft almost 7-m long and 4-m wide, weighing 2150 kg and powered by the radioactive decay of Plutonium-238. Hydrazine rocket thrusters allow the spacecraft to be agile and turn to point its suite of instruments at the variety of targets, but it is this which limits the lifetime of the mission – once the hydrazine supply is exhausted, the mission will be over.
Saturn and its Rings
This lecture gives a whistle-stop tour of some of the discoveries of the Cassini-Huygens mission. Starting from the gas giant itself, we’ll talk about the different atmospheric layers, what we think is present deep down inside the planet, and what forces drive the weather we see. When you observe Saturn through a telescope, you’re seeing light being reflected from the clouds and hazes in the upper atmosphere; possibly fluffy clouds of ammonia ice rather than the water clouds we observe in our own atmosphere. Those clouds are blown around the planet by winds racing east and west. We’ll talk about small storms and lightning (including the southern storm alley), and the seasonal eruptions of globe-encircling storm systems that endure for many months. One such storm erupted in 2010, and its aftermath is still being felt today. Cassini is currently in a high inclination orbit, allowing it to gaze down at the poles of the giant planet to explore the mysterious hexagonal wave around the North Pole, and the twin hurricane-like cyclones churning like giant plugholes at both poles. It has also captured movies of the aurora dancing in Saturn’s high atmosphere in response to pressures from the solar wind and plasmas being injected within the planets magnetic field environment.
Saturn’s delicate and beautiful rings serve as a wonderful laboratory for gravitational interactions. We’ll describe the ring structure, their potential origin and the process of continual renewal and recycling. Discrete features like elusive spokes in the B ring; structures towering above the ring plane and casting shadows back across the main rings; shepherding moons generating wakes in the rings and beautiful gravitationally sculpted structures seen by Cassini’s high-resolution cameras. Beyond the rings, a vast array of unique satellites orbit the giant planet, including Phoebe (a remnant of solar system formation captured by Saturn’s immense gravity); Hyperion (with the appearance of a sponge); Mimas (with the huge Herschel crater making it look suspiciously like a science fiction icon); and Iapetus (with it’s asymmetric brightness and enormous equatorial ridge). Tiny Enceladus, with its four south polar fissures actively venting ice and gas into space, is intricately connected with Saturn’s diffuse E-ring and a truly remarkable discovery.
The jewel in Saturn’s crown is this enormous satellite, shrouded in a thick smoggy orange atmosphere and the second largest moon in our Solar System. Although Voyager had captured images of Titan thirty years ago, the thick hazes were impenetrable, preventing any glimpses of the surface of the moon. What would we discover there, and what might this unexplored terrain look like? The mystery of Titan was one of the driving goals of this mission to the Saturn system. Imaging systems on the Cassini orbiter provided access to wavelengths where the hazes are transparent, providing glimpses through the smoggy atmosphere. Radar swaths have shown the undulating terrain, from dunes to impact scars, mountain ranges and river valleys. And the Huygens probe, designed by the European Space Agency, became the first human-made object to touch down on Titan in January 2005. At that time no one knew what we might be landing on, from solid surfaces, to hydrocarbon sludge, or even into a vast ocean of liquid methane and ethane. The probe descended beneath a parachute, being buffeted by the winds until coming to rest in a dried up river bed; pebbles of water ice appearing rounded by the flow of fluids across the surface, and hints of moisture in the upper layers of the soil. It’s too cold on Titan for that fluid to have been water – instead, methane is a fluid at these temperatures, and a ‘methane-cycle’ on Titan mimics the ‘water-cycle’ on Earth, with methane clouds, methane rain, methane rivers and lakes. Evidence for this methane cycle can now be seen everywhere on Titan, and the river networks and lakes reveal a very Earth-like world.
One of Cassini’s most enticing discoveries is that of lakes and seas of hydrocarbons at the northern pole of the planet. For the first time in the history of our solar system exploration, we have seen standing bodies of liquid on another planet, and can envisage what it would be like to sail those Titanian seas. We have the technology to do so, and could one day see a long-live vessel bobbing on the hydrocarbon sea, floating in and out of different drainage deltas and looking back at the hills, valleys and mountains on the Titanian shore. It’s a lovely idea, and one I hope we’ll one day see with our robotic explorers.
By 2017, the Cassini mission will have completed an in-depth orbital reconnaissance of the whole Saturn system, and it will be time to draw this hugely successful mission to a close. It cannot simply be left in the Saturn system to potentially collide with, and contaminate, any of the pristine environments to be found on the satellites. So it must be disposed of by burning up in Saturn’s atmosphere, a dramatic fireball at the end of the mission. But before that happens, mission planners have designed a dramatic series of final orbits, taking more risks with this grand old spacecraft. It’ll be flying closer to Saturn than ever before, within the rings and skimming just above the cloud tops. It’ll be measuring the strength of the close-in magnetic field, and also the gravitational parameters of the planet itself, using these to probe down to the centre of the planet at depths we’ve never seen before. It’s been described as a new mission, a new lease of life for this ageing spacecraft, and contributing even more to our knowledge of the Saturn system.
As we were preparing to embark on our passage to New York, on Friday July 19th at around 21:30, the Cassini spacecraft trained its powerful cameras back onto the inner solar system, to capture another stunning image of the Earth and moon system, as seen by a lonely robotic explorer a billion miles away in orbit around Saturn. While we’ve been sailing, that image was beamed to Earth, assembled by computers in California and processed to show the results to the world. All of humanity, once again sharing just a few pixels, and showing how small, how fragile, and how precious our home world truly is. It seems like the perfect place to end this series of lectures on board the Queen Mary 2.