As Saturn approaches opposition this month, we have the best chance to view the planet and its moons from Earth. At its closest to Earth and directly opposite the Sun from our perspective, Saturn will be acting as a mirror for our star’s light.
However, the best glimpses of this system have come from Earth’s most distant orbiting outpost, over a billion kilometres away – the ESA/Nasa Cassini mission. Last year, Cassini celebrated its tenth anniversary at Saturn, and over the past decade has captured that planet eclipsing the Sun, explored its icy rings, watched the birth of a new moon, tracked storms and gazed upon the planet’s ethereal aurorae.
Of all the amazing sights captured by the spacecraft, its most perplexing subject has been the mysterious moon Titan. With an atmosphere twice as thick as Earth’s, Titan’s surface is shrouded in a hydrocarbon haze. Cassini came well equipped though, with both infrared vision and radar that are oblivious to the haze. Infrared images collected last year show glistening pools of liquid at the north pole of the moon with clouds above the surface. As idyllic as this may seem, the temperature on Titan is far from balmy, averaging around –180 degrees Celsius: at this temperature water has the consistency of solid rock. Images revealed huge lakes and rivers of liquid methane and ethane on this cold dark alien moon.
Here on Earth we witness a water cycle: clouds condense into rainwater that runs down valleys to join vast oceans, eventually evaporating into the sky above to start the process all over again. On Titan, images of methane clouds building up and dissipating were captured in July last year: as we were experiencing our summer season here, Titan was warming up as it emerged from a seven year long spring. One side of Titan permanently faces Saturn, which means its seasons are dictated by the lengthy 30-year orbit of Saturn around the Sun.
As we are being amazed every year by the wealth of information gathered by the Cassini spacecraft, scientists are still feasting on the data collected by its short-lived partner in crime – the Huygens probe. Just over ten years ago it detached from its mothership to descend through the smog-like atmosphere and parachute onto the surface. During its two and a half hour descent it captured valuable information regarding wind speed and direction in Titan’s sky at different altitudes. This data has recently helped University of Washington planetary scientists solve an intriguing puzzle.
On Titan, sand dunes sweep eastwards, despite prevailing surface winds that blow westward. With climate models in place from Huygens data they turned to terrestrial storms for inspiration. Researchers found that strong methane storms produce downdrafts that cause the eastward sand dunes, similar to some storms on Earth.
One of over sixty moons orbiting Saturn, Titan is just one exciting example of the privilege of exploring the iconic ringed system up close. On 22 and 23 May look low to the south-east for the planet, and if you have large binoculars or a telescope you can spot the unassuming yet otherworldly moon Titan as a pinprick of light to the east of Saturn.