With a ‘space-exploration CV’ big enough to eclipse the achievements of most of the other space probes in its class, the Cassini-Huygens orbiter has revealed much from one of our Solar System’s most fascinating planetary systems, Saturn. Cassini-Huygens’ fact-finding days are not over however, having recently sent back fresh data from Saturn’s largest moon, Titan…
Launched in 1997, after nearly two decades of development, the world can perhaps still recall the dramatic news story of the 2.5 tonne American-European-Italian space agency (NASA/ESA/ASI) space probe, Cassini-Huygens, that had arrived at the greatest of our ringed planets, Saturn, after a 7-year voyage that had seen it flyby Earth, Venus and Jupiter.
Named after the Dutch astronomer, physicist, and mathematician who discovered Saturn’s first moon, Titan in 1655, Christiaan Huygens, the atmospheric probe/lander was developed by the European Space Research and Technology Centre, and armed with six instruments to investigate a moon larger than the planet Mercury, Titan. Respectively the name of the Italian French astronomer who discovered more of Saturn’s moons, Giovanni Cassini, was given to the main flagship-class ASI/NASA orbiter.
The fourth spacecraft to travel to the golden gas giant, Cassini’s main mission began on the 30th June 2004, with it commencing a Saturnian orbit on the 1st July. Its mission took four years to complete. During it, the spacecraft studied the ringed-planet and its many natural satellites and sent a stream of information back to Earth every day. Cassini’s mission also greatly advanced our scientific knowledge of Saturn’s giant moon.
As Titan is the only moon in our solar system with a thick opaque atmosphere and clouds, Cassini’s primary and subsequent missions focused on investigating the complex organic chemistry and structure of these features. With its primary mission complete and with the space probe still functioning, Cassini was commissioned to undertake further observations under the Equinox project, conducted between 2008 and 2010.
However, not only did Cassini study the lunar gases, identifying the air as being composed of 95% nitrogen and 5% methane, but at last it broke through a ceiling of knowledge, or rather, through the moon’s orange haze when it sent an automated probe down through Titan’s clouds. Huygens ‘piggy-backed’ to Titan on Cassini, and then on Christmas Day 2004, Cassini sent its package to Saturn’s largest moon on a ballistic trajectory. This represented mankind’s first attempt to send a visitor to the strange world.
21 days later the Huygens lander parachuted down through Titan’s atmosphere, finding a place in the annals of space exploration history by surviving the hazardous 2hr 17min descent and successfully landing on Titan’s surface. The data and first-of-its-kind footage of this alien world, captured in infra-red by Cassini’s imaging cameras and the Huygens Descent Imager/Spectral Radiometer (DISR) during its downward flight and subsequently from terra firma, was then sent back to Earth via the Cassini orbiter.
From this frigid planet-like world, that is approximately twice the size of Earth’s moon (in terms of diameter), Earth’s robotic emissaries revealed vast methane lakes at the moon’s polar districts and widespread stretches of wind-sculpted hydrocarbon (oil) sand dunes. Titan’s diverse and active atmosphere is also understood to contain organic molecules of hydrogen and carbon in the lower atmosphere which are thought to be responsible for Titan’s ‘would-be’ (to human eyes) amber colour. Despite the fact that Saturn’s largest natural satellite is chiefly composed of ice and rock, researchers responsible for gathering and interpreting the data returned by Cassini, also deduced the presence of an internal, liquid water-ammonia ocean.
Scientists now believe that of the hundreds of lakes scattered across Titan’s surface, those that are chiefly composed of methane, nitrogen and tiny amounts of carbon monoxide or argon are fed by hydrocarbon rainfall. Other lakes however discovered to be made up of a slightly different mixture of methane and ethane or propane and ethane are by contrast thought to be chemically influenced by what lies beneath in ‘alkanofers’ (subsurface reservoirs). Scientists are chiefly interested in the chemical structure of Titan’s purported sub-surface water reserves.
Thus far, gravitational information provided by Cassini suggests the alien world’s underground ocean could be as salty as the Dead Sea. It is perhaps noteworthy though that this potentially high level of salt is unlikely to bode well for any accompanying life that has sought to co-exist in those waters. Not only is Titan famous for being the only known celestial body of the Solar System with “stable bodies” of surface liquid like Earth, frequently other Earth-like atmospheric elements have also been detected, none less than that life-sustaining ingredient, oxygen.
This is one of the reasons perhaps why some in scientific circles even go as far as to say that Titan may not be unlike the early years of our planet’s existence. Last year on the 18th of June, during flyby ‘T-102’, and following on from the similar and highly productive T-101 flyby on May 17th which received data from seas Ligeia Mare and Kraken Mare, the Cassini spacecraft attempted to ping yet more radio signals off Titan’s surface, after which they were reflected back toward NASA’s ground station receiver centres on Earth.
On the second flyby, scientists on Earth attempted to remotely read the structure of the moon’s terrain. Officially known as a ‘Bistatic Scattering Experiment’, the automated deep-space operation sought answers to the following types of questions: Are the large areas of Titan’s surface liquid, slushy or solid? Are they reflective? What might they be made of? Cassini project manager Earl Maize at JPL, California described the level of difficulty of T-102: “This was like trying to hit a hole-in-one in golf…close to a billion miles away, and moving”.
More than 17.5 years since its launch into space, Cassini is still operating, employed on another extended mission known as Solstice, with its current space photography managed by scientists from the USA, England, France and Germany. December 2014 was the date of flyby ‘T-107’ which gave Cassini the opportunity to test the density of Titan’s atmosphere in three different ways. T-108 was the subsequent flyby mission conducted on the 8th January, during which the probe applied its SAR (Synthetic Aperture Radar) to image Titan’s shorelines and channels.
Most recently, on the 12th of February 2015 (flight T-109), Cassini brushed passed Titan again at a distance of 1200km from the huge moon, with the Cassini Visible and Infra-red Mapping Spectrometer compiling among other things, a high-resolution map of a large portion of Titan’s north pole . Cassini’s next flyby of Titan, named T-110 is scheduled for the 16th March 2015.
With the overall extended mission referring in name to Saturn’s summer solstice occurring in May 2017, (by which time it is hoped the spacecraft will still be responsive), Cassini is endeavouring to study a complete seasonal period of the gas giant. With its reputation as an extremely effective and trouble-free automated spacecraft still intact, Cassini’s demise is not predicted until late 2017, its life-span only to be determined by the amount of propellant remaining in its tank.
(Article by Nick Parke, Education Support Officer)