Ever since Galileo pointed his telescope at Saturn and discovered its spectacular rings, scientists have yearned to learn more about the planet and its moons. Until recently, they had little information on which to base their theories and could only speculate on what might be happening around the ringed giant.
- Chelsea Trull
With the arrival of the Cassini-Huygens mission to Saturn in 2004, and the successful touchdown of the Huygens probe on Titan in January of this year, all that has changed. With the new flood of data, scientists are finding Saturn and its moons to be dynamic worlds exceeding every expectation.
Launched in 1997, Cassini-Huygens is a cooperative mission between NASA and the European Space Agency with many other contributing institutions, including the University’s Department of Atmospheric, Oceanic and Space Sciences.
Recent results from the probe show evidence for oxygen in Saturn’s rings, and geysers of water forming an atmosphere on the small moon Enceladus. Even more fascinating are the results from the Huygens probe that landed on the planet-sized moon Titan in January and revealed a primordial world much like a young Earth.
“Cassini is one of the last ‘flagship’ class missions,” said Tamas Gombosi, chair of AOSS and coordinating scientist for the Cassini Magnetosphere and Plasma Science group. “Flagship” missions are huge undertakings designed to learn as much as possible about their destinations. With its powerful suite of 18 instruments, the mission is well on its way to fulfilling that objective.
“When the spacecraft flew just above the rings, INMS detected the presence of a water based ‘ring atmosphere,’” Gombosi said. The Ion and Neutral Mass Spectrometer is an instrument, built in part by University scientists, that measures the chemistry of gasses around Saturn and its moons. The tenuous ring atmosphere includes oxygen, likely formed when solar radiation breaks water molecules apart.
Another unexpected discovery was an atmosphere around Saturn’s small moon Enceladus. This bright white moon shows “very clear signs in plasma and magnetic fields that it has a significant atmosphere,” Gombosi said. Normally, plasma, — positively charged matter — spirals around the magnetic field of Saturn. However, measurements show bending and oscillation in the magnetic field near Enceladus, with the plasma being deflected and slowed as it passes the moon. These effects are the result of interactions with ionized water vapor, giving strong evidence that the moon has an atmosphere.
Because Enceladus is so small — about 250 kilometers in diameter — its gravity is too low to hold onto a permanent atmosphere. This leads scientists to believe that the atmosphere of the moon is being continually replenished by geysers and ice volcanoes. Enceladus may turn out to be Saturn’s icy equivalent to Jupiter’s fiercely volcanic moon Io.
Fascinating new discoveries are being made on Titan as well, using the Gas Chromatograph Mass Spectrometer instrument. “Quite a few (University) engineers were involved in the GCMS for Huygens,” AOSS professor Sushil Atreya said. Atreya is a co-investigator on the instrument, which was built in cooperation with NASA’s Goddard Space Flight Center.
The instrument separates out components of the gas being studied before passing them through a mass spectrometer, allowing it to make a detailed analysis of the chemical composition of Titan’s atmosphere.
The giant moon has a thick atmosphere of nitrogen and methane, with a haze of heavier hydrocarbons that hid its surface from view until Huygens landed on Jan. 14 this year.Hydrocarbons, as their name suggests, are molecules made of hydrogen and carbon.
“We had very little information about what to expect,” Atreya said. “We had to account for all possibilities as the probe was descending.” Huygens detected almost three times more haze than predicted in Titan’s atmosphere, and the GCMS data indicated that there was an extensive cloud layer starting at about 10 kilometers above the surface.
“To our surprise, when we landed, the instrument was still working,” Atreya said. On the surface, the GCMS detected a dramatic increase in the amount of methane, meaning that the normally gaseous compound exists there as a liquid.
Methane is a greenhouse gas that traps heat from the sun, keeping Titan slightly warmer than it otherwise would be, Atreya said. “In the absence of methane, Titan’s atmosphere would collapse,” he added.
The nitrogen that makes up most of the moon’s atmosphere is believed to have come from ammonia, which was abundant in the nebula around Saturn from which Titan formed.
As the newly formed moon radiated away the heat from its own formation, keeping the ammonia from condensing, the sun’s rays broke it down into hydrogen and nitrogen. Hydrogen is extremely light and escaped the moon’s gravity, but nitrogen remained.
Methane however is more mysterious. Researchers know that it must be constantly replenished, but do not yet fully understand how or why. Serpentinization, a process in which low-silica rocks combine with water and release methane, may occur in Titan’s interior and is a promising possibility, Atreya said.
“Titan is a new world,” Atreya said. “It is a perfect laboratory, with almost everything that allowed the molecules that led to life to form.” In that respect, Titan is a relic of what earth was like in the early solar system. “What is remarkable,” Gombosi said, “is how much it looks like earth.”
Despite the similarities, Titan is still a very alien world. It is so cold that water ice is as hard as rock. The Huygens probe found a temperature of -291 degrees Fahrenheit at the landing site. Methane springs feed flowing rivers of natural gas, and heavier hydrocarbons rain down from the upper atmosphere, forming a dark sludge of organic molecules.
The new discoveries being made by the Cassini-Huygens mission are broadening our understanding of how planets and moons form, what processes alter them, and how their atmospheres are created and maintained.
“It’s just unbelievable fun,” Gombosi said of participating in the Cassini-Huygens mission. “It’s extremely rewarding.”The Cassini mission officially extends until 2008, but “it is almost certain that there will be an extended mission,” Gombosi said.