Salty Volcanoes on Jovian Moon
Astronomers at The Johns Hopkins University, the Observatoire de Paris, and other institutions have solved a nearly 30-year-old mystery surrounding Jupiter’s moon Io, showing that volcanoes there appear to be shooting gaseous salt into the moon’s thin atmosphere.
|Jupiter’s moon Io as seen by the passing Voyager 1 spacecraft Credit: NASA/JPL.|
"This gives nice closure to the discovery Bob Brown made in 1974 of sodium in neutral clouds of gas around Io," said Darrell Strobel, a professor of earth and planetary sciences in the Krieger School of Arts and Sciences at Johns Hopkins (pictured at right) and an author of a paper on the new results in the Jan. 2 issue of "Nature."
Further analysis of the results, including modeling how the salt is broken down into sodium and chlorine atoms, could help planetary scientists move closer to determining what kinds of meteoritic materials originally came together to form Io, according to Strobel.
Strobel said Brown, who later became a project scientist at the Space Telescope Science Institute, found the sodium around Io while testing out a spectrograph he had built.
"He told me some years afterwards, ‘This discovery of mine is so simple. I was amazed somebody hadn’t done it 30 to 40 years earlier,’" Strobel said. "Nobody was looking for it; nobody would have guessed it was there."
Astronomers winnowed the list of theoretical suspects for the source of sodium for years before determining the most likely suspect was salt, or sodium chloride. That conclusion was reached after the detection two years ago of chlorine in a doughnut-shaped, electrically charged cloud of gas around Io known as the plasma torus. Based on the new chlorine finding and the theoretical work, astronomers decided to conduct the exacting studies necessary to look for salt.
Strobel said the lead author of the new "Nature" paper, Emmanuel Lellouch of the Observatoire de Paris, had looked previously for salt in Io’s atmosphere and failed to find signs of it. Co-author Nicholas Snyder of the University of Colorado at Boulder, one of the researchers who discovered chlorine in Io’s plasma torus, suggested using millimeter-wavelength radio telescope at the Institut de Radio-Astronomie Millimetrique in Granada, Spain, to perform a definitive search for salt.
Observations with a millimeter-wavelength radio telescope force astronomers to focus on very tiny regions of the spectrum, making it necessary to carefully choose the frequencies they want to observe. But when the team conducted its studies in January 2002, they found the characteristic spectroscopic lines they were looking for.
An examination of potential sources for the salt in the atmosphere pointed to the volcanoes as the most likely point of origin for the salt.
|Observations of Io by Galileo spacecraft. Click image for higher resolution black and white. The image has a resolution of 335 meters (1,100 feet) per picture element. North is to the top of the image. Credit: Arizona/ Galileo images NASA/JPL. False color. BW Image produced by: Zibi Turtle, Planetary Image Research Lab. (PIRL), Lunar and Planetary Lab. (LPL), University of Arizona|
"The bottom line is that there seems to be enough salt in Io’s volcanic atmosphere to supply both the amount of sodium that one sees in the neutral clouds and the chlorine in the plasma torus," said Strobel, who is also a professor of physics and astronomy at Johns Hopkins.
A slightly eccentric orbit around Jupiter and the gravitational fields of two nearby large moons, Europa and Ganymede, subject Io to a great deal of stress, flexing the moon’s crust and heating its core. As a result, Io is hands-down the most volcanically active planetary body in the solar system. Roughly comparable in size to Earth’s moon, Io’s frequently active volcanoes would make it a hell for anyone who might want to visit, but it’s a heaven for scientists eager to watch a planetary body regularly belch up tons of its innards.
"Roughly two tons of volcanic material are tossed into Io’s magnetosphere every second, and then when this material is ionized [electrically charged], the inner magnetosphere starts to resemble a miniature pulsar," Strobel said.
Interactions between the clouds of electrically charged gas around Io and electrically charged particles in Jupiter’s polar atmosphere speed up the rotation of the charged particles around Io but also apply an infinitesmal drag to the rotation of Jupiter, gradually slowing the speed at which the giant planet spins.
"It’s a remarkable, unique system of interaction," Strobel said. "We’ve learned quite a bit since the days when Voyager 1 first swept by the moon in 1979 and revealed eight active volcanoes, but we don’t understand it completely."
Most critical to astrobiologists studying Jupiter’s moons, the eccentricity or oval shaped orbits of Jupiter’s moons are pumped or oscillated by tidal forces as they orbit. This input of Jupiter’s gravitational energy heats up the inner moons particularly like Io without relying only on the Sun’s radiant heat, and thus gives an interesting way to provide one of the three ingredients for life–an energy source–even if far from the Sun.
Jupiter itself seems to be a long shot in the search for extraterrestrial microbial life. True, the planet is warm and has plenty of organic materials. Jupiter may also have wet and dry areas like the desert and tropical regions of Earth. Telescopes on Earth and on the Galileo space probe even have detected areas on Jupiter with clouds of water that could indicate rainfall. But other qualities of Jupiter seem far too extreme to sustain life.
For instance, a great deal of Jupiter is composed of liquid metallic hydrogen, an element only possible at pressures exceeding 4 million bars. (The atmospheric pressure on Earth at sea level is a little over one bar, or 14.7 pounds per square inch.) Any life that could withstand such conditions would still have to face Jupiter’s torrential winds, which clock in at an astounding rate of 400 miles per hour, twice as fast as tornadoes on Earth.
What remains to be found among the Giant Planets like Jupiter and Saturn are some candidates that combine all three ingredients for primitive life: energy, liquid water and some atmosphere. Only Saturn’s moon, Titan, has an appreciable atmosphere, and only Jupiter’s Europa or Ganymede have any indications of water ice. But uniquely powerful tidal forces around the Giant Planets do offer some promising, non-radiant and non-volcanic heat sources.
The Galileo probe has neared the end of its investigations of the Jovian system. The latest series of mission trajectories, now referred to as the Galileo Europa Mission (GEM), conclude its highly focused follow-on to Galileo’s Jupiter system exploration and a precursor for future missions to Europa and Io. Galileo has functioned in orbit more than three times longer than its originally planned mission.
Other authors on the paper were Gabriel Paubert of the Institut de Radio-Astronomie Millimetrique; and Julianne Moses of NASA’s Lunar and Planetary Institute. This research was supported by the NASA Planetary Atmospheres Program.The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA’s Office of Space Science, Washington, D.C. Additional information about Galileo and its discoveries is available on the Galileo mission home page at http://galileo.jpl.nasa.gov/.