SPACE EXPLORATION | Expanding the boundaries of human understanding
01 April 2008
Cassini Spacecraft Finds Basic Conditions for Life on Enceladus
Closest flyby March 12 suggests interior of Saturn moon may be liquid
Washington -- The closest flyby so far of Saturn’s moon Enceladus by NASA’s Cassini spacecraft shows that one of the brightest objects in the solar system may have nearly everything it needs to sustain life.
Traveling about 15 kilometers per second and at an altitude of 50 kilometers, Cassini flew past Enceladus for its fourth encounter with the moon March 12 and its closest flyby to date with any celestial body, passing through the edge of one of the moon’s geyser like jets.
Cassini's instruments discovered evidence for the jets on Enceladus in 2005, finding that continuous eruptions of ice water create a gigantic halo of ice grains and gas around the moon that helps supply material to Saturn's E-ring. Enceladus is embedded in the ring.
In the latest flyby, sophisticated instruments “sniffed” and “tasted” one of the plumes, said Hunter Waite, principal investigator for Cassini’s ion and neutral mass spectrometer at the Southwest Research Institute in Texas, finding “methane, carbon dioxide, simple organics and more complex organics.”
Taken together, heat from the planet’s interior, organic molecules and water give Enceladus what scientists call “astrobiological potential.”
“We see on Enceladus the three basic requirements for the origin of life,” Larry Esposito, principal investigator for Cassini’s ultraviolet imaging spectrograph at the University of Colorado-Boulder, said during a March 26 briefing.
“We see water, although it may not be liquid,” he added, “we see organic compounds detected by the ion and neutral mass spectrometer, and we also have a source of heat, indicated by the composite infrared spectrometer. These three basic ingredients provide a minimum for the origin of life.”
MEASURES OF LIFE
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency (ESA) and Agenzia Spaziale Italiana, the Italian space agency.
On July 1, 2004, the Cassini-Huygens spacecraft (named for NASA’s Cassini orbiter and ESA’s Huygens probe) fired its main engine to reduce speed, allowing the spacecraft to be captured by Saturn's gravity and enter orbit. The Huygens probe was deployed to Titan, Saturn's largest moon, in January 2005. The spacecraft’s four-year mission to explore the ringed planet, its mysterious moons, stunning rings and complex magnetic environment ends in June.
At that time, Cassini will have completed 74 orbits of the planet, 44 close flybys of the hazy moon Titan and numerous flybys of Saturn's other icy moons. From then on, an extended mission will include seven more Enceladus flybys. The next flyby will take place in August.
The scientists used three different measurement methods -- remote, direct and what is called occultation (blocking of light) by a star -- to gather the latest data:
• The composite infrared spectrometer looked at Enceladus’s surface and measured temperatures and heat radiation.
• The ion and neutral mass spectrometer determined the chemical and elemental composition of the moon’s atmosphere.
• The ultraviolet imaging spectrograph watched Zeta Orionis, one of the stars in Orion’s belt, as it passed behind the plume, and used measurements of its starlight behind the plume to measure the plume’s shape, structure and composition.
That measurement confirmed the theoretical analysis performed before the flyby that showed it was safe for Cassini to fly very closely past Enceladus, even through part of the plume, during the March 12 flyby.
New heat maps of the surface show higher temperatures than previously known in the south polar region, with hot tracks running the length of giant fissures. The scientists say the organics on Enceladus are similar to some of those found in comets.
HABITAT FOR LIFE
Enceladus has at least five different kinds of terrain. Parts of the moon show craters no larger than 35 kilometers in diameter. Other areas show regions with no craters indicating major resurfacing events in the geologically recent past. There are fissures, plains, corrugated terrain and other crustal deformations. All of this indicates that the interior of the moon may be liquid today, even though it should have been frozen eons ago.
“We don’t yet see nor can we tell or state whether the interior of Enceladus contains liquid water and if that water might be a habitat for life,” Esposito said.
Esposito added that during future flybys -- in August, October and the following years -- scientists will try to “answer the question, ‘what in the interior makes these jets and plumes and what connection that might be to a possible habitation for life.’”
More information about Enceladus and the Cassini-Huygens mission is available at the NASA Jet Propulsion Laboratory Web site.
An ultraviolet imager watches the star Zeta Orionis pass behind jets of ice and gas spewing from Enceladus's southern region.