Cassini Spacecraft Revisits Icy Saturn Moon

This story was updated at 8:28 pm. EDT.

NASA?s Cassini spacecraft circling the planet Saturn took a new look at one of the ringed world?s icy moons and its geyser-like plumes on Monday.

The probe zoomed past Saturn?s moon Enceladus with its camera eyes wide open to photograph the jets of frozen water vapor that gush from fissures along the satellite?s south pole.

?It's super exciting, because we'll pass just about 50 kilometers (30 miles) from the surface, and fly into the plume at the south pole,? wrote Cassini mission scientist Amanda Hendrix of the Jet Propulsion Laboratory in Pasadena, Calif., in a NASA blog chronicling the event.

The 39,540 mph (63,633 kph) flyby marked Cassini?s fifth swing past Enceladus, an icy moon of Saturn about 310 miles (500 km) wide that has tantalized scientists with geyser-like eruptions of icy water vapor that were first spotted in 2005. The spacecraft last visited Enceladus earlier this year in March, when it skirted through the fringe of the plumes and found signs of organic chemicals similar to those seen in comets.

But during today?s pass, Hendrix noted, Cassini was slated to use its cameras to take detailed images and temperature measurements of the moon?s active jets. The eruptions produce a halo of frozen water vapor and gas that replenishes Saturn?s E-ring as Enceladus circles the planet.

Cassini was expected to survey the source of the jets - prominent fractures that scientists have dubbed ?tiger stripes? - in the infrared, visible and ultraviolet range of the spectrum. Researchers hope the probe was able to measure the size of ice grains within Enceladus? plumes and generate temperature maps to help determine if liquid water or water vapor is present near the moon?s surface.

But despite Cassini?s resolution of about 23 feet (7 meters) per pixel, the probe was not expected to resolve the individual vents spewing the icy water vapor, which may be up to 2 feet (0.5 meters) wide, mission scientists said.

?We won't see such openings even in our highest resolution images,? wrote Carolyn Porco, Cassini?s imaging team leader at the Space Science Institute in Boulder, Colo., in a Sunday blog entry. ?But maybe ... just maybe ... we might see other evidence of eruption, either something geologically unusual or banks of snow where icy particles in the jets fall back to the ground.?

Cassini?s Monday flyby past Enceladus began in earnest at about 3:05 a.m. EDT (0805 GMT), when the spacecraft was due to use its cameras to take the first long-distance view of the moon?s plume of the rendezvous.

The probe was scheduled to make its closest approach to Enceladus at about 6:31 p.m. EDT (2131 GMT), swinging within 30 miles (50 km) of the moon, before moving on. Cassini has orbited Saturn since July 1, 2004 and is now in the midst of a two-year extension of its initial four-year mission.

?Whatever we see, this will definitely be a first and very unusual event,? Porco added.

Cassini was expected to begin beaming home the bulk of its flyby images and data at about 12:00 a.m. EDT (0400 GMT) Tuesday morning and take a parting glance at Enceladus. The spacecraft has two more passes near Enceladus set for later this year, on Oct. 9 and Oct. 31, respectively, that could bring it even closer to the icy Saturnian moon, NASA officials said.

• Video: Enceladus' Cold Faithful
• Cassini's Greatest Hits: Images of Saturn
• Special Report: Cassini's Mission to Saturn