NASA Revives Saturn Probe in Time for Fly-By of Geysers of Enceladus


NASA reawakened the Cassini spacecraft from a forced hibernation while in orbit around Saturn on Wednesday, after three weeks of stalled science work due to a computer glitch. All of the probe's science instruments have been reactivated, and the spacecraft is in good health, just in time to record observations of Saturn's icy moon Enceladus during a close flyby set for Nov. 30. The upcoming flyby will bring Cassini within about 30 miles (48 kilometers) of the surface of Enceladus, Saturn's sixth-largest moon. The frigid world has icy geysers jetting from its south pole and — possibly — a subsurface ocean of liquid water that some NASA experts believe may harbor life.

Cassini had been operating in a protective standby mode — called "safe mode" — since Nov. 2 because of an ill-timed flip of a data bit in Cassini's command and data system computer. The unexpected bit flip prevented Cassini's main computer from registering a vital instruction, and the spacecraft went into standby mode as a result.

"Engineers have traced the steps taken by the computer during that time and have determined that all spacecraft responses were proper, but still do not know why the bit flipped," NASA officials said in an update released Wednesday.

The computer glitch marked the sixth time that Cassini has gone into safe mode since its launch in 1997. During these periods, the probe beams engineering and spacecraft health data to its mission operations center at NASA's Jet Propulsion Laboratory in Pasadena, Calif., but cannot perform science observations. The glitch prevented Cassini from studying Titan, Saturn's largest moon, during a Nov. 11 flyby of the cloud-covered satellite.

Cassini will fly near Enceladus' north polar region on this flyby, and then will perform another rendezvous just like it three weeks later. The twin encounters will mark the spacecraft's second-closest approach to Enceladus (Cassini dipped to within 16 miles, or 25 kilometers, of the moon in October 2008).

During the Enceladus flyby, Cassini will use its instruments to take gravity measurements of the icy moon. The results will be compared with those from an earlier flyby of Enceladus' south pole to better understand the moon's interior structure, according to NASA officials. Cassini will also sample the charged-particle environment around Enceladus and snap images in visible light and other parts of the spectrum during the close encounter, they added.

The Cassini spacecraft arrived at Saturn in 2004 to study the mysterious planet and its rings, and to deliver the European-built Huygens probe to land on Titan. Cassini completed its primary mission in 2008 and is now in a second phase that extends through May 2017.

The mother lode of all discoveries was discovered during the past fly bys at the South Pole of Enceladus, according to Carolyn Porco, director of flight operations and imaging team leader for the Cassini spacecraft in orbit around Saturn, describing Cassini’s findings of elevated temperatures in the moon’s polar region, as well as an enormous plume of icy particles shooting tens of thousands of kilometers into space. Analysis of the icy trail, which includes water vapor and trace amounts of organic materials such as methane, carbon dioxide, and propane, suggests it is fueled by geysers erupting from a pocket of salt water within the moon.

The findings, noted Porco, point to the possibility of “an environment where life itself might be stirring. Should we ever discover that a second genesis had occurred in our solar system, independently outside the Earth,” she added, “then I think at that point the spell is broken. The existence theorem has been proven, and we could safely infer from it that life was not a bug but a feature of the universe in which we live, that it’s commonplace and has occurred a staggering number of times.”

On the surface, Saturn's icy moon Enceladus is one of the oddest places in our solar system to look for extraterrestrial life.  Located in the frigid outer solar system, it should have frozen solid billions of years ago.

Unlike Mars or Jupiter's moon Europa, which give hints that they might harbor liquid water beneath their surfaces. With a diameter only slightly more than 500 miles, Enceladus just doesn't have the mass needed for its interior to stay warm enough to maintain liquid water underground.

Although its surface temperatures hover around 324 degrees below zero Fahrenheit, in 2005 NASA's Cassini spacecraft discovered a giant plume of water shooting up from cracks in the surface over the moon's south pole, indicating that there was a perhaps a global ocean of water beneath the ice.

Analysis of the plume by Cassini revealed that the water is salty, scientists estimate from the Cassini data that the south polar heating is equivalent to a continuous release of about 13 billion watts of energy generated by radiation coupled with tidal heating. During the formation of the solar system, if Enceladus was able to gather greater amounts of rock, which contained radioactive elements, enough heat could have been temporarily generated by the decay of the radioactive elements in its interior to melt the body.

But to  keep Enceladus warm enough for liquid water to remain under its surface it's theorized that Enceladus' slightly oval-shaped orbit generates heat from friction deep within Enceladus, called gravitational tidal forcing.

The gravitational tides also produce stress that cracks open the surface ice  at the south pole, opening and closing the cracks by shearing them back and forth generating friction, which releases heat.

To test the tidal heating theory, scientists with the Cassini team overlaid a map of the gravitational tidal stress on the moon's icy crust to a map of the warm zones created using Cassini's composite infrared spectrometer instrument (CIRS).

"However, they don't exactly match," says Dr. Terry Hurford of NASA's Goddard Space Flight Center, Greenbelt, Md. "For example, in the fissure called the Damascus Sulcus, the area experiencing the greatest amount of shearing is about 50 kilometers (about 31 miles) from the zone of greatest heat."

Hurford and his team believe that this discrepancy is caused by Enceladus' rotation rate, which wobbles slightly as it rotates.  "Cassini observations have ruled out a wobble greater than about 2 degrees with respect to Enceladus' uniform rotation rate," says Hurford.

The team created a computer simulation that made maps of the surface stress on Enceladus for various wobbles, and found a range where the areas of greatest stress line up better with the observed warmest zones.

"Depending on whether the wobble moves with or against the movement of Saturn in Enceladus' sky, a wobble ranging from 2 degrees down to 0.75 degrees produces the best fit to the observed warmest zones," said Hurford.

The wobble also generates about five times more heat in Enceladus' interior than tidal stress alone, and the extra heat makes it likely that Enceladus' ocean could be long-lived, according to Hurford. This is significant in the search for life, because life requires a stable environment to develop.

"Enceladus is not completely spherical, so as it moves in its orbit, the pull of Saturn's gravity generates a net torque that forces the moon to wobble," said Hurford. Also, Enceladus' orbit is kept oval-shaped, maintaining the tidal stress, because of the gravitational tug from a neighboring larger moon Dione. Dione is farther away from Saturn than Enceladus, so it takes longer to complete its orbit. For every orbit Dione completes, Enceladus finishes two orbits, producing a regular alignment that pulls Enceladus' orbit into an oval shape.

The mission is a collaborative effort between NASA, the European Space Agency and the Italian Space Agency.

Casey Kazan via NASA

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