NASA Cassini Image: Tiger Stripes of Enceladus

Pictured here is a high resolution Cassini image of Enceladus' Tiger Stripes, from a close flyby. 

Do underground oceans vent through the tiger stripes (in false-colour blue) on Saturn's moon Enceladus?

The long features, dubbed tiger stripes, are known to spew ice from the moon's icy interior into space, creating a cloud of fine ice particles over the moon's south pole and creating Saturn's mysterious E-ring.

Why Enceladus is active remains a mystery, as the neighbouring moon Mimas, approximately the same size, appears to be quite dead.

An analysis of dust captured by Cassini found evidence of sodium, as expected in a deep salty ocean.

Such research is particularly interesting since such an ocean would be a candidate to contain life.

Conversely however, recent Earth-based observations of ice ejected by Enceladus into Saturn's E-Ring showed no evidence of the expected sodium.

Image: Cassini Imaging Team, SSI, JPL, ESA, NASA

Extreme Space Weather at Mercury Blasts the Planet's Poles

The solar wind sandblasts the surface of planet Mercury at its poles, according to new data from a University of Michigan instrument on board NASA's MESSENGER spacecraft.

The sodium and oxygen particles the blistering solar wind kicks up are the primary components of Mercury's wispy atmosphere, or "exosphere," the new findings assert.

Through interacting with the solar wind, they become charged in a mechanism that's similar to the one that generates the Aurora Borealis on Earth.

The findings are published in the Sept. 30 edition of Science.

The Fast Imaging Plasma Spectrometer (FIPS,) made by U-M scientists, has taken the first global measurements of Mercury's exosphere and magnetosphere in an effort to better understand how the closest planet to the sun interacts with its fiery neighbor.

The measurements confirmed scientists' theories about the composition and source of the particles in Mercury's space environment.

"We had previously observed neutral sodium from ground observations, but up close we've discovered that charged sodium particles are concentrated near Mercury's polar regions where they are likely liberated by solar wind ion sputtering, effectively knocking sodium atoms off Mercury's surface," said FIPS project leader Thomas Zurbuchen, a professor in the Department of Atmospheric, Oceanic and Space Sciences and Aerospace Engineering at the U-M College of Engineering.

Earth and Mercury are the only two magnetized planets in the solar system, and as such, they can somewhat deflect the solar wind around them.

The solar wind is a squall of hot plasma, or charged particles, continuously emanating from the sun. Earth, which has a relatively strong magnetosphere, can shield itself from most of the solar wind. Mercury, which has a comparatively weak magnetosphere and is 2/3 closer to the sun, is a different story.

"Our results tell us is that Mercury's weak magnetosphere provides very little protection of the planet from the solar wind," Zurbuchen said.

Studying Mercury's magnetosphere and space environment helps scientists understand fundamental science about the sun.

NASA Mercury Messenger Finds Surprises

On its third swing past Mercury, NASA’s Mercury Messenger spacecraft discovered an unexpectedly young lava plain, rapid rufflings of the planet’s weak magnetic field and an unanticipated dance of elements in the thin atmosphere.

“I think the biggest surprise for the community is that the planet is turning out to be much more dynamic than people appreciated,” said Sean C. Solomon of the Carnegie Institution of Washington who is the principal investigator for the Messenger mission.

The flyby occurred in September, when the spacecraft swooped within 142 miles of Mercury’s surface at 12,000 miles per hour, but the findings of that flyby just appeared in three papers the journal Science published last week on its Web site.

Within the 180-mile-wide double-ring Rachmaninoff crater, Messenger photographed flat, smooth plains that scientists interpreted as the hardened outflow of lava. Based on the number of smaller impact craters, the age of the volcanic deposits within Rachmaninoff is probably less than two billion years, said Louise K. Procktor of the Johns Hopkins University Applied Physics Laboratory and lead author of the paper that examined the crater.

While not recent, that would still suggest that Mercury was volcanically active well into its middle age, given that it formed 4.5 billion years ago with the rest of the solar system.

“It is quite a big surprise,” Dr. Procktor said.

Mercury, not much more than 3,000 miles in diameter, is by far the smallest of the planets, and thus many thought it would have cooled off in less than a billion years. But Mercury’s mantle may not rise and fall in convective patterns as in larger planets like Earth, and that may have kept the heat inside for more than two billion years. By imparting heat and relieving pressure, the impact of the meteoroid that formed the crater may have helped melt rocks that were already close to melting point.

Messenger’s measurements also detected rapid fluctuations in Mercury’s magnetic field as it shifted from one side of the planet to the other. Such fluctuations occur around Earth in a matter of hours. Around Mercury, the fields shifted in minutes.

“It looked as though we were seeing a complete collapse of the magnetic field,” Dr. Solomon said. “This is an incredibly dynamic magnetosphere.”

The third paper looked at Mercury’s tenuous atmosphere, which consists of atoms knocked off the surface by sunlight, charged particles and dust-size meteoroids. One surprise was that near the equator, the concentration of calcium was higher near sunrise than sunset, but that effect was not seen for sodium or magnesium.

Because only calcium and not the other elements exhibits this behavior, “We can’t explain it,” said Ronald J. Vervack Jr., another scientist at the Johns Hopkins laboratory and lead author of that Science paper. “They just don’t look the same, and they don’t look the same anywhere we look,” he said. “Which is a bit puzzling, because we expected there to be some similarities.”

A tail of sodium that had been seen behind Mercury during the first two flybys, in January 2008 and October 2008, was much diminished in the third flyby. The reason was that Mercury was in a different part of its elliptical orbit, and the resulting change in velocity diminished the power of sunlight that was sweeping the sodium atoms away from Mercury into the tail.

A clearer picture should start to emerge next year. The spacecraft, launched in 2004, has been doing a gravitational do-si-do with the inner planets to slow down as it spirals inward and gets in position to enter orbit around Mercury in March.