Astrosphere for April 15, 2008

Don’t just look inward, look outward. There’s a whole astrosphere out there. Your picture for the day is Saturn (of course), captured by Stargazer 7000.

Astronomy Picture of the Day has a beautiful shot of the night sky over Sweden.

PZ Myers has spoken and I must obey. Here’s all you need to know about expelled.

And here’s a review for another movie, Dark Matter.

Speaking of dark matter, Ethan Siegel explains the practical uses for his research.

Astroprof recalls famed astronomer John Archibald Wheeler; the man who coined the term “black hole” passed away this week.

Bad Astronomer Phil Plait celebrated Yuri’s night, in style.

Cassini Finds Patterns and Rhythm in Saturn’s Rings


Cassini has been orbiting around Saturn for almost four years, and amazingly, the spacecraft keeps discovering new and unexpected features about this world and its system of rings and moons. Recently, in two of Saturn’s rings, Cassini found orderly lines of densely grouped, boulder-size icy particles that extend outward across the rings like ripples from a rock dropped in a calm pond. Surprisingly, the distances between these ring particles stay relatively equal even though their velocities may change. This type of pattern is completely new, as normally, the distances between particles change with their velocity.

The pattern was detected when Cassini sent out three signals toward Earth. The signals crossed Saturn’s rings, and the frequencies were scattered from the passing ring particles. Once the signals were captured by Earth-based antennas of NASA’s Deep Space Network, Cassini scientists saw a regular pattern in the received signal frequencies.

“This particular feature is the smallest and most detailed of anything seen in Saturn’s rings so far,” said Cassini radio science team member Essam Marouf. “In the chaotic environment of the rings, to find such regularity in the most cramped areas is nothing short of amazing.” The regular structure can only be found in locations where particles are densely packed together, such as the B ring and the innermost part of the A ring. The signals were sent to capture a complete view of the rings.

The unexpected pattern within Saturn’s rings may give scientists some new ideas of what to expect from other similar planets and solar systems.

Scientists call this pattern of particles “enormously extended natural diffraction grating.” A diffraction grating has parallel lines like a picket fence; when light hits this fence, it separates according to wavelength, from ultraviolet to infrared light.

“The signals showed that the particle groups were arranged in an unexpectedly regular formation that had rhythm within the rings of Saturn,'” said Marouf. “Each particle is in its own orbit, and sometimes they collide and move apart as their velocities change. As a result, you have particles bunched together into dense groups that extend across the ring in harmony with each other.”

Original News Source: Cassini Press Release

Enceladus: Cold Moon With a Hot Spot


Saturn’s tiny moon Enceladus is a cold and icy place. But somehow, there’s enough heat being generated on Enceladus’ south pole to eject plumes of ice and vapor high above the moon. These plumes are extremely intriguing to the Cassini mission scientists and they want to know more about this hot spot on a very cold moon. In fact, Enceladus has become a major priority for study by the Cassini team and they are anticipating learning more about the moon in an upcoming fly-by.

The temperature at Enceladus’ south pole is about -220 degrees Celsius, but the hot spot is at least 100 degrees warmer. The leading model for the cause of the plumes on Enceladus is that the moon’s tides cause its crust to ratchet, or rub back and forth, in a set of faults near the south pole. The forces between Enceladus, the big planet Saturn and another moon, Dione cause what’s called dynamical resonance, and Enceladus is continually squeezed under this gravity field. This process creates a small hot spot, in relative terms, for an icy satellite.

Cassini has actually flown through the plumes, giving scientists a glimpse of the plume’s make-up.

“The plume particles are like smoke, ice smoke,”said William B. McKinnon, professor of earth and planetary sciences at Washington University in St. Louis. “If you were standing on Enceladus’ surface you wouldn’t even be able to see the plumes. The particles are just larger than the wavelength of light, about one-thousandths of a millimeter. Most icy bodies of this size are geologically inert, but this is a clear indication of geological activity. Cassini has found active venting of water vapor. This leads to scientifically intriguing speculations and questions.”

The scientists are pondering if Enceladus has active ice volcanism, and if so, is it due to ice sublimating, like a comet, or due to a different mechanism, like boiling water as in Old Faithful at Yellowstone. Even though there may be water on the moon, McKinnon doesn’t believe there is the possibility of life on Enceladus. This is because measurements made from Earth don’t indicate there is enough sodium present in the plumes to warrant the “life” question.

“The emerging view is that there’s not obvious evidence for a subterranean ocean in contact with rock, no boiling or venting,” said McKinnon.

The Cassini science team has made Enceladus a major priority and there will be seven additional close fly-bys of the moon by the spacecraft through mid-2010 (provided the mission is extended to that period.) The next fly-by will be on March 8, 2008 and Cassini will approach Enceladus at an incredibly close 25 km in altitude at the low latitudes and fly over the south pole at 580 km altitude. The spacecraft will actually fly through the plumes and should be able to take high-phase images of the plumes, map the temperatures of that region, search for any activity at other latitudes as well as image other interesting features on Enceladus, such as “tiger-stripe”-like fissures found near the south pole.

“We still can’t say how truly ‘hot’ the hot spots are,” said McKinnon. “We’ll probably learn this in March.”

Original News Source: Washington University Press Release