Philae, Active Moons, and a Larger Milky Way

Philae…Phone Home!

On Wednesday, March 11, European Space Agency (ESA) scientists made their first attempt to contact Philae, the spindly, little lander believed to be lying on its side somewhere in the frigid shadow of a cliff on the surface of Comet 67P/Churyumov-Gerasimenko (or 67P to its friends), 300 million km from the Sun (about twice Earth’s distance). At 9:00 p.m. PDT on Wednesday (5:00 a.m. Thursday CET), the Rosetta orbiter sent a signal to the lander in hopes of receiving an answer—with only slim prospects for success, according to Rosetta team scientists. As of this writing the morning after, they have detected no response.

Despite the fact that several mechanisms were designed into the three-legged craft specifically to prevent it from ricocheting off the comet and to attach it firmly to the surface… None of them worked. Contacting the surface at about a meter per second, Philae bounced a kilometer off the surface, halted only by the comet’s extremely weak gravity, which is 1/100,000 Earth’s. This was still strong enough to pull the lander back, albeit very, very slowly. It took about two hours for Philae to fall back to the surface, during which time the comet rotated beneath it. When it finally came to rest, about a kilometer away from its intended landing site, tipped over in a craggy location that controllers believe gets only about an hour of direct sunlight each 12-hour rotation.

As the comet continues approaching the Sun, scientists hope that the angle of sunlight will change enough to illuminate Philae’s solar panels for a longer time, recharging its batteries. For Thursday’s attempt to succeed, the internal temperature of the lander had to be above –45°C (–49°F) so the batteries can charge at all (extremely cold batteries don’t function well), and its solar panels had to be able to generate 5.5 watts, just so Philae could receive the “ping” from Rosetta and start its wake-up routine. Philae will be able to respond to Rosetta as soon as its battery charges up to 19 watts, so there’s a slim chance that it may already have awakened but just can’t tell anyone.

Ironically, mission scientists say that if Philae had successfully landed where it was supposed to, it would’ve overheated in the sunlight by now, so if contact can be reestablished, it may turn out that its wayward bounce to a shady spot may have fortuitously prolonged its usefulness.

ESA will continue attempting to communicate with Philae until March 20—a window determined by orbital dynamics and the odd shape of the comet, which allow Rosetta to pass over the landing area. If ESA detects no response from Philae by then, mission controllers will try again when a similar window of opportunity opens in April. Philae’s mother ship, Rosetta, will continue orbiting Comet 67P through the end of 2015, observing changes taking place on the comet as it passes closest to the Sun in August. –Bing Quock

Underground Ocean on Ganymede

Astronomers have confirmed an extremely large subsurface ocean on Jupiter’s biggest moon, Ganymede, using the most unusual technique.

A team of scientists led by Joachim Saur of the University of Cologne used the Hubble Space Telescope to look at aurorae on the moon to determine the existence and size of the massive body of water.

But what do aurorae have to do with oceans?

“I was always brainstorming how we could use a telescope in other ways,” says Saur. “Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior.”

Jupiter’s powerful magnetic field causes aurorae to form at Europa’s poles, but it also induces a secondary magnetic field in Europa’s saltwater ocean. This “magnetic friction” would suppress the normal “rocking” of the aurorae as the moon orbits Jupiter and moves through its changing magnetic field. Europa’s subsurface ocean fights Jupiter’s magnetic field so strongly that it reduces the rocking of the aurorae to two degrees—instead of the six degrees we would expect if the ocean were not present.

Scientists estimate the ocean is 60 miles (100 kilometers) thick—10 times deeper than Earth’s oceans—and buried under a 95-mile (150-kilometer) crust of mostly ice.

“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said John Grunsfeld, of NASA. “In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth.” –Molly Michelson

Hydrothermal Activity on Enceladus

Speaking of icy moons, more big water-in-the-solar-system news this week comes from Saturn’s small moon Enceladus.

In 2005, the Cassini mission orbiting Saturn sent back stunning images of geysers of frozen water spewing from the south pole of the Enceladus. Now, findings published this week in Nature connect the plumes to activity inside the moon itself, indicating that Enceladus has not only a warm ocean at its southern pole, but ongoing hydrothermal activity. This makes it the first place off Earth found to have active hot-water chemistry.

In the study, a group led by Sean Hsu, a Cassini team member at the University of Colorado in Boulder, used Cassini’s Cosmic Dust Analyzer to analyze the exact contents of Enceladus’s south pole spray. In addition to the expected ice crystals and organic compounds, they found a surprising level of nanometer-sized particles that appear to be made of silica.

The next step was to figure out how they formed. “What we did first was just to know what these particles were, and by doing this, we started to think about how they formed,” Hsu explained.

On Earth, the most common way to form such tiny silica grains is hydrothermal activity. More specifically, they form when salty, alkaline water is supersaturated with silica and then undergoes a big drop in temperature. For example, in a hydrothermal vent like we find at the bottoms of Earth’s oceans, when hot, mineral-rich water bursts into the cooler ocean. Or the vents spewing out of Enceladus into the cold vacuum of space.

“We methodically searched for alternate explanations for the nanosilica grains, but every new result pointed to a single, most likely origin,” said co-author Frank Postberg, a Cassini CDA team scientist at Heidelberg University in Germany.

Other research also supports the team’s findings. Just last year, results were published that strongly suggested the presence of a six mile deep ocean beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick. Additionally, a second paper about the contents of the plumes was recently published in Geophysical Research Letters. It too suggests the presence of hydrothermal vent, not based on the presence of silica, but as one of two likely sources of the high concentration of methane in the plumes.

“These findings add to the possibility that Enceladus, which contains a subsurface ocean and displays remarkable geologic activity, could contain environments suitable for living organisms,” said NASA’s John Grunsfeld. –Elise Ricard

Bigger Milky Way

Our Milky Way galaxy may be at least 50 percent larger than previously thought, according to new findings published this week in Astrophysical Journal.

A ring-like filament of stars, called the Monoceros Ring, which wraps around our galaxy above and below the relatively flat plane of its disk three times, is now thought to be part of the Milky Way’s galactic disk. Its inclusion would extend the span of the Milky Way from 100,000 light years across to 150,000.

Professor Heidi Newberg of Rensselaer Polytechnic Institute established the ring’s existence in 2002. At the time though, it was thought to have been torn from the nearby Canis Major Dwarf Galaxy. Even then, there were questions about the concentration and distribution of stars. Building upon her previous research and revisiting data from the Sloan Digital Sky Survey, Newberg and her team reanalyzed the brightness and distance of stars at the edge of the galaxy.

“In essence, what we found is that the disk of the Milky Way isn’t just a disk of stars in a flat plane—it’s corrugated,” she explains. “As it radiates outward from the sun, we see at least four ripples in the disk of the Milky Way. While we can only look at part of the galaxy with this data, we assume that this pattern is going to be found throughout the disk.”

The Monoceros Ring is associated with the third of these ripples. The ESA’s Gaia telescope may soon give astronomers higher-resolution, three-dimensional views of the Monoceros Ring, and another ring even farther out called TriAnd, which may also be part of the Milky Way. –Elise Ricard

Images: ESA/Rosetta/Philae/CIVA, NASA/ESA, NASA/JPL, Rensselaer Polytechnic Institute