Meeting briefs: European Planetary Science Conference and AAS Division for Planetary Sciences

The snows of Enceladus, evidence for climate change on Titan and planets in threes

By Science News Staff

Web edition : Tuesday, October 11th, 2011

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MERCURY MAPAfter six months in orbit around Mercury, the MESSENGER spacecraft has produced a surface map covering 99 percent of the planet.NASA, Johns Hopkins University Applied Physics Laboratory, Carnegie Institution of Washington

Messages from Mercury
After orbiting the solar system’s smallest, innermost planet for just six months, NASA’s MESSENGER spacecraft is relaying some intriguing observations. For one, MESSENGER team members reported October 5, Mercury’s magnetic field is offset from the planet’s core: it’s centered 480 kilometers to the north of center, a distance roughly 20 percent that of the planet’s diameter. Atmospheric calcium curiously clusters near the planet’s equator at dawn. And a 99 percent complete surface map reveals a complex area of former volcanic activity in the northern hemisphere. It’s also possible for ice to exist on this smoldering world: as is true for Earth’s moon, there are perpetually shadowed hollows in Mercury’s surface. At the poles, these areas are tucked into craters — super-shady places that make perfect hidey-holes for ice. — Nadia Drake
Kepler’s Three Musketeers
One of the 170 planetary systems detected by NASA’s Kepler spacecraft includes three tightly packed planets orbiting a sunlike star. With an orbit within Mercury’s distance from the sun, one of the planets is a super-Earth that whizzes around its star every 3.5 Earth-days. The next planet, Neptune-sized, completes its exo-year in 7.6 days, and the furthest away — a second Neptune — takes a leisurely 14.9 day path. Astronomers used a statistical method to infer the presence of the innermost planet, said Bill Cochran of the University of Texas at Austin, who presented the find on October 4. — Nadia Drake
Three distant dwarfs
Scientists spying on dwarf planets in the Kuiper Belt have used stellar occultations — when a planet passes between Earth and a star — to determine some physical features of the distant worlds Eris, Makemake and Quaoar. Quaoar is bigger, less dense than previously thought, and might be missing a big chunk. Makemake is elliptical, somewhat flattened — perhaps because it spins so quickly — and has two different types of surface terrain, astronomers reported on October 4. Eris, Pluto’s rival for the biggest dwarf, is essentially Pluto’s twin – but very bright, and potentially covered in a collapsed atmosphere. — Nadia Drake
Enceladean snowstorm
It’s snowing on Saturn’s moon Enceladus, and probably has been for tens of millions of years. Snow cover on the icy moon may be 100 meters deep in places — that’s the length of three blue whales — and is produced by the icy particles bursting from the moon’s south polar geysers. Scientists studying the plume determined that some of the heavier particles can re-accrete and snow on the surface, producing observable color asymmetries. The rest fly off into the Saturnian system where they water the giant planet and craft its E ring. The Enceladean snow fields indicate that the south polar geysers may have been erupting for as long as 100 million years, Paul Schenk of the Lunar and Planetary Institute in Houston said October 3. — Nadia Drake
Unlucky Uranus
Getting knocked around in the early solar system is no new idea — ’twas apparently a lot like bumper cars on steroids back then — but Uranus may have suffered a one-two punch that explains its peculiar orientation. The planet is tipped 98 degrees — it’s on its side. And its moons didn’t notice and reorient themselves. They orbit askew, too. On October 6, Alessandro Morbidelli of the Cote d’Azur Observatory in Nice, France, explained this observation by suggesting that it was not one impact but two that toppled the planet. The first got only part of the job done. The second came in and finished it off. — Nadia Drake
Signs of climate change on Titan
Earth may not be the only body in the solar system experiencing significant changes in climate. Saturn’s moon Titan might be, too, Jani Radebaugh of Brigham Young University in Provo, Utah, suggested October 4. While equatorial sand dunes cover millions of square kilometers on Titan, the dunes are missing near the poles. At high latitudes, the moon is covered with landforms resembling older dunes. Radebaugh says that desert conditions may once have allowed dune formation in these areas, but that changing winds and humidity delivered liquid methane and organic cements, which have since anchored the dunes in place and altered their shape. — Nadia Drake
Enceladus’ ocean may be frozen
While it’s tempting to conclude that Enceladus must host a liquid water reservoir, an extraterrestrial salty ocean is not certain. Mikhail Zolotov of Arizona State University in Tempe suggested on October 3 that water must have been present at some point during Enceladus’ history — but that it’s not necessarily there now. In fact, the gases present in the plume are inconsistent with the presence of liquid, even though some of the plume’s ingredients require a watery birth. Zolotov says it will take some work to reconcile these observations, but suggests that a flash-frozen, formerly liquid reservoir could be lurking beneath the Saturnian moon’s crust. — Nadia Drake

Daily Science News...

Astrophysicists have found evidence of black holes destroying stars, a long-sought phenomenon that provides a new window into general relativity. The research, reported in the latest issue of the Astrophysical Journal, also opens up a method to search for the possible existence of a large population of presently undetectable "intermediate mass" black holes which are hypothesized to be precursors to the super-massive black holes at the centers of most large galaxies.

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See Also:

Space & Time

• Black Holes
• Galaxies
• Stars

Matter & Energy

• Albert Einstein
• Quantum Physics
• Physics

Strange Science

Reference

• Astrophysics
• Supergiant
• Stellar evolution
• Red giant

The study was carried out primarily by Glennys Farrar and Sjoert van Velzen at New York University's Center for Cosmology and Particle Physics, and also included the following researchers: Suvi Gezari of Johns Hopkins University's Department of Physics and Astronomy; Linda Ostman of Spain's Universitat Autònoma de Barcelona; Nidia Morrell of the Las Campanas Observatory in Chile; Dennis Zaritsky of the University of Arizona; Matthew Smith of South Africa's University of Cape Town; Joseph Gelfand of NYU-Abu Dhabi; and Andrew Drake of Caltech. Van Velzen is currently a doctoral candidate at Radboud University in the Netherlands.

Cosmologists have calculated that, on occasion, a star's orbit will be disturbed in such a way that it passes very near the super-massive black hole at the center of its galaxy -- but not so close that it is captured whole. Such a star will be torn apart by the extreme tidal forces it experiences: the force of gravity on the near side of the star is so much stronger than that on the far side that the gravitational force holding the star together is overwhelmed, causing the star to simply come apart. While some of the star's matter falls into the black hole, much of it continues in chaotic orbits, crashing into itself and producing intense radiation lasting days to months. These phenomena are called stellar tidal disruption flares, or TDFs.

Although discovering evidence of TDFs has been a high priority of astrophysicists for many years, and several possible examples have been found using X-ray and UV satellites, discovering TDFs in a large-scale, systematic survey using ground-based optical telescopes as has now been achieved, is critical to controlling bias and avoiding misidentifications.

The difficulty in detecting TDFs is largely due to the challenge of distinguishing them from more common types of flares such as supernovae. (For every TDF there are about 1000 supernovae.) In addition, some super-massive black holes have an "accretion disk" surrounding them -- gas and dust, often left from an earlier merger with another galaxy -- which is continuously feeding the hole. Such accreting black holes are usually evident from the bright emission they produce and are known as quasars or Active Galactic Nuclei (AGN). However, a hiccup in the accretion of an undetected active black hole could produce a flare that might be mistakenly identified as a TDF.

The researchers on the Astrophysical Journal study uncovered sound evidence for the presence of two TDFs through a rigorous analysis of archival data from the Sloan Digital Sky Survey (SDSS).

To do so, they sifted through voluminous SDSS data, in which more than 2 million galaxies were repeatedly observed over 10 years. By very carefully registering the images and looking at differences between consecutive images, they obtained a sample of 342 intense and well-measured flares.

Of these, almost all could be classified into supernovae and AGN flares. However, two cases were left that did not fit either classification. By relying on multi-year observations, the researchers could see that the two flares' host galaxies showed no other flaring activity, as would be the case if the flares came from a hidden variable AGN. This means the possibility these two flares were produced by undetected AGNs is extremely small.

In addition, the researchers located these flares at the nucleus of their galaxy with high precision, which reduces the likelihood that they are supernovae to less than 1 percent since supernovae are randomly distributed through galaxies.

Finally, the properties of these flares are very different from flares of AGNs and supernovae -- and their spectra are unlike any supernovae observed to date. Supernovae flares are characteristically very blue at first but become red as they cool and rapidly decay, whereas the TDF flares are very blue throughout -- slowly decaying without changing color. This behavior is consistent with expectations for a TDF -- the debris from the star should rapidly form an accretion disk and look like a short-lived AGN.

Sjoert van Velzen, the study's lead author, was a Dutch first-year graduate student who came to NYU to work under the direction of Glennys Farrar, a Professor of Physics at NYU and senior scientist of the project. Van Velzen is now completing his Ph. D. in Holland.

About his first encounter with real scientific work, van Velzen says, "Searching through 2.6 million galaxies was actually a lot of fun -- there is so much to discover! Based on our search criteria and observing two TDFs that met those criteria, the rate of TDFs is about once per 100,000 years, per galaxy. It's quite thrilling to have been able to make such a measurement."

"The next step is to develop models to explain in detail the flares' properties and duration, and address the question of whether TDFs could be responsible for producing Ultrahigh Energy Cosmic Rays, whose sources have been elusive up to now," says Farrar. "It is very exciting that we are on the verge of obtaining a large and better-observed sample of TDFs to study -- though a more sensitive search of SDSS archival data and the new generation of transient surveys which will observe more flares in real-time and with multi-wavelength follow-up. A large sample will be invaluable to understanding many outstanding questions in astrophysics."

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