My first Magazine Sky & Telescope - 01.2019

NEWS NOTES SOLAR SYSTEM Jupiter’s Magnetic Field Has Weird Structure netic ﬁ eld of any of the planets in the solar system. Like the ﬁ eld that shelters Earth, Jupiter’s shield is essentially dipo- lar, which means it has a north pole and a south pole, similar to the ﬁ eld created by a bar magnet. A really, really big bar magnet. Or, at least it kind of does. Reporting in the September 6th Nature, Kimberly Moore (Harvard) and colleagues have discovered a strange plume of mag- netic ﬁ eld shooting up from a region in Jupiter’s northern hemisphere and reentering the planet at its equator. And it’s three times stronger than the main dipole ﬁ eld. Combining data from eight ﬂ ybys of the Juno probe, the scientists conﬁ rmed the existence of the bizarre magnetic feature, hints of which had shown up in an analysis in 2017 from Juno’s ﬁ rst orbit. The structure looks like a ponytail shooting out from the planet’s fore- t Colorful, swirling cloud belts and vortices dominate Jupiter’s northern hemisphere in this image captured by NASA’s Juno spacecraft. IN BRIEF Voyager 2 Approaches Interstellar Space Since August, the Voyager 2 spacecraft has detected an uptick in cosmic rays, high- energy particles such as protons impinging on the solar system from deep space. Its sister craft, Voyager 1, saw a similar increase in 2012 about 3 months before it punched through the heliopause, the point where the solar wind yields to the tenuous pressure of interstellar gas. Voyager 2 — now roughly 18 billion kilometers (11 billion miles) from home — is inching toward the same milestone. The two Voyager spacecraft left Earth in 1977 for grand tours of the giant planets. Both probes cozied up to Jupiter and Saturn, while Voyager 2 went on to become the i rst (and only) craft to zoom past Uranus and Neptune. Now both are headed for points beyond the solar system. Read more about the Voyager journeys at https://is.gd/V2CosmicRays. ■ CHRISTOPHER CROCKETT 12 JA N UA RY 2 019 • SK Y & TELESCOPE TESS Satellite Spots First Exoplanet Scientists working with data from the recently launched Transiting Exoplanet Survey Satel- lite (TESS) have discovered a super-Earth in a close orbit around the Sun-like star Pi Mensae. Based on its average density, this planet probably has a rocky or iron core sur- rounded by an atmosphere of light molecules such as hydrogen, helium, water, or methane. The planet’s atmosphere is likely too close to the planet’s surface for much starlight to i lter through when the world passes in front of its star, so studying it via transit spectroscopy will be difi cult. But the planet’s proximity to Earth (60 light-years) and the star’s brightness (magnitude 5.7) make the system a promising target for other studies. Chelsea Huang (MIT) and colleagues reported the discovery Sep- tember 16th on the astronomy preprint site arXiv.org. The world is the second discovered in the Pi Men system: The other is a gas giant on a highly elongated orbit. The star is visible to unaided eyes in clear, dark skies. ■ MONICA YOUNG head and reentering through the nose, at a location the team calls the Great Blue Spot (for its color in a map of the planet’s ﬁ eld). There’s nothing like this ponytail in the southern hemisphere. “This was a very unexpected result,” Moore says. “Why is the ﬁ eld so simple in one hemisphere and so complicated in the other?” Earth’s magnetic ﬁ eld is produced by churning liquid iron in the planet’s outer core. But Jupiter doesn’t have an iron core — it’s unclear if it has a core at all. The source of the magnetic ﬁ eld is instead an overlying mantle of metal- lic hydrogen (S&T: Dec. 2017, p. 14). The most likely reason for Jupiter’s weird ﬁ eld is layering in the metallic hydrogen, the team argues. This could naturally arise if the planet’s core is dissolving: Rock and ice mixed in with hydrogen would raise the density, and if that mixing isn’t uniform, it could cre- ate layers of different densities. These could destabilize the mantle’s cyclic convection patterns or spur different convection patterns in distinct layers. ■ CAMILLE M. CARLISLE Dust Storms on Titan Saturn’s moon Titan now joins Earth and Mars among the worlds known to whip up dust into massive storms. In 2009 and 2010, the Cassini spacecraft spied three distinct, short- lived patches of infrared light near the moon’s equator. The time scale and spectra of these regions appear to be consistent with dust whipped up by gusts blowing across dunes, Sébastien Rodriguez (Paris Institute of Earth Physics) and colleagues report September 24th in Nature Geoscience. The researchers considered several explanations, including cryovolcanoes, lava l ows, and methane rain- storms. But none of these were a good match for the infrared spectra or the observed time scales, ranging from 11 hours to 5 weeks. The dust storms point to fairly strong surface winds — or at least, strong for Titan: The roving dust requires gusts of at least 1.4 m/s (3.1 mph), about i ve times the ambient wind speed. The winds could have accompanied methane squalls expected at the equator dur- ing Titan’s equinox. ■ CHRISTOPHER CROCKETT JUPITER HAS THE STRONGEST mag-

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