Fire and ice The MESSENGER spacecraft reveals water ice lurking in Torrid Mercury’s icy poles deeply shadowed craters near the innermost planet’s poles. by James Oberg Color explodes from Mercury’s surface in this enhanced-color mosaic. The yellow and orange hues signify relatively young volcanic plains, while blue represents older terrain. The planet’s equator runs horizon- he saga of water ice hiding in GEochemistry, and Ranging — started Planetary scientist David Paige of the works best when good observations are tally through the center of this image; the poles lie at top and bottom. NASA/JHUAPL/CIW the shadows on Mercury ranks orbiting Mercury in March 2011 and has University of California, Los Angeles, a guided by theory.” among the most fascinating been returning reams of data ever since. participating scientist on the MESSENGER During the past several decades, scien- chapters in the history of plan- The discovery of ice contains its own sur- project, stresses the wider significance of tists have come to realize that a conspiracy to the planet nearly 40 years ago, could not Untangling Mercury’s web etary exploration. And the prises and new mysteries. the ice story. “The Mercury discoveries of freak accidents involving Mercury’s make the observations necessary to prove It took centuries for astronomers to deci- story didn’t end a year ago Scientists had suspected for decades that demonstrate the power of theory and motion and orientation had created small the case. Still, radar observations from pher Mercury’s physical and dynamic when scientists using NASA’s water ice might survive in corners of the imagination in astronomy and planetary regions on the planet’s surface where it Earth did detect unusually reflective characteristics and to understand their MESSENGER spacecraft con- solar system’s innermost world. Theorists science,” he says. “However, just because ought to be cold enough for ice to form and regions in Mercury’s polar regions that implications. Although the planet lies rea- firmed the existence of ice in craters near realized that cold regions could exist in something might be there doesn’t necessar- survive for billions of years. Unfortunately, seemed to overlap the supercold regions sonably close to Earth, it is a difficult world Tthe planet’s poles. MESSENGER — short certain areas, and observers seemingly ily mean that it is. Careful observations instruments on NASA’s Mariner 10 space- theorists had calculated. This match pro- to observe in detail. As the closest planet to for MErcury Surface, Space ENvironment, backed up these theoretical computations. and analysis are required as proof. Science craft, which made the only previous visits vided a tentative hint of what might be. the Sun, Mercury never strays far from our 30 ASTRONOMY • DECEMBER© 2014 Kalmbach 2013 Publishing Co. This material may not be reproduced in any WWW.ASTRONOMY.COM 31 form without permission from the publisher. www.Astronomy.com A chill at the poles But not all coincidences are bad news. With better observations in following years, sci- entists realized that Mercury’s axis tilts 89.99° to its orbital plane — almost pre- cisely perpendicular. Theory suggests that this near-perfect match of axis and orbit is a long-term effect of the planet’s gravita- tional coupling with the Sun, which would mean that Mercury’s rotation axis adjusts to changes in its orbital inclination over time. As a consequence, deep craters near the planet’s poles can remain in permanent shadow and serve as ice traps for a billion years or more. In 1991, researchers beamed radar sig- nals at Mercury. Using the Goldstone radio telescope in California and the Very Large Array in New Mexico, they detected unusu- This oblique view of the inner world’s north polar region shows the surface temperature averaged over ally strong radar returns from the planet’s two Mercury years. Blue and purple represent the coldest areas, where ice can persist. NASA/UCLA/JHUAPL/CIW polar regions. These areas come into view because Mercury’s orbit tilts 7° to Earth’s, NATIONAL ASTRONOMY AND IONOSPHERE CENTER/ARECIBO OBSERVATORY CENTER/ARECIBO IONOSPHERE AND ASTRONOMY NATIONAL NASA/JHUAPL/CIW which allows astronomers to peek over the Deep craters near Mercury’s north pole harbor water ice. The first evidence for A mosaic of MESSENGER images of Mercury’s north polar region appears Maximum temperature (kelvins) these deposits came from Earth-based radar observations in the early 1990s, beneath radar data of the same area seen at left. All of the large deposits sit inner world’s poles. The giant 1,000-foot 50 100 150 200 250 300 350 400 450 500 550 which revealed bright patches that reflected most of the incoming signal. on the floors or walls of impact craters, including Kandinsky and Prokofiev. (305 meters) radio dish at Arecibo Obser- vatory in Puerto Rico later observed these same regions with similar results. star’s glare. The best observing conditions an elliptical orbit. (For the same reason, we First, each Mercury day, from one sun- Photographs taken during the three occur when it lies near one of its greatest see slightly more than half of the Moon’s rise to the next, lasts two Mercury years. Mariner 10 flybys in the mid-1970s showed elongations, which create brief periods when surface during a lunar month.) Astrono- Long phases of heating and cooling are the that, in some cases, the high-return regions the planet climbs well above our earthly mers accepted Mercury as another example rule across the planet’s surface. seemed to coincide with deep polar craters. horizon and appears approximately half-lit. of rotational lock well into the 20th century. Second, despite these long cycles, not all Although water ice seemed the most likely It wasn’t until the late 19th and early But further Earth-based observations longitudes experience the same share of cause, theorists developed several even 20th centuries that astronomers, most suggested that Mercury’s “dark side” was heating. Because the planet has a fairly more exotic explanations, including sulfur notably Giovanni Schiaparelli in Italy and far warmer than it should be. Theorists eccentric orbit, it travels much faster when snow, sodium ions, or an unknown feature Greek-born Eugène Antoniadi in France, proposed various ideas to explain the it lies closer to the Sun. of the supercold surface. (After all, this consistently observed the same markings apparent contradiction, including one Noontime heating in one longitudinal was the coldest surface radar scientists on the planet’s illuminated side. These fea- in which the planet possesses a massive zone can be twice that in another 90° had ever explored.) tures did not move over short periods and atmos phere that away. The 3:2 resonance also means The highest temperature observed over two Mercury years gives a better idea of where ice can survive. The reappeared in the same locations at succes- spawns hurricanes that the same regions experience MESSENGER to the rescue coldest spots in permanently shadowed craters only reached –370° Fahrenheit (50 kelvins). NASA/UCLA/JHUAPL/CIW sive greatest elongations. to carry super- extra heating day after day after Into this uncertainty and mystery, NASA These observations implied that Mer- heated air into day. The two hottest launched the MESSENGER probe in 2004. cury likely is tidally locked with the Sun the perma- zones lie along It was a challenge to get a spacecraft to Ice depth (centimeters) and always keeps the same face pointed nently shad- the equator on Mercury with a speed slow enough that the 0 10 20 30 40 50 toward our star. The Moon suffers a similar owed regions. opposite sides of biggest propulsion module possible could fate, in which Earth’s gravity locks the But none of the MESSENGER the planet. Tem- decelerate the craft into a stable orbit. After NASA/JPL-CALTECH Moon so that it rotates in the same period theories seemed peratures there soar as a seven-year voyage that included three it takes to revolve around our planet. The to fit the obser vations. high as 845° Fahrenheit (725 kelvins). flybys of the inner world, the probe arrived observations of Mercury also suggested Then, in 1965, researchers bounced Third, Mercury’s true rotation period in its planned orbit March 17, 2011. that its rotation axis lines up nearly per- powerful radar beams off Mercury. A sub- relative to the stars (58.8 days) coincidentally After a year of observations and anal- pendicular to its orbital plane. tle shift in the wavelengths of the signals turns out to be approximately half of Mer- ysis, NASA announced the results in In Mercury’s case, being tidally locked returned from the planet’s edges didn’t cury’s 116-day synodic period, the time it November 2012. The ice is real, but it isn’t would create a scorchingly hot Sun-facing match an object rotating at the same rate takes the planet to return to the same orbital what scientists expected. If anything, it hemisphere, a perpetually dark and frigid as it revolved around the Sun — Mercury configuration as seen from our planet. proved even more interesting. region on the opposite side, and a fairly spins faster than anyone had suspected. This means that successive observation MESSENGER Principal Investigator broad twilight zone where the Sun would Even so, the planet is tidally locked, just periods from Earth occur when the same Sean Solomon, director of Columbia Uni- rise and set periodically in response to the not in a 1-to-1 ratio. Mercury rotates three side of Mercury faces the Sun. This dynam- versity’s Lamont-Doherty Earth Obser- planet’s varying orbital speed as it follows times on its axis for every two revolutions ical accident is just bad luck for earthbound vatory, described how the science team it makes around the Sun.