Torrid Mercury's Icy Poles

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 atmos­phere 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 obser­vations. 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. Although this astronomers, who noticed — and misinter- painstakingly developed its case for ice at This map of “permafrost” shows the same area as above and indicates the depths below Mercury’s surface James Oberg, a former NASA “rocket scientist,” so-called 3:2 resonance is stable over long preted — the repeated appearances of the the poles. First, the researchers tested the where water ice should be stable. Gray denotes regions that are too warm for any ice, color reveals areas now works as a space consultant for NBC News. periods, it has some bizarre implications. same surface features. hypothesis that the areas where scientists where subsurface ice can exist, and white shows spots cold enough for surface ice. NASA/UCLA/JHUAPL/CIW

32 ASTRONOMY • DECEMBER 2013 WWW.ASTRONOMY.COM 33 How neutrons point to ice had observed bright radar returns were regolith [the layer of loose rock and soil How ice forms and survives positioned inside craters having the right found on the surfaces of most solar system shape and in locations where ice could sur- bodies] but is instead material with a reflec- North vive. “Imaging of both poles over multiple tance half that of Mercury’s average. The Sunlight Cosmic rays solar days on Mercury confirmed that all specific reflectance and limiting tempera- polar deposits are located in areas of per- ture are best matched by organic-rich mate- manent shadow,” says Solomon. And the rial found in comets and volatile-rich 550K Shadow craters are big enough so that light reflect- meteorites, and on the surfaces of outer 1 350K 80K ing from the Sun-facing rims does not solar system objects.” Solomon adds that flood the shadows with too much heat. these properties strongly suggest that the Second, the scientists used the craft’s water ice and the organic material found Neutrons neutron spectrometer, an instrument that their way to Mercury’s polar craters by a already had proved its worth during ice common process. searches on the Moon, to seek evidence of The researchers found the dark protec- hydrogen, two-thirds of the building blocks tive layer over ice in craters at some distance 2 for water molecules. “Neutron spectrometer from both poles. Its distribution clearly measurements [in the northern hemisphere] avoids the longitudes of the “hot spots” cre- showed that the polar deposits have a hydro- ated by Mercury’s peculiar 3:2 resonance. gen abundance consistent with a composi- The volatile deposits extend farther from Neutrons Ice layer tion dominated by water ice,” says Solomon, both poles along the coldest longitudes — “but only if most ice deposits are buried those 90° off the hot spot locations — pro- 3 beneath several tens of centimeters [at least viding further proof of their icy nature. several inches] of a low-hydrogen material.” But closer to both the north and south The spacecraft’s laser altimeter provided poles, the temperatures drop even lower. In This enhanced-color mosaic reveals Mercury’s south polar region. MESSENGER’s most detailed observa- the third line of evidence. This instrument, these regions, ice can remain stable without tions show the planet’s northern half because the probe flies closer to that hemisphere. But images of developed to measure the precise shape and any protection. The water ice is pure, and it the south pole show craters that lie in permanent shadow, just like those in the north. NASA/JHUAPL/CIW Low-hydrogen layer 4 (4–8 inches [10–20cm] thick) elevation of the planet’s surface, also paid is naked to space. “We had expected to find dividends in the hunt for ice. “Reflectance evidence of ice, but bright ice on the surface measurements showed that most polar and right where the thermal models pre- But where it is cold, water ice is stable planners intend to have it descend within Pure ice

deposits are dark at near-infrared wave- dicted it was a surprise,” says Paige. “Bright for a long time. “There could definitely be 15 miles (25 kilometers) of the surface at JOHNSON RICK : When high-energy cosmic rays strike Mer- lengths [1,064 nanometers in this case], but surface ice requires ongoing processes to billion-year-old ice on Mercury because the specific points, where it will be able to cury’s surface, they liberate neutrons from some of the polar deposits at the highest deliver water to the polar regions faster cold traps [have remained frigid for a major make the most detailed observations yet. 5 ASTRONOMY subsurface atoms. Although these particles latitudes are much brighter than average than it can be buried by impact debris and fraction of the solar system’s history],” says Until then, scientists will have to be con- normally escape into space after traveling Sunlight hits an impact crater near Mercury’s through the surface, hydrogen atoms in water for Mercury,” says Solomon. [ultraviolet] radiation.” Paige. Still, he remains cautious: “We don’t tent with what the orbiter already has deliv- north pole at a shallow angle. Although ice block their route. An insulating layer low in Finally, the team matched the newly know enough about the sources and ered. For Solomon, the water ice detection the sunlit rim grows hot, the opposite side remains in shadow (1). When comets strike the hydrogen protects the ice. ASTRONOMY: RICK JOHNSON ranks high for two reasons. “First, the imaged candidate regions to actual cold Taking a bath destruction rates of the ice to say how old planet (2), they spread water and organic com- areas. “Thermal models derived with topo- Considering NASA’s exobiology imperative any given piece of ice might be.” required measurements were difficult, pounds over a wide area, and some migrate to graphic maps constructed from altimeter to “follow the water” in search of extrater- Could the ice exist in successive layers, given that our spacecraft was far from the the poles and get trapped in craters (3). Water profiles showed that water ice is thermally restrial life, the question arises whether with the oldest at the bottom? If so, it could planet’s polar regions and the polar depos- ice in the warmer areas vaporizes over time (4) but stays stable in the coldest spots, protected Neutrons tell the tale stable at the surface in those areas with water ice on Mercury opens a potential new provide future explorers with a time-lapse its filled only a small fraction of the field of by a layer of organic material (5). bright reflectance, but is stable only if bur- habitat for biology. The answer, so far at history of Mercury and, perhaps, solar view of key remote sensing instruments,” Medium-speed neutrons ied by several tens of centimeters of another least, appears to be no. activity. In much the same way, terrestrial he says. “Second, the confirmation of water 1.02 still-volatile material for most [other] polar “The likelihood of persistent liquid geologists use ice cores from Antarctica and ice required that multiple lines of evidence deposit areas,” says Solomon. This uniden- water is remote,” says Solomon. Water in Greenland to study Earth’s climate history. all pointed in the same direction. The poles are warmer than the lunar poles, he 1.01 tified material appears to be less volatile Mercury’s polar craters either will be a sta- “This is one possibility, but, unfortu- results came in one at a time, like plot points out, but the planet has a lot more Pure ice No ice than water ice, which means it remains ble solid or in vapor form. “There may be nately, we just don’t know the answer,” developments in a mystery novel, and the water in much purer form. “It’s a continuing 1.00 ­stable at higher temperatures. brief intervals where water ice may be Paige admits. He does speculate that the solution came only on the final page.” mystery, the differences in the ice between 0.99 melted by sunlight or subsurface heat,” he apparently well-organized nature of Mer- Paige echoes those sentiments: “Scien- the Moon and Mercury,” he says. “We’ll Relative ux Relative MESSENGER data A dark cover-up continues, “but there is no evidence that cury’s deposits suggests that they might tists have been studying polar ice on the need to go down and sample the surface.” 0.98 MESSENGER scientists didn’t anticipate water has modified the surface or near- have migrated, mixed, and reformed as the Moon and Mercury since the 1960s. The Actually reaching and analyzing Mer- discovering such a dark overcoat: “Finding surface in ways detectable from orbit.” planet’s orbit evolved over millions of years. MESSENGER observations are like a satis- cury’s polar ice is a challenge at the current 0.97 30° 40° 50° 60° 70° 80° 90° unusually dark material in association with “We don’t think there’s any possibility This could jumble any time history. fying end to a good story — or at least one limits of technological speculation. Perhaps North latitude the ice deposits was definitely unexpected,” for liquid water in association with these chapter in a longer story.” a lander could do the job, or an impactor MESSENGER’s neutron spectrometer surveys says Paige. “We hypothesized that these deposits,” adds Paige. “They are way To the next stage David Lawrence of the Johns Hopkins paired with an orbiting collector made of the entire planet looking for neutrons. If no may be dark organic-rich deposits like we too cold. While surface and subsurface The MESSENGER spacecraft continues to University Applied Physics Laboratory, who aerogel that captures the collision’s debris. ice existed on Mercury, the number of neu- find in comets and primitive outer solar temperatures in the soil surrounding the orbit Mercury, dipping closer to the surface led the design team for the neutron spec- In other words, it’s an ideal goal for trons would remain constant with latitude. If the radar-bright deposits were pure ice, system bodies. If this stuff really exists on ice deposits can be in the habitable zone to gather more precise measurements of trometer that detected the hydrogen, stresses young planetary scientists to consider. the plot would follow the yellow curve. The Mercury, that would be pretty amazing.” [where conditions allow liquid water to particularly interesting regions. During the that polar ice on the Moon and Mercury After all, the next chapter on Mercury’s MESSENGER data closely match the pure-ice Solomon agrees: “The surprise was that exist], Mercury has no atmosphere, so any probe’s final year of operation, mission reflect different phenomena. Mercury’s­ polar ice remains to be written. scenario. ASTRONOMY: RICK JOHNSON, AFTER NASA/JHUAPL/CIW the material covering most of those [ice] liquid water in these warmer regions would deposits is not typical soil from Mercury’s quickly boil away into space.” READ MORE ABOUT THE MESSENGER SPACECRAFT’S DISCOVERIES AT MERCURY AT www.Astronomy.com/toc.

34 ASTRONOMY • DECEMBER 2013 WWW.ASTRONOMY.COM 35