Mercury: Planet of Fire And

Planetary science

The MESSENGER spacecraft reveals Mercury to be a world with unexpected composition, an offset magnetic field, and permanently shadowed craters that may harbor ice. by Richard Talcott

MERCURY Mercury’s limb (edge) glows against the blackness Planet of space in this image from early in MESSENGER’s of fire and ice orbital mission. Photos showing the planet’s horizon are rare while the spacecraft is in orbit because the surface typically fills the camera’s field of view. NASA/JHUAPL/CIW

lanetary scientists breathed a Mercury’s surface composition differs ­mission planners chose in part to allow deep sigh of relief March 17. At markedly from what scientists expected, detailed study of the giant Caloris impact 9:00 p.m. EDT, NASA’s MES- and the planet’s magnetic field doesn’t basin, which spans 960 miles (1,550 km) SENGER spacecraft wrapped line up with its equator. and is Mercury’s largest surface feature. up a 15-minute firing of its The new observations also gave scien- (Caloris remained in darkness early in thrusters,P which slowed the probe just tists their first close-up looks at Mercu- the orbital mission; each night on Mer- enough for Mercury’s gravity to snag it. ry’s north and south polar regions. In cury lasts 88 Earth days.) Because the This maneuver marked the end of a each of the six previous flybys (three by Sun’s gravity slowly alters the spacecraft’s 4.9-billion-mile (7.9 billion kilometers) Mariner 10 in the mid-1970s and three orbit, MESSENGER occasionally has to journey and the beginning of an orbital by MESSENGER in 2008–9), the space- fire its thrusters to keep on track. mission to explore the innermost planet craft flew over the planet’s equator and The new images of the planet’s north expected to last at least 1 Earth year. managed only a few low-resolution pho- polar region bring into focus what pre­ Anxiety soon gave way to exhilaration tos of the poles. Initial results from orbit viously had appeared as blurred patches as photographs and other observations support the idea that water ice may lurk of light and dark. They reveal a broad began streaming back from MESSEN- at the bottom of some deep polar craters. expanse of volcanic plains that ranks GER, which stands for MErcury Surface, among the largest on Mercury. These Space ENvironment, GEochemistry, and The view from above ­relatively smooth plains cover approxi- Ranging. By the start of September, MESSENGER orbits Mercury once every mately 1.5 million square miles (4 million NASA’s latest planetary probe had 12 hours on a highly elliptical path that returned more than 40,000 images and brings it within 124 miles (200 km) of the Color explodes from Mercury’s surface when taken millions of measurements of Mer- surface at closest approach but swells to planetary scientists combine images taken through multiple filters. The view at top uses cury’s topography and composition. 9,440 miles (15,193 km) at maximum three filters while the one at bottom combines The flood of data produced the distance. The orbit’s low point comes eight. The yellow-orange hue signifies relatively first two big surprises of the mission: above 60° north latitude, a position young volcanic plains. NASA/JHUAPL/CIW

© 2013 Kalmbach Publishing Co. This material may not be reproduced in any form • 28 Astronomy Decemberwithout permission 2011 from the publisher. www.Astronomy.com www.Astronomy.com 29 A little ice would be nice white at resolutions approaching 33 feet (10m) per pixel. Mission scientists focus on regions of special interest with these high-resolution observations. One of their early targets is the bright, patchy deposits on some cra- Second ter floors discovered during previous 85° Mercury flyby missions. MESSENGER reveals flyby them to be clusters of rimless, irregular pits ranging from several hundred feet to Second a few miles in diameter and concentrated Mercury near central peaks, rings of mountains, 80° flyby Orbital mosaic and crater rims. Diffuse halos of even brighter material surround the pits. MESSENGER built up a color mosaic covering “The etched appearance of these more than 90 percent of Mercury’s surface ­landforms is unlike anything we’ve seen NASA’s latest planetary probe reveals Mercury’s northern latitudes like never before. The best during its first 176 days in orbit. The probe’s previous view of this region came during the spacecraft’s second flyby in October 2008, but it pales wide-angle camera took images like this one before in the solar system,” says Brett in contrast to an orbital mosaic. Note the many “ghost” craters near the image’s center that formed through eight different filters, achieving an Denevi of the Johns Hopkins University 75° when volcanic plains buried pre-existing craters. NASA/JHUAPL/CIW average resolution of 0.6 mile (1 kilometer) Applied Physics Laboratory in Laurel, Deep craters near Mercury’s north pole may harbor water ice. The first evidence came in Earth- per pixel. NASA/JHUAPL/CIW Maryland, and a member of the imaging based radar observations that showed bright deposits. National Astronomy and Ionosphere Center/Arecibo Observatory team. “We are still debating their origin, Using the spacecraft’s laser altimeter, but they appear to be relatively young.” project scientists have begun exploring Some team members suspect they are the depths of some craters near the north volcanic vents, implying that volcanic pole. And, so far at least, the ones that activity on Mercury didn’t die out early coincide with radar-bright deposits dig in the solar system’s history, as some 85° deep enough that their floors remain in ­planetary scientists suspected. complete darkness. As MESSENGER Principal Investigator Sean Solomon of An off-kilter magnetic field the Carnegie Institution of Washington Volcanism at a planet’s surface implies a (D.C.) says, “The case for water ice has molten interior during the period when passed its first test.” the activity took place. But Mercury pres- 80° Other tests will follow, including ones ents strong evidence that part of its core that should reveal the deposits’ composi- is still liquid. Among the inner worlds of This rugged landscape lies outside the giant The crater Praxiteles sports a prominent ring tion and resolve the mystery. If the sub- the solar system, only Mercury and Earth Caloris Basin and stands as testament to the of mountain peaks roughly halfway from center stance does turn out to be water ice, have global magnetic fields. Scientists power of the impact that created it. The force to rim. Near the tops of these peaks lie several Solomon says Mercury has more of it think both fields arise from electrical 75° unleashed by the Caloris impact scoured the irregularly shaped depressions surrounded by than the Moon. (NASA’s Lunar CRater currents flowing in the outer part of the surrounding surface and created these rough bright material that may be sites of previous Observation and Sensing Satellite, or MESSENGER discovered that many of the deposits that appear bright in radar images coincide contours. NASA/JHUAPL/CIW volcanic activity. NASA/JHUAPL/CIW with impact craters. The prominent example circled in red lies at a latitude of 82.3° north and LCROSS, confirmed water ice in lunar spans 15 miles (24 kilometers). NASA/JHUAPL/CIW polar deposits in 2009.) square km), or about half the area of the researchers think they represent water ice United States, and run to depths of a few trapped at the bottoms of craters where Pits of confusion Depth (miles) miles. Their extent helps confirm the key the Sun never shines. However, some sci- During the course of the yearlong mis- –1 role volcanism played in shaping Mer- entists suspect other ices or elemental sion, MESSENGER’s camera system cury’s crust through much of its history. sulfur could produce this radar signature. will image most of Mercury’s globe. A But planetary scientists are even more The fact that astronomers even debate wide-field camera will render the surface 5 miles intrigued by what MESSENGER’s cam- the existence of ice on the planet nearest in black and white at an average resolu- 0 Altitude –1.5 eras cannot show. Two decades ago, radar the Sun points to Mercury’s strangeness. tion of 820 feet (250 meters) per pixel. (miles) 1 observations from Earth revealed several After all, noontime temperatures on This camera also can view the planet 2 20 20 mysterious spots near Mercury’s north ­Mercury can reach 800° Fahrenheit through 11 color filters, each designed 10 10 and south poles. These radar-bright (427° Celsius). But the planet’s axis of to reveal an important aspect of the Distance from crater’s center (miles) 0 Rimless, irregular pits that vary from several 0 –2 deposits appear similar to the icy moons rotation tips nearly perpendicular to its ­planet’s surface composition. These hundred feet to a few miles across dot the floors 10 10 of the outer solar system, and many orbit around the Sun, so deep craters observations will contribute to a global of many impact craters on Mercury. These pits showed up for the first time on MESSENGER’s 20 20 near the poles would remain in perma- color mosaic with a resolution of 0.6 mile Distance from crater’s center (miles) high-resolution orbital images. They appear to Richard Talcott is an Astronomy senior nent shadow. Any ice that found its way (1 km) per pixel. be young and concentrated near crater rims, ­editor and author of Teach Yourself Visually to Mercury, presumably carried there by The spacecraft’s narrow-angle camera central peaks, and rings of mountains. They The crater’s rugged terrain shows in this profile from MESSENGER’s laser altimeter. The Astronomy (Wiley Publishing, 2008). comets, could endure for billions of years. images much smaller areas in black and could be sites of volcanic vents. NASA/JHUAPL/CIW observations reveal that the crater digs deep enough that its floor remains permanently in shadow, where ice could survive for eons. NASA/JHUAPL/CIW

30 Astronomy • December 2011 32 time agoasitsinterior cooled. indicatescarps thewholeplanetshrank along older crater at bottom Mercury’s left. widespread Rupes,Named Beagle itnoticeably deforms the this imageranks amongMercury’s largest. that cutsacross The long, (orcliff) scarp curved however, case, the be remains amystery. must Whyfield offset. be also should this regions inMercury’s core generating the percent of its radius. Solomon says that ringed planet’s is by shifted field just 6 solarthe system is Saturn, although the Inkm). its respect, this closest match in planet’s radius, or some (480 300miles to northby­offset the 20percent of the thatGER revealed Mercury’s is field with planet’s the equator. But MESSEN- field’snetic equator lines up pretty well aminiature tofield be version of Earth’s. Mercury’s scientistsplanetary expected planet’s cores. Before MESSENGER, Degas’ floor. and shrank,itformed thecracks visibleon Asthismaterialmelted thesurface. cooled crater (52kilometers) created the32-mile-wide walls andalarge central peak. thatThe impact Degas Crater (at right) shows finelyscalloped It’s not, by along shot. Earth’s mag - Astronomy NASA/JHUAPL/CIW

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2011 December NASA/JHUAPL/CIW

then flowed out ofthecrater. crater-forming impact. This melted material melted from theheat generated duringthe scientists rocks attribute that itto surface features anunusualdarkpatch. Planetary The bright rayed crater at thisimage’s center core,in the strange is which adecidedly as well.north-south inthese asymmetries MESSENGERthinks ultimately see will ens material surface over time, Solomon and to “space the weathering” that dark- tribute to planet’s the tenuous atmosphere ones.northern such impacts Because con- polar regions far more frequently than the means solar-wind hit particles south the broader area north.This inthe than those to interplanetary spaceover amuch near planet’s the south remain pole open solarthe wind. Thelines magnetic field interacts with chargedcury in particles Mercury’s volatile mix The highpotassiumabundance rulesoutsometheoriesoftheinnermostplanet’s formation. temperatures). alower expected Scientists liessocloseto ratio theSun. Mercury because vaporizes at relatively low temperatures) to thorium(anelement that vaporizes at high The ithasahighratio innermostplanetstandsoutbecause ofpotassium(anelement that 10,000 Mercury’s originates magnetic field The bizarre how affects also field Mer Potassium/thorium ratio 1,000 100 Sun 0 Mercury NASA/JHUAPL/CIW 0.5 Venus Distance fromtheSun(AU) - Moon Earth spectrum of gamma raysspectrum like aDNA acts cause to them emit gamma rays. The neutrons that hit neighboring atoms and cury’s liberate surface, they high-energy from outside solar the system Mer strike gamma rays. When cosmicrays coming trometer (GRNS) measures energetic a lot of data to work with. fairlybeen active in2011,scientists have nating from Sun.our the star Because has get hit they when surface by X-rays ema- X-rays given off by atoms at planet’s the radiation. The spectrometer measures glow illuminated when by ultraviolet process that causes ablack-light poster to trometer works by fluorescence, same the elements on planet. the The X-ray Spec- instruments to measure abundance the of two The carries spacecraft tists expected. and Moon the —and from what- scien cury’s composition surface is from Earth wavetidal of data is how different Mer The other big inMESSENGER’s surprise A compositional conundrum of MESSENGER’s mission. such ahuge core is one of key goals the Understanding why Mercury possesses of way the from center the to surface. the planet’sthe mass and extends 75percent iron-rich core holds at least 60percent of tohalfway surface. the But Mercury’s stretches from center the to abit over one-third of our planet’s total mass and place. Earth’s core contains approximately 1 The Gamma-ray and Neutron Spec- 1.5 Mars 2 - -

NASA/JHUAPL/CIW abundance measurements far so have or Moon. the Earth of Mercury different being of than those but says point they to building the blocks from spectrometers the remain baffling, of Washington admits that results initial Nittler of InstitutionLarry Carnegie the Mercury’s history. geologist Planetary to explosive volcanic activity earlier in Moon could have or Earth, contributed more abundant on Mercury than on the titanium. Sulfur, is which up to 10times and but sulfur low amounts of iron and largepectedly concentrations of sodium little aluminum for effort. the spread melting early on, too, but it shows Mercury must have undergone- wide rockswhen inits mantle melted. partially aluminum asmaller enced enhancement satellite was largely- molten. experi Earth earlysurface inlunar the when history Moon,On the aluminum to floated the face rocks of either Moon the or Earth. sium and far less aluminum than sur the that planet the contains far more magne- sured Mercury’s composition and found sium, thorium, and uranium. rays from decay radioactive the of- potas ments. gamma detects The GRNSalso fingerprint to individual identify ele- darkendust; they over typically time. andpowdery dark halo. smallparticles Rays ejects form whenanimpact Bright rays extend from thecrater Bashoat a lower whichalsosports left, Yet MESSENGER’s most startling The plains of hold Mercury also unex - MESSENGER’s spectrometers- mea NASA/JHUAPL/CIW - metal), butmetal), MESSENGER the results don’t CVchondritescalled contain lots of Suchmetals. exist objects (meteorites havingbodies concentration ahigher of innerthe planet accreting from smaller in different of parts solar the nebula, with The two planetsnever like Earth. formed out hypothesis. this layers. Nittler says findings rule new the blasted Mercury and off its boiled outer core. But young the Sun’s intensity later didand developed an also ironEarth same material solar inthe nebula as innermost planet accreted out of the of its location. In scenario, this the suggest it got that way simply because dinately large iron core. Some scientists that planet the origin. Recall has an inor proposed ideas the to explain Mercury’s The findings cast doubt on some of An unknownorigin lower ­ Sun’s heat, it scientists to have expected a site. to close the Mercury liesso Because temperature, thorium while is oppo the - means that it vaporizes at arelatively low Potassium is avolatile element, which than that on any other inner planet. sium to thorium on Mercury is higher made by GRNS.The the ratio of- potas of those potassiumbeen and thorium A second idea holdsA second idea that Mercury was proportion of potassium. it’s across) big(445miles[715kilometers] andfairlyyoung. with thebright rayed crater Amaral. Rembrandt intrigues scientists because The large, dark Rembrandt Basinonthisimage’s sidecontrasts left nicely - the entirethe solar system focus. insharper to acomplete picture, and to rendering brings scientists planetary one step closer together ajigsaw puzzle. piece new Each back. The process is abit like putting reamsthe of data craft the is beaming But far take it longer will analyze to fully coincides with about 4Mercury years.) 1-year orbital mission. year (One Earth completed of first half the its planned toideas explain observations. the bunch and no doubt develop will new tiny, scientists planetary are aresourceful - impact scru hypothesis further survives fits. Whether the data ifeverything to see instrumentthe more teams need will some of spectrometer the findings, but Mars-sized smashed object into Earth.) of Moon’s the theory formation after a ingly, leading mimicsthe scenario this tle, leaving an outsized iron core. (Intrigu- removed Mercury’s and crust outer man- largeequally body. The impact could have mass but suffered acollision with an Perhaps Mercury out started with more hypothesis bitethis will likewise dust. the match well. Nittler very them suspects By September, MESSENGER had Nittler says matches idea this at least That leaves footing. on one firm theory www.Astronomy.com/toc. For thelatest MESSENGER, visit about www.Astronomy.com NASA/JHUAPL/CIW 33