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.