MESSENGER completed its one-year primary mission on March 17. Since moving into orbit about Mercury a little over one year ago, the spacecraft has captured nearly 100,000 images and returned data that have revealed new information about the planet, including its topography, the structure of its core, and areas of permanent shadow at the poles that host the mysterious polar deposits.
The latest findings are presented in two papers published online in Science Express (http://www.sciencemag.org/content/early/recent) today, and in 57 papers presented this week at the 43rd Lunar and Planetary Science Conference in The Woodlands, Texas. Team members at the meeting will also preview MESSENGER's extended mission, set to run to March 2013. Presentation materials are available on the web at http://messenger.jhuapl.edu/news_room/presscon11.html.
"The first year of MESSENGER orbital observations has revealed many surprises," says MESSENGER Principal Investigator Sean C. Solomon, of the Carnegie Institution of Washington. "From Mercury's extraordinarily dynamic magnetosphere and exosphere to the unexpectedly volatile-rich composition of its surface and interior, our inner planetary neighbor is now seen to be very different from what we imagined just a few years ago. The number and diversity of new findings being presented this week to the scientific community in papers and presentations provide a striking measure of how much we have learned to date."
Ranging observations from MESSENGER's Mercury Laser Altimeter (MLA) have provided the first-ever precise topographic model of the planet's northern hemisphere and characterized slopes and surface roughness over a range of spatial scales. From MESSENGER's eccentric, near-polar orbit, the MLA illuminates surface areas as wide as 15 to 100 meters, spaced about 400 meters apart.
The spread in elevations is considerably smaller than those of Mars or the Moon, notes MESSENGER Co-investigator Maria T. Zuber, author of one of the papers published in Science Express According to Zuber, of the Massachusetts Institute of Technology, the most prominent feature is an extensive area of lowlands at high northern latitudes that hosts the volcanic northern plains. Within this lowland region is a broad topographic rise that formed after the volcanic plains were emplaced.
At mid-latitudes, the interior of the Caloris impact basin -- 1,500 kilometers wide -- has been modified so that part of the basin floor now stands higher than the rim, Zuber says. "The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes," she writes. "Collectively, these features imply that long-wavelength changes to Mercury's topography occurred after the earliest phases of the planet's geological history."
A Surprising Core
Scientists have also come up with the first precise model of Mercury's gravity field which, when combined with the topographic data and earlier information of the planet's spin state, shed light on the planet's internal structure, the thickness of its crust, the size and state of its core, and its tectonic and thermal history.
Mercury's core is huge for the planet's size, about 85% of the planetary radius, even larger than previous estimates. The planet is sufficiently small that at one time many scientists thought the interior should have cooled to the point that the core would be solid. However, subtle dynamical motions measured from Earth-based radar combined with parameters of the gravity field, as well as observations of the magnetic field that signify an active core dynamo, indicate that Mercury's core is at least partially liquid. "MESSENGER's observations of the gravity field have let us peer inside Mercury and get the first good look at its largest component -- the core," says Case Western Reserve University's Steven A. Hauck II, coauthor of one of the papers published in Science Express.
Scientists sought to unravel the mystery of the size and state of Mercury's core by studying its effect on long-wavelength variations in the planet's gravity field, and recent results point to a much different interior structure for Mercury from that expected.
"Mercury's core may not look like any other terrestrial planetary core," Hauck says. "The structure certainly is different from that of Earth, which has a metallic, liquid outer core sitting above a solid inner core. Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, and possibly a solid inner core."
These findings will have implications for how Mercury's magnetic field is generated and for understanding how the planet evolved thermally, Hauck adds.
A chief goal of MESSENGER's primary mission was to understand the nature of the radar-bright deposits at the poles of Mercury. The leading proposal since the deposits were discovered has been that radar-bright material consists dominantly of frozen water ice.
"We've never had the imagery available before to see the surface where these radar-bright features are located," says Nancy L. Chabot, instrument scientist for MESSENGER's Mercury Dual Imaging System (MDIS) at the Johns Hopkins University Applied Physics Laboratory (APL). "MDIS images show that all the radar-bright features near Mercury's south pole are located in areas of permanent shadow, and near Mercury's north pole such deposits are also seen only in shadowed regions, results consistent with the water-ice hypothesis."
This finding is not definitive proof that those deposits are water ice, says Chabot, who is presenting her results at LPSC. And some of the radar-bright deposits are located in craters that provide thermally challenging environments to the water-ice theory. For instance, for the radar-bright material in many of the craters to be water ice would require that there be a thin layer of insulation to keep it colder than the surface, Chabot says. But the MDIS images, combined with ongoing analysis of data from MESSENGER's Neutron Spectrometer and the MLA, will provide a more complete picture of the nature of the deposits.
Extending the Discoveries
MESSENGER's second year at Mercury will build upon these and other results from the primary mission phase, emphasizes MESSENGER Project Scientist Ralph L. McNutt Jr., of APL. "The second year of orbital operations will not be a simple continuation of the primary mission," he says. "Extended mission themes will include more comprehensive measurement of the magnetosphere and exosphere during a period of more active Sun, greater focus on observations at low spacecraft altitudes, and a greater variety of targeted observations."
"MESSENGER has already fundamentally changed our view of this innermost planet," he adds. "With the extension of the MESSENGER mission, many more discoveries can be expected."