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Overview of Observations Made During the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Spacecraft's Flyby of Mercury
Craters in Caloris
As MESSENGER sped by Mercury on January 14, 2008, the Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS) captured this image, which includes the edge of the planet against the blackness of space.

Taken from Abstract by Sean C. Solomon1, Ralph L. McNutt, Jr.2, Peter D. Bedini 2, Eric J. Finnegan 2, David G.Grant 2, and the MESSENGER Team, Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015; 2 Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723,,,,

Mercury in color
Mercury in Color: MESSENGER's Wide-Angle Camera (WAC) part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters, in contrast to the two visible-light filters and one ultraviolet filter that were on Mariner 10's vidicon camera. By combining images taken through different filters in the visible and infrared, the MESSENGER data allow Mercury to be seen in a variety of high-resolution color views not previously possible.

Launched in August 2004, MESSENGER is midway through a complex interplanetary cruise phase that involves six planetary flybys. The first of three flybys of Mercury occurred on January 14, 2008, an event that marked the first spacecraft visit to the innermost planet since Mariner 10 last did so nearly 33 years ago. MESSENGER approached Mercury from the night side and crossed the dawn terminator shortly after closest approach.

Artist's concept of MESSENGER flying through Mercury's magnetoshere
MESSENGER flies through Mercury's magnetosphere.

MESSENGER viewed the sunlit side of Mercury, including about half the hemisphere not imaged by Mariner 10, primarily on departure. Because MESSENGER's second Mercury flyby in October 2008 will be approximately 1.5 Mercury solar days after the first, the opposite hemisphere will be sunlit. MESSENGER thus has the opportunity to view almost the entire surface of Mercury in the course of two flybys.

The MESSENGER mission was designed to answer six questions:

  1. What planetary formational processes led to Mercury's high ratio of metal to silicate?
  2. What is the geological history of Mercury?
  3. What are the nature and origin of Mercury's magnetic field?
  4. What are the structure and state of Mercury's core?
  5. What are the radar-reflective materials at Mercury's poles?
  6. What are the important volatile species and their sources and sinks near Mercury?

The above questions served to shape the science return goals of this mission, which include:

Mercury at closest approach for MESSENGER
This image was acquired about 98 minutes after MESSENGER's closest approach to Mercury, when the spacecraft was at a distance of about 21,000 miles (33,000 kilometers). NASA/JHUAPL/CWI.
  1. Mapping the major element chemistry and mineralogy of the planet's surface;
  2. Imaging the surface at a horizontal resolution of hundreds of meters and make spectral measurements of major geologic units at visible and near-infrared wavelengths;
  3. Measuring the vector magnetic field both near the planet and throughout the planet's magnetosphere;
  4. Measuring Mercury's obliquity, the amplitude of Mercury's physical libration, and Mercury's long-wavelength gravity field;
  5. Carrying out geochemical remote sensing of Mercury's polar surface and exosphere;
  6. Assaying the major neutral species in the exosphere and major charged species in the magnetosphere

The payload consists of seven instruments, including a dual imaging system with wide-angle and narrow-angle cameras; an integrated ultraviolet, visible, and infrared spectrometer that is sensitive enough to detect atmospheric emissions and robust enough to map spectral absorption features on the sunlit surface; gamma-ray, neutron, and X-ray spectrometers for remote geochemical mapping; a vector magnetometer to examine the internal and external field sources; a laser altimeter to examine the topography of surface features and determine whether Mercury has a fluid core; and an energetic particle and plasma spectrometer to characterize ionized species in the magnetosphere.


The MESSENGER spacecraft made a close flyby of the planet Mercury in the first encounter with the planet in almost 33 years. The 1,213 images sent back from the spacecraft conclusively showed that the planet is a lot less like the Moon than many previously thought, with features unique to this innermost world. The puzzling magnetosphere appeared to be very different from what Mariner 10 discovered and first sampled almost 34 years ago. "MESSENGER has shown that Mercury is even more different from the Moon than we'd thought," said Science Team Co-Investigator James Head, professor at Brown University and chair of the mission's Geology Discipline Group.

Nice views of Mercury as Messenger flew by
Rendezvous with Mercury

The tiny spacecraft discovered a unique feature that scientists had dubbed, "the Spider," and has since been officially named by the International Astronomical Union (IAU), as "Pantheon Fossae." This type of formation has never been seen on Mercury before, and nothing like it has been observed on the Moon. It is in the middle of the Caloris basin and consists of over a hundred narrow, flat-floored troughs (called graben) radiating from a complex central region. "The Spider" has a crater near its center, but whether that crater is related to the original formation or came later is not clear at this time.

Unlike the Moon, Mercury also has huge cliffs or scarps, structures snaking up to hundreds of miles across the planet's face, tracing patterns of fault activity from early in Mercury's-and the solar system's-history. The high density and small size of Mercury combine to provide a surface gravity about 38% that of Earth and almost exactly the same as that of Mars, which is some 40% larger than Mercury in diameter (2.7 times Mercury's volume). Because gravity is stronger on Mercury than on the Moon, impact craters appear very different from lunar craters; material ejected during impact on Mercury falls closer to the rim and many more secondary crater chains are present.

Significance to Solar System Exploration

Illustration of Mercury's magnetosphere

As the smallest of the solar system's eight planets, Mercury holds a key place in the evolutionary spectrum for planets. While it looks like the cratered Moon from a distance, up close it displays a range of features that hint at a far different birth and geological history. The neutral sodium in Mercury's exosphere that the scientists measured and the comet-like tail consisting of neutral sodium could have come from a range of processes - meteoroid impact, photon sputtering, diffusion from the interior - as well as from sputtering by solar wind ions. Stay tuned as the science team continues to analyze more and more data returned from the MESSENGER spacecraft as it continues its landmark mission to investigate the innermost planet in our solar system.

Last Updated: 21 January 2014

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Last Updated: 21 Jan 2014