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Matt Ford - Jul 7, 2008 5:30 pm UTC

In mid-January 2008, the MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER) space craft completed the first of three flybys of the innermost planet, Mercury. MESSENGER is the second spacecraft to ever visit this planet, the first being Mariner 10, which flew by Mercury three times in 1974 and 1975. MESSENGER is expected to make two more flybys of the planet before executing an orbital insertion in March 2011. It's hoped that the mission will resolve some of the mysteries that arose from the data sent back by Mariner 10.

Image: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/ Carnegie Institution of Washington

To accommodate this momentous occasion, last week's issue of Science has an entire section devoted to the data sent back from the MESSENGER craft. A total of 11 papers were published based on the data from this single pass of the planet. The papers broke down into three main areas of study: imaging of the planet's surface, detailed study of its geology and geography, and examination of the magnetic field and space weather that surround the planet.

The first group of papers concentrated on the spectroscopic imaging of the planet. During this single pass, MESSENGER took over 1200 pictures1 and managed to image 21 percent more of the planet's surface than Mariner 10 did 34 years prior. Previous observations had suggested that Mercury has an iron-rich core, with iron accounting for over 60 percent of the planet's mass. However, ultraviolet adsorption spectroscopy measurements2 suggested that ferrous oxide (Fe2+) accounted for less than two to three percent (by mass) of the surface material.

Other multispectral images3 of the surface revealed distinct areas of high and low reflectance. The areas of high reflectance are surmised to be a class of smooth plains that are most likely the result of volcanic activity. Lower reflectance areas represent a "distinct crustal component enriched in opaque minerals." While these two areas were determined to be notably different, the bulk of the surface is of an intermediate reflectance.

The next major area of study was the geological past and present of the planet. One of the major questions was the role of volcanoes in forming the current surface of Mercury. As mentioned above, data from MESSENGER supports a volcanic origin for several regions of Mercury's plains. MESSENGER found strong evidence for volcanic activity around the inner margin of the Caloris basin4, which is the youngest known impact basin on the planet but has already been modified by volcanic activity and planet-wide tectonic stresses. Using data sent back by MESSENGER, researchers have been able to describe six major phases in the Caloris basin's geological life5.

Other instrumentation onboard MESSENGER provided high-resolution elevation information about a portion of the planet. Using the Mercury Laser Altimeter6, MESSENGER was able to generate a detailed elevation profile of a swath of the planet approximately 3200km long in the near-equatorial region. Using this equipment to examine craters, the team concluded that the "apparent slope, RMS roughness, and pulse widths are uncorrelated, which implies that the processes that caused tilting and created the roughness of crater floors are complex and do not operate uniformly over different length scales."

A more in-depth study of the craters7 looked into their morphology and size-distribution. This revealed that the plains regions are no older than approximately 3.8 billion years old. In contrast to these older regions, a density of craters was observed near the peak of the Raditladi basin, implying that this area is younger than 1 billion years old.

The subsequent articles turned their attention to the space surrounding Mercury. One of the surprising features of Mercury is that, like Earth, it has a planetary magnetic field—Venus and Mars lack this feature. When Mariner 10 went by, it was only able to determine the field strength to within 10 percent near the equator. MESSENGER, on the other hand, found that the magnetic field is tilted between five and 12 percent from the axis of planetary rotation, assuming a centered dipolar field8.

A simple field geometry may not be the correct shape, though. When researchers tried to fit multipole solutions to the observed data, they found that non-dipole contributions could account for anywhere between 22 and 52 percent of the total field magnitude. According to the authors, "recent simulations of Mercury’s core dynamo suggest that the presence of a stagnant layer at the top of the molten outer core may suppress higher-order structure... We find no evidence for a change in the planetary dipole since 1974 and also find that the planetary field is predominantly and possibly entirely dipolar."

The remainder of the papers dedicated to MESSENGER discussed data about the magnetosphere and exosphere and their interplay with the solar wind. Two papers 9,10 described the ions found in the vicinity of Mercury. Na+ ions were the most abundant, but O+ and K+ also abounded in the region of space near the planet. All of these measurements were consistent with prior neutral species observations. In addition to the singly ionized particle, doubly ionized species were found, which gave scientists further insight into the nature and magnitude of the planet's magnetosphere. The final paper in the series focused entirely on gathering data from Mercury's exosphere. Here there are substantial variations on the seasonal time scale due to the highly elliptical orbit of the planet11. The observations confirm that Mercury is like no other place in the solar system.

All this data came from a single short pass, just over one hour, of one spacecraft near the planet Mercury. Two more passes will occur in the next few years before MESSENGER takes up residence in orbit around the planet closest to the sun. According to Solomon et al. 1, Mercury is a "dynamic planet where the interactions among core, surface, exosphere, magnetosphere, and interplanetary environment are strongly interlinked." Further encounters when solar activity is different are expected to return whole new sets of data for planetary scientists to pore over, teaching us more about the planet closest to the sun.

[1] Science , 2008. DOI: 10.1126/science.1159706 [2] Science , 2008. DOI: 10.1126/science.1159933 [3] Science , 2008. DOI: 10.1126/science.1160080 [4] Science , 2008. DOI: 10.1126/science.1159256 [5] Science , 2008. DOI: 10.1126/science.1159261 [6] Science , 2008. DOI: 10.1126/science.1159086 [7] Science , 2008. DOI: 10.1126/science.1159317 [8] Science , 2008. DOI: 10.1126/science.1159081 [9] Science , 2008. DOI: 10.1126/science.1159040 [10] Science , 2008. DOI: 10.1126/science.1159314 [11] Science , 2008. DOI: 10.1126/science.1159467

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