Just two weeks after MESSENGER's first flyby of Mercury, the mission's science team presented their first impressions from the long-awaited second look at the innermost planet. Prior to the January 14 MESSENGER encounter, the most recent visitor to Mercury was Mariner 10, which swung by Mercury three times in 1974 and 1975, producing images of only 45 percent of the surface. Said Bob Strom, the only member of both the MESSENGER and Mariner 10 science teams: "I was thinking in my mind that I'd see features like Mariner 10 saw, but I was astounded at the quality of these images, and, after looking at them, it dawned on me that this is a whole new planet we're looking at. Every part of this planet, 'seen' or 'unseen,' is new."
Though a small, Discovery-class mission, MESSENGER carries a highly capable suite of science instruments: a wide-angle color camera and high-resolution monochrome imager; a laser altimeter; a magnetometer; and infrared, visible-light, ultraviolet, X-ray, gamma-ray, neutron, and energetic particle spectrometers. All of the instruments returned data during the flyby, with some of that data providing immediate answers to questions left over from Mariner 10's initial reconnaissance. For the most part, though, the data demonstrated that MESSENGER will be capable of learning volumes about Mercury, but not until it has entered orbit and completed its survey of the planet. And, as always happens with every new view of a world in our solar system, MESSENGER's flyby generated new mysteries to solve.
Clues in Caloris
One of the chief lessons from both Mariner 10 and MESSENGER is that Mercury's surface bears the scars of a complex history involving many episodes of different styles of geologic activity. One of the areas of Mercury that the MESSENGER team was most keen to study was the Caloris basin, a gigantic multi-ringed structure seen only partially by Mariner 10, its western half hidden in nighttime darkness. MESSENGER saw nearly all of Caloris, including all of the previously unseen part, and the first thing that the MESSENGER team learned, Strom said, is that it's even bigger than originally thought: 1,550 kilometers (960 miles) in diameter instead of the 1,300 kilometers (810 miles) estimated from the Mariner 10 view.
Strom counted the craters within Caloris to determine that it dates to about 3.8 or 3.9 billion years ago, near (but not at) the end of the violent period of the development of the early solar system known as the Late Heavy Bombardment. But as the team was counting craters, Strom said, they noticed that the western half of Caloris seemed to have fewer of them than the eastern half. In fact, when they worked out the statistics, they found that this impression was true. Since the impact can only have happened at one time in the history of Mercury, that implies that the floor of Caloris has been modified, with some of the craters that must have formed on the western half at the same rate as on the eastern half being obliterated by geologic activity. "That shows that there has been a lot of volcanic activity on Mercury," Strom said.
Inside Caloris, the team spotted a truly bizarre feature. Just a few tens of kilometers from Caloris' geographic center is a 40-kilometer-diameter crater that sits directly on top of a radial spray of 50-odd cracks, together forming a feature referred to informally by the science team as the "spider."
Louise Prockter, instrument scientist for the MESSENGER camera system, explained the science team's struggle to interpret this feature. "What isn't clear is the relationship of the crater to the radial trough complex. Did the crater help to form some or all of the troughs? Did it impact serendipitously right at the bull's-eye? At this point we really don't know. This feature is very close to the center of the Caloris basin. We have seen a number of impact basins in the solar system. We have never, ever seen a feature like this at the center of any of them. And we haven't seen any features like this elsewhere on Mercury or on the Moon. Being scientists we have many theories as to how it formed, but at this point it's anybody's guess; it's a very unexpected find."
Asked to give some examples of the speculative ideas in circulation on how the "spider" formed, Prockter elaborated: "Those polygonal radiating troughs do seem to be related to the other polygons that are found elsewhere in Caloris, close to the margins. They crosscut all the lobate scarps that seem to be in the interior of Caloris. One possible suggestion is that maybe there's some kind of volcanic intrusion below the surface, perhaps related to the formation of the basin. Another possibility is that the basin itself has undergone some kind of isostatic rebound after formation." Isostasy describes the processes that happen within planets to balance out variations in topography of the surface and density of materials below the surface. Isostasy explains why Earth's ocean floors are low: the ocean basins are made of denser rocks than the continents, so the continents "float" higher on Earth's mantle than the oceans do. When a giant impact gouges out a deep crater, isostatic rebound acts to raise the floor of the crater to balance out the loss of mass.
Another area that the science team focused on was this double-ringed impact basin found to the west of Caloris, in an area that was not observed by Mariner 10 (see image at right). Much smaller than Caloris, about 270 kilometers in diameter, the region still bears signs of a complex geologic history. Close to the crater is a rumpled "ejecta blanket" consisting of piles of material that were excavated from within the crater by the impact. The ejecta blanket is about 80 kilometers wide, considerably narrower than the ejecta blankets of similar-sized craters on the Moon, because of Mercury's much higher gravity. Beyond the edge of the ejecta blanket, chains of secondary craters pepper the otherwise fairly smooth plains. The interior of the crater itself is oddly smooth, suggesting that since the crater and its ejecta blanket formed, it has been filled in by volcanic deposits. The volcanic deposits fill in the crater unevenly; in some places (like in the west), the crater's inner ring is entirely obliterated by the flat-lying lavas; to the north, only separated peaks protrude from the flat floor; but to the south, the inner ring coalesces into a coherent chain of mountains.
There was one aspect of the images that the science team was not yet prepared to provide much comment on: the colors across Mercury's surface. Prockter presented a global false-color view of Mercury, showing that MESSENGER does see variations in color (and hence, composition of the rocks) across the surface, but the science team does not yet have detailed explanations for the color variations. Prockter pointed out that the interiors of the large basins like Caloris in the north and Tolstoj in the south are bright, with darker rings surrounding them. "Color variations here, while they are subtle, are almost the opposite of what we see on the Moon. On the Moon we see dark basins surrounded by bright material; here we have the opposite." The mission is releasing enhanced color as opposed to true-color views of Mercury because, to the naked eye, Mercury would appear gray, lacking any color, like the Moon.
Answers Found in the Magnetosphere and Exosphere
There are other instruments on MESSENGER besides the camera, and data from some of them provided some actual answers to questions left over from the days of Mariner 10. MESSENGER principal investigator Sean Solomon provided some very simple answers provided by the magnetometer instrument. Thanks to MESSENGER, we now know that Mercury's intrinsic magnetic field (a discovery that was a surprise from the Mariner 10 mission) is a dipolar field, like Earth's, and that, like Earth's, it is tilted about 10 degrees to the spin axis. The strength of the field appears not to have changed since Mariner 10 flew by. And MESSENGER detected no regional variations in the strength of the magnetic field like those found on Mars, at least in the region of the planet that MESSENGER flew close to.
However, there were also surprises, Solomon said. "This tiny magnetosphere is full of hot plasma, some of it from the Sun, some of it from interactions between the surface and the atmosphere." Yet even though two of the instruments saw a magnetosphere full of plasma, MESSENGER's energetic particle spectrometer did not see a single particle at Mercury, even though Mariner 10 did. "I think that tells us that the magnetosphere of Mercury is very changeable. It's a function of the [activity in the] atmosphere of the Sun; we went by at a particularly quiet period of solar activity, and [we think] it can change in a matter of minutes or hours. To prove that, we'll have to wait for some of the future observations we make during flybys." A movie released today (Quicktime format, 5 MB) demonstrates the geometry of the flyby and shows the measurements of the plasma environment made by MESSENGER.
Solomon also reported results from MESSENGER's investigation of Mercury's "tail," the region behind Mercury as seen from the Sun, where Mercury's atmosphere and magnetosphere stretch off into space. Mercury does have an atmosphere consisting mostly of neutral sodium atoms, so sparse that molecules almost never collide with one another; such a thin atmosphere is usually referred to as an "exosphere." Solomon showed data on both neutral sodium and neutral hydrogen atoms in the exosphere, the highest-resolution images ever taken of this tail region of Mercury. He pointed out that there is an asymmetry in both, where the signal is much stronger from the northern hemisphere than the southern hemisphere. "Hydrogen, we believe, is mostly of solar wind origin. This asymmetry in hydrogen points to some mechanism that involves an interaction between the solar wind and the magnetosphere. The fact that hydrogen and sodium have the same asymmetry suggest that there is a strong contribution of the same solar wind-magnetosphere interactions in the sodium loss, at least at the time we were flying by." Solomon also reported that emissions from calcium atoms were detected by the same methods.
The flyby also yielded one precious swath of altimetric data from the Mercury Laser Altimeter. The most significant conclusion to be drawn from the MLA data is that the altimeter appears to work extremely well, and should provide an excellent map of Mercury's topography once the orbital mission is complete. The geometry of the flyby permitted only 11 minutes of data spanning 3,200 kilometers across a region of Mercury's surface that was not imaged by either Mariner 10 or MESSENGER. Consequently, the data can only be compared to low-resolution Arecibo radar images of Mercury, but there are clear correlations between the altimetric swath and craters visible in the radar data. In total, reported instrument scientist Maria Zuber, MESSENGER found topographic variations of about five kilometers (three miles) from the highest peaks to the lowest crater bottoms.
Today's presentations contained only the most preliminary scientific conclusions from MESSENGER's first flyby of Mercury. Solomon explained the difficult, but exciting, tasks facing the science team: "I want to leave you with the message that all of those aspects of Mercury" -- its surface, topography, magnetosphere, and atmosphere -- "are closely interlinked. It's a very dynamic planet with an awful lot going on." As thrilling as this new data set is for the science team, it represents only a tiny fraction of what MESSENGER should eventually return to Earth. In the end, the most important result of the first flyby is that it has demonstrated that MESSENGER has the capability to gather the data necessary to answer many of the questions left over from Mariner 10. Strom promised that "the best is yet to come. I am very excited not only to see the 'new' part of Mercury, but also the 'old' part, which will be new to my eyes."