15 February 2011
After navigating a seven-year trek across the inner solar system past Venus, Earth and Mercury, NASA’s MESSENGER spacecraft will finally park itself in orbit around Mercury next month. MESSENGER (whose name stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) will be the first probe to orbit the innermost planet, and the space agency hopes to answer long-lingering questions about Mercury’s formation, composition and its dynamic atmosphere. Sean C. Solomon, Principal Investigator for the MESSENGER mission and a former president of the American Geophysical Union, spoke with GeoSpace about the mission’s goals, Mercury’s potential to surprise, and his own path in research. He gave GeoSpace a sneak preview of topics he might be discussing during his talk at the annual meeting of the American Association for the Advancement of Science this Sunday in Washington, D.C.
GeoSpace: You’ve been involved with this mission from the start, and it’s taken many years to get to this point. Next month, MESSENGER will begin a one-year orbit around Mercury. What has it been like to see this project through from the beginning?
Sean Solomon: Exploring the Solar System is not for the impatient. Years can be invested in seeing a mission concept through to acceptance for flight. There are years between mission approval and launch, and some spacecraft spend years en route to their targets. MESSENGER launched more than six and a half years ago, but in that period the spacecraft has accomplished six planetary fly-bys, including one of Venus and three of Mercury when many new scientific observations were made. The year in orbit about Mercury will indeed be the culmination of the MESSENGER mission, but we’ve learned a great deal about the innermost planet on the road to orbit insertion.
GS: MESSENGER will be the first spacecraft to orbit Mercury, and these sorts of missions have always surprised astronomers in the past. Early missions to Venus, Mars, and others overthrew conventional wisdom at the time. What do you expect to find on Mercury, and with as much as we now know about the solar system, how much room is there left for surprise?
SS: Every spacecraft encounter with Mercury has been different, and each has produced surprises. MESSENGER will be the first spacecraft to orbit the innermost planet, a vantage that will permit it to build up global observations of Mercury’s surface, interior, tenuous atmosphere, magnetosphere, and interplanetary environment. During MESSENGER’s fly-bys, we imaged most of the planet that had never before been viewed at close range by spacecraft, but at very uneven lighting and resolution, so we expect that an orbital campaign of imaging at generally higher resolution and under optimum lighting conditions will continue to reveal new secrets about Mercury’s geological history. Our geochemical remote sensing instruments were challenged during the fly-bys by the short periods when the sensors were close to the planet. So, much of what we expect to learn about Mercury’s surface composition, and what that information tells us about how Mercury formed and differentiated, must await orbital observations. From orbit, we will collect observations of topography and gravitational and magnetic fields that will provide new windows into Mercury’s internal structure and dynamics, its exceptionally large iron core, and its enigmatic magnetic field. Mercury’s exosphere [the uppermost layer of its atmosphere] and magnetosphere are extraordinarily dynamic. And, MESSENGER’s magnetometer, energetic particle and plasma spectrometer, and ultraviolet and visible spectrometer will afford us a front-row seat on their interactions with the solar wind, the interplanetary magnetic field, and each other as the Sun becomes increasingly active. There will surely be more surprises from these orbital observations.
GS: The standard Astronomy 101 course explains that Mercury’s high density comes from an enormous impact early on in the solar system’s history. Most of the planet’s exterior would have been stripped away in the impact, leaving a dense molten core with little crust. How will MESSENGER shed light on Mercury’s formation and this theory?
SS: A giant impact that stripped off the crust and much of the mantle of a previously differentiated protoplanet is the leading explanation for Mercury’s anomalously high bulk density and the inferred large mass fraction of its dense metal core. There are alternative hypotheses, however, including vaporization of Mercury’s outer silicate shell in a hot solar nebula [a cloud of gas and dust left over after the sun was created]… These different hypotheses lead to different predictions for the composition of Mercury’s crust, so elemental remote sensing of crustal composition promises tests of competing ideas for how Mercury was formed.
It is not out of the question, however, that none of the competing ideas, each about three decades old, is correct. From fly-by images, MESSENGER showed that on Mercury’s surface there are pyroclastic deposits, products of explosive volcanism attributed on Earth and other bodies to the release of volatiles during the ascent and eruption of magma. The current scenarios for Mercury’s formation all involve high-temperature processes, ones that should not have led to the retention of substantial volatiles even locally in Mercury’s interior. The textbook accounts of Mercury and its formation will need to be rewritten when the MESSENGER mission is complete.
GS: This mission has a number of lofty science goals, all of which are supposed to be answered over the course of a one-year mission. Is that enough time to answer all the questions your team would like to, and is there a potential to extend the mission if you find something that warrants more study?
SS: All of the broad questions that have framed the MESSENGER mission since the first mission concept studies can be substantially addressed from one year of orbital observations with the instruments that MESSENGER carries. That said, the fly-bys have already raised follow-on questions, and the orbital observations will certainly raise others. MESSENGER’s year in orbit will still be more than a full year shy of solar maximum, so we won’t have witnessed the full sweep of dynamic interactions between Mercury, the Sun, and the interplanetary environment. Even before orbit insertion, the MESSENGER team has been planning a proposal for an extended mission, one that will take advantage of the more active Sun, a different cadence for lowering the periapsis altitude [the closest it comes to Mercury during its orbit] to spend more time close to the planet, and a different mix of targeted observations by the full set of payload instruments. [The Sun's gravity is constantly tugging the the spacecraft away from the planet, so the team must fire MESSENGER's thrusters every 88 days to return to the original orbit; an extended mission would fire the thrusters twice as often to maintain better views of the planet.] Once we’ve gained experience from early spacecraft and instrument performance from orbit, we hope that NASA will agree that a new set of observations of Mercury are well worth the modest cost of continuing orbital observations beyond the first year.
GS: You’ve spent an entire career studying the solar system and have been involved in spacecraft sent to Venus and Mars, and even researched Earth’s deep interior. Do you feel like you’ve been fortunate to occupy a place and time that allowed you to have such a broad research career and be present for so many firsts in solar system exploration?
SS: When I was in graduate school, the plate tectonics revolution exploded and the Apollo expeditions launched an intensive exploration of the Moon. Those watershed events set off four decades of extraordinary advances in our understanding of the solid Earth and the planets, and I feel fortunate to have been present when some of those era’s discoveries were made. That said, all of us in science build on the work of those who have preceded us. I have no doubt that the next four decades will be equally remarkable, in different and not altogether predicable ways, for the Earth and space scientists now setting out on their careers.
– Eric Betz, contributing science writer.