Clark R. Chapman Southwest Research Inst. Boulder, Colorado Clark R. Chapman Southwest Research Inst. Boulder, Colorado MESSENGER Fellows Program Awaiting the launch, Cocoa Beach FL July 29th, 2004 MESSENGER Fellows Program Awaiting the launch, Cocoa Beach FL July 29th, 2004 The Planet Mercury and the Science Goals of the MESSENGER Mission

Mercury: an extreme planet Mercury is the closest planet to the Sun Mercury is the smallest planet except for Pluto Mercury is like a “Baked Alaska”: extremely hot on one side, extremely cold at night Mercury is made of the densest materials of any planet: it is mostly iron Mercury’s size compared with Mars

Mercury is Difficult (but Possible) to See for Yourself Mercury is visible several times a year just after sunset (e.g. tonight, but it will be tough!) just before sunrise (the week after Labor Day weekend is best); Mercury will be near Regulus in Leo It is always close to the Sun, so it is a “race” between Mercury being too close to the horizon and the sky being too bright to see it…use a star chart to see where it is with respect to bright stars and planets Through a telescope, Mercury shows phases like the Moon Tonight, Mercury is to the lower right of Jupiter at dusk

Mercury’s Strange “Day” Mercury does not keep one face to the Sun like the Moon does to the Earth… but it is trapped by huge solar tides into a 2/3rds lock: its DAY is 2/3rds of its 88-(Earth)day YEAR, or 59 days. But that’s its “day” (time it spins) with respect to the stars. Its “solar day” (time between two sunrises) takes two Mercurian years (176 Earth-days). This was explained 4 decades ago by the Italian physicist, Bepi Colombo Bepi Colombo A prospective ESA mission to Mercury is named after him Bepi Colombo A prospective ESA mission to Mercury is named after him {Interesting Fact: Over Mercury’s “hot pole,” when Mercury’s closest to the Sun (like 10 suns!), the Sun stops moving west overhead, reverses back east, then moves west again, shrinks in size, and finally sets.}

First (and last, so far) Mission to Mercury: Mariner 10 This early spacecraft made 3 flybys of the same side of Mercury in 1974 and 1975 It took what are still the best pictures we have of its surface and made many discoveries: Mercury has a magnetic field Mercury’s crust has buckled Mercury’s geology is much like the Moon’s

Other Mariner 10 Views of Mercury Artist’s view of Discovery Scarp [extreme right]

MESSENGER: A Discovery Mission to Mercury MESSENGER is a low-cost, focused Discovery spacecraft, built at Johns Hopkins Applied Physics Laboratory It will be launched within days It flies by Venus and Mercury Then it orbits Mercury for a full Earth-year, observing the planet with sophisticated instruments Designed for the harsh environs MErcury Surface, Space ENvironment, GEochemistry and Ranging Important science instruments and spacecraft components

MESSENGER’s Trajectory

Some MESSENGER Science Goals Determine if Mercury’s polar ice deposits are made of ice or sulfur Study Mercury’s interaction with the nearby Sun: magnetic field, “atmosphere” Study structure of core

Mercury’s Surface and Interior: Clues to How and Where it Formed Can we learn Mercury’s bulk composition from observing its surface? Where did planetesimals accrete to form Mercury, what were they made of? Core Mantle Optical surface Regolith probed by long- wavelength sensing Crust [Not to scale]

Is there or isn’t there: ferrous iron? Or is Mercury’s surface reduced? Putative 0.9μm feature appears absent Other modeling of color/albedo/near-to-mid-IR-spectra yield FeO + TiO 2 of 2 - 4% (e.g. Blewett et al., 1997; Robinson & Taylor, 2001) Warell (2002): SVST data (big boxes) compared with earlier spectra Vilas (1985): all glass

Recent Color Processing of Mariner 10’s Images Although Mariner 10’s vidicon system was primitive, enhanced colors (reflecting different minerals) provide clues about whether volcanism has occurred on Mercury. MESSENGER has many state-of-the-art instruments sensitive to composition. MASCS instrument will map Mercury’s surface in the IR; also X-ray, gamma-ray, neutron spectrometers

Introducing Mercury’s Craters Craters seen by Mariner 10 look superficially like Moon/Mars But morphologies differ (high g, fewer erosive processes, etc.) Origins of craters Near-Earth/Inter-Earth asteroids Comets Vulcanoids (hypothetical: could have cratered Mercury post-LHB & moved observable history closer to the present, which would be compatible with still-active interior) Secondary cratering

Images of Mercury Cratering Rays Secondaries 90m/pix Primary Rays Secondaries 90m/pix Primary Cluster?

Secondary Craters on Europa, Moon & Mars… and Mercury? (B. Bierhaus PhD, 2004) Spatial clustering and size distributions of ~25,000 craters on Europa shows that >90% (perhaps all) of them are secondaries (Bierhaus, 2004)! Extrapolation to the Moon (if craters in ice behave as in rock) shows that secondaries could account for all small craters < few hundred meters diameter. McEwen (2004) finds that a single 10 km crater on Mars produced a billion secondaries > 10m diameter!

Concluding Remarks MESSENGER’s six science goals Why is Mercury so dense? Why is Mercury so dense? Why is Mercury so dense? Why is Mercury so dense? What is the geologic history of Mercury? What is the geologic history of Mercury? What is the geologic history of Mercury? What is the geologic history of Mercury? What is the structure of Mercury's core? What is the structure of Mercury's core? What is the structure of Mercury's core? What is the structure of Mercury's core? What is the nature of Mercury's magnetic field? What is the nature of Mercury's magnetic field? What is the nature of Mercury's magnetic field? What is the nature of Mercury's magnetic field? What are the unusual materials at Mercury's poles? What are the unusual materials at Mercury's poles? What are the unusual materials at Mercury's poles? What are the unusual materials at Mercury's poles? What volatiles are important at Mercury? What volatiles are important at Mercury? What volatiles are important at Mercury? What volatiles are important at Mercury? But I think that serendipity and surprise will be the most memorable scientific result of MESSENGER The history of past planetary spacecraft missions teaches us to expect surprise MESSENGER has superb instruments, it will be so close to Mercury, and it will stay there a full year