Clark R. Chapman Southwest Research Inst. Boulder, Colorado

1. The Planet Mercury and the Science Goals of the MESSENGER Mission Clark R. Chapman Southwest Research Inst. Boulder, Colorado MESSENGER Teacher Workshop Denver Museum of Nature & Science August 1, 2005 (awaiting Earth fly-by)

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

3. Mercury is Difficult (but Possible) to See for Yourself (Tonight, Mercury is very near the Sun and cannot be seen.) • Mercury is visible several times a year • just after sunset (especially in the spring) • fall: just before sunrise (around Aug. 22nd would be best, about 5:30 a.m.; Mercury will be about 12 Moon diameters below Saturn, in the constellation Cancer) • 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 http://btc.montana.edu/messenger/wheremerc/wheresmerc.php

4. 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 {Interesting Fact: Over Mercury’s “hot pole,” when Mercury is 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.}

5. 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

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

7. MESSENGER: A Discovery Mission to Mercury MErcury Surface, Space ENvironment, GEochemistry and Ranging • MESSENGER is a low-cost, focused Discovery spacecraft, built at Johns Hopkins Applied Physics Laboratory • It was launched one year ago, flies by Earth tomorrow • It flies by Venus twice (2006 - 2007) and Mercury 3 times (2008 - 2009) • Then, starting 2011, it orbits Mercury for a full Earth-year, observing the planet with sophisticated instruments • Designed for the harsh environs Important science instruments and spacecraft components

8. http://messenger.jhuapl.edu/ Caloris Basin

9. MESSENGER’s Trajectory

10. MESSENGER’s Earth Fly-by • Closest approach: 1:13 pm MDT, Tuesday, 2 Aug. 2005 • Will take images of South America, look-back movie • Calibrate instruments (Moon) • Gravity-assist (orbit change) toward Venus Launch on Delta II from Cape Canaveral, Florida, 4 August 2004 Image of Earth taken by MESSENGER narrow-angle-camera on 24 July 2005

11. 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

12. 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? Optical surface Regolith probed by long-wavelength sensing Crust Mantle Core [Not to scale]

13. Is there or isn’t there: ferrous iron?Or is Mercury’s surface reduced? • Putative 0.9μm spectral feature appears absent (spectra of reflected sunlight observed with telescopes on Earth) • Other modeling of color/albedo/near-to-mid-IR-spectra yield FeO + TiO2 of 2 - 4% SVST data (big boxes) compared with earlier spectra Vilas (1985): all glass

14. 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

15. Introduction to Impact Cratering on Mercury • Only direct evidence is from Mariner 10 images of mid-70s (and recent radar) • Theoretical and indirect studies • Comparative planetology (Moon, Mars, …) • Calculations/simulations of impactor populations (asteroids, comets, depleted bodies, vulcanoids) • Theoretical studies of cratering physics, how ejected material is distributed, evolution of the surface soil, etc. • Clearly, impact cratering dominates Mercury’s geology today, was important in the past • Impact processes range from solar wind and micrometeoroid bombardment to huge basin-forming impacts • MESSENGER will address cratering issues

16. Origins for Mercury’s Craters • Primary impact cratering • High-velocity comets (5x lunar production rate) • Sun-grazers, other near-parabolic comets • Jupiter-family (short-period) comets • Crater chains may be made by fragments of comets disrupted by solar tides • Near-Earth, Aten, and Inter-Earth asteroids • Ancient, maybe now-gone, impactor populations • Late Heavy Bombardment (3.9 billion years ago) • Outer solar system planetesimals (outer planet migration) • Main-belt asteroids (planetary migration, collisions) • Trojans and other remnants of terrestrial planet accretion • Left-over remnants of inner solar system accretion • Vulcanoids (bodies that primarily impact Mercury only) • Secondary cratering • Craters <2 km diam. from larger impacts • Basin secondaries up to 30 km diam. (?) • Endogenic craters (volcanism, etc.)

17. Images of Mercury Cratering Cluster? Rays Secondaries 90m/pixel Primary

18. Possible Role of Vulcanoids ? • Zone interior to Mercury’s orbit is dynamically stable (like asteroid belt, Trojans, Kuiper Belt) • If planetesimals originally accreted there, they may or may not have survived mutual collisional comminution • If they did, “Yarkovsky” drift of >1 km bodies in to Mercury could have taken several billion years and impacted Mercury alone long after LHB • Telescopic searches during last 20 years have so far failed to set stringent limits on current population of vulcanoids (but absence today wouldn’t negate earlier presence) • Vulcanoids could have cratered Mercury after the Late Heavy Bombardment, with little leakage to Earth/Moon zone; that would compress the timescale for Mercury’s geological history toward the present (e.g. thrust-faulting might be still ongoing, more consistent with molten interior)

19. 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 >95% (perhaps all) of them are secondaries! • 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. • A. McEwen finds that a single 10 km crater on Mars produced a billion secondaries > 10m diameter!

20. Concluding Remarks • MESSENGER’s six science goals • Why is Mercury so dense? • What is the geologic history of Mercury? • What is the structure of Mercury's core? • What is the nature of Mercury's magnetic field? • What are the unusual materials at Mercury's poles? • 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