Clark R. ChapmanSouthwest Research Inst.

Boulder, Colorado

Clark R. ChapmanSouthwest Research Inst.

Boulder, Colorado

MESSENGER Fellows ProgramMESSENGER Fellows ProgramAwaiting the launch, Cocoa Beach FLAwaiting the launch, Cocoa Beach FL

July 29th, 2004July 29th, 2004

MESSENGER Fellows ProgramMESSENGER Fellows ProgramAwaiting the launch, Cocoa Beach FLAwaiting the launch, Cocoa Beach FL

July 29th, 2004July 29th, 2004

The Planet MercuryThe Planet Mercuryand the Science Goals of the MESSENGER Missionand 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 MarsMercury’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

http://messenger.ciw.edu/WhereMerc/WhereMercNow.php

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 {Interesting Fact: Over Mercury’s “hot pole,” when Mercury’s closest to the Sun (like 10 suns!), the Sun stops moving west overhead, reverses the Sun (like 10 suns!), the Sun stops moving west overhead, reverses back east, then moves west again, shrinks in size, and finally sets.}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 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 sulfurDetermine if Mercury’s polar ice deposits are made of ice or sulfur

Study Mercury’s interaction with the nearby Sun: magnetic field, “atmosphere”

Study Mercury’s interaction with the nearby Sun: magnetic field, “atmosphere”

Study structure of core

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?

CoreCore

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 + TiO2 of 2 - 4% (e.g. Blewett et al., 1997; Robinson & Taylor, 2001)

Warell (2002): SVST data (big boxes) compared with earlier spectra

Warell (2002): SVST data (big boxes) compared with earlier spectra

Vilas (1985): all glassVilas (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 MASCS instrument instrument will map will map Mercury’s Mercury’s surface in the surface in the IR; also X-ray, IR; also X-ray, gamma-ray, gamma-ray, neutron neutron spectrometersspectrometers

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? 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