Transcript Clark R. Chapman Southwest Research Inst. Boulder, Colorado

The Planet Mercury
and the Science Goals of the MESSENGER Mission
Clark R. Chapman
Southwest Research Inst.
Boulder, Colorado
MESSENGER Fellows Program
Awaiting the launch, Cocoa Beach FL
July 29th, 2004
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 on
one side, extremely cold
at night
 Mercury is made of the
densest materials of any
planet: it is mostly iron
Mercury is Difficult (but Possible)
to See for Yourself
Tonight, Mercury is to the
lower right of Jupiter at dusk
http://messenger.ciw.edu/WhereMerc/WhereMercNow.php
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Mercury is visible several
times a year
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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
Mercury’s Strange “Day”
 Mercury does not keep one face to the Sun
Bepi Colombo
A prospective ESA
mission to Mercury
is named after him
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
{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
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Other Mariner 10 Views of Mercury
Artist’s view of Discovery
Scarp [extreme right]
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 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
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?
Optical surface
Regolith probed by longwavelength sensing
Mantle
Crust
Core
[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
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
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Images of Mercury Cratering
Cluster?
Rays
Secondaries
Primary
90m/pix
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
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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
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