Transcript The Planet Mercury and the Science Goals of the MESSENGER

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)
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
Mercury is Difficult (but Possible)
Mercury is very near
to See for Yourself (Tonight,
the Sun and cannot be seen.)
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Mercury is visible several
times a year
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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
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 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.}
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 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
http://messenger.jhuapl.edu/
Caloris Basin
MESSENGER’s Trajectory
MESSENGER’s Earth Fly-by
 Closest approach: 1:13 pm
MDT, Tuesday, 2 Aug. 2005
Launch on Delta II from
Cape Canaveral, Florida,
4 August 2004
 Will take images of South
America, look-back movie
 Calibrate instruments (Moon)
 Gravity-assist (orbit change)
toward Venus
Image of Earth taken
by MESSENGER
narrow-angle-camera
on 24 July 2005
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 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
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
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.
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 Clearly, impact cratering dominates Mercury’s
geology today, was important in the past
 Impact processes range from solar wind and
micrometeoroid bombardment to huge basinforming impacts
 MESSENGER will address cratering issues
Origins for Mercury’s Craters
 Primary impact cratering
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High-velocity comets (5x lunar production rate)
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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
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Late Heavy Bombardment (3.9 billion years ago)
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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
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Craters <2 km diam. from larger impacts
Basin secondaries up to 30 km diam. (?)
 Endogenic craters (volcanism, etc.)
Images of Mercury Cratering
Cluster?
Rays
Secondaries
Primary
90m/pixel
Possible Role of Vulcanoids
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 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)
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!
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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