Transcript Slide 1

Mercury
Basic Properties of Mercury
• Average Distance from Sun: 58,000,000 km (0.39 AU).
•
•
•
•
•
•
•
•
– But: Orbital eccentricity = 0.206.
– Therefore distance varies from 46,000,000 to 70,000,000 km.
Orbital period: 88 days.
Period of Spin around axis: 58.65 days
Mass = 3.3x1023 kg = 0.055 M
Radius = 2436 km = 0.38 R
Density = 5.4 g/cm3 (recall that Earth ~5.5 g/cm3).
Surface Gravity = 3.7 m/sec2 (38% of Earth's).
Surface Temperatures: -185°C (night), +430°C (day)
Surface superficially lunar, but important differences.
Global Properties
only about 45% of Mercury’s surface has been seen
Albedo = 0.12
Mass
= 0.055 of Earth’s
Density = 5.43 gm/cm3
[2nd most iron-rich object in
solar system]
Surface gravity = 0.38 Earth’s
[twice the Moon’s]
Axial tilt = 0 degrees
ORBIT:
0.39 AU
0.207
7 deg
88 day
= semimajor axis
= eccentricity
= inclination
= period
Mercury’s southern hemisphere
Density
Mercury's density vs.
size is distinctly
different from
that of the other
terrestrial planets.
Implies a large
metallic core !
Planet
Earth
Moon
Mercury
%Core
~54%
~10%?
~74%
Largest temperature range (~1100 F) in solar system
800 F (700 K) at
noon at perihelion
(lead would melt!)
-280 F (100 K) at
midnight
Telescopic Observations
Difficult because:
– Mercury low in the sky.
– Mercury is small.
– Contrasts are weak.
Result: Very little was
known based on visible
telescope data.
Observations
Mercury’s small orbit causes
it always to appear close to
the Sun.
At best you can see Mercury
just after sunset or just
before sunrise toward the
horizon through a lot of
blurring atmosphere.
Early astronomers thought
Mercury would be in
synchronous rotation like our
Moon.
Thus, the Sun-facing side
would always be hot!
Transit of Mercury
It's hard to view surface details
Early maps showed
imagination only!
Synchronous Rotation?
1962
Radio astronomers found
that the backside wasn't
cold.
1965
Dyce & Pettengil bounced
radar off Mercury and
learned its rotation period =
58.7 days
Mercury’s year is about 88
days
A Solar Day on Mercury
Giuseppi Columbo (1920-1984)
Mercury’s solar day is 176
days long
Sun USUALLY rises in east
& sets in west:
UNLESS Mercury is near
perihelion when it swings
by Sun more quickly & Sun
seems to reverse
directions
This “resonance” is caused
by tides, just as we
discussed for the Moon.
Mercury rotates exactly 3 times
during two orbits around Sun.
RATIO = 87.97/58.65 = 3:2
[“3:2 spin-orbit resonance”]
The Sun from Mercury
At 0.38 AU from Sun,
Mercury is 2.5 times
closer than the Earth
Sunlight is (2.5)2x = 6.25x more intense
Mercury
2nd highest density of any
planet.
Its ancient surface records
processes from the earliest
part of planetary formation.
Its exotic atmosphere is the
thinnest among those of all
the terrestrial planets.
Mystery: It is the only
terrestrial planet besides
Earth to possess a global
magnetic field.
Mariner Venus-Mercury Flyby Mission
1974-1975
The first-ever gravity-assist trajectory used Venus’s gravity & orbital
motion to help Mariner 10 fall towards Mercury. Such a maneuver
avoids the need for large braking rockets. Mariner 10 went into orbit
around the Sun and flew by Mercury three times before exhausting its
positioning fuel.
Mercury's Surface Properties
• Surface appears generally Moon-like,
covered by dark gray regolith.
• Brightness, roughness similar to lunar
highlands.
• No water!
• Small, localized mare deposits.
• Many craters, several large impact
basins.
Mariner 10
Mercury studied by just one space mission: Mariner 10
– Flew past Mercury 3 times in 1974 and 1975 (after gravity
assists from the Earth/Moon and Venus...)
– Only about 50% of the surface imaged by Mariner.
Geology of Mercury
• Crater shapes different from the Moon:
• Gravity on Mercury is ~ twice that on theMoon.
• Crater bowls shallower than on the Moon.
• Ejecta blankets less extensive than on the Moon.
Mercury’s Surface Features
Transition from heavily
cratered area to smooth lava
plains. About 490 km across.
“Discovery scarp” 350 km
long. Intersects two craters
with diameters 35 & 55 km.
Geology of Mercury
• Evidence of tectonic forces at work in the past.
• Lobate Scarp indicates compression of the
planet's crust.
Volcanoes on Mercury
• Mercury appears to be
geologically dead and is heavily
cratered. There are no large
volcanoes like Mars’ Olympus
Mons, but there are many smooth,
flat plains with few craters
• Scientists have debated whether
these ancient plains were formed
by erupting volcanoes driven by
internal heat, or simple melting
associated with impact processes
• The latest close-up images by
NASA’s MESSENGER support the
volcano theory
MESSENGER false color image of Caloris impact basin
(light orange is the basin interior). Extinct volcanoes
were imaged in several of the bright orange regions just
inside the southern crater rim.
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider [email protected] http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
Direct & Indirect Evidence for Volcanoes
• MESSENGER has found
shield volcanoes and vents
suggesting explosive
volcanism inside the large
Caloris basin
partly filled
crater
• The Mercury volcanoes
may be similar to the
Hawaiian Islands or
Olympus Mons on Mars
• Lava appears to have partly
filled impact craters both
inside and far from Caloris
basin (not shown)
vents
MESSENGER image (left) of a shield-like volcanic dome, multiple
vents and associated bright deposits, and partially buried nearby
features. Shield volcanism formed the island of Hawaii (right).
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider [email protected] http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
The Big Picture
Alaska
Hawaii
• Volcanism appears to be
responsible for formation
of Mercury’s widespread
plains
• Mercury’s ancient plainsforming, crater-filling
volcanic style was more
similar to the Moon than
Mars or Earth
• MESSENGER will enter
orbit around Mercury in
2011, offering abundant
opportunity to image
volcanic features and place
Mercury’s volcanism in a
solar system context
Mars
Venus
Mercury
Volcanic features in the inner solar system
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider [email protected] http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009
Moon or Mercury?
Why would craters differ between Mercury & Moon?
A large crater is shown at higher resolution. It is ~100
km diameter and shows radial ejecta, secondary craters,
central peaks, etc.
Consequences of a Gravity Difference
Between the Mercury & Moon
Mercury’s Atmosphere
10-14 of Earth’s pressure: an “exosphere”
So thin that molecules are more likely to bounce around
the surface than to hit each other.
Hydrogen & helium come from solar wind.
Hydrogen & oxygen may come from comets that
evaporated nearby as they approached the Sun.
Potassium, sodium, & oxygen may come from surface
rocks that have been vaporized by impacts.
Mercury’s Magnetic Field
More like Earth’s than Venus, Moon, Mars
Only other terrestrial planet besides Earth with
an overall magnetic field.
Strength ~ 1% of Earth’s
Could Mercury still have a molten core?
Radar Observations
• Arecibo radio telescope used to discover the 59
day spin period of Mercury (1965).
• More recently: Evidence for polar ice caps
found from radar data!
– Something very "radar bright" near the poles.
– Ice reflects radar very strongly.
– But it sounds crazy--ice on such a hot planet?
Water Ice at Mercury’s Poles?
Since 1992, radio astronomers have bounced radar beams
off Mercury and detected possible water ice at its
poles and in numerous areas with circular shapes.
Water Ice at Mercury’s Poles?
• Radar-reflective deposits
appear to occur in shadowed
craters.
• Mercury's axis tilt is ~0° (no
seasons!)
• Some deep craters near the
poles never see sunlight!
• Hypothesis: ice from
cometary impacts never
evaporated (primordial!)
• Other hypotheses: sulfur,
silicates.
Planetary Evolution
• Mercury has a weak magnetic field:
– Metallic core may still be partially molten?
– Or is the magnetic field frozen in the crust?
• Surface evidence for compression, but
no extension.
–
Mercury's crust appears to have shrunk from a
once-larger size:
– Contraction after cooling from a molten state?
The Shrinking Planet
NASA’s Return to Mercury
• Many questions will be
answered by NASA’s
MESSENGER Mercury orbiter
–
–
–
–
–
Launched in 2004.
Orbit insertion in 2011.
Orbits, maps for 1 Earth year.
Imaging, spectroscopy,
magnetic fields, altimetry.
• One of the new NASA “better,
faster, cheaper” space
missions…
• Hope to confirm the water ice at
the poles
For more information…
Press Releases
•
space.com - 7/3/08 - “Volcanoes on Mercury Solve 30-year Mystery”
http://www.space.com/scienceastronomy/080703-mercury-messenger.html
Images
•
Global view of Caloris basin and Mercury shield volcano courtesy of Science / AAAS
http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/caloris_color_MB.jpg
http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/Head_Fig1.jpg
•
Aerial view of Hawaii courtesy of NASA/JSC STS61A
http://tinyurl.com/maunaloashieldvolcano
•
Aerial view of erupting Mauna Loa in Hawaii courtesy of HVO/USGS
http://hvo.wr.usgs.gov/
•
Image of Alaska’s Redoubt Volcano courtesy of AVO/USGS, taken by Heather Bleick
http://www.avo.alaska.edu/image.php?id=17872
•
Image of Olympus Mons on Mars and Maat Mon on Venus courtesy of NASA/JPL
http://pds.jpl.nasa.gov/planets/captions/mars/olympus.htm
http://photojournal.jpl.nasa.gov/catalog/PIA00106
Source Article
•
(on-campus login may be required to access journals)
Head et al., ‘Volcanism on Mercury: Evidence from the First MESSENGER Flyby’, Science,
321(5885), p. 69, DOI: 10.1126/science.1159256, 2008.
http://www.sciencemag.org/cgi/content/abstract/321/5885/69
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider [email protected] http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009