Mercury
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Almost everything we know about Mercury comes from the single mission we had
to that planet:
Mariner 10 (March 23 - April 3) 2000 high-res television pictures
(Two more flybys: 9-21-74, and 3-16-75) mapped 45% of Mercury’s surface
We’re going back: MESSENGER (2004)
MErcury Surface, Space ENvironment, GEochemistry and Ranging
closest planet to the Sun, very similar to the Moon
Feature
Named After
Craters
authors, artists, and musicians
Valleys
radio observatories (Arecibo and Goldstone)
Scarps
ships associated with scientific exploration
Plains the word “mercury” in various languages
with some exceptions: Caloris Basin (basin of heat!)
Outline of today’s presentation:
I. Overall characteristics
II. Atmosphere
III. Polar Deposits
IV. Magnetic field
V. Terranes
VI. Chemical and Mineralogical Composition
VII. Geologic History
I. Overall characteristics
Characteristic
Mass
Radius
Mean Density
Rotation Period
Mean Orbital
Distance
Orbital Period around Sun
Mean Orbital
Relative to Earth
(Earth = 1)
0.0553
0.382
Earth: 5.52 g/cm3
58.65 Earth days
0.3871 AU
3.303 x 1026 g
2,439 km
5.43 g/cm3
---57.91 x 106 km
87.9 Earth days
1.6
----47.89 km/s
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Absolute Value
Velocity
Orbital eccentricity
Inclination to
ecliptic (degrees)
Temperature range
(112,000 mi/hr)
0.2056
7.004
--------------
360 F (-183C) to
840F (450C)
Huge eccentricity in its orbit, so orbital velocity changes a LOT
4.6 x 107 km at perihelion
6.98 x 107 km at aphelion
Rotation period = 58.646 Earth days
Orbital period = 87.969 Earth days
3 rotations on its axis for every 2 orbits around the Sun
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See Mercury’s Orbital Resonance (under Atmosphere of Mercury)
Obli
quit
y
very
clos
e to
0, so
ther
e are
no seasons
polar regions are always frigid
(allowing ice to be preserved at crater bottoms near poles)
Not the hottest planet (Venus is hotter), but has greatest T variation: 90-740 K
Too dense to be explained by current condensation/accretion models
still a mystery... maybe lost its “crust” and got knocked into present orbit
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II. Atmosphere
very tenuous
a trillion times less atmosphere than the Earth!
H, He, O, Ar detected by Mariner 10's UV spectrometer
Na and K not detected until 1985, from Earth-based observations
highly variable abundances on short time scales (hours or years)
also variable by factor of 5 day/night!
Global distribution also changes!
Consistent with time scales for photoionization of feldspars:
Na (90 min) and K (120 min)
? Na and K photoionization caused by
1. Interactions between surface minerals and solar radiation
2. Impact vaporization of micrometeorites
Mercury’s Atmosphere
III.
Pola
r Deposits
Polar ice does exist on Mercury
found in 1991 by radar images
Goldstone radar transmitter in Mohave sent radar pulses to Mercury
Very Large Array (VLA) in New Mexico picked up the return pulses
“Delay Doppler mapping”
mapped surface reflectivity, and found polar ice deposits
ice may be H2O, CO2 (dry ice), or other ices - controversial
could also be vapor-deposited S
interpreted to be ice in small, permanently shadowed craters within 6.5_ of poles
with high depth:diameter ratios
T = 100 to 60 K (coldest spot inside Saturn’s orbit!)
Mass of ice required to explain observations is as low as a few km3
a lot more than that would be deposited over time by comets
Short period comets may be the source of the ice
Asteroids also possible, but we don’t know much about Mercury-crossing asteroids
because they are so low in the sky.
IV. Magnetic Field
Mercury is the only terrestrial planet (other than Earth) that has a significant
magnetic field
magnetic sphere is about 7.5 times smaller than Earth’s
3
suggests that Mercury has a fluid outer core, solid inner core
Not well characterized because only 2 passes (1st and 3rd) through magnetic field
Densest known silicate is fayalite (Fe2SiO4) D = 4.2 g/cm3, DMercury = 5.43 g/cm3
so, Mercury must have a larger fraction of Fe than anywhere else
in solar system - 70% Fe, 30% silicate
we expected core to be Fe due to high density:
Earth’s uncompressed density = 5.52 g/cm3
Earth’s core is 54% of diameter, 16% of volume
Mercury’s uncompressed density = 5.3 g/cm3
Mercury’s core is 75% of its diameter and 42% of its volume
crust is only 100 km thick
Problems with an outer Fe core that’s liquid:
1. Mercury rotates too slowly to have a circulation-driven dynamo in core
2. Mercury has been geologically inactive for so long
that the core should have cooled off by now!
Either you have to enrich the core with radioactive elements
OR put something other than pure Fe in the core that
would lower its melting point
Between 0.2 and 7% sulfur would keep the core liquid based on
current thermal models!
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See structure (under Mercury’s Interior and Surface)
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V.
1. Impact Craters and Basins
We know from the Moon the time scales of meteorite bombardments in our
solar system; there was particularly heavy meteoroid bombardment ending about
3.8 m.y. on Moon
Bombardment was of objects left over from terrestrial planet formation
OR from the formation of Uranus and Neptune
DIFFER FROM LUNAR CRATERS IN THREE WAYS:
A. For a constant rim diameter, Mercury ejecta blankets are uniformly smaller (by
0.65x) than on Moon AND maximum density of secondary impact craters is closer
to crater rims than on Moon
(2-2.5 crater radii on Moon vs. 1.5 crater radii on Mercury)
This is due to differences in their gravity:
Mercury: 3.70 m/s2
Moon: 1.62 m/s2
B. The densely cratered terrain is not saturated with craters
?different densities of meteorites at different places in the solar system - no
?maybe the crust was warmer during impacts (i.e., Mercury cooled more
slowly than the Moon), so impacts were erased - maybe
?maybe this bombardment erased all the earlier craters - maybe
?early heavy bombardment craters got covered up by intercrater plains - yes!
intercrater plains formed 4.0 -4.2 by ago – end of heavy bombardment
C. Mercurian craters are very shallow and ill-defined
- they’ve degraded, primarily from ejecta being thrown on them
- ejecta doesn’t travel as far because of stronger gravity (2.5x) than on Moon
SIMILAR TO LUNAR CRATERS IN THESE WAYS:
A. Small craters are all bowl-shaped
B. With increasing size, interiors become flatter, craters get rims
C. fresh craters all have haloes (bright or dark) and rays
Mercury has 22 multiring basins
Caloris Basin is 1340 km, ridge is 2 km high
ridged floor 2.5 km higher than surrounding plains
interior fractures are the only extensional fault on the planet
interior covered by smooth plains material
Van Eyck Fm. is radially spreading ridges and grooves
extends 1,000 km beyond Caloris Mtns.
nd
2 largest impact on any of the terrestrial planets
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(S. Pole Aitken basin on Moon, 2,600 km, is biggest)
1 trillion 1-megaton hydrogen bombs
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See Caloris Basin (under Surface Features of Mercury)
2.
Hill
y
and
Line
ated
Terr
ain
antipodal p
directly opposite Caloris Basin, 500 km across
disrupt preexisting landforms such as crater rims
hills are 5-10 km wide, 0.1-1.8 km tall
similar, smaller features exist on the Moon opposite Orientale and Imbrium
3. Intercrater Plains
either impact basin ejecta or lava plains
clusters of impact craters common
shallow secondary craters, often aligned in long chains
topographically very complex
oldest surface on Mercury; very similar to lunar highlands
records the same bombardment flux as lunar suface
dated by analogy with lunar samples
widespread volcanism occurred at the same time
as heaviest bombardment
most extensive terrane on Mercury
few craters older than plains flows
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planet was resurfaced during this time
fill and are superposed by craters — contemporaneous with craters
dated 4-4.2 by
high population of craters <15 km diameter — late emplacement
source basins are lacking; believed to be fissure flows
global distribution implies volcanic origin
4. Smooth Plains/Lowland Plains
40% of Mariner 10 image area
associated with large impact basins
heaviest concentration in northern hemisphere (like the Moon)
fill and surround Caloris Basin — smooth plains are clearly younger than
basins —> late volcanic eruptions
dated to end of late heavy bombardment (3.8 b.y.)
BUT no evidence of fissures or vents
?relatively thin flows – mysterious because no volcanic features
no lobes, no domes, no cones
?could possibly be giant sheets of impact melts
Compositionally similar to surrounding rocks
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See Wrinkle Ridges (under Surface Features of Mercury)
5.
Shie
ld
volc
ano
may
also
be a
large shield volcano on the unimaged side of Mercury
6. Lobate scarps
unique to Mercury
20-500 km in length, 100 m - 3 km high
random spatial distribution
transect fresh and degraded craters
extend from pole to pole, trend roughly north-south
uniform distribution suggests contraction
thrust faults resulting from compressional stress in the crust
caused by cooling 1-2 km decrease in radius when core/mantle cooled
use them to estimate amount of decrease in radius due to cooling
7
assume fault plane inclination = 25_, ave. Ht = 1 km,
count up all the faults and their lengths...
but mantle/core solidification should have shrunk radius by 6-10 km
?maybe some contraction happened before current features formed
?maybe core has not yet solidified (which is why it has magnetic field)
postdate intercrater plains and smooth plains
roughly comparable in age to Caloris basin fm.
VI. Chemical and Mineralogical Composition
surface composition not well known
color relatively homogeneous, suggesting homogeneous distribution of elements
that reflect
color suggests that Ksp is present
Fe-poor pyroxene
all the Fe on Mercury seems to have gone in its core
implies lack of extrusive volcanism on its surface
also plag (labradorite/bytownite) > highly differentiated lavas
about 6.0 wt% FeO in crust
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VII. Geologic History
1. Core formation and differentiation (4.5 by)
metallic Fe is denser, moves toward center due to gravity
movement toward core generates friction, heat
no H2O; apparently moved further out in nebula
DIFFERENTIATION occurred, crust+mantle = 600-700 km
2. Portion of mantle was stripped away by giant impact
“Post-accretionary vaporization”
proto-Mercury was 2.25 x present Mercury
Fe-cored projectiles of 20 km size could vaporize silicates
but not Fe (Fig. 21, p. 143)
3. Intercrater plains erupt (?) about 4 by
Heavy bombardment
4. Caloris Basin formed hilly and lineated terrains
5. Thrust faulting due to cooling
6. Further eruption of smooth plains 3.8 by
heavy bombardment decreased dramatically
smooth plains form as final stage of volcanism (2-3.6 by)
lithosphere cools and thickens
some light cratering
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