Lecture 6: Mercury
(God of travel, commerce & thieves)
History
• Oldest observation (in 235 B.C.), noted by Ptolemy.
• History
• Greeks at first observed two planets – “Apollo” and
“Hermes” – the “morning” and “evening” stars.
• Orbit
• Schiaparelli recorded “markings” (1881-1889).
• Structure
• Lowell (1896) claimed “canal-like features”.
• Surface
• Atmosphere
• Evolution
Mercury - vital statistics
Orbital semi-major axis
0.387 AU
Orbital period
87.969 days
Mercury’s orbit “precesses” by 574" per century.
Newtonian mechanics only predicts 531".
Rotational period
58.65 days (3:2 spin/orbit
resonance)
Difference of 43" per century far above observational error.
Eccentricity of orbit
0.206 at 7o tilt (both large)
Why?
Inclination of rotation axis
0.0º
• Another planet – Vulcan (Le Verrier – predicted Neptune)
Diameter (at equator)
0.383 dEarth (4880 km)
• General Relativity gives right value ⇒ Einstein a genius!
Mass
0.055 MEarth
Mean density
5430 kg m-3 (0.98 Earth)
Orbit precession
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Seasons on Mercury
Orbit, axial inclination and lack of a dense atmosphere
result in a uniquely large range in surface temperature:
Day : Tequator ~ 700 K (825 K at perihelion)
Night : Tequator ~ 100 K
Hard to see details from Earth
Best possible spatial resolution from Earth ~ 100 km.
In practice, typical images have a resolution ~ 200 km
(so no “canals” could have been seen…)
Mariner 10
Until 2011 most of our best data for
Mercury came from Mariner 10,
which imaged 45% of the surface.
Launched: 3/11/73
Arrival(s):
20/3/74 (703 km)
Interior of Mercury
• Observed density of Mercury (5430 kg m-3) is 98% of
Earth. But, we must allow for gravitational compression.
• Earth’s uncompressed density is 4400 kg m-3 compared
to Mercury’s 5300 kg m-3. Mercury is the densest planet.
• Thus, Mercury must have a very large iron core and a
silicate outer shell.
21/9/74 (48069 km)
16/3/75 (327 km)
First mission to use gravity assist –
also flew by Venus (5/2/74).
Now we have Messenger.
(c.f. Earth’s core 1/6 by volume)
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The surface of Mercury
103 kg/m3
Density of Mercury
Mercury lies far from the trend of other inner worlds.
Heavily cratered with some “plains/basins”
(False) Colour imaging
from Messenger
Craters
Multiple impacts
Degas “ray crater”
Volcanic vent
Large “double-ring”
• Scarcity of impact craters > 50 km size implies soft crust
during the period of most massive bombardment.
• Large craters subsided through viscous relaxation.
• Ejecta closer to crater and lower crater walls cf. Moon.
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The shrinking planet – lobate scarps
Discovery Rupes
Messenger
550 km long, 1.5 km high
See many examples of scarps and ridges (rupes): thought
mainly due to planet shrinkage as Mercury cooled. The
radius may have decreased by 1-2 km.
Caloris Basin
Caloris Basin: Mariner 10 & Messenger
Huge impact structure with outer rim diameter ~1550 km.
Ejecta reach as far again. Created three concentric mountain
ranges. Surrounding smooth plains may be 2.5 km lower.
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Interior of Caloris Basin
“Weird” terrain
Messenger
Young (smoother) surface ridges/fractures suggest lava
flooding followed by contraction and settling.
Messenger data suggest many smooth plains – more
than on the Moon – so more volcanic/tectonic activity.
Present-day “Atmosphere” of Mercury
Directly opposite the Caloris basin (the antipodial point) is
a jumbled, hilly region.
Mercury’s large core acts like a lens to focus the seismic
waves, generating huge forces which fractured the surface.
Similar basin/hill pairings exist on Moon and Mars.
The Atmosphere of Mercury
Any initial atmosphere was lost due to thermal escape and
was not replenished due to lack of recent tectonic activity.
O
44000 (no cm-3)
Na
30000
He
6000
Balance between planetary escape speed and gas speed:
K
500
v(escape) = 11.2
H
23000
Mercury has a very tenuous atmosphere – column mass
of 10-10 kg m-3 (about 10-14 Earth atmospheres!).
Composition very different as it is due to solar wind,
ejection from surface (and volatile capture from comets?).
Thermal escape:
√
M (in Earth masses)
km s-1
R (in Earth radii)
For gas, mean speed of Maxwellian speed distribution:
v(ms) = 0.157
T (Gas temp in K)
km s-1
√
m (molecular mass in H units)
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Maxwellian speed distribution
If v(ms) < v(escape)/6 then the total fractional escape
of that gas will be small over the age of Solar System.
Only 1 molecule in 2x106 has speed >3 x mean
Only 1 molecule in 1016 has speed >5 x mean
Water Ice on Mercury?
The present and the future
Strong reflection of radio
waves suggest water ice
near both poles covered
with a thin dust layer.
How can ice survive on
the hottest planet?
Mercury has a zero axial inclination to its orbit and no
atmosphere. Thus, can stay very cold (<100-150 K) at
the poles, particularly if in shadow inside a crater rim.
Messenger+Arecibo data (red=shadow, yellow=radio).
Messenger
Nasa mission – 1 orbiter
Launched: 3rd Aug. 2004
Arrival: in orbit March
2011 after flyby of Venus
and of Mercury (3 times)
BepiColombo
Esa/Japan mission – 2
orbiters (MPO, MMO)
Launch: 2015
Arrives: 2022
(Leicester X-ray instrument)
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BepiColombo MIXS + SIXS
Will measures X-ray emission from the
surface of Mercury:
– X-ray Flourescence induced by solar coronal X-rays
– Bremsstrahlung (and fluorescence) induced by electron and charged
particle impacts on the surface
– Sensitivity range ~ 0.5 – 7 keV
The End
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