1B11 Foundations of Astronomy Mercury, Mars and Venus

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1B11 Foundations of AstronomyMercury, Mars and
Venus

• Liz Puchnarewicz
• emp_at_mssl.ucl.ac.uk
• www.ucl.ac.uk/webct
• www.mssl.ucl.ac.uk/

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1B11 Mercury
This image is a composite of photos taken by the
Mariner 10 spacecraft, which flew past in 1973
and 1974. It showed a world that resembles the
Moon. It is pocked with craters, contains huge
multi-ring basins, and many lava flows. The
craters range in size from 100 metres to 1,300
kilometres. The Caloris basin is 1,300 kilometers
in diameter, and was probably caused by a
projectile larger than 100 kilometers in size.
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1B11 Venus
This beautiful image of Venus is a mosaic of
three images acquired by the Mariner 10
spacecraft on February 5, 1974. It shows the
thick cloud coverage that prevents optical
observation of the surface of Venus. Only through
radar mapping is the surface revealed.
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1B11 Mars
This image is a mosaic of the Schiaparelli
hemisphere of Mars. It was taken by the Viking
Orbiter in 1976. The center of this image is near
the impact crater Schiaparelli, 450 kilometers
(280 miles) in diameter.
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1B11 Mercury, Mars and Venus
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1B11 Mars essential information
Radius 3398 km Mass 0.11 x mass of the
Earth Mean density 3.9 g cm-3 Average
temperature -58OC Atmosphere 95 CO2 with a
pressure of 6 mbar No magnetic field today No
evidence for plate tectonics Abundant volcanism
in recent past Two small moons (Phobos and
Deimos)
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1B11 Interior of Mars
Mars is believed to have a thin crust, mantle and
core. Core radius is between 1300-2000 km. It has
polar caps in the north and south. In the winter,
the caps cover 30 of the hemisphere and are
made up of CO2 frozen out of the atmosphere. They
retreat to a few in the summer the north cap
is H2O ice, the south a mixture of CO2 and H2O.
Layered deposits indicate a cyclical deposition
of dust and ice.
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1B11 Mars surface features
Mars is smoother in the north it is younger,
has fewer craters and is relatively low (-1 to 2
km wrt 6mbar pressure level) with volcanic
plains. In the south it is much older (4 billion
years), cratered and high (2-3km). It has 3
major impact basins, Hellas (2200 km across),
Argyre and Isidis. These date from the age of
heavy bombardment. Olympus Mons is a giant shield
volcano, 600 km across, 26 km high and less than
500 million years old. The Tharsis Ridge contains
Olympus Mons and three similar shield volcanoes -
there are also many smaller volcanoes.
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1B11 More surface features on Mars
The Vallis Marineris is 5000 km long, 100s of km
wide and up to 9 km deep. Its believed to be
caused by tectonic activity possibly faulting
associated with Tharsis.
This topographical image of Mars shows the Vallis
Marineris, Tharsis Ridge and the Hellas impact
basin. The crust is very thick (120 km) so
inhibited tectonics.
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1B11 Water channels
Mars was once believed to have canals. We now
interpret many surface features to be the result
of erosion by water.

• Outflow channels Caused by catastrophic floods
  from the southern highlands and canyons to the
  north plains, due to the sudden release of ground
  water.
• Sinuous Valleys Mostly in the southern
  hemisphere. These are short, stubby tributaries
  thought to be due to ground-water sapping.
• Dendritic channels Maybe due to rain but
  ground water is considered the most likely cause.

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1B11 Evidence for water
High resolution images show features possibly due
to very recent (possibly ongoing) discharges of
water from canyon and crater walls.
How much water is there? Erosional features
account for more than 6 million km3 (equivalent
to 40m spread over Mars.) In total, this is
probably more than 200-400m. gt Suggests a
permafrost and groundwater layer which is several
km thick!
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1B11 Water phase diagram
1.0
LIQUID
10-1
SOLID
10-2
Pressure (bar)
GAS
MARS
10-3
200
273
373
Temperature (K)
Liquid water is not stable - to remain liquid it
requires a higher temperature and pressure than
on Mars. There is evidence for climate change on
Mars however, possibly a denser CO2 atmosphere in
the past.
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1B11 Is there Life on Mars?
Before space exploration, Mars was considered the
best candidate for harboring extraterrestrial
life. In 1976, three biology experiments aboard
the Viking landers discovered unexpected and
enigmatic chemical activity in the Martian soil,
but provided no clear evidence for the presence
of living micro organisms in the soil near the
landing sites. But 3.5-3.8 billion years ago,
Mars had a thicker CO2 atmosphere so the average
temperature was probably higher than 0OC, so
water would have existed in liquid form. Earth
was in the same state at the time. So did life
evolve? The search is on for fossil remains
and the question of life on Mars at some time in
the distant past remains open.
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1B11 Venus
Venus is the Earths sister planet it has a
similar size, mass, density and volume. However
it is very different from the Earth. It has a
heavy CO2 atmosphere with almost no water vapour.
Its clouds are composed of sulphuric acid and at
the surface, the pressure is 92 times that of the
Earth at sea level. Heat is trapped in the
atmosphere by the greenhouse effect and raises
the temperature to about 482OC. One day on Venus
243 Earth days One year 223 days, so its day
is longer than its year. And it rotates from east
to west.
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1B11 Venus the essential facts
Radius 6050 km (0.95x the radius of the
Earth) Mass 0.82x the mass of the Earth Density
5.25 g cm-3 (Earth, 5.52 g cm-3) Rotation
period 243 days (retrograde) Atmosphere
pressure 90bars, 90 CO2, 3.5 N2 Severe
greenhouse effect gt surface temp 460OC Total
cloud cover (composed of H2SO4) No magnetic
field No moons
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1B11 Venus revealed
This image compares the surface image of Venus
with the clouds image. The clouds are mostly
sulphuric acid and reach 65km above the surface
(cf 16km on the Earth). The surface shows
mountains, plains, high plateaux, canyons,
volcanoes, ridges and impact craters.
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1B11 Venus
Only 3 of sunlight reaches the surface. An
extreme greenhouse effect traps the surface heat
to prevent heat from escaping. Heat is carried by
strong winds so that day-night temps vary by less
than 10K. Wind speeds in the cloud deck reach
240km/h
tops of clouds
-50OC
10OC
1bar upper cloud deck (H2SO4)
60
haze layer
95OC
height (km)
40
clouds thin out
20
clear atmosphere
0
460OC
winds 1-3km/h
surface
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1B11 Venus surface features

• Highlands
• Two continental-sized regions
• Aphrodite equatorial, approx 10,000 x 2000 km
  with a maximum altitude of 4-5km
• Ishtar lies 60O-75O North, is the size and
  shape of Australia and reaches an altitude of
  11km (Maxwell Mountains)
• 2. Impact Craters
• Uniform distribution gt surface age of 400-500
  million years, gt global re-surfacing. Small
  craters (lt3km) dont form because small meteors
  are vaporized.

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1B11 Venus North-South divide
South Flat, rolling terrain. Seems to consist of
vast lava plains.
North Mountainous with uncratered upland
plateaux, resembling continents on Earth. Ishtar
Terra is the great northern plateau and measures
1000km x 1500km.
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1B11 Volcanoes and craters

• Venus is almost completely covered in flat,
  volcanic plains with 10,000s volcanic plains and
  shields.
• Plains are punctuated with approx 1000 craters
  from 10km to over 100km in diameter.
• Impact craters have a great deal of structure
• Central peaks
• Terraced walls
• Shocked surfaces
• Flooded floors.
• Dense atmosphere gt no extensive ray systems

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1B11 Volcanism and tectonics
Volcanoes are generally large shield volcanoes
(eg Maat Mons). Thousands of smaller volcanoes
cover the lowlands. It is not clear whether these
volcanoes are active (Magellan orbiter) but
there is evidence for lava channels. Sulphur
compounds in the atmosphere suggest on-going
release of gases from volcanoes (outgassing, cf
the Earth). Tectonics The surface of Venus is
relatively smooth, (cf ocean basins and
continents on Earth) suggesting no planetwide
crustal plates. Any tectonic activity seems to
concentrated in the Highlands. Maybe the crust is
more plastic and heat losses are local and
volcanic.
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1B11 The greenhouse effect
Calculating the equilibrium temperature. Planetary
surfaces are heated by sunlight
Planet, radius R
Sun, Luminosity LSUN
R
D
Total energy intercepted by the planet
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1B11 Venus
Albedo, A is the fraction of energy incident on
a planet that is reflected back to space
So total energy absorbed by the planet is
This energy will warm the planet. Assuming the
planet then radiates as a blackbody of effective
temperature Teff
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1B11 Equilibrium vs measured temp
Putting in the numbers and measuring D in AU
gives

• For Venus, A 0.76 and D 0.72 AU.
• Teff 230K
• But the measured surface temperature is approx
  730K!
• The difference of 500K is due to the CO2
  greenhouse effect. CO2 transmits visible light
  but absorbs the near IR, trapping heat which is
  radiated by the surface.

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1B11 Planetary greenhouse effects
Remember Wiens law
For T 230 K,
which is absorbed by CO2.
NB the Earth has 60 bars of CO2 locked up in
carbonate rocks.
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1B11 Mercury
This image is a composite of photos taken by the
Mariner 10 spacecraft, which flew past in 1973
and 1974. It showed a world that resembles the
Moon. It is pocked with craters, contains huge
multi-ring basins, and many lava flows. The
craters range in size from 100 metres to 1,300
kilometres. The Caloris basin is 1,300 kilometers
in diameter, and was probably caused by a
projectile larger than 100 kilometers in size.
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1B11 Mercury the essential facts
Mass 0.06x the mass of the Earth Radius 0.38x
the radius of the Earth Density 5.4g cm-3 NO
atmosphere Surface temperature - daytime 190OC,
night -180OC Weak magnetic field (1/60th of the
Earths) probably from surface rocks High mean
density gt large (solid?) iron core 35 of
surface imaged by Mariner 10 (1973-4) gt heavily
cratered surface, approx 4 billion years old
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1B11 Is there ice on Mercury?
Ground-based radar images of the poles give
echoes characteristic of water ice. Impact
craters near the poles are in perpetual shadow,
so have a temperature of approx 170OC which is
cold enough for ice. The thickness of the ice
must be at least the wavellength of the radar
(12.5cm).