1B11
Foundations of Astronomy
Mercury, Mars and Venus
Liz Puchnarewicz
emp@mssl.ucl.ac.uk
www.ucl.ac.uk/webct
www.mssl.ucl.ac.uk/
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.
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.
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.
1B11 Mercury, Mars and Venus
Planet
Diameter
(Earth=1)
Mass
(Earth=1)
Density
(water=1)
Surface
pressure
(Earth=1)
Mercury
0.38
0.055
5.4
10-15
Venus
0.95
0.82
5.2
100
Earth
1.0
1.0
5.5
1.0
Mars
0.53
0.11
3.9
0.01
Moon
0.27
0.012
3.3
10-15
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)
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.
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.
1B11 More surface features on Mars
The Vallis Marineris is 5000 km long, 100s of km wide and up
to 9 km deep. It’s 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.
1B11 Water channels
Mars was once believed to have “canals”. We now interpret
many surface features to be the result of erosion by water.
1. Outflow channels : Caused by catastrophic
floods from the southern highlands and
canyons to the north plains, due to the
sudden release of ground water.
2. Sinuous Valleys : Mostly in the southern
hemisphere. These are short, stubby
tributaries thought to be due to groundwater sapping.
3. Dendritic channels : Maybe due to rain –
but ground water is considered the most
likely cause.
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.
=> Suggests a permafrost and groundwater layer which is
several km thick!
Pressure (bar)
1B11 Water phase diagram
1.0
10-1
10-2
SOLID
MARS
10-3
200
LIQUID
GAS
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.
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.
1B11 Venus
Venus is the Earth’s “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.
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 => surface temp ~ 460OC
Total cloud cover (composed of H2SO4)
No magnetic field
No moons
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.
1B11 Venus
tops of clouds
height (km)
60 1bar
upper cloud deck (H2SO4)
-50OC
10OC
haze layer
95OC
40
clouds thin out
20
clear atmosphere
0 winds: 1-3km/h
surface
460OC
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 daynight temps vary by
less than 10K. Wind
speeds in the cloud
deck reach 240km/h
1B11 Venus’ surface features
1. Highlands
Two continental-sized regions:
(a) Aphrodite – equatorial, approx 10,000 x 2000 km
with a maximum altitude of 4-5km
(b) 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 => surface age of 400-500 million
years, => global re-surfacing. Small craters (<3km)
don’t form because small meteors are vaporized.
1B11 Venus North-South divide
North
South
Mountainous with
uncratered upland plateaux,
resembling continents on
Earth.
Flat, rolling terrain. Seems
to consist of vast lava
plains.
Ishtar Terra is the great
northern plateau and
measures 1000km x
1500km.
1B11 Volcanoes and craters
Venus is almost completely covered in flat, volcanic plains
with 10,000’s 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 –
1. Central peaks
2. Terraced walls
3. Shocked surfaces
4. Flooded floors.
Dense atmosphere => no extensive ray systems
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.
1B11 The greenhouse effect
Calculating the equilibrium temperature.
Planetary surfaces are heated by sunlight:
Sun, Luminosity
LSUN
Planet,
radius R
D
Total energy
intercepted by the
planet:
L SUN
2



R
2
4D
R
1B11 Venus
Albedo, A
is the fraction of energy incident on a planet that is
reflected back to space
2
So total energy
absorbed by the
2 SUN
planet is:
R
 (1  A ) 
L
4D
This energy will warm
the planet. Assuming
the planet then
radiates as a
blackbody of effective
temperature Teff:
R2
2
4
(1  A )  2 L SUN  4R σTef f
4D
L SUN (1  A )
4
 Tef f 

2
16σ
D
1B11 Equilibrium vs measured temp
Putting in the numbers and
measuring D in AU gives:
Teff
 (1  A) 
 280  
2

D


1
4
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.
1B11 Planetary greenhouse effects
Remember Wien’s law:
For T = 230 K,
λMAX
λMAX
3000

μm
T
3000

μm  13 m …which is
230
absorbed by CO2.
planet
Temp increase by
greenhouse effect
Venus
Earth
+500K
+35K
Mars
+5K
NB: the Earth has ~60
bars of CO2 locked up
in carbonate rocks.
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.
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 Earth’s) – probably from
surface rocks
High mean density => large (solid?) iron core
35% of surface imaged by Mariner 10 (1973-4) => heavily
cratered surface, approx 4 billion years old
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).