lecture 4: venus historical record (goddess of love & beauty)pto/lecture4-handout.pdf ·...
TRANSCRIPT
1
Lecture 4: Venus
(Goddess of love & beauty)
• Vital Statistics
• Orbit & rotation
• Interior
• Surface
• Atmosphere
• Greenhouse effect
Historical record
• Third brightest object
• First record of phases by
Galileo in 1610
• Transits observed: 1761,
1769, 1874, 1882, 2004
• Last one June 5-6 2012 –
next 11 December 2117)
• Transits used to measure
AU and detect atmosphere
Copernican
Ptolemaic
Venus - vital statistics
Orbital semi-major axis 0.723 AU
Orbital period 224.70 days
Rotational period 243.019 days (retrograde)
Eccentricity of orbit <1%
Inclination of rotation axis 177.4º
Diameter (at equator) 0.949 dEarth (6052 km)
Mass 0.815 MEarth
Mean density 5240 kg m-3 (0.95 Earth)
Most planets in the Solar System have prograde spin
Exceptions: Venus, Uranus (& Pluto). As they formed from
a single disk the planets should spin in the same direction.
Venus, Uranus (& Pluto) may have been hit early on – this
would explain their unusual tilt and slow rotation-rate.
2
A Venusian Solar day (noon to noon) is 116.8 Earth days.
This is almost exactly 1/5 its synodic period (583.92d, the
orbital period viewed from Earth). This means Venus
presents nearly the same face to us on closest approach.
Is this near 5:1 resonance
a coincidence?
Yes.
No known way for
Earth to influence
Venus significantly.
Exploration of Venus
• Spectroscopy/imaging from Earth – CO2 found in 1932.
Imaging shows cloud “features”. Surface temp. ~750 K !!
• Radar data dominate, particularly Magellan (1990-94).
• Landers – all Russian. Venera 3 (1965). Crushed!
Venera 9 (1975) sent first picture. Others followed.
Venera 9 –
20-50 cm rocks.Appear young.
Venera 13 –
True-colour.
Similarity in size and density
to Earth implies a silicate-rich
mantle and an iron rich core.
Venera landers confirmed
Venus has a silicate surface.
No seismic data but we do
known it has no detectable
magnetic field (<0.1% Earth).
Interior of Venus Pioneer (1978) view of Venus
3
Purple = low altitude
Green/Red = high altitude
25% “lowland” (≤0 km), 65% “rolling plains” (0-2 km),
only 10% “highland” (>2 km). Altitude range ~15 km.
(Earth: altitude range 20 km. 25% of surface continental.)
The Surface of Venus
Magellan: radar and gravity
mapped 99% of Venus at 100 m
resolution during 1990-94.
Venus gravity map (Magellan)
Very good correlation between the two.
Impact craters on Venus
No small craters (<3 km)
due to thick atmosphere.
Surface lightly covered –
relatively few craters
suggests recent resurfacing.
Surface age ~ 300 Myr.
Dickinson crater
diameter 69 km
Volcanic features - abundant
2 km shield volcano (middle)
+ coronae (front)700 km shield volcano
– but only 2 km high
4
Coronae – roughly circular, can be very large (~600 km).
Arachnoids – smaller than coronae – precursor?
These features are unique to Venus and tend to be along
tectonic belts. Thought due to magma uplift.
Tectonic features
See mountain chains and fracture patterns in highland
areas. Here (dark) lava fills a 20 km wide valley.
Small fluctuations in atmospheric levels of SO2 indicate
that there may also be active volcanoes on Venus.
Active today?
Some evidence for
surface changes.
Landslide � Venus is much hotter than Earth
or Mercury
Cannot be explained by their
relative distances from the Sun.
Highly reflective atmosphere.
Sulphuric acid clouds!
The Atmosphere
5
CO2 96.5 %
N2 3.5 %
SO2 150 ppm
Ar 70 ppm
CO 30 ppm
H2O 20 ppm
Present-day Atmosphere of Venus
Very different to Earth (N2 : O2 dominated – 78% : 20%).
Venus has a CO2 column mass x100 Earth’s atmosphere..
Structure of atmosphere on Venus
Profile simpler than Earth –
no stratosphere (no O3).
Mesosphere dominated by
sulphuric acid (H2SO4).
High cloud deck (30-70km).
Volcanic gases in the
atmosphere. Very high
surface temperature (mean
≈735 K).
Surface pressure ≈90x
Earth.
Thermosphere
Mesosphere
Runaway Greenhouse Effect
More CO2 – less heat escapes – warmer planet
∆Tgreenhouse ≈ +500°C on Venus, cf. +35°C on Earth.
Mean surface temperature on Venus ~ 460°C !!
Our only overall view of Venus
Radar maps (Magellan+Pioneer+Venera+Arecibo)
East at centre North at centre
6
The End