Unformatted text preview: Grand Tour of the Grand Tour of the Solar System, Part 2 Discussion Questions from Last Class
Discussion Questions from Last Class If the Moon was formed by an impact on Earth knocking some of the Earth’s mantle into orbit, why is its density ~40% less than the Earth’s? Why is Mercury’s density about the same as Earth’s What’s wrong with Pink Floyd’s album title “The Dark Side of the Moon?” What’s the chain of logic that is used to infer that there was a period of very frequent impacts on the Earth early in its history? Why is the night side of Mercury one of the coldest places in the solar system? What does the density of a planet tell you about its bulk composition? Discussion Questions for Today
Discussion Questions for Today How can we see through the clouds of Venus, and what is actually imaged? What kind of structures occur in the Venusian crust, how are they distributed, and what do they signify? Why did Venus turn out so different from the Earth? What does the population of impact craters on Venus suggest about its history? Why do we think the pressure and temperature environment on Mars was different in the past from what it is today? Venus
Venus Venus, Earth’s Hot Sister Venus
Earth Venus: Basic Properties
Venus: Basic Properties
6,051 km Comments
0.95 REarth Mass 4.87x1024 kg 0.815 MEarth Density 5.25 g/cm3 < ρEarth ~ 5.5 g/cm3 Composition Rocks, metals Gravity 8.60 m/s2 0.91 gEarth Distance from Sun 108.2x106 km 0.723 AU Orbital Period 224.7 Earth days 0.615 years Obliquity 177.3o Retrograde rotation Rotation (or Spin) 243.0 Earth days Slower than Venus’s year! Atmosphere CO2, ~90 bar Thick atm. w/ H2SO4 clouds Surface Temperature 753 K (480oC) Lead would melt!... Moons None Venus: Atmospheric Composition
Venus: Atmospheric Composition
Venus’s atmosphere is composed mainly of CO2.
Atmospheric composition determined by spectroscopy from ground based
telescopes and direct sampling and analysis from robotic balloons and landers. Gas
0.03% N2 3.5% 78.1% Ar 0.006% 0.93% O2 0.003% 21.0% Ne 0.001% 0.002% Venus: Why All That CO2?
Venus: Why All That CO
Earth: CO2 cycling “sources” Venus: No cycling
No oceans or plate
tectonics (?) on Venus,
so no "sinks" for CO2.
CO2 stays accumulated
in a thick atmosphere. “sinks” Venus: Greenhouse Heating
Venus: Greenhouse Heating
Greenhouse Effect on Venus:
The top clouds of Venus’s atmosphere
reflect about 75% of the incident sunlight;
25% filters through and heats the surface.
The heat radiated by the surface (IR
radiation) then tries to escape, but it is
absorbed by CO2 and by the lower cloud
deck, and is re-radiated downwards.
Temperature builds up until an equilibrium
is reached at T ~ 480oC!
480 Venus: Runaway Greenhouse
Venus: Runaway Greenhouse
Early Venus… Venus’s surface (not to mention interior) was probably too hot from the beginning to ever have had large bodies of liquid H 2O.
Most of Venus’s original endowment in H2O likely turned to vapor and was progressively photodissociated and lost by exospheric escape.
With no significant bodies of liquid H2O to absorb and dissolve atmospheric CO2 and lock it up as carbonate rocks, CO2 was allowed to build up massively in Venus’s atmosphere. Because CO2 is an effective greenhouse gas, temperatures on Venus rose progressively higher until liquid H 2O was no longer even possible on Venus.
Venus experienced a Runwaway Greenhouse Effect from which it never recovered. Venus: Topography Two "continents": Aphrodite (Africa-sized)
& Ishtar (Australia-sized) w/ highest peak
at 11 km: Maxwell Montes. Venus: Magellan Mission
Venus: Magellan Mission US radar orbiter mapping mission. Launch from Space Shuttle, May 1989 Mapped Venus Sept. 1990 Oct. 1994: “Imaged” 98% of Venus' surface with
a resolution of ~ 1 km.
Measured surface topography to
~100 m vertical resolution. Venus: Magellan Results
• Nearly 1000 impact craters
> 1 km in size. Crater density
implies young ~500 MY age.
Some small areas may be
much more recent.
No "ancient" surface.
• Much evidence of tectonism,
but no clear evidence of plate
tectonics like on Earth.
• Abundant evidence of past
volcanism: volcanoes and
vast low-lying lava plains.
• Some evidence for erosion:
dunes, wind streaks, lava
channels. Radar return depends on surface roughness and dielectric properties.
•Smooth areas (like basalt flows) have low radar reflectivity, whereas crater ejecta or one side of fractures reflect radar back to the spacecraft.
•Highlands at altitudes >2.4 km have high radar emissivity, indicating presence of semi
conducting materials with high dielectric constants (iron sulfides or metallic frosts formed from volcanic exhalations?). Impact Craters Limited number and wide distribution of craters used to argue for global volcanic resurfacing at ~500 million years ago, but resurfacing could have been spread over
a billion years instead. Heat loss (mostly by conduction) is slower than on Earth, so periodically the crust may founder and sink into the overheated mantle (repeatedly?).
Small size (3 km) cutoff of craters is due to atmospheric disruption of smaller impactors. Ejecta extend out ~2.5 crater radii, farther than predicted from ballistic emplacement in the dense vensusian atmosphere. Dickinson
(D ~70 km) Three
D~50 km) Golubkina
(D ~ 30 km) Venus: Tectonics
Numerous patterns of ridges and fractures, including mountain belts and rift valleys. Evidence of powerful tectonic forces likely driven by convection in Venus’s interior No direct evidence for Earthlike plate tectonics. Fractured region.
Area is 40 km wide. “Tessera” - folds that appear to predate the largescale volcanic plains that embay them Impact crater (D=37 km) split apart by tectonic forces. Venus: Volcanism
Venus: Volcanism Hundreds of thousands of volcanoes. Volcanism is widespread and planetwide. Vast “marelike” lava plains dominate. Many individual cones and mountains, “pancake domes” Coronae formed by upwelling of mantle, downwelling of lithosphere (or both). No direct evidence for presentday volcanic activity (?). Sapas Mons volcano “Pancake Domes” Cinder cone field.
Each cone D ~ 1 km “Miss Piggy’s Face” Corona Venus: Erosion
Venus: Erosion Evidence for surface erosion includes:
Wind streaks Dune fields Long channels carved into bedrock No H2O! So erosion must indicate the action of wind and molten lava Surface winds very weak: long timescales Deep, long lava channels: long timescales Lava channels, some 1000s of km long Wind streak behind 5 km volcano Venera surface photo Venera 13 Venera 14 Mars Earth/Mars Comparison
Earth/Mars Comparison Martian Moons Phobos (Fear) Deimos (Panic) Global Topography
Global Topography Surface Ages from Crater Counting
Surface Ages from Crater Counting Mars has a thin (~6 mbar) atmosphere of mostly carbon
dioxide Liquid Water Not Stable on Mars Today Layering
Layering Watercarved features
Watercarved features Watercarved features
Watercarved features Watercarved features
Watercarved features Geologic Composition from Remote Sensing
Geologic Composition from Remote Sensing Geologic Composition from Remote Sensing Geologic Composition from Remote Sensing Hematite An Ancient Wet Mars?
An Ancient Wet Mars? Liquid Water Not Stable on Mars Today Modern Subsurface Water
Modern Subsurface Water Discussion Questions for Today
Discussion Questions for Today How can we see through the clouds of Venus, and what is actually imaged? What kind of structures occur in the Venusian crust, how are they distributed, and what do they signify? Why did Venus turn out so different from the Earth? What does the population of impact craters on Venus suggest about its history? Why do we think the pressure and temperature environment on Mars was different in the past from what it is today? ...
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