1. Lecture 14
Venus, Earth and Mars

2. Announcements Telescope Observing lab following class
Maps are available at the front of the classroom
You may want to stop for a snack on the way, we probably won’t do a lot of observing before about 7:30 due to sunlight (although we may look at the Sun if we get set up early enough).
If weather (ie, clouds) prevents us from seeing enough tonight, we will likely try again later in the semester.

3. Comparative Planetology
Comparative Planetology is the study of planets as groups, comparing their similarities and differences. We will take this approach to studying the solar system, starting with the planets most similar to our own.
We Will compare several features of these planets:
Interiors
Surfaces
Atmospheres
Magnetospheres
Moons

4. The Early History Four main stages of evolution:
The terrestrial planets formed 4.6 billion years ago from the inner solar nebula.
Four main stages of evolution:
Two sources of heat in planet’s interior:
Potential energy of infalling material
Decay of radioactive material
Most traces of bombardment (impact craters) now destroyed on Earth by later geological activity

5. Earth’s Interior Two types of seismic waves:
Direct exploration of Earth’s interior (e.g. drilling) is impossible.
Earth’s interior can be explored through seismology:
earthquakes produce seismic waves.
Two types of seismic waves:
Pressure waves:
Shear waves:
Particles vibrate back and forth
Particles vibrate up and down

6. Seismology
Seismic waves do not travel through Earth in straight lines or at constant speed.
They are bent by or bounce off transitions between different materials or different densities or temperatures.
Such information can be analyzed to infer the structure of Earth’s interior.

7. Venus and Mars Two most similar planets to Earth:
Similar in size and mass
Atmosphere
Same part of the solar system
Similar interior structure
Yet, no life possible on either one of them.

8. Interior Basic structure: Solid crust Solid mantle Liquid core
Solid inner core
Basic structure:
interior gets hotter towards the center.
Earth’s core is as hot as the sun’s surface; metals are liquid.
Melting point increases with increasing pressure towards the center
Melting point = temperature at which an element melts (transition from solid to liquid)
=> Inner core becomes solid

9. The Rotation of Venus
Almost all planets rotate counterclockwise, i.e. in the same sense as orbital motion.
Exceptions: Venus, Uranus and Pluto
Venus rotates clockwise, with period slightly longer than orbital period.
Possible reasons:
Off-center collision with massive protoplanet
Tidal forces of the sun on molten core

10. Mars Diameter ≈ 1/2 Earth’s diameter Very thin atmosphere, mostly CO2
Rotation period = 24 h, 40 min.
Axis tilted against orbital plane by 25o, similar to Earth’s inclination (23.5o)
Seasons similar to Earth  Growth and shrinking of polar ice cap
Crust not broken into tectonic plates
Volcanic activity (including highest volcano in the solar system)

11. The Active Earth
Earth’s surface geology is much more dynamic than Venus and Mars.
About 2/3 of Earth’s surface is covered by water.
Mountains are relatively rapidly eroded away by the forces of water.

12. Tectonic Plates
Earth’s crust is composed of several distinct tectonic plates, which are in constant motion with respect to each other  Plate tectonics
Evidence for plate tectonics can be found on the ocean floor
… and in geologically active regions all around the Pacific

13. Plate Tectonics Tectonic plates move with respect to each other.
Where plates move toward each other, plates can be pushed upward and downward  formation of mountain ranges, some with volcanic activity, earthquakes
Where plates move away from each other, molten lava can rise up from below  volcanic activity

14. Earth’s Tectonic History

15. History of Geological Activity
Surface formations visible today have emerged only very recently compared to the age of Earth.

16. The Surface of Venus
Early radar images already revealed mountains, plains, craters.
More details from orbiting and landing spacecraft:
Venera 13 photograph of surface of Venus:
Colors modified by clouds in Venus’s atmosphere
After correction for atmospheric color effect:

17. Radar Map of Venus’s Surface
Surface features shown in artificial colors
Scattered impact craters
Volcanic regions
Smooth lava flows

18. Lava Flows
Young, uneven lava flows (shown: Lava flow near Flagstaff, AZ) show up as bright regions on radar maps.

19. Surface Features on Venus
Highland regions:
Maxwell Montes are ~ 50 % higher than Mt. Everest!
Smooth lowlands

20. Craters on Venus Nearly 1000 impact craters on Venus’s surface:
 Surface not very old.
No water on the surface; thick, dense atmosphere
 No erosion
 Craters appear sharp and fresh

21. This type of volcanism is not found on Venus or Mars.
Volcanism on Earth
Volcanism on Earth is commonly found along subduction zones (e.g., Rocky Mountains).
This type of volcanism is not found on Venus or Mars.

22. All volcanoes on Venus and Mars are shield volcanoes
Found above hot spots:
Fluid magma chamber, from which lava erupts repeatedly through surface layers above.
All volcanoes on Venus and Mars are shield volcanoes

23. Example: The Hawaiian Islands
Shield Volcanoes
Tectonic plates moving over hot spots producing shield volcanoes  Chains of volcanoes
Example: The Hawaiian Islands

24. Volcanism on Venus Sapas Mons (radar image) ~ 400 km (250 miles)
2 lava-filled calderas
Lava flows

25. Volcanic Features on Venus
Aine Corona
Baltis Vallis: 6800 km long lava flow channel (longest in the solar system!)
Coronae: Circular bulges formed by volcanic activity
Some lava flows collapsed after molten lava drained away
Pancake Domes:
Associated with volcanic activity forming coronae

26. Lakshmi Planum and Maxwell Mountains
Radar image
Wrinkled mountain formations indicate compression and wrinkling, though there is no evidence of plate tectonics on Venus.

27. Tales of Canals and Life on Mars
Early observers (Schiaparelli, Lowell) believed to see canals on Mars
This, together with growth/shrinking of polar cap, sparked imagination and sci-fi tales of life on Mars.
We know today: “canals” were optical illusion; do not exist!
No evidence of life on Mars.

28. Reddish deserts of broken rock, probably smashed by meteorite impacts.
The Geology of Mars
Giant volcanoes
Valleys
Impact craters
Reddish deserts of broken rock, probably smashed by meteorite impacts.
Vallis Marineris

29. The Geology of Mars Possibly once filled with water.
Northern Lowlands: Free of craters; probably re-surfaced a few billion years ago.
Possibly once filled with water.
Southern Highlands: Heavily cratered; probably 2 – 3 billion years old.

30. Volcanism on Mars Olympus Mons:
Volcanoes on Mars are shield volcanoes.
Olympus Mons:
Highest and largest volcano in the solar system.

31. Volcanism on Mars Tharsis rise (volcanic bulge):
Nearly as large as the U.S.
Rises ~ 10 km above mean radius of Mars.
Rising magma has repeatedly broken through crust to form volcanoes.

32. Hidden Water on Mars No liquid water on the surface:
Would evaporate due to low pressure.
But evidence for liquid water in the past:
Outflow channels from sudden, massive floods
Collapsed structures after withdrawal of sub-surface water
Splash craters and valleys resembling meandering river beds
Gullies, possibly from debris flows
Central channel in a valley suggests long-term flowing water

33. Gusev Crater and Ma’adim Vallis:
Hidden Water on Mars
Gusev Crater and Ma’adim Vallis:
Giant lakes might have drained repeatedly through the Ma’adim Vallis into the crater.

34. Ice in the Polar Cap
Polar cap contains mostly CO2 ice, but also water.
Multiple ice regions separated by valleys free of ice.
Boundaries of polar caps reveal multiple layers of dust, left behind by repeated growth and melting of polar-cap regions.

35. Evidence for Water on Mars
Galle,
the “happy face crater”
Meteorite ALH84001:
Identified as ancient rock from Mars.
Some minerals in this meteorite were deposited in water  Martian crust must have been richer in water than it is today.
Large impacts may have ejected rocks into space.

36. Atmospheres
Terrestrial planets had primeval atmospheres from remaining gasses captured during formation
Atmospheric composition severely altered ( secondary atmosphere) through a combination of two processes:
1) Outgassing: Release of gasses bound in compounds in the interior through volcanic activity
2) Later bombardment with icy meteoroids and comets

37. The Structure of Earth’s Atmosphere
Composition of Earth’s atmosphere is further influenced by:
The ozone layer is essential for life on Earth since it protects the atmosphere from UV radiation
Chemical reactions in the oceans,
Energetic radiation from space (in particular, UV)
Presence of life on Earth
The temperature of the atmosphere depends critically on its albedo = percentage of sun light that it reflects back into space
Depends on many factors, e.g., abundance of water vapor in the atmosphere

38. Human Effects on Earth’s Atmosphere
1) The Greenhouse Effect
Earth’s surface is heated by the sun’s radiation.
Heat energy is re-radiated from Earth’s surface as infrared radiation.
CO2, but also other gases in the atmosphere, absorb infrared light
 Heat is trapped in the atmosphere.
This is the Greenhouse Effect.
The Greenhouse Effect occurs naturally and is essential to maintain a comfortable temperature on Earth,
but human activity, in particular CO2 emissions from cars and industrial plants, is drastically increasing the concentration of greenhouse gases.

39. Global Warming
Human activity (CO2 emissions + deforestation) is drastically increasing the concentration of greenhouse gases.
As a consequence, beyond any reasonable doubt, the average temperature on Earth is increasing.
This is called Global Warming
Leads to melting of glaciers and polar ice caps ( rising sea water levels) and global climate changes, which could ultimately make Earth unfit for human life!

40. Human Effects on the Atmosphere
2) Destruction of the Ozone Layer
Ozone (= O3) absorbs UV radiation, (which has damaging effects on human and animal tissue).
Chlorofluorocarbons (CFCs) (used, e.g., in industrial processes, refrigeration and air conditioning) destroy Ozone.
Destruction of the ozone layer as a consequence of human activity is proven (e.g., growing ozone hole above the Antarctic);

41. The Atmosphere of Venus
4 thick cloud layers ( surface invisible to us from Earth).
UV image
UV image
Very stable circulation patterns with high-speed winds (up to 240 km/h)
Extremely inhospitable:
96 % carbon dioxide (CO2)
Very efficient “greenhouse”!
3.5 % nitrogen (N2)
Rest: water (H2O), hydrochloric acid (HCl), hydrofluoric acid (HF)
Extremely high surface temperature up to 745 K (= 880 oF)

42. Very thin: Only 1% of pressure on Earth’s surface
The Atmosphere of Mars
Very thin: Only 1% of pressure on Earth’s surface
95 % CO2
Even thin Martian atmosphere evident through haze and clouds covering the planet
Occasionally: Strong dust storms that can enshroud the entire planet.

43. The Atmosphere of Mars Most of the Oxygen bound in oxides in rocks
 Reddish color of the surface

44. History of Mars’s Atmosphere
Atmosphere probably initially produced through outgassing.
Loss of gasses from a planet’s atmosphere:
Compare typical velocity of gas molecules to escape velocity
Gas molecule velocity greater than escape velocity  gasses escape into space.
Mars has lost all lighter gasses; retained only heavier gasses (CO2).

45. Earth’s Magnetic Field
Earth’s core consists mostly of iron + nickel: high electrical conductivity
Convective motions and rotation of the core generate a dipole magnetic field

46. The Role of Earth’s Magnetic Field
Earth’s magnetic field protects Earth from high-energy particles coming from the sun (solar wind).
Surface of first interaction of solar wind with Earth’s magnetic field = Bow shock
Region where Earth’s magnetic field dominates = magnetosphere
Some high-energy particles leak through the magnetic field and produce a belt of high-energy particles around Earth: Van Allen belts

47. The Aurora (Polar Light)
As high-energy particles leak into the lower magnetosphere, they excite molecules near the Earth’s magnetic poles, causing the aurora

48. A History of Venus Complicated history; still poorly understood.
Very similar to Earth in mass, size, composition, density,
but no magnetic field  Core solid?
Heat transport from core mainly through magma flows close to the surface ( coronae, pancake domes, etc.)

49. Solar Wind Interaction
Solar wind interacts directly with the atmosphere, forming a bow shock and a long ion tail.
CO2 produced during outgassing remained in atmosphere (on Earth: dissolved in water).
Any water present on the surface rapidly evaporated → feedback through enhancement of greenhouse effect

50. The Moons of Mars Probably captured from outer asteroid belt.
Two small moons: Phobos and Deimos.
Too small to pull themselves into spherical shape.
Typical of small, rocky bodies: Dark grey, low density.
Phobos
Very close to Mars; orbits around Mars faster than Mars’ rotation.
Probably captured from outer asteroid belt.
Deimos

51. Earth’s Moon
Earth has an unusually large moon… details next time.

52. For Next Time We will discuss the Moon and Mercury
Read Units 37 and 38