Venus and Mars: Earthlike Worlds Unveiled

1. Note that the following lectures include animations and PowerPoint effects such as fly ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

2. Venus and Mars Chapter 22

3. Guidepost The previous chapter grouped Earth’s moon and Mercury together because they are similar worlds. This chapter groups Venus and Mars together because we might expect them to be similar. They are Earthlike in their size and location in the solar system, so it is astonishing to see how different they actually are. Much of this chapter is aimed at understanding how Venus and Mars evolved to their present states. Neither Venus nor Mars can tell us much about the formation of the planets. Both planets have evolved since they formed. Nevertheless, we find further hints that the solar system was a dangerous place, with major impacts smashing the surfaces of the planets, a process we first suspected when we studied the moon and Mercury.

4. Guidepost (continued) This chapter concludes our exploration of the Earthlike worlds. In the next two chapters, we will visit planets that give new meaning to the word “unearthly.”

5. Outline I. Venus A. The Rotation of Venus B. The Atmosphere of Venus C. The Venusian Greenhouse D. The Surface of Venus E. Volcanism on Venus F. A History of Venus II. Mars A. The Canals of Mars B. The Atmosphere of Mars C. The Geology of Mars D. Hidden Water on Mars E. A History of Mars III. The Moons of Mars A. Origin and Evolution

6. 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.

7. Planetary Atmospheres (SLIDESHOW MODE ONLY)

8. 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

9. 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)

10. 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:

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

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

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

14. 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

15. 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.

16. 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

17. Shield Volcanoes (2) Tectonic plates moving over hot spots producing shield volcanoes Chains of volcanoes Example: The Hawaiian Islands

18. Hotspot Volcanoes (SLIDESHOW MODE ONLY)

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

20. 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

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

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

23. 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)

24. 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.

25. 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.

26. The Atmosphere of Mars (2) Most of the Oxygen bound in oxides in rocks Reddish color of the surface

27. 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).

28. 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 (2) 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 Volcanoes on Mars are shield volcanoes. Olympus Mons: Highest and largest volcano in the solar system.

31. Volcanism on Mars (2) 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. Hidden Water on Mars (2) 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. The Moons of Mars 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

37. New Terms subsolar point composite volcano shield volcano corona outflow channel valley network

38. Discussion Questions 1. From what you know of Earth, Venus, and Mars, do you expect the volcanoes on Venus and Mars to be active or extinct? Why? 2. If humans someday colonize Mars, the biggest problem may be finding water and oxygen. With plenty of solar energy beating down through the thin atmosphere, how might colonizers extract water and oxygen from the Martian environment?

39. Quiz Questions 1. Why do larger and more massive planets take longer to cool? a. Larger planets have a greater mass-to-surface area ratio and therefore cool more slowly. b. The more mass a planet has, the greater is its heat of formation. c. The more mass a planet has, the more heat-producing radioactive elements it has. d. The more mass a planet has, the thicker is its insulating blanket. e. All of the above.

40. Quiz Questions 2. Why might we expect Venus and Earth to be similar? a. Both planets are about the same size and density. b. Both planets have about the same chemical composition. c. Both planets have about the same atmospheric composition. d. Both a and b above. e. All of the above.

41. Quiz Questions 3. Why might Venus have the slow retrograde rotation we see today? a. A large off-center impact may have set Venus to spinning backwards. b. The long-term effect of solar tides on its dense atmosphere may be responsible. c. The tidal effect of Earth on Venus may have spun Venus backwards. d. Both a and b above. e. All of the above.

42. Quiz Questions 4. What evidence do we have that Venus once had more water than at present? a. We find dry lakebeds on the surface of Venus. b. We find dry riverbeds on the surface of Venus. c. The deuterium hydrogen ratio in the atmosphere of Venus is about 150 times that of Earth. d. Both a and b above. e. All of the above.

43. Quiz Questions 5. Which gas is most abundant in the atmospheres of Venus and Mars? a. Oxygen (O2). b. Nitrogen (N2). c. Argon (Ar). d. Carbon dioxide (CO2). e. Methane (CH4).

44. Quiz Questions 6. Why is the surface temperature of Venus higher than that of any other planet? a. Its period of rotation is longer than that of any other planet. b. Its orbit is more nearly circular than that of any other planet. c. It has an extreme greenhouse effect. d. It is the closest planet to the Sun. e. It has a retrograde rotation.

45. Quiz Questions 7. Why are the surface temperatures of Venus and Earth so very different? a. Venus is too close to the Sun to have liquid water oceans. b. Earth is far enough from the Sun to have liquid water oceans. c. The ozone layer of Earth shields the surface from ultraviolet radiation. d. Both a and b above. e. All of the above.

46. Quiz Questions 8. How do we know what the surface of Venus looks like? a. Large Earth-based optical telescopes have photographed the surface. b. The high resolution of the Hubble Space Telescope reveals the surface details. c. Radar mapping at radio wavelengths allows us to determine surface elevations. d. Orbiting spacecraft have imaged the surface at ultraviolet and infrared wavelengths. e. Two spacecraft that entered the atmosphere of Venus released balloons that floated just beneath the cloud deck and mapped the surface.

47. Quiz Questions 9. Why do you suppose that the smallest impact craters on the Moon are microscopic, the smallest on Earth are dozens of meters in diameter, and the smallest on Venus are more than one kilometer in diameter? a. The Moon has no atmosphere and Venus has an atmosphere much more dense than Earth's. b. Atmospheric gases easily slow the speed of smaller incoming meteoroids. c. Venus is farther away from the asteroid belt. d. Both a and b above. e. All the above.

48. Quiz Questions 10. All of the dormant volcanoes on Venus and Mars are the shield type, and many are much larger than any shield volcano on Earth. What does this tell us about Venus and Mars? a. Venus and Mars both have plate tectonics b. Neither Venus nor Mars has plate tectonics. c. Their interiors are at a higher temperature than Earth's interior. d. Their interiors are at a lower temperature than Earth's interior. e. Venus and Mars both have carbon dioxide atmospheres

49. Quiz Questions 11. Planets that have strong magnetic fields also have rapid rotation and convective fluid interior zones that are good electrical conductors. The magnetic field of Venus is so weak that it has not yet been detected, which makes it at least 25,000 times weaker than Earth's magnetic field. Examine the Venus data file on page 473. What information here may indicate why Venus is lacking a strong magnetic field? a. The eccentricity of orbit = 0.0068. b. Inclination of orbit to ecliptic = 3 degrees, 23 arc minutes, 40 arc seconds. c. Period of rotation = 243.01 days (retrograde). d. Average density = 5.24 grams per cubic centimeter. e. Surface temperature = 745 K (472ºC or 882ºF).

50. Quiz Questions 12. What determines whether an atmospheric gas can be retained by a planet, rather than be lost to space? a. The escape speed of the planet. b. The mass of the gas atoms or molecules. c. The temperature of the planet's upper atmosphere. d. Both a and b above. e. All of the above.