Venus vs. Earth

1. Venus vs. Earth • In bulk, Venus and Earth are twin worlds. • nearly the same size • nearly the same density • This similarity leads to an expectation of similar evolutionary histories. • They are made of the same mix of rock and metal, must have had similar outgassing and impact histories...

2. Venus vs. Earth • In bulk, Venus and Earth are twin worlds • nearly the same size • nearly the same density • This similarity leads to an expectation of similar evolutionary histories. • One look from the outside suggests that they have achieved substantially different outcomes.

3. Venus: Evidently Not a Twin • Venus has an atmosphere mostly composed of Carbon Dioxide (96% CO2 4% Nitrogen) with a surface pressure nearly 100 times that of Earth. • Clouds composed of droplets of sulfuric acid hide 100% of the surface 100% of the time. • The surface temperature is 750K (900 F)

4. Venus: Evidently Not a Twin • Venus has an atmosphere mostly composed of Carbon Dioxide with a surface pressure nearly 100 times that of Earth. • Clouds composed of droplets of sulfuric acid hide 100% of the surface 100% of the time. • The surface temperature is 750K (900 F)

5. Venus: Hotter than Expected • The temperature of a planet is a balance between the incoming solar radiation and the outgoing blackbody “glow”, primarily at infrared wavelengths. • Ignoring subtleties like reflectivity (albedo) and infrared emission efficiency, the temperature of a rotating planet is 280K divided by the square root of the planet's distance from the sun in AU. • The Earth's equilibrium temperature should be right around freezing. Venus should be about 330K or 140F. • Accounting for Venus' high reflectivity this simple calculation suggests a more Earthlike temperature. • 1950's science fiction movies depicted a tropical Venus.

7. Venus: The Greenhouse on Steroids • Currently, on Earth, we are concerned about an atmospheric abundance of Carbon Dioxide of a fraction of a percent. • The Earth's greenhouse is essential for life on Earth. • Accounting for albedo, the Earth's equilibrium temperature would be well below freezing without it. • Contributors include water vapor, carbon dioxide and methane among others.

8. The Greenhouse Effect • Carbon Dioxide is transparent to visible light. • Light enters the atmosphere and warms the surface • The warm surface emits infrared light • Carbon dioxide traps infrared • Temperatures increase until enough infrared leaks out to balance incoming sunlight energy.

9. Why the Difference between Earth and Venus?? • Liquid water on Earth an lots of it! • Earth was initially a twin to Venus with a crushing Carbon Dioxide atmosphere, but oceans removed Earth's CO2 leaving Nitrogen. • Venus being closer to the Sun may • have received less water initially • had a hard time forming oceans due to the higher temperatures (and the greenhouse effect of the water vapor itself). • Where is the water today? • Ultraviolet light from the Sun can break up water molecules • The hydrogen escapes due to its low mass. • “Heavy” hydrogen (deuterium) lingers • On Venus deuterium is 10 times more abundant than on Earth, suggesting that it indeed has forever lost an ocean worth of water.

10. Radar Mapping • The Venusian clouds forever hide the surface of Venus from prying eyes at visible wavelengths • But radio light can penetrate the clouds.

11. Radar Mapping • The Venusian clouds forever hide the surface of Venus from prying eyes at visible wavelengths • But radio light can penetrate the clouds. • An Earth-based or Venus-orbiting radio transmitter can send out a pulse of radio light (like a flash picture). • The time delay for the pulse coming back reveals surface topography. • The brightness of the returned signal reveals surface roughness. • A rotating planet combined with the Doppler effect permits a reconstruction of a detailed picture.

16. Venus does not display the clear distinction between “highlands” and “lowlands” seeing on the Earth.

17. Landers • Spacecraft from Earth have visited the Venusian surface • Given the temperature, survival is brief – a few hours

18. Landers • Spacecraft from Earth have visited the Venusian surface • Given the temperature, survival is brief – a few hours

19. Landers • Spacecraft from Earth have visited the Venusian surface • Given the temperature, survival is brief – a few hours

20. Venusian Surface History • Large (>35 kilometer) impact craters uniformly pepper the surface of Venus. 1000 of them, compared with about 200 on Earth. • Venus' thick atmosphere stops smaller impactors. • The uniformity points to a global resurfacing event ½ billion years ago.

21. Venusian Volcanoes • Volcanoes are common on Venus. • Due to the lack of weather, erosion is slow. The volcanism may look “recent” simply because of this lack of erosion. • Most impact craters are undisturbed. • Some astronomers argue that Venus is currently geologically dead.

22. The Venusian Surface • Overall, the surface of Venus looks quite young, consistent with a global resurfacing ½ billion years ago. • At the same time, much of the “action” seems to have occurred at this distant time, and Venus is likely not quite so active today.

23. The Venusian Surface • Overall, the surface of Venus looks quite young, consistent with a global resurfacing ½ billion years ago. • At the same time, much of the “action” seems to have occurred at this distant time, and Venus is likely not quite so active today.

24. One theory: Earth dissipates its internal heat through plate tectonics and volcanism at plate boundaries. Venus does not have a crust that accommodates this process. Heat builds up leading to catastrophic resurfacing at billion-year intervals.