2. Life on other Solar System Planets George Lebo 29 October 2012 AST 2037 1

3. Life on Mercury Can the closest planet to the Sun support life? 2

4. 3 Mercury’s Orbit and Rotation Astronomers initially thought Mercury’s rotational and orbital periods were the same - same side always faces sunAstronomers initially thought Mercury’s rotational and orbital periods were the same - same side always faces sun Radar observations showed rotational period = 59 days (orbital period = 88 days!)Radar observations showed rotational period = 59 days (orbital period = 88 days!) Mercury is not tidally locked to the sun in the same way as the moon-earth systemMercury is not tidally locked to the sun in the same way as the moon-earth system Sun’s gravity and Mercury eccentric orbit brought it into semi-synchronous orbitSun’s gravity and Mercury eccentric orbit brought it into semi-synchronous orbit Mercury presents the same face to the sun every OTHER time around!Mercury presents the same face to the sun every OTHER time around!

5. Mercury’s 3:2 Resonance Once thought to be synchronous Mercury rotates 3 times for every two orbits One Mercury “day” lasts two Mercurian years!

6. 5 Mercury’s Atmosphere Almost non-existent - high surface temperature and low escape velocityAlmost non-existent - high surface temperature and low escape velocity no atmosphere means drastic temperature changes 700 K (day) = 800 F 100 K (night) = -280 Fno atmosphere means drastic temperature changes 700 K (day) = 800 F 100 K (night) = -280 F what little atmosphere there is comes from solar wind particles (Hydrogen and Helium)what little atmosphere there is comes from solar wind particles (Hydrogen and Helium)

7. 6 Mercury’s Surface Similar to the moon - craters!Similar to the moon - craters! Old surfaceOld surface No plate tectonicsNo plate tectonics Craters flatter and have thinner ejecta rims than lunar craters due to higher gravity on Mercury than the moonCraters flatter and have thinner ejecta rims than lunar craters due to higher gravity on Mercury than the moon Craters not as dense as on the moon - filled by volcanic activity - but not dark like “maria”Craters not as dense as on the moon - filled by volcanic activity - but not dark like “maria” Mariner 10 image from mid-1970s

8. Messenger Spacecraft Orbit Summary The spacecraft flew by Earth once, Venus twice, and Mercury itself three times, allowing it to decelerate relative to Mercury with minimal fuel. It also executed 5 Deep Space Maneuvers (DSM’s).

9. Messenger Spacecraft Orbit www.youtube.com/watch?v=GXEuQtpreXE

10. Messenger Spacecraft Mosaic

12. 11 Mercury’s Surface Scarps (or cliffs) are seen on the surface Scarps (or cliffs) are seen on the surface NOT seen on the moonNOT seen on the moon appear to be about 4 billion years oldappear to be about 4 billion years old not the result of plate tectonicsnot the result of plate tectonics probably the result of the surface cooling, shrinking and splitting at this timeprobably the result of the surface cooling, shrinking and splitting at this time

13. Water on Mercury? It’s WAY HOT there – don’t be stupid!! Besides, no atmosphere (water would boil off, right?) We thought so … But Messenger’s FIPS (Fast Imaging Plasma Spectrometer) Experiment has now found traces of water in its atmosphere 12 Radar map made in 1991 shows large reflections at North Pole Similar to radar signature from ice on Mars polar caps Believe Mercury has ice at the bottom of craters near the pole Permanent shade  permafreeze!

14. Why Venus? Duh … 13

15. OK … Really Physical properties of Venus: Diameter = 0.95 * Earth Mass = 0.8 *Earth Distance from Sun = 0.72 * Earth Solid surface, with atmosphere (incl. clouds) Venus is commonly known as our “twin” planet! 14

16. 15 Venus’s Rotation Rotation direction is retrograde (opposite that of other terrestrial planets)!Rotation direction is retrograde (opposite that of other terrestrial planets)! 243 day rotation period243 day rotation period Axis is almost exactly perpendicular to orbitAxis is almost exactly perpendicular to orbit Why? Possibly hit by large body during formation altering spin directionWhy? Possibly hit by large body during formation altering spin direction

17. 16 Venus’s Atmosphere Much more massive atmosphere than Earth’s (surface pressure is 90x Earth!)Much more massive atmosphere than Earth’s (surface pressure is 90x Earth!) Surface temperature is 730 K (!!!)Surface temperature is 730 K (!!!) Carbon dioxide (96.5%), Nitrogen (3.5%)Carbon dioxide (96.5%), Nitrogen (3.5%) No water - the clouds are made of sulfuric acid dropletsNo water - the clouds are made of sulfuric acid droplets Pioneer UV image taken 1979

18. 17 Venus’s Atmosphere Fast moving clouds 50-70km above surfaceFast moving clouds 50-70km above surface Haze 30-50 kmHaze 30-50 km Clear air below 30 kmClear air below 30 km Upper atmosphere is very windy - 400 km/hourUpper atmosphere is very windy - 400 km/hour Wind speed decreases lower in the atmosphereWind speed decreases lower in the atmosphere Temperature and pressure increase closer to the surfaceTemperature and pressure increase closer to the surface

19. Why is Venus So Hot? F = L/4  d 2  Venus receives about twice the solar radiation as Earth per unit area F_emitted =  T 4  equilibrium temperature scales up by (2) 1/4 power  about 20% higher temperature This is 20% of 300K, or about 60K higher Expect Venus to be at about 170  F (but really is more like 900  F) Atmosphere is largely CO 2 (a greenhouse gas) This traps solar radiation more efficiently Temperature much higher this way What about “cool spots”? Not likely (why?) 18

20. Runaway Greenhouse Effect On Earth, most CO 2 from atmosphere has been locked up in limestone (CaCO 3 ), slowly getting released/replenished via combo of rain and plate tectonics If temps higher (i.e. move Earth closer to the Sun, by magic), more water vapor in the air Water vapor increases infrared absorption from the ground (it’s an EXCELLENT greenhouse gas!) Temps rise some more, more water evaporates, more greenhouse effect, temps rise more, etc. Once hot enough, water vapor rises to top of atmosphere, and solar UV light dissociates it into H + O Lighter H drifts off into space (which is why Venus has 150x the deuterium of Earth!) 19

21. 20 Venus’s Surface Radar (radio waves) echoes reveal the surface topology Elevated “continents” make up 8% of the surface Mostly rolling plains with some mountains (up to 14 km) No tectonics Buckled and fractured crust with numerous lava flows

22. 21 A few Soviet spacecraft have landed on Venus in 1970s Survived only an hour before burning up little evidence of erosion - young surface rocks are basaltic and granite some craters (very few) caused by meteoric impact

23. Venus: Magellan Mission Satellite orbiting Venus in 1990s Precision radar mapping of entire surface with ~150- yard resolution (better map of all Venus than of all Earth!) – not so now 22

24. 23 Venus’s Surface: Volcanoes and Craters Largest volcanic structures are called coronae - upwelling in the mantle which causes the surface to bulge out - not a full-fledged volcano. Usually surrounded by other volcanoes Venus is thought to still be volcanically active today (Magellan lava flows) Volcanoes resurface the planet every ~300 million years Shield volcanoes are the most common (like Hawaiian Islands) A caldera (crater) is formed at the summit when the underlying lava withdraws

25. How did Venus get this way? Runaway greenhouse raises temps, gets rid of surface water early on Early plate tectonics brings up internal H 2 O and CO 2 from early limestone into atmosphere No surface water  no way of trapping CO 2, so it stays in the atmosphere now (huge pressure, greenhouse high temps) Internal water lost  important lubricant for plate tectonics; plate tectonics stops Volcanoes continue 24

26. Summary Mercury has no atmosphere, little water (frozen in caps), and extreme hot/cold temps Venus is Earth’s twin superficially, but has huge pressure, no water, sulfuric acid for rain, and temps that can melt lead (pretty much everywhere) Reasons for Venus situation indicate Earth would look like that too if it was at Venus distance from Sun So … in general it is unlikely that planets this close to a star like the Sun will be able to support life (!) 25

27. The Jovian Planets Jupiter Jupiter, Saturn, Uranus, Neptune much larger than terrestrial planets not solid - gaseous all have rings all have many moons

28. Jupiter Named after the most powerful Roman god third-brightest object in the night sky (after the Moon and Venus) Atmospheric bands are very different than inner planets Many moons – four largest called Galilean Moons

29. Saturn Named after the father of Jupiter in Greco-Roman mythology Almost twice Jupiter’s distance from the Sun Similar banded atmosphere Uniform butterscotch hue Many moons Spectacular ring system

30. Uranus Discovered by William Herschel in 1781 while looking for comets Named after father of Saturn Barely visible to naked eye Featureless atmosphere Deviations in the expected orbit of Uranus pointed to the possibility of another planet influencing its motion

31. Neptune There had to be another planet influencing Uranus 1845 - John Adams determined the planet’s mass and orbit 10 months later - Urbain Leverrier, independently came up with the same result 1846 - Johann Galle (Berlin Observatory) found the new planet Neptune Cannot be seen with naked eye “Bluish” Jupiter atmosphere

32. Space Craft Exploration of Jovian Planets Voyager 1and 2 left Earth in 1977 reached Jupiter in March and July of 1979 gravity assist Used Jupiter’s strong gravity to send them on to Saturn - gravity assist Voyager 2 used Saturn’s gravity to propel it to Uranus and then on to Neptune Studied planetary magnetic fields and analyzed multi-wavelength radiation Both are now headed out into interstellar space!

34. Space Craft Exploration of Jovian Planets Galileo - launched in 1989 and reached Jupiter in December 1995 Gravity assists from Venus and Earth Two components: atmospheric probe and orbiter Probe descended into Jupiter’s atmosphere Orbiter went through moon system Cassini mission to Saturn, launched in 1997 Gravity assists from Venus (twice), Earth and Jupiter Studying Saturn’s moon Titan in much the same way as Galileo studied Jupiter

35. Jovian Planet Properties Most of their mass is Hydrogen and Helium – light elements = low densities High surface gravity allows their atmospheres to retain these light elements Dense compact core at the center But, NO SOLID SURFACE – gaseous atmosphere becomes denser (eventually liquid) at core Differential Rotation – outer regions rotate slower than inner regions

36. Jovian planets - axis tilt and magnetic fields Uranus has the most inclined rotational axis - extreme seasons! All appear to have strong magnetic fields - rapid rotation and liquid conductive cores or mantles

37. Jupiter’s Atmosphere Two main features: colored bands and Great Red Spot molecular hydrogen – 86% helium – 14% small amounts of methane, ammonia, and water vapor Darker colored belts lie atop downward moving convective cells Lighter zones are above upward moving cells Belts are low-pressure, Zones are high pressure As on Earth, wind moves from high to low But rotation causes wind patterns to move East/West along equator Temperature difference between bands is main reason for color difference

38. Jupiter’s Atmosphere Haze lies at the upper edge of the troposphere Thin layer of white ammonia clouds – 125 – 150 K Colored clouds below that Warmer - 200 K clouds are mostly droplets or crystals of ammonium hydrosulfide At deeper levels, clouds of water ice or water vapor The Galileo probe survived for about an hour before being crushed at this altitude.

39. Weather on Jupiter Main weather feature – Great Red Spot! swirling hurricane winds has lasted over 300 years! diameter twice that of Earth rotates with planet’s interior the spot appears to be confined and powered by the zonal flow Smaller storms look like this (this one is over 40 years old) Why do the storms last so long? On Earth, hurricanes lose power when then come upon land No continents on Jupiter – nothing to stop them once they start

40. Saturn’s Atmosphere molecular hydrogen 92.4% helium 7.4% traces of methane and ammonia Layer of haze Troposphere contains 3 cloud layers Overall temperature is cooler than Jupiter Atmosphere thickness is 3 times that of Jupiter (caused by lower surface gravity on Saturn) Thicker clouds result in less varied visible colors ammonia ice ammonium hydrosulfide ice water ice

41. Weather on Saturn Computer enhanced image shows bands, oval storm systems, and turbulent flow patterns like those seen on Jupiter

42. Atmospheres of Uranus and Neptune molecular hydrogen 84% helium 14% methane 2% (Uranus) 3% (Neptune) Abundance of methane gives these planets their blue color Methane absorbs longer wavelength light ( and reflects short wavelength light

43. Weather on Uranus and Neptune Uranus Few clouds in the cold upper atmosphere – featureless Upper layer of haze blocks out the lower, warmer clouds Neptune Upper atmosphere is slightly warmer than Uranus (despite its further distance from Sun) More visible features (thinner haze, less dense clouds lie higher) Storms – Great Dark Spot Seen in 1989 – gone in 1994

44. Internal Structures – models that fit the data Jupiter Saturn Uranus/Neptune Increasing temperature and pressure deeper in core Jupiter bulges at radius (7% larger) Saturn less assymetric – larger core – same basic overall structure on a smaller scale Uranus/Neptune have a high density “slush” below cloud level - compressed water clouds Metallic hydrogen is like liquid metal

45. Internal Heating Primordial Heat Generation of Heat Effect of internal heating - raises the temperature of the interior and atmosphere to higher values (2.5X) than expected from the Sun’s heating alone -Jupiter’s heat source results from strong heating during formation by the collapse of material onto the core -Saturn generates some heat due to the gravitational contraction of helium gas

46. Life on the Giant Planets? We see many “life chemicals”! (Water, methane, etc.) Lightning observed too Similar to early Earth atmosphere (?) Expect complex hydrocarbons (Urey-Miller) (?) At some depth, have warm temps (~300K) But … these temps at high pressure Methane is not a sign of life here – just a sign of LOTS of free hydrogen, some carbon, relatively little oxygen No solid surface – no oceans, no tidal pools, no clay matrix – in short, no (Earth-like) places for life formation

47. Life on the Giant Planets? What about gas matrix life? (i.e. no solid surface, but life in the air) Wind speed turbulence problem – try to put together a house of cards outdoors in a hurricane That’s easier than forming life in Jupiter’s atmosphere! Convection Causes any chemicals from warm lower layers to rise This gives exposure to solar UV radiation, which breaks it down

48. Does this mean NO life here? Nope Floater possibility But, this is pure speculation

49. Moons Almost all moons in the Solar System orbit the Jovian planets Jupiter’s 4 major moons are the Galilean satellites: Io, Europa, Callisto, and Ganymede Jupiter has 100’s of smaller moons

50. Io Innermost Galilean satellite Reddish color Smooth “young” surface Extreme volcanic activity Interior tidally heated by Jupiter (& Europa)

51. Io – Tidal Heating

52. Europa Second Galilean satellite Whitish, highly- reflective color Smooth surface with patterns similar to ice caps on Earth Young surface May be a liquid “waterworld” with ice crust

53. Life on Europa? Possible view of Europan ocean Note thermal segregation

54. Life on Europa? Europa “seabed” may resemble “black smoker” environment on Earth (!) Probably little O 2 in the water (where would it come from?) But anaerobic bacteria are the basis for life in Earth vents (even if larger life O 2 -dependent) NASA developing preliminary plans for a drilling/submarine exploration mission

55. Callisto & Ganymede Rock/ice moons Evidence for some “glacial” flows This is Ganymede

56. Saturn’s Titan Largest moon Atmosphere more than 95% nitrogen, The rest is methane “Smooth” surface due to thick atmosphere (!) Infrared images show evidence for “continents” beneath

57. Titan’s Surface Lakes of liquid hydrocarbon imaged from orbit via Cassini; T ~85K (probably ethane/methane) Huygens lander mission in 2006

58. Life on Titan? Solid/liquid phase environment similar to Earth’s surface Totally different chemicals and temp ranges; will not be H 2 O- based like on Earth But … still a possibility

59. Titan’s Surface Lakes of liquid hydrocarbon imaged from orbit via Cassini Huygens lander mission in 2006

60. Other moons

61. Enceladus Saturn moon Water here too?

62. Enceladus Saturn moon Water here too?

63. Summary The Giant Planets lack solid surfaces and have extreme wind speed, turbulence and convection in their atmospheres Still.. we cannot rule out life there (floaters?) Moons such as Europa and Enceladus may have liquid water oceans with Earth-like temperatures and geothermal vents; these may be promising places to search for life The moon Titan has a thick atmosphere and (apparently) lakes/rivers of liquid hydrocarbons; temps are COLD, but we can imagine some low- temperature, slow-reaction life developing here too (?) 62