ASTR/GEOL 3300: ET Life

1. ASTR/GEOL 3300: ET Life • Logistics: • Paper proposals are nearly finished (lots of reading for me…) • The next homework will be released this Monday. The MT1 is ready to return to you. • Don’t be discouraged by the MT grade (average was <65), but consider this as a challenge to do better next time. • MT#2 is November 8 • Plan for Today: • The search for life on Mars Next time: • Life on Venus?!?(or Mercury, or the Moon?)

2. Life on Mars???

3. Mars Geology & Climate: Summary • Some "recent" water; lots of past water. • Mars has “died out”: • Was active, warm, & wet during Noachian; activity waned during Hesperian. • Abundant ground ice today, with water deep underground. • Consistent with the intermediate size of Mars: • Lost most of its heat early, so activity level waned. • Lost greenhouse over time. • Could life have arisen on Mars during the Noachian? • Could it have retreated underground, or into rocks (endoliths)? • Could “recent” climate change (tilt changes) bring it to surface?

4. Viking: Search for Life • Twin Viking landers searched for life (1976).

5. Viking Life-Detection Experiments • Carbon Assimilation Experiment: • “Fed” soil 14CO & 14CO2, and found 14C was incorporated by the soil! • BUT heating to 175°C (which should kill any organisms) did not eliminate the positive response, so probably just geochemical reactions incorporated the C-rich gases. • Gas Exchange Experiment: • “Fed” soil wet nutrients and found O2 and other gases released! • BUT positive response continued upon heating, and in the dark, and when exposed to H2O only, so again probably just geochemical reactions.

6. Viking Life-Detection Experiments • Labeled Release Experiment: • “Fed” soil organics labeled with radioactive isotopes, and found radioactive gases were released! • Amount of released gas decreased as nutrients were used up, and stopped when soil was heated to 160°C! • This seems a positive result! BUT probably doesn’t really indicate life, considering the next result… • GCMS (Gas Chromatograph / Mass Spectrometer): • Heated soil to vaporize organics & measure abundance. • No organic materials found in soil to parts per billion! • Organics on Mars surface are readily destroyed by UV light & oxidants in the soil.

7. Viking Results: Summary • 3 Viking experiments gave results consistent with life. • For two, “positive” results continued at temps high enough to kill life. • Chemical reactions by soil oxidants can explain the results. • In hindsight, the Viking life detection experiments were not well designed!

8. Meteorites from Mars • Some meteorites all have similarly strange isotope ratios: • Not from asteroids, Earth, or Moon. • Chemistry of air bubbles trapped in one of these matches Viking measurements of Mars atmosphere! • These “SNC” meteorites must be from Mars!

9. Mars Meteorite ALH84001 • The Mars meteorite that changed planetary science! • 1996: thought to possibly show signs of life! • Evidence of life within now severely weakened.

10. ALH84001: 4 Lines of Possible Evidence for Life • Carbonate Globules: • Alternating layers of Fe-, Mg-, Ca-rich carbonates: minerals out of equilibrium, which have biological origin. • BUT could be deposited by high-temperature fluids. • PAHs (Polycyclic Aromatic Hydrocarbons): • Complex C-molecules, on Earth typically remnants of dead organisms. • BUT can form inorganically, or could be terrestrial contamination of meteorite.

11. ALH84001: 4 Lines of Possible Evidence for Life • Magnetite crystals: • On Earth, observed hexagonal crystals, sometimes aligned in chains, are characteristic of biological origin. • BUT these have now been produced in the lab inorganically. • Nanobacteria?: • Segmented, rod-shaped structures that resemble terrestrial bacteria. • BUT these may be artifacts of sample preparation. ALH84001 made by terrestrial bacteria

12. Discussion Question • Should we continue the search, or does the weight of the Viking and ALH84001 evidence say that there is no (and has not been) any life on Mars?

13. If yes, then where would YOU search for evidence of past life?

14. Martian “Frozen Sea”? • Ice now protected by volcanic ash? • A good place to search for preserved life?

15. Methane in Mars’ Atmosphere • Ground-based & Mars Express orbiter indications of CH4. • Life (methanogens) is a possible source: • CO2 + 4H2 CH4 + 2H2O or 2H2O + 3CO  CH4 + 2CO2 • BUT may just be minor volcanic outgassing… (how?)

16. Ancient Mars???

17. In the not too distant future…

18. That’s it for Mars… you will hear much more on Oct. 18 (next week) from Prof. Hynek (I hope, he might still be in Africa) Now, there’s the rest of the solar system, and beyond!

19. Venus

25. Venus: Earth’s Evil Twin "Venus Is the Planet Most Like Hell."-Carl Sagan • Similar size to Earth: • Similar internal heat (WHY?). • Thick CO2 atmosphere: • Pressure = 90 bar (WHY?). • Searing hot from greenhouse warming: • ~460 °C (~860 °F) • SO2 clouds: • sulfuric acid rain. • Only traces of water. • Why no CO2 cycle? • no oceans! • Why no oceans? • runaway greenhouse! • Why no continents?

26. Magellan radar(bright = rough or metallic) Soviet Venera lander

28. uplands • highlands Venus Physiography • lowlands

29. Varied volcanic styles: Some extremely runny lava. Some more stiff. Lowland plains volcanism: Basaltic (?) Covers much of surface. Volcanism on Venus

33. Tectonics on Venus • Tesserae (Highlands-like): Intense compression & extension. • Coronae: Mantle plumes upwarp lithosphere from below. • No plate tectonics: Very dry lithosphere is too stiff & strong. • Mantle convection may be sluggish (WHY?). Alpha Regio

34. Artemis

37. Impact Craters on Venus • Thick atmosphere shields smallest impactors. • Some large impacts have melted the surface rocks. • Craters are near-randomly distributed. • Surface age is nearly uniform: ~750 Myr old. • Did volcanism “burp” then halt? • Catastrophic overturn of lithosphere? Episodic overturn? 4 km 24 km 90 km

39. Degradation on Venus Wind streak • Elusive signs of erosion: • Sluggish winds (WHY?). • Limited (75 m) resolution. Mass wasting Dunes

40. Is Venus Geologically Active? • 750 Myr surface age • Long-term cooling… • …or episodic overturn? • Is Venus still active? • Atmospheric gases imply volcanoes still outgas... • ... no eruptions witnessed. • No magnetic field • Is its core frozen… • …or dynamo inhibited? • Atmospheric gases stripped by solar wind.

42. Earth Venus Venus: Why No Magnetic Field? • “Dynamo” magnetic field requires: • Internal conductor (e.g. molten Fe). • Rotation (Venus is slow, but okay). • Core convection (not in Venus?!?). • Earth: • Plate tectonics cools mantle. • Temp gradient across core drives core convection, powering magnetic field. • Venus: • No plate tectonics. • Interior more insulated: stays very hot. • Insufficient core temperature gradient to drive core convection. • No protective magnetic field!

44. Atmospheric History of Venus • Outgassing: lots of H2O and CO2 into Venus' atmosphere early on, dust & comets brought SOME H2O: • Venus should have similar water supply as Earth! • ~30% dimmer early Sun means Venus may have had oceans! • Because of Venus' distance from Sun, it experienced a runaway greenhouse: • Closer to Sun, early Venus was hotter than early Earth. • So, more H2O in the early Venus atmosphere than early Earth's (more evaporation; & more water can be held in a warmer atmosphere). Catastrophic loss of Hydrogen (vs. Deuterium) • More atmospheric H2O meant Venus had a stronger greenhouse effect. • More greenhouse warming put even more water into Venus atmosphere.... AND SO ON: POSITIVE FEEDBACK!

45. Highway to Hell • With H2O in atmosphere, most then lost from Venus: • By atmospheric stripping(no magnetic field!). • By thermal escape(following UV dissociation). • Venus has no CO2 cycle: • Most of Earth's CO2 is in carbonate rocks! • Most of Venus' CO2 remains in its atmosphere!

46. Runaway Greenhouse: Could It Happen Here? As Sun continues to brighten: Probably some day! Could human activity trigger this? We'll see!

47. Life on Venus?!? • Could life have existed on Venus early on? • Sun was cooler: Venus may have had oceans. • Meteorites travel between planets: maybe carry life? • Why not life today? • Too hot (organics destroyed). • No water (solvent needed). • No ozone to protect from UV (organics destroyed). • Can life possibly overcome these problems?