Is There Life Beyond Earth?

1. Is There Life Beyond Earth? Philip Hughes Department of Astronomy University of Michigan phughes@umich.edu www-personal.umich.edu/~phughes/ (Material For Download)

2. Life Beyond Earth • Some important distinctions: • Simple (single-celled), Complex (multi-celled) • Intelligent (dolphins....), Technological (humans) • Current or Past • Finding evidence of current or past, simple or complex, life beyond Earth would be remarkable • Finding evidence of current or past, intelligent/technological, ditto would be more socially significant • Where do we look?

3. Carbon & Water?? • What seems to matter for life are (as judged by life on Earth): • organic chemistry – involving simple compounds of Carbon • H20 • Is this universal?

4. Carbon-based Life • Carbon is versatile; H has one bond, O two, C four: can get a vast array of C-based scaffoldings, to which other simple atomic groupings may attach, making complex organic molecules such as • Lipids which store energy & form membranes • Carbohydrates which provide cell energy & structures (cellulose) • Amino acids which are the building blocks of proteins, catalyzing reactions & forming structures

6. Carbon-based Life contd. • Is there a substitute for Carbon? • Silicon has four bonds, but... • the bonds are weak • Si-based molecules will not survive long in water • only single, not double bonds, so • fewer chemical reactions than for C-molecules • less rich set of molecular structures • There is a high probability that “life elsewhere” is Carbon-based

7. The Importance of H2O • Liquid water is an invaluable solvent: • it facilitates reactions by bringing together the chemical components • in ice, no transport occurs • in vapor the chemicals are dispersed • it transports chemicals to and from cells • Water actually participates in key reactions • Water is a polar molecule (see later), that can pass through cell boundaries

8. The Importance of H2O contd. • Is there a substitute for H2O? • A substitute must be common, and liquid, over a wide range of temperature and pressure • Organics like ethane and methane can be liquid but are less plentiful, and liquid only when the temperature is so low that chemical reactions will be very slow – maybe 10-20 x slower than in Earth's primeval oceans

9. The Importance of H2O contd. • Is there a substitute for H2O? • Water is unique in that ice is lighter than liquid water, so floats: under cold conditions, an ice layer forms an insulating sheet above a body of liquid water (freezing throughout occurs only under the most extreme conditions)

10. The Importance of H2O contd. • Is there a substitute for H2O? • Water is a polar molecule, with + and – charge at either end: hydrogen bond, a key feature in the organic chemistry of life

11. The Importance of H2O contd. • water dissolves other polar molecules easily (some organic compounds & salts) • but does not dissolve non-polar molecules, such as the stuff of cell walls • water is one of the few simple molecules that can cross a cell membrane allowing osmosis – critical in living organisms • There is a high probability that “life elsewhere” will not develop in the absence of H2O

12. Warning: Extremophiles • Don’t exclude acidic, alkali, salty, hot…. • or lithophiles….

13. WithinThe Solar System

14. Mars: Past “Blueberries”: hematite concretions maybe formed under water (Opportunity, 2004)

15. Mars: Past (Speculative)

16. Mars: Search For Past Life • ExoMars – astrobiology project (ESA/Roscosmos) • Part II launch 2020, ExoMars Rover deploys 2021 • Pasteur Analytical Laboratory – biosignatures of past life • OxiaPlanum most favored site

17. Europa

18. Enceladus

19. Outside The Solar System

20. Exoplanets! Total 18 June, 2019: 4086 planets in 3048 systems

21. Discovery: We Can’t Just “Look” • From beyond the Solar System, the Sun outshines Jupiter by a billion, and the Earth by 10 billion • We have the sensitivity, but.... • compare viewing a firefly next to a search-light

22. Transit Method

23. Kepler Mission • Launched by NASA, 2009 • Photometer monitored brightness of >145,000 stars • Periodic dimming reveals planet(s)

24. Stellar Habitable Zone • Simple definition (liquid water) is naïve but practical

25. But Are The Exoplanets Habitable? (Conservative sample: 13; potential: 52)

26. The Rise Of Oxygen • In early, Oxygen-free atmosphere, simple organisms would have been anaerobic; they were probably • chemoautotrophs – getting energy from inorganic compounds • modern Archaea in hot springs get their energy from H/S/Fe compound reactions • Photosynthesis evolved from light absorbing pigments, that eventually allowed • photohetero(auto)trophs – getting energy from sunlight • release oxygen

27. Oxygen contd. • Oxygen is highly reactive: • Oxidizes surface rock & Iron minerals in oceans • Rocks > 2 billion years old have 1% modern oxygen levels • No more than 10% current until about 1 billion years ago • Then reaches current level • Look for oxygen in exoplanet atmospheres!

28. Aside: Spectra

29. The Sun’s Spectrum

30. Detecting Exoplanet Atmospheres I • First direct detection: David Charbonneau et al. in 2002, using Hubble Space Telecope

31. Detecting Exoplanet Atmospheres II • To date, more than 50 atmospheres have been studied • We are just beginning to probe structure, as for planets in our Solar System: • day/night temperature differences & winds • We are beginning to probe composition: • TiO, CO, C02, H2O, CH4 …no oxygen as yet!

32. Beware False Positives • NASA Astrobiology Institute: Virtual Planetary Lab has simulated thousands of “atmospheres”, allowing for many reactions • O2/O3 could come from CO2 – broken down by ultraviolet light • Most terrestrial methane is biological but could come from volcanic activity • Both ozone & methane would be a good indicator of life, because a burst of methane from volcanism doesn’t last long in atmosphere with oxygen

33. Kepler Shadowgrams • Recall: the Kepler satellite monitored stars for the telltale periodic dimming of starlight as a planet transits • Suppose an alien civilization has constructed a light weight “gossamer” billboard orbiting their star; it's shape would be evident to us in the shadowgram: (rotating triangle)

34. Shadowgrams contd. • This could be used passively – for generations • Or actively, like semaphore, sending information as binary digits (multiple transits by groups)

36. Civilizations Need Energy • A data center can use as much as medium sized town • Globally, “data warehouses” use 30 billion watts – 30 nuclear power plants; the USA accounts for about 1/4-1/3 of that • Up to 70% of the power is used for cooling/air handling • This is just one example of how an advancing civilization's energy use rises dramatically as technology develops

37. We Borrow Energy • Energy is not created or destroyed, it just changes form • eg, Potential ⇒ Kinetic ⇒ Heat (falling object) • eg, Electrical ⇒ Heat (electronics) • An advanced civilization with vast energy needs will generate a vast amount of waste energy • Use 'degrades' energy; we can expect the waste energy to show up as heat – radiation in infra-red

38. Dyson Sphere/Swarm • Freeman Dyson, British/American physicist, b. 1923

39. A Search For Waste Energy • Jason Wright at Penn State • Funded by the John Templeton Foundation • Using WISE (Wide-field Infrared Survey Explorer): radiation from solar-system sized objects at -100 oF to +100 oF, in infrared • Search for ‘astronomically anomalous’ infrared emission from the vicinity of (unseen?) stars or even from whole galaxies – a web of stars enshrouded in ‘industrial megastructures’

40. Essential Points: • Does life exist beyond Earth? We don’t know. But… • Mars and moons of the gas giants will be explored • Exoplanets are common and today we could detect • Oxygen in exoplanet atmospheres, an almost certain indicator of (maybe only primitive) life • Signals from “billboards” orbiting stars – an indicator of life a little more advanced than us • Evidence of vastly more advanced civilization via their waste energy We don’t have to wait for ET to visit!