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Is There Life Beyond Earth?
Philip Hughes
Department of Astronomy
University of Michigan
phughes@umich.edu
www-personal.umich.edu/~phughes/
(Material For Download)
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?
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?
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
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
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
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
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)
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
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
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
➢ Oxia Planum most favored site
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
Kepler Mission
➢ Launched by NASA, 2009
➢ Photometer monitored brightness of >145,000 stars
➢ Periodic dimming reveals planet(s)
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
Oxygen contd.
➢ Oxygen is highlyreactive:➢ 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!
Detecting Exoplanet Atmospheres I
➢ First direct detection: David Charbonneau et al. in 2002, using Hubble Space Telecope
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!
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
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)
Shadowgrams contd.➢ This could be used passively – for generations
➢ Or actively, like semaphore, sending information as binary digits
(multiple transits by groups)
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
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
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 oreven from whole galaxies – a web of stars enshrouded in ‘industrial megastructures’
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 certainindicator 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!
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