Astronomy190 - Topics in Astronomy Astronomy and Astrobiology Lecture 20 : Biosignatures Ty Robinson

Questions of the Day

• What signs of life does our own planet exhibit when viewed from space? • How do we search for signs of life on distant planets?

How can we tell if a planet is inhabited?

Hi!

Without direct contact with an alien civilization, or traveling to the nearest planetary system, our best chance for finding life in the Universe is to look for

global changes

in the atmosphere and surface of a terrestrial planet.

Detecting Biosignatures on Extrasolar Planets

• we will have no spatial information • signs of life must be global and on/above the surface

Distant Signs of Life

•

astronomical biosignatures

are global-scale photometric, spectral or time-varying features indicative of life – Earth shows us that life can provide global-scale modification of: • a planet’s atmosphere • a planet’s surface • a planet’s appearance over time – biosignatures must always be identified in the context of the planetary environment • e.g. Earth methane and Titan methane • technosignatures – TV signals or other radio transmissions, pulsed lasers, planetary or solar system-sized structures, are also signs of life – arguably easier to detect than biosignatures, but also less likely to be present on any given habitable planet

Biological Modification of the Atmosphere 10000 1000 100 10 1 0.1

0.01

Hydrogen Methane Isoprene Dimethyl Sulphide Ammonia Nitrogen Nitrous Oxide Carbon Monoxide Carbon Dioxide Oxygen

Tim Lenton, Nature, 1998

Earth Without Life • life modifies the atmosphere via production of gaseous by-products of metabolism (e.g. O atmosphere in 2 from photosynthesis) • because there is an active source, life’s gases are often seen in the

chemical disequilibrium

Signs of Life: Atmosphere

• metabolic byproducts: O 2 , O 3 (ozone), CH 4 (methane) • anything out of chemical equilibrium – O 2 and CH 4

together

– CH 4 on Mars (no obvious geologic source)

O 3 CH 4

Signs of Life: Surface

characteristic changes in the surface reflectivity due to photosynthetic plants

Signs of Life:

Time

NOAA-CMDL

gas or surface signatures that change with day-night, or seasons

Types of Technosignatures

• beacons, messages or communication sent using electromagnetic radiation –

radio

, visible or infrared waves would likely be easiest • “rocket trails” from interstellar travel • astroengineering – artifacts left in our own Solar System – structures that gather energy from the parent star

Detecting Life on Earth: The

Galileo

Flyby Galileo observed the Earth’s biosignatures during a flyby on its way to Jupiter (Sagan et al., 1993).

Spectra of Earth

Atmospheric Biosignatures

Sagan et al., 1993

Surface Biosignatures

Technosignatures Sagan et al., 1993

Questions of the Day

• What signs of life does our own planet exhibit when viewed from space? • How do we search for signs of life on distant planets?