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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 5: The Pale Blue Dot


Key Ideas

Direct detection of "ExoEarths" is the ultimate goal
Direct detection is very hard!
Earth's spectrum is dominated by two 'bumps':
--Bump #1: Reflected light from the Sun (Visible)
--Bump #2: Thermal emission from the warm planet (Infrared)
The Earth/Sun flux ratio is very small ~10-9
Measuring the visible light gives planet size
Measuring the IR light gives indication of temperature
Spectra allow one to determine the 'composition'
--Look for biomarkers - signs of life!
Measuring variability tells about surface structures


What is the ultimate goal?

Indirect detection of Earth-mass, habitable planets:
--Kepler, other future missions

Direct detection of Earthlike planets
--Measurement of light is very powerful

Determining the properties of the planets
--Spectra allow detection of biomarkers
--Variability allows inference of surface properties


The Pale Blue Dot

The Solar System from the Outside In

On February 14, 1990, Voyager 1 imaged the solar system planets when it was 4 billion miles from the Earth.

Carl Sagan (1934-1996)

Trained as an astronomer
Strong science advocate
Writer
One of the first "astrobiologists"
Advocate of SETI
Part of the Voyager team

Studying Earth as an Extrasolar Planet

Earthshine:
--Light from the Sun reflected off the Earth.
--Can use this light to measure spectrum of the "unresolved" Earth

Spectrum and variability of light from the Earth tell us about the constituents of the atmosphere and composition of the surface.


Light From the Earth

Why does the Earth shine?

Two basic sources of light form the Earth:
Reflected light from the Sun.
--Closely traces the spectrum of the Sun
Thermal emission from the Earth's internal heat.
--Mostly infrared radiation (heat!)

Earth's Reflected Light

Flux ratio is incredibly small

Measuring the flux ratio gives an estimate of the planet's size (modulo albedo).

Thermal Emission

Depends on how sunlight is absorbed and reradiated
Equilibrium temperature of the Earth, assuming an albedo A:

Emits thermal radiation at a wavelength of,
Earth emits in the infrared
Sun emits in visible
Contrast better for thermal emssion

Thermal emission gives the temperature of the planet.


Spectrum of the Earth

Spectrum of Reflected Light

Basically the Sun, but?
--Light must pass through the atmosphere (twice!)

Spectrum of Thermal Emission

Infrared radiation must pass through atmosphere (once)

Features in Earth's Spectrum

Many different features, but most important:
H20, i.e. Water
--One of the most obvious features
Oxygen, in two forms:
--O2 (molecular oxygen)
--O3 (ozone)
Carbon dioxide
CH4 Methane


Signatures of Life? Biomarkers

Relevance to life:
Water
--Requirement for life?
Oxygen
--On Earth, created by biological processes
Carbon dioxide
--Moderator of greenhouse effect
Methane
--On Earth, mostly produced by biological processes
--Methanogens, and?


Extrasolar Plants?

The "Red Edge"

Most plants on Earth are highly reflective at infrared wavelengths
How they stay cool
Spectral feature relatively unique to plants


Extrasolar oceans, continents, deserts, beaches?

Flux variability of the unresolved Earth may allow us to determine surface features.


Spectrum and variability of "pale blue dots" tell us about the constituents of their atmosphere and compositions of their surfaces.


See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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