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Rendezvous with Earth
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Rendezvous with Earth

What might (and might not) passing aliens be able to observe that might lead them to conclude that there is life on Earth?

The ideas and quotations in the table below are from Carl Sagan, et. al., "A search for life on Earth from the Galileo spacecraft" (Nature, vol. 365, 21 October 1993).

Potential Observations

 Comments

Departures from thermodynamic equilibrium.

As compared to Mars and Venus, for example.

Abundant surface liquid water.

 

Implies greenhouse effect because the equilibrium temperature (Te) obtained using the planetary energy balance equation is -20° C.

Water is an ideal medium for life.

Abundance of O2 in the atmosphere.

 

Observed at long-wavelength & near-infrared wavelengths, which detect spectral emission lines of O2 (also H2O, O3, CO2, N2O, CO).

O2 is a highly reactive gas.  A steady-state abundance of O2 in the atmosphere implies a steady rate of renewal.

There is much more O2 in the atmosphere than can be accounted for by UV photodissociation of H2O and Jeans escape of H to space over geological timescales. Compare to Venus and Mars, where UV photodissociation of H2O continues today.

Lack of impact craters & pervasive wind and water "suggest continuing exposure of fresh,oxidizable regolith" (i.e. a large O2 sink driven by plate tectonics).

Observed abundance of O3 ("which is related approximately logarithmically to the O2 abundance"). "Therefore a train of argument may exist from abundant O3 to abundant O2 to life."

Presence of N2O at high disequilibrium abundances.

 

N2O is lost to photodissociation, with an atmospheric lifetime of c. 50 years.

"Ground truth" based on direct observation on the Earth's surface  shows that nitrogen-fixing bacteria & algae that convert soil and oceanic nitrate to N2 & N2O are the main sources of N2O.

Abundance of CH4.

 

"At thermodynamic equilibrium there should not be a single methane molecule in the Earth’s atmosphere."

"The disparity between observation and thermodynamic equilibrium is about 140 orders of magnitude."

"It has long been suggested that an extreme disequilibrium abundance of a reduced gas such as CH4 in an O2-rich atmosphere could be evidence for life on Earth."

One needs to discount non-biological origins of methane (volcanoes, etc.). Ground-truth Earth shows these to be negligible.

Waveband imaging may reveal chlorophyll.

 

Combining the wavebands in global imaging (RED, VIO, GRN) in different ways reveals absorption at wavelengths "inconsistent with any known rock or soil types on terrestrial planets of iron silicate surface composition."

RED wavebands are especially significant:  "the possibility naturally arises that the strong RED absorption is the signature of a light-harvesting pigment in a photosynthetic system…"

Substantial land surface areas return RED spectra.

Radio emission at 4-5Hz is strong indication (but not conclusive) of intelligent life.

Radio signals can only be observed on the nightside of the Earth, which suggests an origin below the ionosphere (in daylight, the ionosphere propagation cutoff frequency blocks radio emissions from below).

Topographic images at > 1km pixels fail to reveal evidence of life.

 

No structures which can be interpreted unequivocally as evidence of life can be detected.

Artifacts of life cannot be observed without high resolution imaging.

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Timeline of Earth History

 

Geological Era

Major Events

Events

Related Topics

Pre-Hadean

Synthesis of chemical elements in stars/supernovae

    

Hadean

Formation of the Earth (4.6 Ga)

    
 

Formation of the Moon

    
 

Loss of the primary atmosphere

    
 

Differentiation of the Earth

    
 

Major bombardment period (4.0 - 3.8 Ga)

    
 

Plate tectonics begins

    
 

Possible origin of life before 3.8 Ga

  

RNA, DNA, proteins & amino acids

Archean

Evolution of prokaryotes (by 3.6 Ga or earlier)

Photosynthesis (autotrophy) begins

  
 

Organic carbon burial begins

  

d13C & Schidlowski Diagram

Carbonate system (TDE, pp. 84-85)

Global carbon cycle (TDE, ch 3)
 

Build-up of atmospheric O2 begins

BIFs

  
 

Drawdown of atmospheric CO2 begins

  

Kasting’s Energy Balance Model

Proterozoic

Ice Ages

Huronian (2.3 Ga)

Late Proterozoic (c. 800 Ma)

Renewed BIF formation

 

Varanger Ice Age (610 - 590 Ma)
 

Atmospheric O2 reaches 0.2% (2 Ga)

Red Beds

Paleosols

Mantle oxidation state

Uraninite deposition

PAL calculation

Holland’s model of Earth’s oxygen budget (C:P ratio)
 

Increasing rate of carbon burial (at 2 Ga & 1 Ga)

    
 

Evolution of eukaryotes (1.7 Ga)

    

Vendian

Ediacaran fauna

(565 Ma)

Origin of animals?

Threshold of atmospheric O2 of 1% reached

Breakup of Rodinia

    

Phanerozoic

Continuing increase in atmospheric O2

Continuing drawdown of atmospheric CO2

  

Berner & Canfield model

Process-based model of atmospheric CO2

Climate models (ring world, etc.)

Cambrian

Cambrian Explosion (540 Ma)

Origin of multi-tiered trophic pyramids

Increasing shift in favour of Ccarb burial

Ordovician - Devonian

Origin and spread of land plants

    

Permo-Carboniferous

Development of coal swamps

Ice Age

Enhanced organic carbon burial

Atmospheric O2 reaches 35%

Atmospheric CO2 reaches lowest point in the Paleozoic

 

 

 

 

GEOCARB model

Permian

Permian mass extinction

Formation of Pangea

 

 

End-Permian marine regression

  

Triassic

Spread of monsoon conditions

Breakup of Pangea begins

    

Cretaceous

Greenhouse world

Carbonate platforms

Pacific superplume & increased volcanism

High sealevels

Arctic paleoflora

Deep water carbonate factories

CCD

Cretaceous/ Tertiary boundary

End-Cretaceous mass extinction (65 Ma)

Deccan Traps

Marine regression

Meteorite impact

Flood basalts - outgassing & the atmosphere

Tertiary

Global cooling over last 50 Ma

Initiation of Antarctic Circumpolar Current

Rise of Tibetan Plateau & Himalayas

Initiation of South-West Monsoon

 

 

Van Cappellen / Ingall model

Raymo-Ruddiman hypothesis

87Sr/86Sr ratio

Pleistocene

Ice Age (2 Ma to 10 ka)

  

Milankovitch Cycles

Holocene

Appearance of modern human societies

Agriculture

Modern industrial civilization

Dating techniques (14C, etc.)

Techniques of paleo-environmental reconstruction

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Feedbacks

 
Some examples of positive feedback
Runaway greenhouse on Venus.
Higher level of atmospheric O2 in the Late Proterozoic as a trigger for evolutionary diversification.
The evolution of multi-tiered trophic pyramids in the Cambrian Explosion (related to the evolutionary "arms race" between predators & prey).
The effect of increased sediment-churning during the Cambrian Explosion on Corg burial and atmospheric/oceanic oxygen.
Appearance of carbonate shells in the Cambrian shifts the balance of carbon burial towards carbonate deposition.
Spread of land plants and elevation of the boundary layer (ELE p. 85).
Evolution of leaves (ELE p. 87).
Spread of land plants in the Palaeozoic had a warming effect by reducing the Earth’s albedo.
Evolution of trees (lignin) coupled with numerous continental sedimentary basins Ù  increased Corg burial Ù elevated atmospheric O2 levels (Carboniferous).
Evolution of trees (lignin) coupled with numerous continental sedimentary basins Ù  increased Corg burial Ù  elevated atmospheric O2 levels (Carboniferous).
End Permian marine regression contributed directly to the mass extinction in shallow shelf seas, and increased oxidation of buried Corg, drawing down atmospheric oxygen levels, increasing oceanic anoxia, and damaging pelagic and deep-ocean life-forms.
Growth of ice caps during ice ages, with increase in the Earth’s albedo and increased cooling.
Formation of tropical cyclones (convection of moist air and release of latent heat through precipitation).
ENSO events and global climatic instability.
Anthropogenic CO2 and enhanced greenhouse warming.
Some examples of negative feedback
James Lovelock’s model of fire regulation of atmospheric O2 (AEL p. 88).
Holland’s model of O2 regulation based on phosphorous, as applied to the Carboniferous [High O2 levels Ù  High level of dissolved O2 in the oceans Ù  Increased oxidation of phosphorous to apatite & ferric phosphates Ù  Reduced C:P ratio (less P for biomass production) Ù  Reduced burial of organic carbon Ù  Reduced level of atmospheric O2.
Increased rate of carbon burial in the Cretaceous (cycle of carbonate platform drowning & rebuilding) linked to volcanic emissions of CO2.
Increased rate of weathering of silicate rocks by H2CO3 and increased marine carbonate deposition as temperatures and rainfall increase, leading to increased rate of sequestration of carbon and reduction in atmospheric CO2.
Cooling effect of increased cloud cover as temperatures and rainfall increase.

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