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Detecting Biosignatures of an Evolving Earth-like

Atmosphere via New Worlds Observer

Julia DeMarines

University of Colorado

newworlds.colorado.eduAbGradCon 2008

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Abstract• 200 + extrasolar planets have been found

– 1 has been imaged

• New Worlds will be able to image and take spectroscopy of the atmospheres of extrasolar planets

• What would an Earth look like if viewed at different points in its history via NWO?

• Biosignatures (such as CH4 and O2) could be detected as early as ~1.0 Gyr after planets formation!

• This is useful because we will be looking at stars of all ages, this planets in different stages of evolution

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• Early life may have been Methanogens– Early organisms may have utilized methanogenisis as a

metabolic process which produces CH4 (Schindler and Kasting 2000, Walker 1977).

– The major influence of methane on the atmosphere may have begun almost as soon as life originated more than 3.5 billion years ago (Kasting & Siefert 2002; Ueno et al. 2006; Canfield 2006). Methanogens are believed to have produced methane levels roughly 1000 times that of today.

• Most of the methane in our atmosphere is produced by biological activity– The primary sources of Methane in our atmosphere is biological

(Watson et at. 1990), although a small amount of CH4 is from volcanic outgassing at mid ocean ridges (Schindler and Kasting, 2000, Welhan 1988, Kasting and Brown, 1998).

• Life may have began as early as 3.9 billion years (Mojzsis et al. 1996), possibly even photosynthetic life, but any O2 would have reacted with volcanic gases (Schindler and Kasting, 2000).

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Planet Detection Methods

• Doppler shift• Transit• Gravitational

Microlensing• Direct imaging

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New Worlds Observer

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New Worlds Observer• As of February 2008, New Worlds was

selected for an Astrophysics Strategic Mission Concept Study by NASA– With the purpose of preparing for the decadal

review

• Mission conceived by Dr. Webster Cash of CU

• Starshade:– Occulter that suppressed the light from the star so the light reflected from

the small terrestrial planets are visiable

• Current parameters:– 16 petal 50m starshade

– 4m diameter telescope (or greater)

– Over a hundred really good target stars in 20 parsecs

– Spectra graph in the visible wavelength

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Planet Hunting

www.sharoma.com/frontierverse/stars.htm

Candidate Star

Region

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4m telescope 10m telescope 16m telescope

Courtesy of Phil Oakley

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Lenton et al. 2004

A) no oxygen produced by biosphere

B) oxygen produced, but absorbed in oceans and by seabed rock

C) oxygen starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer.

10Kaltenegger et al. 2007

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The Data• I chose to use the atmospheric

compositions at six different points (or epochs) in Earths history

• Data from Kaltenegger et al. 2007

Epoch

Time (Gyr) N2 CO2 O2 CH4

O3, N2O and trace elements

0 3.989.90

% 10% 0% 0.00% 0.10%

1 3.5 98% 1% 0% 0.20% 0.98%

2 2.4 98% 1% 0.02% 0.70% 0.28%

3 2 98% 1% 0.20% 0.40% 0.40%

4 0.8 97% 1% 2% 0.04% 0.46%

5 0.3 78% 0.04% 21% 0.00% 0.96%

EpochTime (Gyr) N2 CO2 O2 CH4

O3, N2O and trace elements

0 3.9 89.90% 10% 0% Trace 0.10%

1 3.5 98% 1% 0% 0.20% 0.98%

2 2.4 98% 1% 0.02% 0.70% 0.28%

3 2 98% 1% 0.20% 0.40% 0.40%

4 0.8 97% 1% 2% 0.04% 0.46%

5 0.3 78% 0.04% 21% Trace 0.96%

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Atmospheric Constituents

75%

80%

85%

90%

95%

100%

3.9 3.5 2.4 2 0.8 0.3

Billions of Years

Per

cent

O3, N2O and trace elementsCH4O2CO2N2

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The Code• In collaboration with Giada Arney and Phil

Oakley, supervised by Web Cash, we generated codes that simulate planets atmospheres via NWO

• Used spectral signatures of Sun and planets– reflectance of Earth

• Modeled at 10 parsecs

• Added exo-zodiacal light – subtracted background

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White dotted line – Spectrum from planet

Green line – Actual spectrum

Red line – Fit to methane line at .725 microns

CH4

Based on current amounts of CH4 in Jupiter ~ 0.3%

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CH4 band at 730nm

Resolution 100

Time 106

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CH4 ?

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False Positives

• Venus-like planet – Losing water due to run away greenhouse– Hydrogen escapes over <107 years

– O2 could build up to 10-100 bars before reacting with surface

– Would occur just before the inner edge of the habitable zone

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False Positives

• Mars-like planet– Too small to maintain volcanism and too cold

to have liquid water

– O2 production from photodissociation of H2O

– Lighter H2 escapes atmosphere leaving O2

– Would occur just beyond the outer edge of the habitable zone

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False Positives

• Ice World– “Oxygen having a nonbiological origin can

accumulate on a large ice-bound planet.” (Kasting, 1997; Des Marais, 1997)

– Ice prevents it from being absorbed by surface rock

– However, larger planets likely have volcanic exhalations with chemicals that reduce

– Such gasses would remove nonbiological oxygen

Des Marais et al. 2002

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Conclusion • We can see the Methane band at .725 microns at

only 0.2% of the total atmosphere and the Oxygen band at .761 microns at >2% of the total atmosphere – This corresponds to being able to detect a biotic

planet at a lower limit of ~1.0 billion years after formation!

• “There may be only a very small range of planetary conditions that might produce a false positive answer for oxygen.” (Des Marais et al. 2002)

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