Detecting Biosignatures of an
Evolving Earth-like
Atmosphere via New Worlds
Observer
Julia DeMarines
University of Colorado
1
AbGradCon 2008
newworlds.colorado.edu
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
2
• 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).
3
Planet Detection Methods
• Doppler shift
• Transit
• Gravitational
Microlensing
• Direct imaging
4
New Worlds Observer
5
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
6
Planet Hunting
Candidate Star
Region
7
www.sharoma.com/frontierverse/stars.htm
4m telescope
10m telescope
Courtesy of Phil Oakley
16m telescope
8
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.
Lenton et al. 2004
9
10
Kaltenegger et al. 2007
The Data
•
•
Epoch
Time
(Gyr)
I chose to use the atmospheric
compositions at six different points
(or epochs) in Earths history
Data from Kaltenegger et al. 2007
N2
CO2
O2
CH4
O3, N2O
and trace
elements
O3,
0
3.9
89.90%
10%
0%
Trace
0.10% N2
O
a
n
d
t
r
a
c
e
e
l
e
11m
e
n
t
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%
Trace
0.96%
Time
21%
(
G
y
r
Atmospheric Constituents
100%
95%
Percent
90%
85%
80%
75%
3.9
3.5
2.4
2
Billions of Years
0.8
0.3
O3, N2O and trace elements
CH4
O2
CO2
N2
12
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
13
Based on current amounts
of CH4 in Jupiter ~ 0.3%
CH4
White dotted line – Spectrum from planet
Green line – Actual spectrum
Red line – Fit to methane line at .725 microns
14
CH4 band at 730nm
Resolution 100
Time 106
15
CH4 ?
16
17
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
18
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
19
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
20
oxygen
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)
21
22