Grand Challenge

_{JPL GRAND CHALLENGE INITIATIVE 1999

Identifying chemical signatures
of life from in situ measurements of biogenic gases & their isotopic ratios

Dr. Christopher R. Webster (Team Leader) Jet Propulsion
Laboratory Atmospheric science, in situ measurements

Professor Tobias Owen University of Hawaii Planetary atmospheres, isotope ratios

Professor Yuk L. Yung California Institute of Technology Planetary atmospheres,
modeling

Dr. Mark A. Allen Jet Propulsion Laboratory Planetary atmospheres, modeling

Dr. Lisa Y. Stein California Institute of Technology Biogenic gases, microbial
physiology

Professor Geoffrey A. Blake California Institute of Technology Optical
spectroscopy, isotopic ratios

Dr. Ara Chutjian Jet Propulsion Laboratory Mass spectroscopy

Professor John Eiler California Institute of Technology Mass spectroscopy

Dr. Robert L. Herman Jet Propulsion Laboratory Mineralogy, laser spectroscopy

Ms. Elisabeth L. Moyer California Institute of Technology Laser spectroscopy,
isotope ratios

Geochemical activity of microorganisms in the Earth’s biosphere

Comparable to the production of organic matter during photosynthesis

Greatly exceeds that of the whole human population

CO₂: fossil fuel combustion~5 x 10⁹ ton/year

microbial decomposition of plant residues ~5 x 10¹⁰ ton/year

CH₄:6-8 x 10⁸ ton/year arrives in atmosphere,

of which 60-80% is of microbial origin

SO_x:fuel combustion and sulfide ore smelting~ 10⁸ ton S /year

sulfate reduction in oceans ~4 x 10⁸ ton of reduced S /year

BIOLOGICAL MARKERS

BIOGENIC GASES:

Biogenic gas detection: CH₄, C₂H₆, N₂O, NH₃,
CO₂, H₂S, etc

Discriminate against non-biogenic sources and sinks

ISOTOPIC RATIOS:

Life’s catalysts (enzymes) preferentially use lighter isotopes during metabolism:
interested in N, C, S, and O biogeochemical cycles

e.g. ¹³C/¹²C of organic C is 2-4% lower than that of inorganic C
(carbonates) because organisms preferentially fix ¹²C during organic
biosynthesis.

Iron-reducing bacteria (that produce ferrous iron) have different isotopic composition
to abiological terrestrial iron.

Early Earth’s rock record shows onset of sulfate reduction and the divergence of
the isotopic compositions of sulfates and sulfides through ³⁴S/³²S.

MOLECULAR CHIRALITY:

Chirality: amino acids, building blocks to proteins. Life on Earth based on left-handed
L-amino acids (non-terrestrial could be L- or D-, but not both).

Table 1: KNOWN BIOGENIC GASES (after
Stein/Yung/Fegley)

Earth Mars
Earth troposphere

Mixing Ratio
Sources Sinks

H₂
500 ppb
50 ppm
N₂-fixing bacteria
H₂-utilizers

O₂
20.946 %
0.13 %
Plants and cyanobacteria
Aerobic metabolism

CO₂
350 ppm
95.3%
Respiration
Autotrophy

CO
125 ppb
0.07%
Methanogenesis
Carbooxydotrophic bacteria

CH₄
1.7 ppm
£ 20 ppb
Methanogenesis
Methylotrophy

N₂
78 %
~3 %
Denitrification
N₂ fixation

N₂O
320 ppb
£ 100 ppb
NitrificationDenitrification
Denitrification

NO_X
30-300 ppt
50 ppt
NitrificationDenitrification
Denitrification

DMS
5-60 ppt
None
Marine algae
Bacteria

COS
500 ppt
£ 10 ppb
CS₂-oxidizers
Thiobacillus, P. denitrificans

H₂S
30-100 ppt
£ 200 ppm
Sulfur metabolism
Sulfur Metabolism

CH₃Br
10-15 ppt
?
Plants, algae, & fungi
CH₄-and NH₃- oxidizers
& anaerobic bacteria

CH₃Cl
612 ppt
?
Plants, algae, & fungi
CH₄-and NH₃- oxidizers
& anaerobic bacteria

CHCl₃
16 ppt
?
Dechlorinators of CCl₄
CH₄-and NH₃- oxidizers
& anaerobic bacteria

Fundamental modeling questions

Are chemical species produced by potential Martian life forms distinct from atmospheric
constituents resulting from abiogenic processes?

Given source fluxes from putative Martian life community, or sink fluxes consumed by an
active life community:

what are the consequent constraints for detection strategies?

what is the impact on atmospheric composition?

Unique biologically-derived
atmospheric constituents

Expand successful model of Martian atmosphere abiogenic photochemical composition to
include biological compounds

Compare speciation of abiogenic atmosphere with atmospheric products and derivative
species of potential life forms

Identify potentially unique signatures of biological activity

Atmospheric composition with
prescribed biogenic sources & sinks

Define surface sources/sinks for a putative life community

Compute perturbation of atmospheric abundances resulting from surface sources/sinks on
various spatial scales in 1D, 2D, and 3D atmospheric models

Define required sensitivities for detection strategies

A strategy for identifying chemical signatures

a broad search for minor, disequilibrium constituents

a directed search for specific gases

an investigation of anomalous isotope ratios

Laboratory measurements

Low temperature mass spectrometry (Eiler Group)

Study the relative fractionation of H, C, N, and O compounds at temp<273 K.
Non-biological fractionations accompanying phase transitions

Mass spectrometry and laser pyrolysis (Chutjian Group)

interfaces to either GC’s or laser/thermal pyrolysis oven

Tunable laser spectroscopy (Webster Group)

Study the sensitivity limitations to measuring isotopic ratios.
Pulsed (QC laser) vs. cw (near-IR and mid-IR) detection schemes

IR laser –induced fluorescence (Blake Group)

optimum laser pumping and detection wavelengths.

Field measurements

Microbial mat community measurements (Nealson/Stein Group)

Measurement of production and release of biogenic gases in microbial mats
Comparison of techniques: laser and mass spectrometry, GC, flame photometry

Airborne demonstrations (Webster Group)

Measurement of ¹³CH₄/CH₄ and HDO/H₂O in
Earth’s stratosphere

_{JPL’s Atmospheric Laser Spectroscopy
Group}}
_{TUNABLE LASER SPECTROMETERS
FOR EARTH}

_{ALIAS (ER-2)}

ALIAS-II (balloon), BLISS (balloon)

Laser hygrometer (ER-2, DC-8, WB57F)

WISP (WB57F)

JPL’s Atmospheric Laser Spectroscopy Group

TUNABLE LASER PLANETARY SPECTROMETERS

_{PIRLS TDL spectrometer for Cassini Mission (proposed for Titan)}
_{MVACS TDL spectrometers (en route to Mars)}

Mars Organic Detector (MOD) TDL spectrometer

QC laser spectrometer for Mars: MICROBE, Mars Airplane

QC laser spectrometer for Titan probe/balloon

_{Mars Organic Detector (MOD) TDL spectrometer

QC laser spectrometer for Mars: MICROBE, Mars Airplane
QC laser spectrometer for Titan probe/balloon}

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	Earth Mars		Earth troposphere
	Mixing Ratio		Sources Sinks
H₂	500 ppb	50 ppm	N₂-fixing bacteria	H₂-utilizers
O₂	20.946 %	0.13 %	Plants and cyanobacteria	Aerobic metabolism
CO₂	350 ppm	95.3%	Respiration	Autotrophy
CO	125 ppb	0.07%	Methanogenesis	Carbooxydotrophic bacteria
CH₄	1.7 ppm	£ 20 ppb	Methanogenesis	Methylotrophy
N₂	78 %	~3 %	Denitrification	N₂ fixation
N₂O	320 ppb	£ 100 ppb	Nitrification Denitrification	Denitrification
NO_X	30-300 ppt	50 ppt	Nitrification Denitrification	Denitrification
DMS	5-60 ppt	None	Marine algae	Bacteria
COS	500 ppt	£ 10 ppb	CS₂-oxidizers	Thiobacillus, P. denitrificans
H₂S	30-100 ppt	£ 200 ppm	Sulfur metabolism	Sulfur Metabolism
CH₃Br	10-15 ppt	?	Plants, algae, & fungi	CH₄-and NH₃- oxidizers & anaerobic bacteria
CH₃Cl	612 ppt	?	Plants, algae, & fungi	CH₄-and NH₃- oxidizers & anaerobic bacteria
CHCl₃	16 ppt	?	Dechlorinators of CCl₄	CH₄-and NH₃- oxidizers & anaerobic bacteria