Measurement Protocols for In situ Analysis of Organic Compounds at Mars and Comets

2005·0.24 MB·English

by  Mahaffy, P. R.

MEASUREMENT PROTOCOLS FOR INSITU ANALYSIS OF ORGANIC COMPOUNDS AT MARS
AXD COMETS. P. R. Mahaffyl, W. B Brinckrrhoff?,A . Buch', M. Cabane', P. Coil', j. kmick'. E. P. Giavin',
R. Navarro-Gon~alez.~ 'NASA Goddard Space Flight Center (GSFC), Greenbelf MD 20771,
[email protected], 'The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723,
'Service d'Aeronomie, IPSL. Universite Pierre et Mane Curie, Paris, 'LISA. Universite de Pans VI1 et XII, Creteil,
'University of Mexico.
Introduction: The determination of the abun- or by the chemistry of dust devil induced discharges)
dance and chemical and isotopic composition of or- or by ultraviolet or particle radiation. For this reason, it
ganic molecules in comets and those that might be is important not only to locate sites of high preserva-
found in protected environments at Mars is a first step tion potential. but to implement organic analysis ex-
toward understanding prebiotic chemistries on these periments with the highest possible sensitivity and
solar system bodies. While future sample return mis- breadth of chemical analysis. We have explored the
sions from Mars and comets will enable detailed techniques of laser desorption mass spectrometry
chemical and isotopic analysis with a wide range of (LDMS) for analysis of insoluble refractory organics.
analytical techniques, precursor insitu investigations chemical derivatization for analysis of soluble organ-
can complement these missions and facilitate the iden- ics, and pyrolysis. The pyrolysis approach is to heat a
tification of optimal sites for sample return. Robust few milligrams of sample from ambient temperature to
automated experiments that make efficient use of lim- approximately 1200°C using a linear temperature
ited spacecraft power, mass, and data volume re- ramp. Gases are rapidly removed from the sample by
sources are required for use by insitu missions. Within an inert helium camer gas. They are first analyzed
these constraints we continue to explore a range of directly by a mass spectrometer and then again after
instrument techniques and measurement protocols that completion of the pyrolysis by rapid release of organic
can maximize the return from such insitu investiga- molecules trapped on a high surface area adsorbant
tions. into a GC column. The controlled sample heating also
Mars: A significant step toward resolving the enables analysis of simple inorganic gases, such as
question of the habitability and potential for past or CO:, H,O, SO2 that provide information on mineral
present life on Mars will be the detailed characteriza- type and the degree of mineral alteration by weather-
tion of organic compounds that might be preserved in ing. The derivatization technique also employs the
rocks, ices, or sedimentary layers such as those re- separation technique of multicolumn gas chromato-
cently revealed by the Mars Exploration Rovers. The graph mass spectrometer (GCMS) analysis for detec-
Scout Phoenix mission [I] will target an analysis of tion of a wide range of organic compounds. A subset
lolatiles in ices and it is likely that the mobility of the of these techniques was recently selected for inclusion
planned Mars Science Laboratory (MSL) will enable it on the Mars Science Laboratory. Figure 1 illustrates a
to locate and sample sedimentary layers. It is not yet chemical derivatization reaction that enables detection
clear how well near surface environments at Mars of a wide range of molecules that are relevant to terres-
might preserve organic molecules produced in an ear- trial life such as carboxylic acids. amino acids, and
lier warmer and wetter epoch or delivered to Mars by nucleobases to be analyzed by the GCMS technique.
meteoritic infall. These may be destroyed by oxidants Such derivatization transforms polar organic molecules
such as hydrogen peroxide (produced photochemically into highly volatile species that readily elute from the
Figure 1. The chemical reaction that we have used on a wide variety of Mars analogue samples to transform polar molecules
such as carboxylic acids and amino acids to volatile products that can readily be analyzed by the GCMS technique. The mass
spectrometer, in this case, must have a mass range sufficient to reach the parent peak of the dcrivatized product. The deri-
vatization agent is N,N-Methyl-tert.-butyl (dimethylsilyl) trifluoroacetamide (MTBSTFA) assisted by the solvent dimethyl-
formamide (DMF). Lacking derivatization, the Viking GCMS would not have detected carboxylic acids.
GC columns. Mars analogues that we have examined measure the mixing ratios in homologous carbon chain
inciude ~tacamad esert [Zj samples, and weathered series such as GI4S, &, C&, . .. C,JH1Va-n2d similar
volcanic material that is often selected for remote sens- series for the more oxidized alcohols, aldehydes, or
ing analogue studies. carboxylic acids. Other parameters of interst include
Comets: The role of the exogeneous delivery of the degree of branching of organic chains, the fraction
organic material to the terrestrial planets in the origin of aliphatic vs aromatic molecules, and the isotopic
of life motivates the detailed exploration of the organic composition of C and H as a function of chemical
chemistry of comets. Simple organic molecules C&, complexity. These measurements may distinguish
C2H6,C 2HZ,C H30H, HICO and others have been ob- thermodynamically driven abundance distributions
served by remote sensing in comets, but the fast flyby from those that are kinetically controlled and hetero-
of Comet Halley by the Giotto and Vega spacecraft did geneous catalytic formation mechanisms from ho-
not yield a detailed understanding of the composition mogoneous processes.
of more complex organic molecules present in the Insitu Laboratories: The analytic power of the
coma. Since the cold ices of the cometary nucleus may individual techniques described is greatly enhanced
never have reached liquid water temperatures, it is when several of these techniques can be combined to
appropriate to interrogate these samples with a variety interrogate the same sample. The organic composition
of techniques such as the direct ionization (LDMS). should be analyzed in the context of the host mineral-
the dry pyrolysis processing, as well as the liquid ogy and highly complementary elemental and minera-
based derivatization methods. As in the case of the logical techniques are those that resolve tine scale spa-
Mars exploration, our primitive knowledge of the or- tial variations.
ganic composition of comets suggests that these sam- References: (1) Smith, P. “The Phoenix Scout Mis-
ples are best explored with a broad chemical analysis. sion”, 34th Annual Lunar and Planemy Science Con-
For missions that flyby the nucleus or rendezvous with ference, 1855 (2003). (2) Navarro-Gonzalez, R,
a comet without touching the surface, both the gases of Rainey, F.A., Molina, P., Bagaley, D.R., Hollen, B.J.,
the coma and the collected dust particles can be ana- Rosa, J., Small, A.M., Quinn, R.C., Gmnthaner, F.J,
lyzed by the protocols described The best cometary Ciceres, L., Gomez-Silva, B., and McKay, C.P.,
analogues to evaluate the measurement protocols may “Mars-Like Soils in the Atacama Desert, Chile, and
be the chemically complex residues produced by labo- the Dry Limit of Microbial Life”, Science 7, 1018-
ratory irradiation of ices formed from simple combina- 1021 (2003).
tions of gases such as H20, CH4, and NH3. Primary
goals of the analysis of cometary organics will be to
5.E+07
4.E+ 07
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3.E+07
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6
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18 22 26 30 34 38 42 ypol
Retention time (min)
Figure 2. The derivatization agent MTBSTFA allows a variety of polar molecules that are not seen in pyrolysis GCMS to be
identified. This sample is from a dry region of the Atacama desert where there are few organic molecules produced by dry
extraction techniques.