Exobiology and the case forlife on Mars: ALH84001

I. Exobiology

definition: Exobiology is the study of life outside that known on today's Earth. As a discipline it is pretty much defined by a funding program with NASA's Space Sciences which includes studies in early life on earth, abiogenesis (origins of life studies) , SETI (the search for extraterrestrial intelligence, and more recently, the study of martian meteorites.

II. the Viking Mission

In the 1970's two very successful NASA missions sent 2 surface landers to Mars. What they found was a desert-like landscape strewn with volcanic ejecta and shifting sands. The Viking landers were equipped for three experiments designed to test for signs of life on the planet's surface. However, since the GCMS found no organic compounds at its detection limits (a few parts per billion), the initial positive results of the Viking experiments soon became moot.

A. The experiments


    1. Carbon assimilation experiment (PR): 14C labeled CO2is admitted to a martian soil culture, any biological uptake is measured by pyrolytic analysis of o.m. after an incubation period and flushing of excess labeled gas. (Performed under martian conditions)


    2. labeled release experiment (LR): dilute solution of 14C-labeled nutrient broth (formate, glycolate, glycine, D-alanine, L-alanine, D-lactate, and L-lactate) is added to regolith to see if uptake occured.


    3. gas exchange experiment (GEX): detect release of gasses from Martian regolith upon the addition of water, the assumption being that the martian microbes might simply be dormant.

B. The Results

    1. PR detected carbon

    2. LE showed CO2, thought to be due to the reaction between formic acid in the broth and oxygen peroxide resulting from the reaction between the superoxides and introduced moisture.

    3.GEX Initial results showed 15 to 30 times expected Oxygen.

C. Conclusions

    1. Reactions not indicative of extant life - all could be explained by reactions of moisture with superoxides or water gas displacing adsorbed CO2

    2. UV flux is very high on Martian surface, no life could exist there. (JPL Mars box is lethal to bacteria.)

    3. still possibility of prior life or proto-bionts

    4. no o.m. in the regolith (down to 0.1 to 1 ppb)

III. The Alan Hills 84001 meteorite

photo: Johnson Space Center

A. background

    1. preservation and recovery of meteorites in the Antartic ice fields

    2. SNC-meteorites (there are 12 of them known)

      a. Shergotty, Nakhla and Chassigny meteorites in the 1960s

      b. petrographic features differ from other meteorites leading scientist to believe that the SNC group are derived from planets, not asteroids

      c. e.g. the 14N/15N ratios from trapped gasses in the SNCs are exactly equivalent to the 14N/15N ratios measured by Viking lander

      d. ages: mostly 150-1300 Ma, corresponding to probable periods of volcanic activity on the martian surface

    3. carbon isotope fractionation:d13C

      a. Carbon in earth is a mixture of 12C+ 13C + 14C called isotopes

        i. same atomic # = 6 = # of protons

        ii. # of neutrons can be 6, 7 or 8 resulting in carbon atoms weighing in at 12C, 13C or 14C

        iii. 14Cis short-lived, generated by cosmic-ray bombardment of normal, 14N in the upper atmosphere

        iv. normal 13C:12C is 1.11:98.63

      b. some reactions are mass dependant, thus there is a tendency in biological reactions to preferrentially accumulate 12C over 13C

        i. residual differentiation yields enriched (positive) d13C in the surrounding CO2 source and depletion (negative) d13C

      c. d13C measured against a standard usually PDB carbonate

        i. PeeDee Belemnites from the PeeDee formation in S. Carolina

        ii. formula : [(13C/12C standard) - (13C/12C sample) ] / (13C/12C standard) * 1000 yields °/°° (Parts per thousand)

        iii. the general assumption is that d13C = 0 is about the equilibrium value of sea water; organic matter tends to be depleted in 13C, usu. around -27

B. petrography of ALH-840001

    1. a cumulate of pyroxene (iron-magnesium silicates) which could only have formed in the presence of a substantial gravitational field

    2. crack-fillings

      a. carbonate (principle mineral phase)

      b. magnetite

      c. iron sulfide

C. age of ALH-84001

    1. Phase I: primary crystallization age = 4500Ma

      a. based on 238U and 235U in relation to Pb

    2. Phase II: shock at 4000Ma

      a. based on 40Ar/40K clock reset

      b. volcanic event, probably brought cumulate to surface

    3. Phase III: shock at 15Ma

      a. possible ejection event

      b. based on exposure to mineralogic changes due to exposure to cosmic ray bombardment

D. evidence for biogenicity

    1. carbonate cumulates

      a. shapes of carbonate globules: thought to be biogenic

      (photo McKay et al., 1996)

      b. temperature of formation

        i. Oxygen isotopic evidence indicates 0-80°C

        ii. petrographic & compositional features indicate 500-700°C

      c. d13C values range

        i. -17 to +42

        ii. wider range than is normally seen in physical systems

        ii. indicates formation from aqueous precipitation in a CO2 atm

    2. magnetites and Fe-sulfides

    (photo McKay et al., 1996)

      a. associated minerals indicate conditions far from equilibrium which is "characteristic" of living systems

      b. shapes of magnetite grains mimic some known to be biogenic on earth, although the sizes are quite different .

      (photo McKay et al., 1996)

      (photo NASA)

      d. oxidation sequence within carbonate is found in biological systems on earth: Mg-Oxide, Fe-Oxide, Fe-Sulfides

      e. probably due to "changing" redox conditions during the life of the mineral formation

    3. polycyclic aromatic hydrocarbons PAH

      a. based on benzene rings

      b. result from the "cooking" of prior organic compounds

      c. similar to PAHs found in carbonaceous chondrites and interstellar space

      d. d13C values: extreme fraction of 12C relative to the +42 carbonate fractionation would imply a 6% relative d12C enrichment rather than a more modest 2% found in modern biogenic fractionation

E. conclusions

    1. the nature of "changing redox conditions" combine with the very presence of the carbonate mineral phase in the cracks indicate the possibility that these minerals formed out of an aqueous chemical phase and are further evidence for 0 to 100° C temperatures existing as late as 15 Ma on the Martian surface

    2. sum total of evidence leads to the liklihood that this meteorite contains evidence of prior biogenetic activity; however, each line of evidence in and of itself can be disputed

IV. Life on Mars? The current search

A. Evidence of water on the martian surface

1. Dendritic valley networks. These occur on the ancient surfaces and show the dendritic pattern that is similar to valleys carved on Earth by the runoff of liquid water. As such, these are inferred to have formed slowly, and are therefore thought to have required a climate in which water was more stable than it is at present. Even at their densest, these valleys are much less abundant than in almost any region on Earth.

2. Eroded ancient craters. The impact craters on these same old surfaces are very heavily eroded and degraded, and those that are smaller than about 15 km across have been removed entirely. Some partially degraded craters have gullies on their interior walls that suggest that erosion by liquid water may have played a role. Again, this suggests that water was more stable prior to about 3.5 billion years ago.

3. Catastrophic flood channels. These are large, water-carved features that formed when tremendous amounts of water were released from within the crust and flowed over the surface. Individual flood regions can be more than 100 km across, and flowed for up to 1000 km length. These formed more or less throughout geologic time. Because of their catastrophic nature, forming in a matter of weeks or months, they would not have required a climate different from today's climate--water can flow a long ways even in sub-freezing temperatures before it freezes.

4. There are a large number of additional features that formed from the actions of water or ice, including what appear to be rock glaciers (cemented or underlain by ice), the polar ice caps, and a type of patterned ground similar to what is seen in permafrost regions on the Earth.

from Bruce Jakosky (1997), http://quest.arc.nasa.gov/mars/ask/water-ice/Features_left_by_water_on_Mars.txt

B. Looking for ice

V. S.E.T.I.

A. The Drake (or now, more correctly) the Greenbank equation

1. This is an attempt to assign a probability to the liklihood of life on other planets in the universe. The concept is simple, but in practice, there are many unknowns here

N = R* fp ne fl fi fc L

Where,

N = The number of civilizations in The Milky Way Galaxy whose radio emissions are detectable.

R* = The rate of formation of stars suitable for the development of intelligent life i.e. rate of formation of stars similar to our own sun.

fp = The fraction of those stars with planetary systems. There has been much debate in recent years about the possibility of detecting the presence of planets circling nearby stars. Their detection involves large planets that change the way light is emitted from the principal star.

ne = The number of planets, per solar system, with environments suitable for life. This usually means those with water availability. Recent theorizing about the Martian atmosphere now indicates that this "water window" could be somewhat enhanced due to CO2 ice clouds which act to reflect infrared radiation (heat) back into the atmosphere, causing warming on colder planets.

fl = The fraction of suitable planets on which life actually appeared.

fi = The fraction of life bearing planets on which intelligent life emerges.

fc = The fraction of civilizations that develop a technology capable of releasing detectable signs of their existence into space.

L = The length of time such civilizations release detectable signals into space.

B. Radio Astronomy

    1. Radio frequencies are scanned on the so-called "water hole" at frequencies of about 1,420 MHz (MegaHertz). Lower frequencies are subject to interference from galactic background noise. Higher frequencies are blocked by H2O absorption in Earth's atmosphere. But the 1,420 MHz frequence corresponds to the absorption lines for H and is near the OH- ine as as well. So it is surmised that other civilizations interested in sending/receiving long-range signals would be utilizing a similar band.

    2. The radio telescope at Greenbank West Virginia has been used for this and other purposes.

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update 01 December 2005