Lecture 1

A Great Debate: Is There Life on Mars?

What is the history of the debate?

• in 1984, meteorite ALH 84001 was found on Antarctic ice

  

  

• fusion crust, interior looked green at first!

• only recently recognized as one of 12 meteorites believed to have come from Mars -- how do we know?

  • contains gases trapped in small glassy ``bubbles'' (carbonate mineral globules)

    

    

  • careful analysis revealed that gases have same composition and abundance as gases in Martian atmosphere as measured by two Viking landers in 1976

    

  • carbonate mineral globules are martian and formed by liquid water

  • if globules are 3.6 billion years old as claimed, they formed when Mars' atmosphere may have been much thicker than it is now, when temperatures were much warmer, and when liquid water could have been abundant

  • now seems likely that ALH 84001 formed in Martian volcano and later was blasted off surface of Mars when comet or asteroid struck planet

    

    

What is the evidence for life?

• McKay and his co-workers describe three kinds of features in ALH 84001 that they interpret as evidence for ancient martian bacteria (features are all in and near carbonate mineral globules)

  • microscopic shapes that resemble living and fossil bacteria on Earth

    

    

    • elliptical, rope-like, and tubular structures in fractures in the carbonate mineral globules

    • structures are so small, each only 20 to 100 billionths of a meter (or nanometer) across, that they could only be seen with an advanced electron microscope

    • to show that shapes could really be fossil bacteria, McKay and co-workers have tried to show

      • that the bacteria shapes are actually the sizes and shapes of known living organisms
        • observation: similar ``nanobacteria'' do live in igneous rocks on Earth

        • problem: inorganic processes might have produced structures, and there is no evidence for cell walls, reproduction, growth, cell colonies

      • that the bacteria shapes are really part of the rock and were not produced accidentally while they prepared the sample for study
        • observation: no structures in other rocks treated same way

        • problem: could still be artifacts

      • that the bacteria shapes are not Earth bacteria that somehow wiggled into ALH 84001 while it was in Antarctica
        • observation: no evidence of Earth bacteria in other meteorites in same area

        • problem: other rocks did not contain carbonate minerals --- what if Earth bacteria need those minerals?

  • microscopic mineral grains like some produced by living and fossil bacteria on Earth

    

    • McKay and co-workers suggest that these mineral grains in ALH 84001 may have been produced by martian bacteria by arguing that

      • the mineral grains formed on Mars
        • observation: grains lie inside carbonate globules, which formed on Mars

      • the mineral grains have chemical compositions, crystal structures, sizes, and shapes like biologically produced grains on Earth
        • observation: some mineral grains on Earth are products of life (waste)

      • microscopic mineral grains are not likely to have formed by inorganic processes, without assistance from life
        • observation?

        • problem: not sure of proper identification of minerals, and inorganic processes might produce these mineral grains

  • organic chemical compounds, or polycyclic aromatic hydrocarbons (PAHs), that resemble the decay products of bacteria on Earth

    Carbon Rings of PAHs, Hydrogen Connections Not Shown

    • form from heating carbon-rich material (burnt meat, mothballs)

    • H = hydrocarbon (gas, oil), A = aromatic (stinky, flat rings of carbon, usually 6 in hexagon), P = polycyclic (more than one aromatic ring)

    • McKay and co-workers argue that PAHs in ALH 84001 are derived from ancient martian bacteria by trying to show that

      • they are not contamination from laboratory procedures
        • observation: no PAHs introduced into rocks without them in lab

      • they are not Earth PAHs that entered the meteorite while it was in Antarctica
        • observation: amount of PAHs increase toward center of meteorite, no sulfur-bearing PAHs (like exhaust on Earth), too many PAHs to result from pre-industrial Earth

      • they are not like PAHs in other meteorites (which have nothing to do with life)
        • observation: different than other meteorites, so must have originated via particularly martian processes

      • they are consistent with decomposition of simple bacteria
        • observation: limited variety of PAHs consistent with breakdown of simple organisms

        • problem: no experiment has shown direct connection between these PAHs and simple life, and experiments have shown that inorganic processes might be responsible

• traces of amino acids found in the meteorite support idea that building blocks of life are common in Universe

• doesn't prove, however, that there was past life on Mars and it doesn't prove that there was not either

• nevertheless, it leads to interesting corollary -- panspermia, did life on Earth come from Mars or a comet or the Leonid meteor shower?

What is the Scientific Method?

• example: how the motions of the planets were finally understood

  

• example: life on Mars debate

  • observation: life on Earth exists

  • analysis: life on Earth exists in volcanic rocks

  • hypothesis: life exists (or existed) on Mars in volcanic rocks

  • prediction: living or dead bacteria similar to Earth bacteria might be found in Martian meteorites that have landed on Earth

  • test: show that structures, grains, and PAHs not from Earth and are consistent with only organic origin

  • more observations, more problems (see notes above)

What is Astronomy?

Astronomy is not just about constellations...

• study of light and matter beyond Earth

• inter-disciplinary science (as we've seen in the life on Mars example above!)
  • physics: how material interacts and behaves

  • geology: composition of planets

  • chemistry: elements combine in rocks and space

  • biology: can life evolve elsewhere?

  • mathematics: language of science

Our Cosmic Address:

Classroom N210, Tucson, US, Sol 3 (Earth), Solar System, Milky Way Galaxy (2/3's of the way outside center), Local Group of Galaxies, Virgo Supercluster of Galaxies, Universe

Go here and click on the play arrow for a brief, cosmic journey.