Life on Mars?

Life on Mars?

Objectives:

(1) learn about previous efforts to detect life on Mars (the Viking landers of 1976)
(2) consider the evidence presented in 1996 by NASA researchers who became convinced that they had discovered good evidence for life on Mars.

"Extraordinary claims require extraordinary evidence."
"Science is organized skepticism."

The Viking Life-Detection Experiments, 1976, were designed to look for signs of life or organic material in the soil at two different landing sites

There were three experiments: one looked for breathing, one for eating, and one for corpses.

Breathing: scoop of soil, add a liquid that is tagged with radioactive carbon. Monitor gas over the soil for radioactive CO₂. Negative result, because even though CO₂ was generated, there was still the possibility that it was nonbiological. Repeating the experiment yielded no more CO₂.
Eating: add a liquid nutrient broth and monitor for respired gases. Negative result, because although water added to the soil generated oxygen, it was realized that this could be nonbiological. So soil was heated to destroy any life, then the experiment run again, and more oxygen was generated.
Corpses: roast the soil at high temperature and monitor for evolved gases. Gases were given off, but repeating the experiment after preheating the soil to 200^oF and then 350^oF did little to decrease the amount of evolved gases, so they must have been produced nonbiologically, too.

20 years later ... August, 1996 - NASA group publishes in Science and holds a news conference to announce evidence for fossilized life on Mars in a martian meteorite.
Arguments FOR:

Globules of Fe- and Mg-carbonate - much carbonate on earth is biogenic, and most carbonate on earth is precipitated from water (but not all)
In the carbonates are PAHs (polycyclic aromatic hydrocarbons), which are produced on earth as a product of decay of living organisms. They can be produced by nonbiological processes, too. The PAHs in the meteorite are unevenly distributed, which could result from microbial activity
Tiny iron oxide (magnetite) and iron sulfide minerals are present in the carbonate, which suggest a possible analogy with magnetotactic bacteria on earth
Electron microscope images of the globules show tiny ovoid structures resembling bacteria, although they are very small (20-200 nm long, 5-10 nm wide) and much smaller than terrestrial bacteria (typically = 1000 nm)

Questions???
How do we know the rock came from Mars?
How do we know it's not contaminated?
How do we know nonbiologic processes didn't produce the globules, PAHs, magnetite and ovoids?
Meteorites - Antarctica, 1984, the Allan Hills, and the discovery of meteorite ALH84001.

Radiometric age (Rb/Sr method) of 4.5 Ga.
The K-Ar method gives a re-melting age of 4.0 Ga, probably due to impact melting.
Carbonates found within the rock could be younger, but are not easily dated.
Cosmic ray bombardment studies show that the rock spent 15 Ma travelling in space, Followed by 13 Ka on earth.

ALH84001 was identified as the 12th martian meteorite based on
(a) its content of Fe(III), the oxidized form of iron,
(b) the presence of carbonate, and
(c) the oxygen isotopic fingerprint which matches Mars', determined by the two Viking spacecraft landers of 1976.

Carbonate globules: One group says the carbonates formed at 50-80^oC, thus possibly in the presence of water. Another group says 650^oC, thus during a meteorite impact event.
Magnetic minerals: Magnetotactic bacteria on earth produce a magnetic Fe-bearing protein called ferritin, and some produce magnetic minerals magnetite or greigite.
The iron minerals in ALH84001 are magnetite, pyrrhotite and greigite (possibly). They are 40-50 nm large. Note that Mars' magnetic field is very weak (but may have been strong in the past).
The MVA group in Norcross, GA, claims the magnetite is a high-temperature variety.
Barber and Scott (2002) show that the planes of oxygen atoms in the magnetite crystals are aligned with those in the surrounding carbonate crystal. This proves, they contend, that the magnetite crystals could NOT have formed elsewhere and then been deposited in the carbonate.
PAHs: PAHs not present in living matter, but present in decay products. (E.g., the black stuff on barbecued chicken is full of PAHs.)
But PAHs in ALH84001 do not mean there was life present, because PAHs appear in many places, including interstellar space and interplanetary dust particles. In 1868, it was discovered that when you pass methane gas through a hot gun barrel, you smell napthalene (the PAH in mothballs). It's the localization of PAHs within the carbonate globules that might suggest a biological origin.

All of the arguments for evidence of life simply point out possibilities.
Does the sum of possibilities = probability?
"Extraordinary claims require extraordinary evidence."
"Science is organized skepticism."