The Viking labelled release experiment Evidence for microbial life on Mars? An introduction to Mars The Viking mission Biological experiments on Viking Is/was there life on Mars?

The Viking labelled release experiment

Evidence for microbial life on Mars?

An introduction to Mars

The Viking mission

Biological experiments on Viking

Is/was there life on Mars?

A comparison of Mars and Earth 23.9 24.6 Rotation period (h) Yes (71%) ??? Surface water Yes No Magnetic field Yes No (ancient) Tectonic activity 1 2 Moons N 2 78% O 2 21% Ar, CO 2 , noble 1% CO 2 95.3% N 2 2.7% Ar 1.6% Atmospheric composition 1 014 7 (1-9) Atmospheric pressure (mb) 287 218 (140-300) Surface temperature (K) 1 0.38 Gravity 1 1.52 Distance from Sun (AU) 12 756 6 794 Diameter (km) Earth Mars Property

Astronomical observations of Mars 1877: Schiaperelli describes “canali” 1906: Lowell writes “Mars and its Canals” 1997: HST view of dust storms on Mars

Mars, life and science fiction

The history of Mars exploration

The Mariner 4 flyby, 1965

The Viking Missions: 1975-1982 Viking orbiter Viking lander Weighed 900 kg Viking 1: June 1976-August 1980 Viking 2: July 1976-July 1978 Acquired over 52 000 images Weighed 600 kg Viking 1: July 1976-August 1982 Viking 2: August 1976-August 1980 Acquired images, weather readings and biological/chemical data

Weighed 900 kg

Viking 1: June 1976-August 1980

Viking 2: July 1976-July 1978

Acquired over 52 000 images

Weighed 600 kg

Viking 1: July 1976-August 1982

Viking 2: August 1976-August 1980

Acquired images, weather readings and biological/chemical data

Orbiter images 1: Valles Marineris

Orbiter images 2: Olympus Mons

Orbiter images 3: dust storm

Orbiter image 4: the Cydonia region Viking orbiter image Close up of “face” MGS image “ Happy face” crater from MGS

Lander image 1: Viking 2 landing site

Lander image 2: frost at Viking 2 landing site

GEX/LR/PR: gas exchange experiment Soil sample incubated in complex organic medium for 12 days under CO 2 , He and Kr Head space analysed by GCMS for CH 4 , CO 2 , H 2 , N 2 , O 2 Results: O 2 was evolved. However, this occurred in a heat-sterilised sample. An inorganic oxidant is the preferred explanation.

GEX/LR/PR: pyrolytic release experiment Soil sample incubated with 14 CO and 14 CO 2 for 5 days under xenon lamp illumination Sample pyrolysed at 625 °C and 14 C detected in gas sample Results: Some 14 C was fixed. However, some fixation was also seen in a heat-sterilised sample. An inorganic oxidant is the preferred explanation.

GEX/LR/PR: labelled release experiment Soil sample incubated with 14 C-labelled formate, glycine, D/L alanine, D/L lactate and glycolate 14 C in evolved gases is detected Second application of labelled medium. Both responses are compared to a heat-sterilised control. Results: 14 C-labelled gas was evolved. Levels fell initially after the 2 nd application, but then increased. No gas evolution in heat-sterilised samples.

Typical LR response using Martian soil

Comparison of LR experiments on Earth and Mars

The official NASA interpretation of the biology experiments The LR result might be compatible with biological activity The GEX and PR experiments are not compatible with biological activity The GCMS experiment failed to detect any organic compounds in the soil at all UV light at the Martian surface produces peroxides and superoxides. These serve to sterilise the surface, destroy organic matter and act as inorganic oxidants in the LR experiment

The LR result might be compatible with biological activity

The GEX and PR experiments are not compatible with biological activity

The GCMS experiment failed to detect any organic compounds in the soil at all

UV light at the Martian surface produces peroxides and superoxides. These serve to sterilise the surface, destroy organic matter and act as inorganic oxidants in the LR experiment

The principal objections and refutations Current thinking on this differs to that of 1970s Independent origin of life Gas was reabsorbed-terrestrial soils have show similar response Decrease in gas evolution on 2 nd injection Kinetics are similar to several terrestrial soils from extreme environments “ Too much too soon” Recent physical data suggest liquid water can form and persist at the surface No liquid water at surface Samples taken from under rocks show identical LR response Effect of UV light at surface Recent data suggest H 2 O 2 levels much lower than postulated Presence of inorganic oxidants, especially H 2 O 2 Viking GCMS shown not to detect organic compounds in some Antarctic soils with a positive LR response. Martian meteorites contain organic compounds. No organic matter detected by GCMS Counter-argument Objection

Is there microbial life on Mars? Consensus opinion has to be “most probably not” However, there may be less extreme niches on Mars (with heat and liquid water), where micro-organisms might survive This is largely speculation Is it reasonable that life could just be “hanging on”, rather than modifying and maintaining the biosphere as on Earth? (Lovelock and Margulis, Gaia hypothesis)

Consensus opinion has to be “most probably not”

However, there may be less extreme niches on Mars (with heat and liquid water), where micro-organisms might survive

This is largely speculation

Is it reasonable that life could just be “hanging on”, rather than modifying and maintaining the biosphere as on Earth?

(Lovelock and Margulis, Gaia hypothesis)

Was there life on Mars? Meteorite ALH84001 is one of 18 known meteorites from Mars

Evidence for ancient microbial activity in meteorite ALH84001 Although this image gained the most publicity for ALH84001, the best evidence for biological activity is geochemical, including: Carbonate globules, formed from CO 2 and water at low temperature Presence of polycyclic aromatic hydrocarbons (PAHs) Carbon isotope studies indicating a martian origin for the carbonate Magnetite crystals, indistinguishable from those formed by terrestrial bacteria However, there is much scientific debate and controversy about this meteorite

Although this image gained the most publicity for ALH84001, the best evidence for biological activity is geochemical, including:

Carbonate globules, formed from CO 2 and water at low temperature

Presence of polycyclic aromatic hydrocarbons (PAHs)

Carbon isotope studies indicating a martian origin for the carbonate

Magnetite crystals, indistinguishable from those formed by terrestrial bacteria

However, there is much scientific debate and controversy about this meteorite