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

Property Mars Earth

Diameter (km) 6 794 12 756

Distance from Sun (AU) 1.52 1

Gravity 0.38 1

Surface temperature (K) 218 (140-300) 287

Rotation period (h) 24.6 23.9

Atmospheric pressure (mb) 7 (1-9) 1 014

Atmospheric composition CO2 95.3%

N2 2.7%

Ar 1.6%

N2 78%

O2 21%

Ar, CO2, noble 1%

Moons 2 1

Tectonic activity No (ancient) Yes

Magnetic field No Yes

Surface water ??? Yes (71%)

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

Spacecraft Year launched Spacecraft Year Launched Mars 1960A 1960 Mars 5 1973 Mars 1960B 1960 Mars 6 1973 Mars 1962A 1962 Mars 7 1973

Mars 1 1962 Viking 1 1975 Mars 1962B 1962 Viking 2 1975

Mariner 3 1964 Phobos 1 1988 Mariner 4 1964 Phobos 2 1988

Zond 2 1964 Mars Observer 1992 Mariner 6 1969 Mars Global Surveyor 1996 Mariner 7 1969 Mars 96 1996 Mariner 8 1971 Mars Pathfinder 1996

Kosmos 419 1971 Nozomi (Planet B) 1998 Mars 2 1971 Mars Climate Orbiter 1998 Mars 3 1971 Mars Polar Lander 1999

Mariner 9 1971 Deep Space 2 1999 Mars 4 1973

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

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 organicmedium for 12 days under CO2, He and Kr

Head space analysed by GCMS for CH4, CO2, H2, N2, O2

Results: O2 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 14CO and 14CO2 for 5 days under xenon lamp illumination

Sample pyrolysed at 625 °C and 14C detected in gas sample

Results: Some 14C 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 14C-labelled formate, glycine, D/L alanine, D/L lactate and

glycolate

14C in evolved gases is detected

Second application of labelled medium. Both responses are compared to a heat-sterilised

control.

Results: 14C-labelled gas was evolved. Levels fell initially after the 2nd 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 principal objections and refutations

Objection Counter-argument

No organic matter detected by GCMS Viking GCMS shown not to detect organic compounds in some Antarctic soils with a positive LR response.

Martian meteorites contain organic compounds.

Presence of inorganic oxidants, especially H2O2

Recent data suggest H2O2 levels much lower than postulated

Effect of UV light at surface Samples taken from under rocks show identical LR response

No liquid water at surface Recent physical data suggest liquid water can form and persist at the surface

“Too much too soon” Kinetics are similar to several terrestrial soils from extreme environments

Decrease in gas evolution on 2nd injection Gas was reabsorbed-terrestrial soils have show similar response

Independent origin of life Current thinking on this differs to that of 1970s

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)

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 CO2 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