PHYS Life? So far we have discussed the existence of: organic molecules in interstellar dust clouds possible conditions for life in Europa & Titan organic molecules in comets and meteorites evidence for liquid water on Mars in the past The Drake Equation Suppose intelligent civilisations last for L ic years and they form at a rate of R ic per year, then the number in existence at any one time is N ic = R ic  L ic where = rate of star formation averaged for the galaxy, stars in years = 10 per year. P p = probability that a star has a planet = 0.5 ? P e = probability that star will be stable long enough for life to form. Need a star of 0.75 to 1.5 solar masses, not in a binary system = 0.1 ?

PHYS The Drake Equation continued... N e = number of planets in the ecosphere = 1 ? Fig. Z9.12 & K page 588 P l = probability that planets evolve life = 1? P i = probability that biological evolution leads to intelligent life = 1 ? Thus, N ic = 10  0.5  0.1  1  1  1  L ic = 0.5 L ic What is L ic, lifetime of a civilisation? 300 years or 300,000 years or 3,000,000,000 years? Pessimistically, N ic = 150 super-optimistically, N ic = 1,500,000,000 Choose your value.

PHYS Taking the optimistic view of L ic, the separation of habitable planets is tens of light years. How do we find them? Listen for radio emissions. Radio is not easily absorbed by Earth’s atmosphere, interstellar gas or interstellar dust. Stars are quiet at some radio frequencies. Radio should therefore be a natural candidate for beacons and communication. Which frequencies? Not much background noise in range 0.5 GHz to 10 GHz, so try this. Concentrate on sun-like stars (spectral type G) and also do a full sky survey. Look for patterns in the signal. SERENDIP 4 examines 168 million channels every 1.7 seconds in a band of frequencies centred on 1.42 GHz, using the 305 metre diameter Arecibo radio telescope. SETI -Search for ExtraTerrestrial Intelligence

PHYS SETI - Communication Why 1.42 GHz? A fundamental frequency associated with hydrogen which it is assumed any civilisation would discover - the electron flips over and emits a photon. We can transmit a message via radio using a characteristic frequency that is likely to be known throughout the Galaxy. Form of message - simple code such as binary (two states - either on or off) used to send universal information, such as the ratio of proton mass to electron mass,  (pi) etc. Also have radio frequency leakage from TV and radio. Absence of evidence is not evidence of absence - Frank Drake

PHYS The Viking Lander Programme Martian soil samples examined for life assuming basic similarity to Earth bacteria e.g. put sample in nutrient broth to look for gases given off due to metabolic activity or pyrolytic-release experiment: add carbon dioxide containing radioactive C 14 to a sample of Martian soil and atmosphere. Micro-organisms should absorb CO 2 and incorporate C 14 in their cells. After 11 days of incubation, remove ambient gas and heat soil sample to release any carbon compounds as a gas. Pass released gas over a detector to check for the radioactive C 14. Results could all be explained by inorganic (non-biological) chemical reactions.

PHYS Martian Fossils? ~ 12 meteorites have chemistry matching that found on Mars by the Viking Landers. Radioactive dating gives formation dates. Oldest Earth rocks are 4 Gy old, first Earth life 3.85 Gy ( 1 Gy = 1 gigayear = 1000,000,000 years ). Around 4 Gy ago rock was fractured and heated by intensive bombardment. Between 4 Gy and 1.4 Gy ago Mars was warmer and wetter; ground water saturated with CO 2 penetrated fractures and carbonate materials were deposited there. Living organisms may have assisted the formation of the carbonates and these carbonates may contain their remains. When the climate of Mars became cold and dry, liquid water disappeared from the surface leaving carbonate materials and “microfossils” (?) behind.

PHYS Meteorite ALH84001 Figs. Z22.10 & K kg, 17 cm across, found in Antarctica in Martian origin recognised in Gy old, compared with Mars’ 4.55 Gy. History: 15 to 16 million years ago, an asteroid impact blasted ALH84001 into a solar orbit. Its exposure time in space can be judged from cosmic ray pitting of the surface layers of the meteorite. It fell on Antarctica 13,000 years ago, as estimated from snow accumulation and ice flow rates. Its Martian origin was found from analysis of trapped gases - relating them to the composition of the Martian atmosphere.

PHYS Martian Meteorites - Life Evidence I Carbonate granules made of siderite (iron carbonate) and magnesite (magnesium carbonate) similar in size to bacterially formed carbonates on Earth. From measurements of carbon and oxygen isotopes in granules and surrounding rock, carbonates formed at 0 o to 80 o C - much hotter and things would have been more evenly mixed.OK for life. BUT Others say carbonates could have formed at much higher temp., even up to 700 o C depending on source of oxygen - too hot! If formed at low temperature there should be clay minerals present - not found. Meteorites from the asteroid belt also contain carbonates, but almost certainly never had life on them. Carbonate globules are NOT proof of life.

PHYS Martian Meteorites - Life Evidence II Magnetite Particles about 10 nanometres* long are found inside carbonate granules. Dark spots are magnetite (Fe 3 O 4 ) and iron sulphide. Similar in terms of chemical composition, crystal structure, size and shape, to those produced by Earth bacteria to act as compasses to align themselves with the Earth’s magnetic field. BUT Such grains can occur without the involvement of living organisms, except that magnetite and iron sulphide would not normally exist together. * 1 nm = m = one thousand millionth of a metre.

PHYS Martian Meteorites - Life Evidence III Polycyclic Aromatic Hydrocarbons (PAHs) resemble neither common terrestrial ones nor those found in other meteorites. They do resemble PAHs formed by the breakdown of organisms that once lived in ancient rock. BUT PAHs can form without the intervention of life and are found throughout the Solar System and in interstellar material. They could have formed from simpler compounds on Mars that never evolved chemically into living organisms. They could be Earth contaminants, but they are less abundant on the surface of the meteorite - the opposite to what one would expect if they had diffused in.

PHYS Martian Meteorites - Life Evidence IV Bacterial shapes: rod shaped objects about 10 nanometres long, similar to some kinds of micro-organisms found on Earth - could be fossils? BUT Most Earth micro-organisms are more than 500 nanometres long, but there do exist fossilised “nanobacteria” in young Earth rock which are similar in size and shape. No discovery yet of fossilised cell walls or cavities - crucial indicators of life, but have more work to do on ALH Some mineral grains including clays and asbestos have the same size and shape. Martian meteorite 1.3 Gy old discovered in Sahara in analysis in 1999 showed similar shapes - evidence for life throughout Mars’ history??

PHYS How Common Are Civilisations? Fermi Paradox : if the universe is so old, there must be extra-terrestrial civilisations around, but we have no evidence of any (unless they seeded the Earth with life!). Consider a civilisation beginning colonisation. Assume travel speed 1 / 10 th of the speed of light, colonies 10 light years apart and consolidation time 5000 years. All suitable planets in our galaxy, which is 120,000 light years across, can be colonised in 60 million years - about 0.4% of the age of our galaxy. Reasons for non-appearance? Violent conditions in the the galaxy’s past e.g. gamma-ray bursts, many supernovae civilisations destroy themselves before developing technology for interstellar travel they have no wish to colonise the galaxy a strong ethical code of non-intervention!