An Atmospheric Test of Cometary Panspermia What'sNEW
by N.C. WICKRAMASINGHE, F. HOYLE
and J.V. NARLIKAR
AbstractExperiments currently under way could settle once and for all the beleaguered question of the existence or otherwise of microbial life on comets. A program of research planned by Indian scientists under the auspices of the Indian Space Research Organisation (ISRO) seems set to preempt results that may be expected from NASA's Stardust Mission to Comet Wild-2.
A direct way to test the theory of panspermia is to examine a sample of cometary material under the microscope and search for cometary microorganisms (Hoyle and Wickramasinghe, 1981). Such thoughts are being currently voiced in the context of the launch of NASA's Stardust Mission (launch date February 6, 1999) as well as in relation to other space missions being discussed for the new millennium.
There seems to be a growing sense of optimism about possibly resolving a longstanding scientific question: Do comets carry the seeds of life in the Universe? If by "seeds" one means prebiotic chemicals, an affirmative answer is already at hand from the wealth of remote sensing data that is available for Comet Halley (Kissel and Krueger, 1987; Wickramasinghe, 1993). To get further affirmation of this restricted position from a comet sample return would seem an extravagant use of resources to say the least. Until a couple of years ago a more literal interpretation of the word "seeds" to include viable microorganisms was fraught with prejudice. All the early arguments and evidence that two of the present authors had presented for cometary microorganisms (e.g. Hoyle and Wickramasinghe, 1981) had been consistently ignored. This situation changed quite dramatically in 1996 when a claim of microfossils in a Martian meteorite (ALH84001) came to be discussed (McKay et al, 1996). Almost instantly investigations of panspermia came to be elevated to the status of legitimate scientific inquiry. Unfortunately, however, this long overdue change of attitude may have come too late to have had an influence in the planning of experiments connected with the Stardust Mission.
2. Stardust Mission
The main object of the Stardust Mission is to capture a sample of dust from the well-preserved Comet Wild-2 on January 2, 2004 and to return this material safely to Earth on January 15, 2006. The comet dust is to be captured in a "particle catcher" filled with aerogel, the lowest density material known to exist. The hope is that the aerogel would act as a soft landing cushion to slow particles from an initial relative speed of 6.1 km/s to rest fairly gently without significantly modifying original chemical structures.
At the time of planning the package of experiments that was to go on Stardust the concept of microbes on comets was still considered heretical and so "way out" as not to merit serious investigation. No experiment was explicitly planned to search for viable microoganisms, as far as we are aware. It is not clear that the integrity and viability of a bacterial spore, for example, would be preserved after a crash into the gel. In these circumstances one may still hope for the intervention of serendipity. Discoveries in the year 2006 might have a greater bearing on the question of cometary life than Stardust's planners expected.
3. Early Balloon Experiments
Historically the earliest experiments to search for microbes in the upper atmosphere using balloons were conducted in the early 1960's with the aim of determining the microbial content (if any) of the near space environment, presumably as a preparation for manned space flights. Although microbiological techniques at this time were primitive compared to what is available today there were already some dramatic indications of extraterrestrial microbes in air samples collected at altitudes of 30 km and above (Bruch, 1967; Lysenko, 1979). Positive detection of microorganisms at 130,000ft (39km) and a population density that increased with height pointed to a possible extraterrestrial source. However, the smallness of samples collected as well as uncertainties in experimental procedures did not lend much confidence to believe what was "found". In consequence this early program of work was not pursued beyond an initial stage.
4. Plans for Indian Balloon ExperimentA series of balloon experiments using the most modern microbiological techniques is being planned by the Indian Space Research Organisation (ISRO) and the Inter-Universities Centre for Astronomy and Astrophysics, Pune (IUCAA), with collaborative UK links in Cardiff. This program is explicitly directed to testing cometary panspermia at a minute fraction of the cost of the Stardust Mission. It has been known for several decades that cometary dust is present at our very doorstep, and all that is needed is to collect such dust non-destructively and without biological contamination. The sample collections are to be made using a balloon-borne cryogenic pump comprised of many sterilised chambers fitted with valves and cooled to liquid neon temperatures. When the valves are open at predetermined heights ambient air is sucked into the cryogenically cooled chambers. Such air samples, that would include cometary aerosols, are to be recovered and will be subject to careful chemical and microbial examination under contaminant free conditions.
The Principal Investigator of the project and overall co-ordinator is Professor J.V. Narlikar, Director of the Inter-University Centre for Astronomy and Astrophysics in Pune. He will be assisted in this task by Professor S. Ramadurai of the Tata Institute of Fundamantal Research in Mumbai. Other scientists in the team on the Indian side are as listed below:
Professor P. Rajaratnam, ISRO, Bangalore will direct operations relating to the Cryosampler experiment
Sterilisation techniques that are to be used in both sample retrieval and experimental preparation will be expected to achieve levels of microbial sterility that can essentially eliminate even the presence of single contaminant microorganism. At the same time the use of fluorescent dyes sensitive to membrane potential would permit the detection of single viable cells in the collected samples. Professor David Lloyd of Cardiff University has used this latter technique successfully in a number of other applications, and it is hoped that satisfactory results could be obtained in the present instance as well. Professor Lloyd will act as Project Director of Biological Sciences in the UK. Because isotope ratios (C, O, and H) will differ between extraterrestrial and terrestrial bacterial material, isotope analyses are to be carried out with a view to identifying extraterrestrial bacteria.
5. Cost of Project and Additional Support RequiredSeveral of the operational components of the project are already in place in various Indian Research Institutes. For instance a prototype cryogenic sampler has been recently used by the ISRO-PRL Group in their investigation of greenhouse gases in the stratosphere (Shyam Lal et al 1996), and cosmic ray physicists at the Tata Institute have used balloon-launching facilities over several decades. It is expected that the Indian Government will bear the major cost of the project, which is estimated at about £150,000. A contribution of £25,000 is being sought from UK sources to facilitate the purchase of experimental components that require foreign exchange, which is precious to the Indian Government. The Cardiff based part of the program also needs funds to about the same extent (£25,000), towards which a grant of US$8000 has been awarded. Further grants to fund the remainder will be sought from PPARC and NERC.
6. Estimates of Microbial Counts in Collected SamplesThere have been various estimates of the total input of cometary debris to the Earth, which is mainly in the form of microscopic dust. A plausible daily average flux is given to be F=500 metric tonnes (Chyba et al, 1990) or about 5x103 g/s. Of this let us suppose that a fraction x is in the form of cometary bacteria. Under steady state conditions the downward flux of such bacterial particles must balance the rate of infall from space. This gives an equilibrium number density N (per unit volume)
N » 1000 x/mvS litre-1 (1)
Where m is the average mass of a cometary microorganism, v is the average speed of fall through the stratosphere at say a height of 30km and S is the surface area of the Earth ~ 5x1018 cm2 . Earlier estimates of N assumed an average bacterial radius of 5x10-5 cm (mass ~ 10-12 g) and a corresponding value of v ~ 0.1cm/s (Narlikar et al, 1998). There is now growing evidence to suggest that microorganisms in a non-vegetative, nutrient-starved condition have significantly smaller sizes (see pictures in Pflug, 1984; Pflug and Heinz, 1998; Hoyle et al, 1985). A value of radius a » 10-5 cm would appear to be an appropriate average value, from which we get a mass m of ~ 10-14 g and a corresponding terminal velocity at 30km of v=0.01cm/s (Kasten, 1968). Together with F = 5x103 g/s we thus obtain from (1) a volume density
N » 10,000 x litre-1 (2)
(This number could be still higher if nanobacteria of average radii ~ 10-6 cm are considered to be an important component of the cometary microbial flora (Folk and Lynch, 1998). Increases of N by factors of over 1000 would be possible in this case.)
Although it is impossible to arrive at a fully reliable estimate for x , a value close to 0.01 could be justified. If one argues that the flux of organic dust in the outer coma of Comet Halley (as measured by space probes in 1986) is predominantly bacterial for particle masses of the order 10-14 g, the data of McDonnell et al (1986) could be interpreted to give a mass fraction of such particles of nearly 1%. Thus (2) yields N~ 1000 litre-1 . For an anticipated air sample equivalent to 50-100 litres at NTP the bacterial count according to these estimates could be as high as ~ 100,000. As noted earlier this would be well above the detection thresholds of the experiments being planned.
The above considerations are valid for the average population density of bacterial particles. The number N could be significantly higher on occasions when the Earth crosses major cometary meteor streams such as the Leonids. Over several days of such crossings the value of N could be enhanced by factors of the order of thousands or tens of thousands. If balloon flights and sample collections are scheduled for days coinciding with such enhancement, the statistical significance of the collected data could also be correspondingly enhanced.
7. Concluding RemarksWhilst it is now possible to deal effectively with most problems relating to equipment and sample purity that had proved difficult in the past, an outstanding problem to be resolved concerns the separation of extraterrestrial and terrestrial bacteria in the stratospheric collections. Several independent criteria could be used to show that stratosphere contains a mixture of two such distinct components. The microbial density profile with altitude when it is accurately determined could lead to an initial diagnostic showing a combination of infalling and outflowing components. This could show up for instance in a U-shaped density curve with a definite minimum occurring at some altitude.
Decisive evidence of an extraterrestrial bacterial component must, however, come from laboratory experiments. Microscopic studies may show distinctive morphologies that are either unknown or rare in a terrestrial environment. Biochemical studies including determinations of the D/L ratios of amino acid enantiomers could lead to further diagnostic and distinctive criteria being discovered. Isotopic analyses could also lead to decisive results in relation to the distribution of the C12 /C13 isotope ratio in terrestrial and extraterrestrial organisms.
A positive detection of cometary microorganisms and a vindication of panspermia theory would obviously have far reaching scientific consequences. The existence of extraterrestrial life and its relationship to terrestrial life, once established, would surely prove a fitting finale to an eventful century of science. It would inevitably open the doorway to new scientific vistas, which it would be the privilege of future generations to explore.
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