Updating Comets and Magic Universe
Did comets spark life on Earth?
Part 3 Initiating biochemical action
Pascale Ehrenfreund rides again (as in Part 2) in the story in Magic Universe called “Life’s origin: will the answer to the riddle come from outer space?”. But please focus first on Wlodzimierz Lugowsky.
‘I can trace my ancestry back to a protoplasmal primordial atomic globule,’ boasts Pooh-Bah in The Mikado. When Gilbert and Sullivan wrote their comic opera in 1885 they were au courant with science as well as snobbery. A century later, molecular biologists had traced the genetic mutations, and constructed a single family tree for all the world’s organisms that stretched back 4 billion years ago, to when life on Earth probably began. But they were scarcely wiser than Pooh-Bah about the precise nature of the primordial protoplasm.
In 1995 Wlodzimierz Lugowsky of Poland’s Institute of Philosophy and Sociology wrote about ‘the philosophical foundations of protobiology’. He listed nearly 150 scenarios then on offer for the origin of life and, with a possible single exception to be mentioned later, he judged none of them to be satisfactory. Here is one of the top conundrums for 21st Century science. The origin of life ranks with the question of what initiated the Big Bang, as an embarrassing lacuna in the attempt by scientists to explain our existence in the cosmos.
After discussing possible “home cooking” of life by hypercycles, RNA catalysis or lipid catalysis, and touching on the possibility of false starts, the tale turns back to the sky in pursuit of the only hypothesis acceptable to Lugowsky.
… astrochemists took the view that many materials directly useful for starting life arrived ready-made from space. They would have come during the heavy bombardment, when comets filled the sky. Even from those that missed the Earth entirely, huge quantities of carbon compounds would have rained gently to the primordial surface in the form of small grains strewn from the comets’ tails.
Are we children of the comets?
Whether it was a joke or a serious effort to deceive, no one knows. Someone took a piece of a meteorite that fell from the sky at Orgueil near Toulouse in 1864, and stuck lumps of coal and pieces of reed on it. The jest flopped. It went unnoticed for a hundred years, because there were plenty of other fragments of that meteorite to examine. In 1964, Edward Anders and his colleagues at Chicago disclosed the hoax in a forensic examination that identified even the 19th-Century French glue.
In reality the Orgueil meteorite had a far more interesting story to tell. A 55-kilogram piece at France’s Muséum National d’Histoire Naturelle became the most precious meteorite in the collection. It contains bona fide extraterrestrial tar still being quarried in the 21st Century, with ever more refined analytical techniques, for carbon compounds of various kinds that came from outer space and survived the heat and blast of the meteorite’s impact.
Rapid advances in astrochemistry in the closing decades of the 20th Century led to the identification of huge quantities of carbon compounds, of many different kinds, in cosmic space and in the Solar System. They showed up in the vicinity of stars, in interstellar clouds, and in comets, and they included many compounds with rings of carbon atoms, of kinds favoured by living things.
Much of the preliminary assembly of atoms into molecules useful for life may have gone on in space. Comets provide an obvious means of delivering them to the Earth. Confirmation that delicate carbon compounds can arrive at the planet’s surface, without total degradation on the way down, comes from the Orgueil meteorite. In 2001, after a Dutch-US re-examination of the Paris specimen, the scientists proposed that this lump from the sky was a piece of a comet.
‘To trace our molecular ancestors in detail is now a challenge in astronomy, space research and meteoritics,’ said the leader of that study, Pascale Ehrenfreund of Leiden Observatory. ‘Chemistry in cosmic space, proceeding over millions of years, may have been very effective in preparing useful and reactive compounds of the kinds required for life. Together with compounds formed on the Earth, those extraterrestrial molecules could have helped to jump-start life.’
Comets now figure in such a wide range of theories about life’s origin, that a checklist may be appropriate. The mainstream view in the late 20th Century was that, when comets and comet tails delivered huge quantities of loose carbon-rich material to the Earth’s primordial soup, its precise chemical forms were unimportant. In Ehrenfreund’s interpretation the molecules did matter, and may have influenced the direction of subsequent chemistry on the Earth.
Quite different scenarios included the proposal that comets might be vehicles on which spores of bacteria could hitchhike from one star system to another, or skip between planets. Or, as Hoyle suggested, the comets might themselves be the scene of biochemical action, creating new life aboard them. Finally, according to a German hypothesis, comet grains may have directly mothered living cells on the Earth.
In 1986, Jochen Kissel analysed the dust of Halley’s Comet with three instruments, carried in the spacecraft that intercepted it most closely, the Soviet Vega-1 and Vega-2, and Europe’s Giotto. He found grains containing an astonishing mixture of carbon compounds that would be highly reactive on the Earth. After analysing the results, Kissel and his colleague Franz Krueger, an independent chemist in Darmstadt, promptly proposed that life began with comet grains falling into the sea.
Following 15 years of further work on the hypothesis, they saw no reason to change their minds. Theirs was the only scenario among 150 that won approval from Wlodzimierz Lugowsky in 1995. Beside the carbon-rich component of comet grains, possessing the raw materials and latent chemical energy needed to drive the chemistry, Kissel and Krueger stressed the part played by mineral constituents. These provided surfaces with catalytic properties, to get the reactions started.
‘What impresses us is that the carbon compounds in comets are in an ideal chemical state to react vigorously with water,’ said Kissel at the Max-Planck-Institut für extraterrestrische Physik. ‘Also, the grains they come in are of just the right size to act as temporary cells, keeping the materials together while the crucial chemical reactions proceed. So our recipe for life is rather simple: add comet dust grains to water.’
The recipe book
For an example of how materials present in comets could make key biochemicals, here is one of the recipes suggested by Kissel and Krueger. React five molecules of hydrogen cyanide together and that gives you the ring molecule called adenine. Take polyacetylene, a carbon chain depleted in hydrogen, and its reaction with water can make the sugar called ribose. When metal phosphides in comet dust meet water they will make phosphate. Adenine plus ribose plus phosphate combine to form one of the units in the chain of an RNA molecule. As a by-product, adenine also figures in a vital energy-carrying molecule, adenosine triphosphate.
Kissel and Krueger did not dissent from the view that life began more than once. Indeed with so many comets and comet grains descending on the young Earth, it could have happened billions of times. That gave plenty of scope for biochemical experimentation, for survival amidst later impacts, and for competition between different lineages.
Two new space missions to comets would carry Kissel’s instruments for further investigation of the primordial dust grains that they contain. Stardust, launched in 1999, was an American spacecraft intended to gather samples from the dust around Comet Wild and eventually return them to the Earth, where they could be analysed thoroughly in laboratories. Analysis on the spot, but with ample time, was the aim in Europe’s Rosetta (2003). Kissel’s dust analyser is one of many instruments on Rosetta intended to reveal a comet’s constitution in unprecedented detail, while the spacecraft slowly orbits around Comet Wirtanen for 17 months.
By 2014, when the Rosetta mission comes to a climax as Comet Wirtanen makes its closest approach to the Sun, the Cassini-Huygens mission to Saturn and Titan will be long-since concluded and the results from Stardust and Comet Wild will be in. Meanwhile new infrared and radio telescopes, on the ground and in space, will have added greatly to the inventory of chemicals in the cosmos, available for the recipe book. That may be a time to judge whether the switch to space has paid off, in the search for a solution to the mystery of life, and whether Pascale Ehrenfreund was right to look for her molecular ancestors in interstellar space.
For updating this story in Magic Universe the first points concern space missions to comets. Since ESA’s Giotto went on from Halley’s Comet in 1986 to Comet Grigg-Skjellerup in 1992, extended missions for comet chasers have become habitual.
- NASA’s Deep Impact must be added to the earlier chronicle, carrying an impactor with which Comet Tempel 1 collided in 2005 (this mission was mentioned in the “Comets and asteroids” story in Magic Universe, but not in “Life’s Origin” quoted above)
- NASA’s EPOXI mission recycles the Deep Impact spacecraft to observe Comet Hartley 2 during a close flyby in November 2010
- NASA’s Stardust-NExT mission recycles the Stardust spacecraft for a close inspection of Deep Impact ‘s crater on Comet Tempel 1 in February 2010
- ESA’s Rosetta now targets Comet Churyumov-Gerasimenko, not Comet Wirtanen, in 2014
As for my account of the Kissel-Krueger hypothesis – that comet grains fell in the sea and reacted with water to make the first cells – I wish now that it had included a clearer explanation of the reactive state of the raw materials. Life relies on cool but concentrated chemical energy, like that supplied nowadays by sunlight or food. The cosmic carbon compounds found in comets are chemically hungry, and ready to react energetically with water and other materials that may happen to be around. They could thus provide concentrated energy of a cool, life-friendly kind.
But can I still justify putting my reporter’s bet so heavily on the Kissel-Krueger hypothesis? To be frank, it seems to have dropped off the astrobiological radar in recent years, but Jochen Kissel stands by it. He comments to me now, “If one doesn’t keep showing up at conferences, things tend to get forgotten.”
Although formally retired, Kissel is still active in space projects, including StardustNExT and EPOXI. When, with Franz Krueger, Kissel reported the detection of a few PAH’s by an onboard mass spectrometer during the Stardust misison’s cruise in interplanetary space, he provocatively called the molecules “Possible seeds of life on Earth”. The circumstances led them to label the PAHs “interstellar” molecules, which provokes the thought that grains from interstellar space might short-circuit the building and dismantling of comets and deliver the kiss of life directly to the Earth. A word of caution, though, from Kissel: “Well, the spacecraft pointed us in the direction from where interstellar dust is expected to enter the solar system. Even if we see dust there, it is no proof that the dust is really interstellar, since we cannot measure speed and direction of individual impacts.”
Be that as it may, as author of Magic Universe, I stand by the Kissel-Krueger hypothesis too, in the sense that I see nothing that needs updating, except perhaps to note advances in the chemistry. The versatility of the ready-made aromatic materials of the PAHs, extolled by Ehrenfreund in Part 2, could only facilitate the Kissel-Krueger process. And Kissel remarks, “Since we published our scenario much more knowledge has become available especially in terms of nanochemistry – i.e. in ‘directed catalysis’ using chemistry on very small-scale surfaces.”
Certainly no other old or new hypothesis for life’s origin has won the day, nor satisfied the Lugowsky criteria for a satisfactory scenario. The way of driving complex chemistry fast enough to beat the dilution problem remains Kissel and Krueger’s trump card. “I still think,” Kissel assures me (2010), “that our scenario is the only one with a realistic thermodynamical background.”
Given that the origin of life remains the biggest gap in the entire explanatory chain from the Big Bang to our existence as human beings, it’s exasperating that this story seems rather neglected. Also that any real progress still depends on instruments on distant spacecraft. Among the microscopic samples brought home by Stardust, anything delicately carbonaceous had been smashed by the high-speed collection process.
When a bigger harvest of material, scooped from beneath a comet’s surface, is eventually delivered vacuum-packed and undamaged to the Earth, someone will drop a few grains into a flask of water and look for the speedy biochemistry. ESA’s Rosetta was originally conceived as a sample-return mission jointly with NASA. Alas, NASA pulled out and ESA had to “descope” Rosetta – unwittingly abandoning to a later generation of chemists the task of verifying or falsifying the Kissel-Krueger hypothesis by experiment.
You might wonder, why not just test those carbon-rich micrometeorites from Antarctica, which provoked my initial post (Did comets spark life on Earth? – Part 1)? They won’t work. Although Jean Duprat and his colleagues went to great pains to obtain their UCAMMs [ultra-carbonaceous Antarctic micrometeorites] from deep-lying and very clean snow, the material lost its extra-terrestrial chemical pizzazz long ago, by exposure to the air.
For Krueger and Kissel on interstellar dust: see