The Molecules that Fell to Earth

(Introduction)

The Molecules that Fell to Earth

NASA/JPL/Caltech

On 6 February, Nasa will launch a small, unremarkable spacecraft that could help to answer one of the big questions - how life began on Earth. Could we really be about to learn where we all came from, asks Jon Evans

Stardust may be a rather fanciful name, but for Nasa's forthcoming spacecraft (right), due to be launched next month, it is surprisingly apt; for part of Stardust's seven-year mission is to collect samples of the interstellar dust that fills the cosmos and that is believed to be, at least partly, made up of the remnants of stars. Supporters of the mission hope that this stardust could help to shed light on the secret of the origins of life.

Scott Sandford, astrophysicist and co-leader of the astrochemistry laboratory at Nasa's Ames Research Center in California, US, is one of the scientists who may prise open this secret. He has been heavily involved in the Stardust mission from the start, helping to develop and test the sample return equipment. He will also be one of a number of researchers analysing the collected samples once they are returned to Earth.

Their analysis should allow scientists to discover just what kind of complex molecules exist in space. This could confirm and put into context previous laboratory findings that complex organic molecules, even lipid membranes, could have formed in space, and might have 'seeded' the early Earth. The implications of the mission are huge and, as Sandford says, 'the pay-off is potentially enormous because even if we only find three such grains, it will be all three of the ones we have - all that science has to study'.


Littering the cosmos

Interstellar space is not actually empty, but is filled with astronomical debris. Supernovae in particular, but also stellar winds, comets and asteroids, all disperse material into the interstellar medium (ISM) as a result of explosions, collisions or the normal ejection of material. In the case of ejecta from a star, the nature of the material depends on the nucleosynthesis taking place in that star and on the nature of the ejection (supernovae or stellar winds). Nevertheless, in most stellar ejecta, there is an abundance of carbon, hydrogen and oxygen, as well as nitrogen, sulphur and phosphorus - in the form of carbon monoxide, oxides, carbon grains and even organic molecules such as cyanopolyacetylenes and polycyclic aromatic hydrocarbons (PAHs).

This debris floats through space, perhaps combining with the hydrogen and helium left over from the big bang to form dense molecular clouds, or sometimes just remaining as solitary molecules, thereby becoming part of what is known as the diffuse ISM.

Space is a pretty harsh environment, however, and many of the molecules in the diffuse ISM are quickly destroyed as a result of ultraviolet (UV) photolysis (in the case of gaseous molecules) and through erosion, vaporisation and shattering by interstellar shock waves (in the case of the solid grains). Dense molecular clouds provide greater protection to their constituent particles, enabling them to survive longer. In fact, in this environment the energy from cosmic shock waves and the UV light that radiates from background stars causes constructive reactions. And with molecules containing carbon, hydrogen, nitrogen, oxygen, sulphur and phosphorus mixing together in the cloud, the conditions are right for the creation of increasingly complex organic molecules.


Frozen solids

The temperature of dense molecular clouds (10-40K) is usually too low for chemical reactions to take place amongst those molecules in the gas phase. However, UV light can provide the energy needed to overcome the activation barriers, through the production of ions. Most of the gas-phase chemistry taking place in dense molecular clouds is, therefore, the result of ion-molecule reactions.

By using radio and infrared (IR) telescopes, astronomers have so far identified over 100 molecular species in dense molecular clouds and they think that there are bound to be many more (see Box 2). However, a great number of these species could not have been produced simply by gas-phase chemistry - other processes must also be involved.

Laboratory experiments have shown that another class of process involved would be gas-grain reaction. Dense molecular clouds are so cold that any gas hitting a grain of solid matter (such as the silicates and silicon carbide detected in interstellar dust) will freeze out of the gas phase onto the grain, forming an icy mantle. This freezing-out process is so efficient that the products of gas-phase reactions spend most of their lifetime as icy grain mantles and any species observed in the gas phase, including PAHs, should also be present in the grain mantles.

The grain provides a nucleus onto which those molecular species already synthesised in gas-phase reactions are brought into close contact with one another and react to form species that could not be created by gas-phase chemistry alone.

The same UV light that powers gas-phase reactions will also irradiate the constituents of the icy mantles, instigating further reactions. Laboratory experiments, including some undertaken by Sandford and his team, have shown that when the kinds of ices thought to exist in the dense molecular clouds undergo such irradiation, a whole host of new molecules is created.

Astronomers have identified two basic types of ice mantle that should exist in dense molecular clouds, their composition being dictated by the local H:H2 ratio (see Fig 1). Hydrogen is the major constituent of interstellar clouds, being three to four times more abundant than heavier elements such as carbon, nitrogen and oxygen.

Fig 1. Grain mantle growth and evolution in dense molecular clouds
(Source: Scott Sandford)

Polar ices are produced when the H:H2 ratio is greater than one and excess atomic hydrogen is free to react with other elements. The main component is water, but polar ices also contain methanol (which drives much of the interstellar ice photochemistry and gas phase chemistry), carbon monoxide, ammonia, carbon dioxide, formyl radicals, formaldehyde, carbonyl sulphide and dihydrogen. Non-polar ices are created when the H:H2 ratio is less than one and elements other than hydrogen are able to react together. So non-polar ices comprise O2, N2, CO and CO2, with minimal amounts of H2O. These two types of ice mantle may well exist simultaneously in different parts of the same cloud.

Not surprisingly, perhaps, the photochemistry in non-polar ices, which is driven by hot oxygen atoms liberated by photolysis of O2 or CO, produces no interesting products, at least in terms of organic molecules. The principal species produced are carbon dioxide, nitrous oxide, ozone, carbon trioxide, with some evidence for minor amounts of nitrogen oxide and nitrogen dioxide. Formyl radicals and formaldehyde may also be produced if there is some water in the ice.

Polar ice irradiation results in the destruction of some species (such as methanol) and the synthesis of others, such as dihydrogen, formyl radicals, formaldehyde, methane, carbon dioxide, and an as yet unidentified isonitrile, termed XCN. Of these products, only XCN synthesis specifically needs irradiation; all the others could also be, and probably are, produced by gas-phase or gas-grain chemistry.

Things really start to get interesting, however, at least in the laboratory, when the polar ices are warmed up. The additional energy allows new bonds to be formed as reactive species become mobile.

At a temperature of 200K, many of the parent species, as well as the new photoproducts, sublime out of the ice and, at this point, moderately complex organic molecules, such as ethanol (CH3CH2OH), formamide (HC(=O) NH2), acetamide (CH3C(=O)NH2) and nitriles and isonitriles (R-C(integral)N and R-N(integral)C), including XCN, are detectable spectroscopically.

At room temperature, species with even greater complexity are produced in the ice, such as hexamethylenetetramine (HMT, C6H12N4), polyoxymethylene-related species (POM, (-CH2O-)n), ketones (R-C(=O)-R) and amides (R-C(=O)NH2). On Earth many of these species are biologically important, with formaldehyde, nitriles and ethanol all identified as necessary precursors in the production of proteins, phospholipids, and RNA and DNA. Encouragingly, several of these complex organic molecules have recently been detected in clouds by IR telescopes.

Furthermore, experiments performed by Sandford and his co-researchers have shown that when HMT is hydrolysed in acid, amino acids are spontaneously produced (a simpler method for forming amino acids than the extended exposure of methane, ammonia and water vapour to an electrical discharge in the presence of excess hydrogen as described by Miller and Urey in 1953). And when the residue left over from warming polar ices to room temperature is placed in water, insoluble lipid-like droplets are formed, which show self-organising, membrane-forming behaviour.

This is what happens in the laboratory, but how might the molecular cloud be warmed in space? Well, short lived, transient heating may occur as a result of cosmic shock-waves or grain collisions. A more continuous source of heat might be provided by a protostar forming as the dense molecular cloud undergoes gravitational collapse.


Evolving clouds

Dense molecular clouds are transient astronomical features; they exist for between one million and 100 million years - not long at all on cosmic timescales. Astronomers believe that sections of the clouds eventually collapse in on themselves under gravity, to form a star or stars. However, for a certain period of time in its early life, a star will be a protostar embedded in the dense molecular cloud. The protostar warms the cloud and may instigate the formation of the kind of complex organic molecules created by Scott Sandford in the laboratory.

Interestingly, the XCN feature has been detected in protostellar environments, but not in molecular clouds, indicating that it is in these protostellar environments that the complex organic molecules are to be found.

 
Dense molecular clouds photographed by the Hubble space telescope

Emptier space

Although the diffuse ISM is a much less forgiving environment than that found within dense molecular clouds, there are still several organic molecular species able to tough it out. An example of these is the PAHs, which are very stable organic molecules made up of only carbon and hydrogen, with each carbon having three neighbouring atoms, much like graphite. PAHs are relatively large and have a range of vibrational modes, which explains their ability to survive in harsh radiation fields that destroy most other molecules.

These molecules are very common on Earth, being standard combustion products. Some PAHs are also highly potent carcinogens, and therefore biologically interesting; they have the ability to interfere with DNA.

Other organic molecules in the diffuse ISM include aliphatic hydrocarbons and microdiamonds. Not surprisingly, all these species are also found in dense molecular clouds.


Forming the planets

There does, then, seem to be convincing evidence that biologically- interesting molecules exist throughout the cosmos. The question being raised now is whether these molecules could have seeded the Earth and provided an impetus in the development of life. Specifically, while there may very well be complex organic molecules floating about all over the ISM, how could they have safely travelled to, and arrived on, Earth?

 
Fig 2. The solar system (not to scale)

As indicated earlier, dense molecular clouds don't remain as clouds for very long and when a star eventually forms within the cloud, so may an accompanying solar system, although astronomers still don't know how common it is for the latter to occur. Nevertheless, theory states that once a star has been created within the cloud, the rest of the cloud begins to orbit the star, eventually forming a flat, planar mass, known as an accretion disc. Because the constituent particles in the disc are much closer together than in the dense cloud, they begin to agglomerate, forming larger and larger particles. This is the process that eventually creates the planets.

It is also one of the ways that biologically-active species could have found their way to Earth; by helping to form it. Not surprisingly, most astronomers consider that the extremes of temperature and pressure involved in the formation of a planet such as Earth would quickly destroy any complex structure that a molecule may possess. The molecules would simply add to the elemental composition of the planet.

This is not always the case, however. There is a temperature gradient within the accretion disc, depending on the distance from the central star. During the formation of our solar system, for particles in the accretion disc that were closer to the sun than 5AU (about where Jupiter is now), the heat would have caused the icy mantles to sublime away from the grain particles. According to theory, the remaining solid grains then combined to form the rocky planets of Mercury, Venus, the Earth and Mars, as well as the 1020 asteroids that today form the belt between Mars and Jupiter.

This process would have destroyed most complex organics, but some of the tougher organic molecules, such as microdiamonds, aliphatic hydrocarbons and PAHs, have been identified in meteorites that have landed on Earth. Even more intriguing is the fact that the PAHs detected are deuterium-enriched. Astronomers theorise that PAHs will become so enriched because of unimolecular photodissociation in the ISM, by which a UV photon breaks a carbon-hydrogen bond, the hydrogen atom then being replaced by either hydrogen or deuterium. The resulting carbon-deuterium bond is not as easily broken by a UV photon and, because hydrogen and deuterium are both available, interstellar PAHs should gradually become deuterium- enriched. The detection of deuterium-enriched PAHs in meteorites thus suggests an interstellar heritage.


Seeding the Earth

Further away from the sun (more than 5AU), the environment was not so harsh and the icy mantles would not have sublimed away. These larger particles formed agglomerations larger and faster than the grains nearer to the sun. Thus, the snowball-like cores of the 'gas giants' Jupiter, Saturn, Uranus and Neptune formed more quickly than the masses of the rocky planets. When the cores became sufficiently large to capture the gas in the accretion disc, the gas giants were formed. An offshoot of this process was the development of a huge number of smaller, snowball- like planetesimals - the comets - which came to inhabit the Kuiper belt (orbiting just beyond Neptune) and the Oort cloud (a spherical collection of comets that extends to about 50000AU from the sun).

Because the accretion process would have been much less severe at the distances where comets formed, scientists hope that any complex organic molecules may have been able to survive incorporation into a comet intact. Tantalising glimpses have been obtained by studying passing comets, such as Halley and, more recently, Hale-Bopp, using radio and IR telescopes.

Almost all the parent molecules detected in dense molecular clouds have been identified in these comets, including carbon dioxide, water and carbon monoxide. However, it is the spectra for methanol and cyanide (-CN) detected in comets that are most important in terms of complex organic molecules. Methanol is a precursor for many of the biologically-relevant molecules thought to reside in the interstellar clouds, while the CN could well be related to the XCN feature detected in dense molecular clouds. Experiments have already shown that the ultraviolet photolysis of HMT frozen in H2O ice produces this XCN band.

The evidence so far does support, albeit tentatively, the idea that comets may contain complex, biologically-relevant molecules, like HMT. But scientists still don't know this for sure, and cannot prove it, until they have a bit of comet to study in the laboratory. HMT would be a very interesting molecule to discover in a comet: a whole host of biogenically interesting species are produced along its synthesis pathway and by its photolysis, hydrolysis and thermolysis (see Fig 3).

 
Fig 3. The proposed hexamethylenetetramine (HMT) interstellar ice photochemical synthesis route and decomposition products
(Source: Scott Sandford)

But assuming that comets do contain biologically-interesting molecules, there are two ways in which these molecules could arrive on Earth. The first, and most dramatic, way is by crashing into it.

At the dawn of the solar system, comets are thought to have formed in the region of the gas giants, but over time their orbits are believed to have been perturbed by the gas giants such that they either left the solar system altogether, became part of the Kuiper belt or Oort cloud, or were sent careering into the inner solar system, occasionally colliding with one of the forming planets. For the accreting Earth this period of heavy bombardment, as it is known, would have lasted for the first 1000m years of its existence (roughly 4500m-3500m years ago).

Most scientists now accept that it was this cometary bombardment that gave the Earth its atmosphere and water. What is more contentious is the suggestion that it also provided the organic material needed to kick- start life on Earth.

The alternative, and maybe ancillary, way for cometary material to reach the Earth is in the form of cometary dust. As comets come closer to the sun, ice sublimes away, forming the distinctive tail. This process leaves a trail of dust in the comet's wake. If the Earth's orbit then passes through that dust, particles may reach the Earth's surface.

These dust particles are so small that they are unlikely to be heated too much by collision with the Earth's atmosphere - indeed, samples of interplanetary dust (from both asteroids and comets) have been collected from the upper atmosphere and shown to contain PAHs.

PAHs, although biologically-interesting, are only a small part of the story; a lot more than one species of organic molecule would be needed to initiate life. Thus comets, potentially containing a whole host of 'prebiotic' molecular species, could be a major part of the story. But to determine how much of part would require a comet sample. As Sandford says, 'if you want to really understand in detail the composition of something, and how things are in relation to each other, nothing beats having a sample in hand.' Getting hold of a sample is what Nasa hopes Stardust will do.


Trawling the cosmos

Stardust's mission is to collect samples of both interstellar dust and cometary ejecta. The mission will be launched on 6 February and will last for just under seven years, during which time there will be two periods of interstellar dust collection (from March to May 2000, and July to December 2002) and an encounter with comet Wild 2 on 2 January 2004. Stardust is expected to return to Earth on 15 January 2006.

   
  Interstellar gas and dust in M16, the Eagle Nebula

The spacecraft will fly to within 100km of Wild 2 at a speed of about 6.1km s-1. It will collect samples by exposing sheets of aerogel to the flow of cometary and interstellar particles. Aerogel is a low-density, silica-based material composed of individual features only a few nanometres in size, linked in a highly porous dendritic structure.

Because many organic molecules are very sensitive, some will probably not survive the collection process, so an on-board mass spectrometer will also provide compositional information. However, Nasa has stated that it will make the strongest effort possible to preserve organic components in an analysable form.

Those organic samples that are returned safely will be analysed by Sandford and others, using a range of analytical techniques, including mass spectrometry, gas chromatography, chemical reaction analysis, Auger electron analysis and infrared spectroscopy.


Bringer of life

So what are the implications should Sandford discover the kind of organic molecules in the cometary samples that have been created in the laboratory? For his part, Sandford is not really interested in the implications. 'In our lab we don't really have an attitude about [it], we're interested in discovering what kind of things were dropped on the early Earth and then we'd like to know what, if any, roles that material could have played', he says.

But if Sandford does discover convincing evidence that complex organic molecules, like HMT and the kind of lipid membranes he created in his laboratory, would have been dropped on the early Earth, then it is fair bet that they were somehow involved in the origin of life. The level of that involvement will still be open to interpretation, however. For instance, David Williams, professor of astronomy at University College, London, told Chemistry in Britain that even though the creation of complex organic molecules in space sounds reasonable, it seems even more likely to him that the prebiotic molecules responsible for life developed on Earth. The interstellar molecules may simply have acted as foodstuff for early organisms.

Other, more extreme, possibilities also exist. Sandford has speculated that the inside of a comet - with its potential cargo of prebiotic molecules and lipid membranes, together with the periodic warming it receives as it orbits the sun - might well be the ideal environment for the creation of life, or at least the beginnings of life.

The truth, unfortunately, may not be so clear cut. As Sandford says, 'life is a fairly complex thing, even in simple organisms, and it's quite possible that for life to get started it had to beg, borrow and steal compounds from anywhere it could get them'.

Further reading

Source: Chembytes E-zine, January 1999


Box 1

(Box) 1. Glossary of terms

Asteroid a rocky planetesimal (tiny planet) that orbits in a belt between Mars and Jupiter; probably the remnant of a larger object that was destroyed as a result of multiple collisions.
Astronomical unit (AU) a unit of distance equal to that from the sun to the Earth.
Big Bang the theoretical beginning of the universe, in which hydrogen and helium were synthesised.
Comet a 'snowball-like' planetesimal that formed in the outer solar system from an agglomeration of icy grains.
Dense molecular clouds a mixture of hydrogen, helium and the ejecta from supernovae and stellar winds. They are between 10 and 100 light-years in diameter and the densest clouds contain millions of particles per cm3.
Galaxy an autonomous collection of stars, gas and dust. There are thought to be about a thousand million galaxies in the observable universe.
Interstellar dust a component of the interstellar medium (see left), includes polycyclic aromatic hydrocarbon (PAH) molecules, silicates, graphite and microdiamonds.
Interstellar medium (ISM) the gaseous and dusty matter permeating interstellar space, which forms one-tenth of the mass of our galaxy.
Stellar wind outflow of material from the atmosphere of a star.
Supernova the final explosion of a massive star, which for several months shines brighter than a whole galaxy. As a result of this process most of the elements in the Periodic Table are synthesised and dispersed into the interstellar medium.


Box 2

(Box) 2. Tools of the trade

Astronomers' knowledge of the interstellar medium (ISM) is almost entirely the result of observations with radio and infrared (IR) telescopes. These devices are the only means that astronomers have of probing the depths of space, and of determining the molecular species and chemical processes that might occur there.

Both radio and IR telescopes detect the narrow electromagnetic frequency bands produced by the rotational transitions of molecular bonds of species in the ISM. Different molecules with different chemical bonds produce characteristic frequency band spectra; by matching those spectra with examples from known molecular species studied in the laboratory, identification can often be achieved.

Radio telescopes can only detect transitions involving a rotating dipole moment and so they are restricted to studying gaseous molecules. IR telescopes, meanwhile, can detect the vibrations of virtually all the molecules in the ISM, both gaseous and solid, although they are mostly used for detecting solids.

Dense molecular clouds are usually studied using IR telescopes and most of the infrared spectral features are due to the absorption (as opposed to emission) of infrared energy at specific frequencies by the constituent molecules of the cloud. The infrared source can either be a combination of an embedded protostar and the dust heated by that star, or a background star behind the cloud.

IR astronomical techniques do have their limitations and the data generated are by no means unequivocal. For instance, it is difficult to distinguish between molecules with similar molecular bonds, and thus molecular identifications based on only a couple of observed spectral features are often uncertain. In addition, any spectra produced from the study of a molecular cloud are actually an amalgamation of all absorptions along the line of sight from the infrared source. It is therefore difficult to assess whether the constituents identified within the cloud are in separate, distinct groups or all mixed together; and it is also not always clear which spectral features are the result of the cloud and which are intrinsic to the source.


Box 3

(Box) 3. Could it have happened twice?

Earth may not have been the only beneficiary of cosmic deliveries of organic materials, Mars may have been equally blessed. That Mars was bombarded with comets and meteorites is not in doubt; the scars, in the form of impact craters, are clearly evident. But exactly what those foreign bodies may have brought down with them has a bearing on the arguments raging over the most famous meteorite to have landed on Earth - ALH84001, the Martian meteorite that may or may not contain the remnants of alien life.

The battle over this small chunk of rock has been fought for more than two years (Chem. Br., September 1996, p20 and July 1997, p17), with much of the original evidence now looking far from convincing; for instance,supposed fossils of primitive microorganisms could result from crystallisation of shock-melted rock. Scott Sandford, co-leader of Nasa's astrochemistry laboratory and one of the expedition members that found ALH84001 in Antarctica, says that even he isn't convinced by the evidence.

The discovery of PAHs in the meteorite was also claimed as proof of the existence of life on Mars, with Nasa researchers arguing that these were the remnants of Martian biological activity. This conclusion has been heavily contested, with Luann Becker from Scripps Institute of Oceanography, California, US, suggesting that most of the PAHs in ALH84001 are due to terrestrial contamination by ice melt water. Becker also argued that if a small percentage of the PAHs are of Martian origin then they were simply brought to Mars by meteorites or interplanetary dust.

However, new research led by Simon Clemett of Stanford University, California, US, and reported in Faraday Discussions (July 1998, 109, 417), found no evidence of terrestrial contamination and concluded that all the PAHs in ALH84001 were probably of extraterrestrial origin. While these findings don't exactly support the notion of a biological basis for the PAHs, they do give credence to the idea that complex organic molecules fell on Mars. Whether, at some point, they developed into life is still entirely open to question.


http://www.biology-online.org/articles/molecules_fell_earth/introduction.html