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Simulating Low Temperature Photochemistry in Space - The Photoproduction of Organic Residues in Laboratory Interstellar Ice Analogs

New stars and planets form in dense interstellar molecular clouds. These clouds are sufficiently dense to screen out outside starlight and the dust grains in them can get as cold as 10 K (-263 oC). At these temperatures, most gas phase materials other than hydrogen, helium, and neon, will freeze out onto these cold dust grains, much as the water vapor in your breath freezes onto a cold window. As a result, most of the grains in dense clouds are coated with a mantle of ice that contains a variety of molecules.

Figure 1: M16.

Dense clouds are the birthsites of new stars and planetary systems.

The composition of these ices varies with local dense cloud conditions, but infrared spectra taken from these clouds indicates that the ices are generally dominated by H2O, the same material in ice cube trays in our refrigerators. However, these ices often also contain a host of other simple molecules, including methanol (CH3OH), carbon monoxide (CO), carbon dioxide (CO2), ammonia (NH3), methane (CH4), formaldehyde (H2CO), and so on.

While these ices exist in interstellar dense clouds, they are exposed to various forms of high energy radiation in the form of cosmic rays (high speed particles) and ultraviolet (UV) photons. This radiation can break many of the chemical bonds of the molecules in the ices, resulting in the production of ions, radicals, and other highly reactive chemical species. These can react within the ice, either immediately or later when the ice is warmed (say, by a star forming nearby), to form new chemical compounds that did not previously exist in the ice. Similar processes likely occur in exposed ices in our own Solar System, for example in the surfaces of comets and icy bodies in the outer Solar System.

Figure 2: The temperatures in interstellar dense clouds are low enough that most gas phase molecules freeze into ice mantles on dust grains.

High energy radiation can convert some of these icy materials into more complex organic compounds Slim Films.com.

We can simulate these chemical processes in our laboratory by mimicking the conditions of these astrophysical environments, namely, low temperatures, high vacuum, and high radiation. We do this by using cryo-vacuum systems of the type shown in the picture below. Each of these units consists of a vacuum system capable of attaining pressures as low as 10-8 millibars in a sample chamber. Suspended in the sample chamber of each apparatus is a sample head that can be cooled to temperatures of 4-15 K (depending on the system). Mixed gases can be sprayed onto the cold head to produce ices of the desired composition. These ices can then be irradiated with high energy photons from a variety of different lamps. We frequently use hydrogen plasma lamps that produce UV photon in the Lyman α range.

Figure 3: An example of one of the cryo-vacuum systems in which we irradiate astrophysical ice analogs.

Figure 4: A schematic of a sample chamber.

The sample chambers of these systems are equipped with windows that allow us to use UV, visible, and infrared spectrometers to study the ices before, during, and after irradiation, and during any subsequent warming and sublimation of the ice.

When these ices are warmed after irradiation, the original volatile materials sublime away into the gas phase, leaving behind an organic residue on the sample head. This residue consists of a host of more complex chemical species created by the irradiation process. These new materials can then be removed from the system and examined using a host of analytical techniques (HPLC, GC-MS, L2MS, XANES, etc.).

Figure 5: An example of an organic residue resulting from the UV irradiation of a simple astrophysical ice analog.

The composition of the organic residues produced when astrophysically-relevant mixed molecular ices are exposed to high energy radiation is not yet fully understood. Even in simple ices that contain only H2O, CH3OH, and NH3, the mass spectra of the residues produced upon irradiation show the presence of hundreds (probably thousands) of new compounds.

Many of these new species are simple, volatile molecules like methane (CH4), carbon dioxide (CO2), ethanol (CH3CH2OH), formamide, acetamide, ketones, and alcohols. However, many of the new molecules that are produced are more complex and remain on the sample head at room temperature, long after the more volatile starting materials are gone. A significant fraction of these more complex compounds have been identified in the residues, including a number of species that have potential importance for astrobiology and the issue of the origin of life. Some of the compounds of astrobiological significance that have been identified in our ice photolysis residues include:

  • Amino acids (the building blocks of proteins)
  • Amphiphiles (the building blocks of membranes)
  • Polyoxytheylene (POM) and related compounds
  • Hexamethylenetetramine (HMT; precursor reactant to make amino acids, cyanides, etc.)

In addition, if polycyclic aromatic hydrocarbons (PAHs) and related N-containing aromatic compounds are present in the ices, we also see the formation of:

  • Quinones (oxidized aromatic molecules used for a host of processes in biochemistry)
  • Hn-PAHs (PAHs with extra peripheral H atoms) (***same link as Quinones***)
  • Nucleobases (the building blocks of RNA and DNA)

You can click on the links to learn more about each of these specific types of compounds we find in our residues.

The production of prebiotic molecules in the interstellar medium is of little consequence to the origin of, and search for, life unless these molecules can be delivered intact to habitable planets where they can potential play some role in getting life started. This requires that these molecules survive the transition from the dense cloud into the protostellar nebula of a forming star, followed by subsequent incorporation into planetesimals that deliver these materials to a planetary surface.

The presence of excess deuterium (heavy hydrogen), an indicator of the presence of extraterrestrial materials, in certain molecules in meteorites and cometary and asteroidal dust particles demonstrate that some interstellar organic species do, in fact, survive planetary accretion, and are delivered to the Earth’s surface. Indeed, meteorites are known to be currently be delivering extraterrestrial organics in the form of amino acids, nucleobases, and sugars to the Earth.

Figure 6: Extraterrestrial materials, including organics, some of which have an interstellar heritage, are delivered to the surface of planets via meteorites and cosmic dust.

The observation that the irradiation of astrophysically-relevant ices results in the abiotic (non-biological) production of a number of compounds of biological significance, many of which are seen in meteorites, has important astrobiological implications. First, it suggests that the early Earth was almost certainly seeded with these types of compounds and that these materials would likely have been available to play key roles in the origin of life on our planet. Second, since the irradiation of ices is a process that is expected to occur universally in all interstellar dense molecular clouds, where new stars and planets form, these types of materials should be present at the birth of most planetary systems. Thus, insofar as these materials may have played an important role in the formation of life on the Earth, they would be expected to be present to play a similar role on any newly formed planets that have conditions conducive to the formation and evolution of life. The ubiquity of these starting materials suggests life may not be uncommon elsewhere in the Universe.

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