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Our research group is part of the Astrochemistry
Laboratory in the Solar System Exploration Division at NASA's Goddard
Space Flight Center, and is a member of the Goddard Center for Astrobiology. We
specialize in studying the spectra, the chemistry, and the
physical properties of ices relevant to comets, icy satellites and planets, and the
coatings of dust grains in the interstellar medium. Although many cosmic ices are dominated by
H2O, they also contain "prebiotic" molecules such as CO, CO2, CH4,
NH3, and CH3OH. In studying these molecules we are probing the early, ancient
chemistry which eventually led to the origin of life.
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In our laboratory we prepare ices by using a cryostat to
condense gas-phase mixtures to temperatures as low as 10 K. The ices
are made in a vacuum system to simulate the low pressure of outer space, and also to avoid
undesirable contamination from the Earth's atmosphere. An infrared spectrometer
is used to record spectra of an ice during experiments.
Our laboratory set-up is unusual because it is interfaced not only to a
Van de Graaff accelerator, that can produce protons at
energies up to about 1 million electron volts (1 MeV), but also to
a hydrogen-discharge lamp that supplies ultraviolet (UV) photons
(energy ~10 eV). The high-energy protons simulate the magnetospheric
or cosmic ray radiation exposure expected for planetary, cometary, and interstellar
ices, while the UV photons from our lamp simulate the Solar or interstellar UV field.
When either the protons or UV photons strike an ice sample they produce ionizations
and excitations, resulting in chemical reactions
to make new molecules. By comparing infrared spectra taken before
and after this processing of an ice, we can identify molecules formed
by either radiolysis or photolysis.
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Comet Hyakutake
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The Eagle Nebula is an interstellar
cloud of gas and dust. |
Ices in interstellar environments often are accompanied by dust
grains. With comets, both dust grains and gases subliming from ices
are observed. In the Cosmic Ice Laboratory we use silicate
materials to simulate cometary, and other, grains. Ices formed on these
silicates are used to investigate ice-grain interactions.
When warmed to room temperature, many irradiated icy mixtures
leave an organic residue, which can be analyzed by chromatography and mass
spectrometry. We used these methods to show that
hexamethylenetetramine (HMT) is synthesized when H2O + CO + NH3 + CH3OH
mixtures are processed with either photons or protons. HMT is known
to produce amino acids if hydrolyzed.
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Results from our
experiments explain and predict the existence of specific molecules
on space objects. For example, since SO3 is formed in
laboratory irradiations of SO2 ice then SO3 is also
expected on the Jovian satellite Io, whose surface SO2 experiences intense
radiation from Jupiter. Our experiments on H2O + CH3OH mixtures suggested
ethylene glycol as an interstellar molecule, and indeed this molecule was later detected
in interstellar space. In other work we showed that the oft-debated
"XCN" infrared feature in interstellar ices is due to the cyanate ion (OCN-), and that
the anomalous HCN-HNC ratio in comets can arise from radiation exposure to cometary molecules.
Still other experiments have concerned the formation of H2O2 on Europa,
an icy Jovian satellite, and the synthesis of C2H6 and C3O2
in comets.
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Europa, a moon of Jupiter, has ices such as H2O, H2O2,
CO2, and SO2 on its surface. |
