Low temperature spectroscopy and irradiation effects on Solar System ices--Kuiper belt objects as a case study.

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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13,15747–15748 15747
Citethis: Phys. Chem. Chem. Phys.,2011,13,15747–15748
Low temperature spectroscopy and irradiation
effects on Solar System ices – Kuiper belt
objects as a case study
Ralf I. Kaiser,*aWeijun Zheng,bYoshihiro Osamuracand Agnes H. H. Changd
DOI: 10.1039/c1cp90129j
Since their discovery in 1992,1Kuiper
Belt Objects (KBOs)—small planetary
bodies beyond the orbit of Neptune and
‘precursors’ to short periodic comets2—
havereceivedconsiderable
from the planetary sciences, astronomy,
and physical chemistry community.3,4
At the low temperatures of KBOs of
30–50 K, a chemical modification of
their surface ices such as water, nitrogen,
carbonmonoxide,
methanol, and possibly ammonia5is pre-
dominantly induced via non-equilibrium
chemistry through ionizing radiation
from solar wind particles, solar wind
photons, andgalactic
(GCR) particles.6,7Laboratory experi-
ments simulating the interactions of
ionizing radiation with icy surfaces at
ultralow temperatures offer a remark-
able opportunity for understanding the
chemical processing of primitive bodies
and allow for an establishment of a
‘chemical’ time-line dating back to the
origin of our Solar System. Since pre-
biotic molecules like amino acids and
their precursors can be synthesized in
attention
carbondioxide,
cosmicray
these ices as well,8those studies also
explore scenarios where in our Solar
Systemastrobiologically
molecules might have been synthesized
since the formation of the Solar System
4.6 billion years ago.
The symposium ‘Kuiper Belt Objects –
LaboratoryStudies,
Observations’ at the Pacifichem 2010 in
Honolulu, Hawaii, focussed on the inter-
play between laboratory spectroscopy,
theoretical investigations, and funda-
mental laboratory studies on specific
molecular processes, which lead to a
chemical modification of KBO surfaces
with the overall goal to extract general-
ized concepts on the chemical processing
of KBO surfaces and related bodies in
the outer Solar System. Four ‘radiation’
sources are considered ranging from high
energy electrons and MeV nuclei (simu-
lating GCR particles) via solar photons
to thermal chemistry of open shell reac-
tants like hydroxyl radicals and hydro-
gen atoms at low temperatures. In detail,
Zheng et al. (DOI: 10.1039/c1cp20528e)
demonstrated that an investigation of the
chemistryof(suprathermal)
atoms, formed in the decomposition of
water molecules upon interaction with
energeticelectrons
galactic cosmic ray particles (GCRs) yields
vital laboratory data on the interacting
of non-equilibrium species with water-
rich KBO ices. Kim et al. (DOI: 10.1039/
c1cp20658c) proposed that non-equilibrium
effects of triplet carbon monoxide in
nitrogen-rich ices lead to the formation
of the diazirinone molecule (N2CO),
important
Models,and
oxygen
inthetrackof
which was recently characterized for the
very first time spectroscopically.9These
results were amplified by Pilling et al.’s
study (DOI: 10.1039/c1cp20592g) on the
processing of water–carbon dioxide ices
by heavy MeV particles of the galactic
cosmic radiation field. Non-equilibrium
effects are also induced via the photo-
lysis of water-bearing ices as outlined by
Kinugawa et al. (DOI: 10.1039/c1cp20595a)
and Andersson et al. (DOI: 10.1039/
c1cp21138b) utilizing sophisticated time-of-
flight spectroscopy of the photofragments
produced at 157 nm. Oba et al. (DOI: 10.
1039/c1cp20596j), Hidaka et al. (DOI:
10.1039/c1cp20645a), and He et al. (DOI:
10.1039/c1cp21601e) showed nicely that
reactions of hydroxyl radicals and hydrogen
atoms can also result in a rich chemistry
on KBO surfaces. Finally, Kayi et al.
(DOI: 10.1039/c1cp20656g) demonstrated
via electronic structure calculations that
the astrobiologically relevant glycine amino
acid is thermodynamically less stable by
31–37 kJ mol?1compared to its methyl-
carbamic acid isomer thus raising questions
of a possible photon-induced isomerization
of both structures.
We hope that the timeliness of this topic
in light of the currently operating New
Horizon Mission (NASA) on its way to
(ex-planet) Pluto and the Rosetta Mission
(ESA) launched in 2004, which is currently
on its route to comet 67P/Churyumov-
Gerasimenko, triggers further laboratory,
spectroscopy, and theoretical investiga-
tions on the stability and chemical proces-
sing of ices of Kuiper belt objects in the
next decade.
aDepartment of Chemistry,
University of Hawaii at Manoa, 2545
McCarthy Mall, Honolulu, HI 96822-2275,
USA. E-mail: ralfk@hawaii.edu
bState Key Laboratory of Molecular Reaction
Dynamics, Institute of Chemistry, Chinese
Academy of Sciences, No. 2, North 1st Street,
ZhongGuanCun, Beijing 100190,
P. R. China
cKanagawa Institute of Technology,
Atsugi 243-0292, Japan
dDepartment of Chemistry, National Dong
Hua University, Hualien, 974, Taiwan
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