There are at least 3 separate photochemical self-shielding models with different degrees of commonality. All of these models rely on the selective absorption of (12))C(16)O dissociative photons as the radiation source penetrates through the gas allowing the production of reactive O-17 and O-18 atoms within a specific volume. Each model also assumes that the undissociated C(16)O is stable and does not participate in the chemistry of nebular dust grains. In what follows we will argue that this last, very important assumption is simply not true despite the very high energy of the CO molecular bond

Johnson, Natasha M.

Nuth, Joseph A., III

NASA Technical Reports Server

43rd Lunar and Planetary Science Conference (2012) 1495.pdf 
WILL ORGANIC SYNTHESIS WITHIN ICY GRAINS OR ON DUST SURFACES IN THE PRIMITIVE 
SOLAR NEBULA COMPLETELY ERASE THE EFFECTS OF PHOTOCHEMICAL SELF SHIELDING ? 
Joseph A. Nuth III & Natasha M. Johnson, Solar System Exploration Division, Code 690, NASA's Goddard Space 
Flight Center, Greenbelt MD 20771, USA (joseph.a.nuth@nasa.gov). 
Introduction: There are at least 3 separate photo-
chemical self-shielding models with different degrees 
of commonality. All of these models rely on the selec-
tive absorption of 12C160 dissociative photons as the 
radiation source penetrates through the gas allowing 
. f . 170 d 180 . h' the productIOn 0 reactIve an atoms WIt III a 
specific volume. Each model also assumes that the 
undissociated C160 is stable and does not participate in 
the chemistry of nebular dust grains. In what follows 
we will argue that this last, very important assumption 
is simply not true despite the very high energy of the 
CO molecular bond. 
Self-Shielding Models: Stellar radiation penetrat-
ing into the natal cloud from which the solar system 
collapsed produces a gas enriched with 170 and 180 
towards the interior of the cloud (1) that could be trans-
ferred into the silicates during grain processing events 
such as shocks. Such a model produces regions within 
the cloud where, on average, the oxygen isotopic com-
position of the dust would be distinctly heavier than the 
oxygen isotopic composition of the gas. In particular 
the model assumes that most ofthe heavy oxygen com-
bines with abundant hydrogen to produce water or ice 
and that the CO in these regions would be significantly 
enriched in C160. (Note that this CO could be trapped 
in icy grain coatings.) If dust does not separate from 
gas during collapse, then this would represent the start-
ing composition of the nebular material. 
In the other two versions of the self-shielding mod-
el, the source of the C160 dissociative photons is the 
energetic young sun. In one model (2) the radiation 
preferentially dissociates Cl70 and C180 at the margins 
of the flared nebular disk. The resulting oxygen atoms 
react with hydrogen to produce water or ice that must 
then react with dust grains following convection deeper 
into the nebula. As in the first model, the gas is en-
riched in C160 that should remain with or could be 
trapped in coatings on the solids, barring a mechanism 
for dust-gas separation. In the second model (3), solar 
radiation penetrates directly into the disk near the 
midplane producing heavy oxygen atoms that could 
react directly with under-oxidized dust that might form 
via the vaporization and recondensation of refractory 
materials (4) in the high temperature regions near the 
sun. As in the previous versions, the gas is enriched in 
C160 that is implicitly assumed to be an extremely sta-
ble long-term reservoir for 160. 
Fischer-Tropsch-Type Reactions: CO is not stable 
in a dusty, hydrogen-rich environment such as the solar 
nebula. In fact, such environments present ideal condi-
tions for the Fischer-Tropsch type reactions that will 
convert CO into hydrocarbons, releasing the enriched 
160 back into the gas phase as water (see Figure 1). 
2. 
6. 
Dust 
Grain 
Catalysis 
5. 
3. 
4. 1 
Figure 1: These reactions are often generalized as the 
production of hydrocarbons and nitrogen compounds but 
the specific "name-brand" reactions are as follows: 
Fischer-Tropsch Reaction: CO+3H2(+grain)=>CH4+H20 
Haber-Bosch Reaction: N2 + 3H2 (+grain) =>2NH3 
The Fischer-Tropsch mechanism has long been seen 
as a potential route to meteoritic organics (see, e.g.5) 
and such surface mediated reactions are still under ac-
tive experimental investigation (6,7). A series of pa-
pers (8,9) has recently shown that any solid surface 
(not just iron grains) can promote Fischer-Tropsch 
Type (FTT) reactions, even graphite (10). A more 
interesting finding is that the FTT reactions appear to 
leave an organic film on the grain surface that is a more 
effective catalyst than the original material, thus greatly 
increasing the efficiency ofFTT reactions in convert-
ing CO and H2 into organics. 
Organic Synthesis in Irradiated Ice: Ciesla and 
Sandford (11) demonstrated that the thermal and radia-
tion environment in the outer solar system is ideal for 
the conversion of CO trapped in ices into complex or-
ganics. Such CO-rich ice coatings could form on the 
outer edge of the solar nebula and be transported out-
ward for processing or they may have formed in the 
pre-collapse molecular cloud. The organics thus 
formed would eventually filter back into the inner neb-
ula where the organic coated grains would be incorpo-
rated into growing planetesimals. As the planetesimals 
evolve due to internal heating or collisional processes, 
the organics will react with oxidized metals to produce 
CO that will be released back into the gas. 
https://ntrs.nasa.gov/search.jsp?R=20120009281 2019-08-30T20:24:57+00:00Z
43rd Lunar and Planetary Science Conference (2012) 
A Carbon Cycle in the Inner Nebula: From the 
above we can deduce that there should be a significant 
carbon cycle operating in the inner nebula. Organic 
coated grains incorporated into larger planetesimals 
can react with silicates after radiogenic heating to gen-
erate copious quantities of CO or CO2 that escapes 
back to the nebula. Once back in the gas phase these 
oxides have at least two paths (FTT or irradiated ice 
synthesis) that can reform organics on grain surfaces to 
start the cycle over again. The C160 left intact by the 
photochemical self shielding mechanism should mix 
with CO generated by planetesimal evolution that is 
recycled back into organics via FTT or reactions in 
irradiated ices, greatly diminishing the effectiveness of 
the self shielding process in altering the isotopic com-
position of nebular dust. 
Summary: Operation of the photochemical self 
shielding mechanism in CO allows the production of 
two distinct reservoirs. The first is the CO that is not 
photodissociated and which contains highly enriched 
C160. The second reservoir is generally water or ice 
that is enriched in 170 and 180. Even ice-coated dust 
grains are small enough that they remain with the gas, 
thus ensuring that the two reservoirs remain intimately 
mixed as they fall into the nebula where an unspecified 
series of chemical reactions might occur to incorporate 
the 170 and 180 into the dust. During these reactions, 
the CI60 is generally assumed to remain unreactive in 
the gas phase. 
Unfortunately, CO will not remain unreactive in the 
gas phase in the presence of dust, and will not remain 
unreactive even when buried in ice subject to cosmic 
radiation processing in the outer nebula. In both cases 
the CO is converted to hydrocarbons plus water, thus 
returning the 160 back to the gas phase to react with the 
dust in exactly the same form as the heavier 170 and 
180 produced earlier by photochemical selfshieIding. 
While the conversion of CO into organics might erase 
all traces of the oxygen isotopic fractionation caused 
by photochemical self shielding, the devil is indeed in 
the details. While there is little doubt that the CO will 
be cycled into organics plus water, the question comes 
down to the potential separation of dust from gas in the 
solar nebula. Separation of dust from gas will mean 
the existence of heavy oxygen rich volumes in dust rich 
zones, together with volumes rich in 160 where organic 
molecules are forming, whether via FTT processes or 
as the result of radiation processing in ice. 
The remaining question is the relative size of the 
isotopically fractionated reservoir of CO compared to 
the size ofthe unfractionated reservoir participating in 
the larger carbon cycle. It has been hypothesized that 
all interstellar dust has an organic coating, yet only 
some meteorites contain as much as a few percent car-
bon. This implies the loss of carbon - probably as CO 
1495.pdf 
or CO2 - from planetesimals following the accretion of 
such grains during heating events. 
Did the CO generated from heated planetesimals 
constitute a larger or smaller reservoir of CO than the 
CO that participated in photochemical self shielding at 
any given time? By how much would the isotopic ef-
fects of converting C 160 into organics and water be 
diluted by the presence of normal planetesimal CO? 
Does any trace ofC I60 generated via photochemical 
self shielding survive in the inner solar nebula? These 
questions must be addressed quantitavely before we 
can estimate the effect of photochemical self shielding 
on the oxygen isotopic composition of dust in the prim-
itive solar nebula. 
References: [1] Yurimoto, H. and Kuramoto, K., 
2004, Molecular cloud origin for the oxygen isotope het-
erogeneity in the solar system, Science 305, 1763 - 1766. 
[2] Lyons 1. R. and Young E. D. 2005, CO self-shielding 
as the origin of oxygen isotope anomalies in the early 
solar nebula Nature 435: 317-320. [3] Clayton R. N. 
2002, Self-shielding in the solar nebula, Nature 415: 
860-861. [4] Nuth, 1. A. Rietmeijer, F. 1. M. and Hill, H. 
G. M., 2004. Condensation Processes in Astrophysical 
Environments: The Composition and Structure of 
Cometary Grains, Meteoritics & Planetary Science 37, 
1579 - 1590. [5] Hayatsu R. and Anders, E., 1981, Organ-
ic compounds in meteorites and their origins, Topics in 
Current Chemistry, 99, 1-37. [6] Liorca, 1. and Casanova, 
1., 2000, Formation of carbides and hydrocarbons in 
chondritic interplanetary dust particles: A laboratory 
study, Meteoritics and Planetary Science 33, 243 - 251. 
[7] Hill H. G. M. and Nuth, 1. A., The Catalytic Potential 
of Cosmic Dust: Implications for Prebiotic Chemistry in 
the Solar Nebula and Other Protoplanetary Systems, 
2003, Astrobiology, 3,291- 304. [8] Nuth, 1. A., John-
son, N. M. and Manning, S. 2008a, A Self-Perpetuating 
Catalyst for the Production of Complex Organic Mole-
cules in Protostellar Nebulae in Organic Matter in Space, 
Proceedings IAU Symposium No. 251, 2008 (S. Kwok & 
S. Sandford, eds.) pp. 403 - 408. [9] Nuth, J. A., John-
son, N. M. and Manning, S. 2008b, A Self-Perpetuating 
Catalyst for the Production of Complex Organic Mole-
cules in Protostellar Nebulae Astrophysical Journal Let-
ters 673 L225 - L228 [10] Nuth, 1. A., Kimura, Y. Lucas, 
C., Ferguson, F. T. and Johnson, N. M., 2010, The For-
mation of Graphite Whiskers in the Primitive Solar Nebu-
la, Astrophysical Journal Letters. 710, L98-LlO1. [11] 
Ciesla F. 1. and Sandford, S. A., 2011, Organic Synthesis 
on Ice Grains in the Solar Nebula, preprint. [12] Nuth, J. 
A. 2009, "A Simple Mechanism for Fractionating Oxygen 
Isotopes in the Solar System" Ann. Meeting Meteoritical 
Society, Nancy France, July 2009 Abstract# 5086. [13] 
Kimura, Y., Nuth, 1.A., Chakraborty, S., and Thiemens, 
M.H., 2007. Non-Mass-Dependent Oxygen Isotopic Frac-
tionation in Smokes Produced in an Electrical Discharge 
Meteoritics and Planetary Science. 42, 1429-1439. 


Will Organic Synthesis Within Icy Grains or on Dust Surfaces in the Primitive Solar Nebula Completely Erase the Effects of Photochemical Self Shielding?

Will Organic Synthesis Within Icy Grains or on Dust Surfaces in the Primitive Solar Nebula Completely Erase the Effects of Photochemical Self Shielding?

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server