Triton's volatile budget provides important links to planetary formation processes in the cold outer solar nebula. However, the budget has been modified by processes subsequent to the accretion of this body. It is of interest to assess whether certain formation environments can be ruled out for Triton on the basis of its current volatile abundances, and also to quantify some of the post-accretional processes by which the abundances have been modified

Lunine, J. I.

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EVOLUTION OF TRITON'S VOLATILE BUDGET; J.I. Lunine, University of Arizona, Tucson AZ 85721
Triton's volatile budget provides important links to planetary formation processes in the cold outer solar nebula.
However, the budget has been modified by processes subsequent to the accretion of this body. It is of interest to assess
whether certain formation environments can be ruled out for Triton on the basis of its current volatile abundances,
and also to quantify some of the post-accretional processes by which the abundances have been modified.
Current surface abundances: A combination of Voyager UVS, IRIS and RSS data strongly point to the nitrogen at-
mosphere being in saturation equilibrium with surface frost on the sunlit hemisphere. Since Triton is approaching
southern summer solstice, a minimum surface abundance can be computed based on the continued presence of nitro-
gen in the southern hemisphere. The energy-limited mass flux of nitrogen northward is ~ 10 .7 gcm 2 sl; over 50
years a layer 2 meters thick can be sublimated. An upper limit to the nitrogen abundance is obtained by considering
limits to the thickness of nitrogen frost deposits. The scale of topography on Triton is limited to of order kilometers in
cliffs, ridges, etc. The edges of the polar frost deposits do not seem to exhibit such relief. More significant is that heat
flow models of the nitrogen cap yield a thermal gradient of 15K per kilometer [3]; a 2-3 km-thick cap would reach the
melting point of nitrogen at its base. Before then, viscous spreading of the cap would decrease its thickness [4], so
that 1-2 km is a good upper limit to the cap thickness. Averaged over the surface, an upper limit of 0.5-1 km thickness
is obtained for the amount of nitrogen ice; this is converted in the accompanying table to an equivalent mass.
Carbon monoxide has a saturation vapor pressure roughly an order of magnitude below that of nitrogen at 38K. Ap-
plication of Raodt's law to the atmospheric upper limit of 1% mixing ratio CO/N2 from UVS data [2] yields a ratio in
the mixed ice of 15%. Alternatively, one could postulate that carbon monoxide ice is physically separate from the ni-
trogen ice, but the CO vapor is compelled to move northward in the general nitrogen vapor flow. Then the areal cov-
erage of CO frost in the summer hemisphere can be computed to be <1% to produce an undersaturation of at least
10% in the atmosphere. Thus whether the CO is mixed with the nitrogen or separate, a reasonable upper limit to its
abundance range is 1-10% of the surface nitrogen abundance. Recent work [14] suggests that the atmospheric mixing
ratio limit might be 3 orders of magnitude smaller than given in [2], leading to a surface abundance commensurately
smaller. Methane is present in Triton's atmosphere and is within a factor of 10 of saturation at 38 K. Because it is
close to or at saturation the methane abundance is not constrained by the nitrogen abundance in surface frosts. An up-
per limit to the methane frost abundance can be obtained by similar arguments to those presented above for nitrogen,
though the higher melting point of methane allows a thicker layer. A lower limit for methane is obtained by consider-
ing the amount of methane photolyzed over the age of the solar system, which is the equivalent of roughly a meter-
thickness of productsspreadover the surface.
The photolysis rate implies that the volatile frosts are underlain by a meter or so of photolysis products. This raises
the question as to how carbon dioxide frost has remained exposed to view; perhaps impact stirring has allowed some
CO 2 to be present at the optical surface, or perhaps recent internal activity has allowed fresh CO 2 to be outgassed.
More intriguing is the idea that the CO2 seen in the spectra is the product of photolysis of CO and subsequent addition
of oxygen through infalling ice particles. Such a photochemistry is predicted for Titan, although the rate is computed
to be only 0.1% of the methane photolysis rate [13]. If the same holds on Triton, the photochemically-produced CO2
could be highly diluted in the detritus of higher hydrocarbons.
Models for initial volatile inventory. The first two models use the computations presented in [12] to compute molecu-
lar abundances in the gas phase of a model solar nebula and proto-satellite nebula. The molecules are then assumed to
either condense or become trapped in water ice as clathrate hydrate, with abundances determined from the computa-
tions of [6]. The gas phase molecular abundances in [12] aredetermined by equilibration reactions with finite rates
which are allowed to proceed for a disk lifetime fixed at 10 '° seconds, for concreteness. Likewise for concreteness
the proto-satellite disk model is that for Jupiter; a detailed temperature-pressure profile for a nebula around Neptune
has not been published, but the circum-Jovian disk case illustrates sufficiently well the effect of a more reducing envi-
ronment. For both nebular models we consider results for two different types of chemistry: A fully gas-phase chemis-
try with no catalysis by grains, and a much more rapid set of reaction rates appropriate for efficient grain catalysis.
Given the molecular abundances, we assume that ammonia and carbon dioxide condense out directly and mix with
the water ice either at the molecular or grain level. This is appropriate for ammonia, but a small amount of the carbon
dioxide may instead be incorporated in clathrate hydrate [61. Nonetheless, even for the small mixing ratios in the pro-
to-satellite nebula, the partial pressure of carbon dioxide is sufficient to allow condensation below 60-70K. At 30 AU
from the sun, the environment of Triton was colder than this during planet formation. Methane, carbon monoxide and
molecular nitrogen were assumed to be trapped in clathrate hydrate; we use the published results at 80 K [6], though
the fractionation effects are likely more severe at lower temperatures.
These cosmochemical models am undoubtedly oversimplified. The gas phase molecular abundances at 30 AU, in the
solar nebula, are almost certainly not determined by chemical quenching much closer to the center of the nebula, be-
cause radial mixing may be severely limited [15] (but see [11]). Instead, infalling material from the surrounding col-
lapsing cloud core will be partially processed but still retain a strong signature of molecular cloud abundance [7].
Additionally, volatile trapping in water ice was equally complex, may have involved several stages, and probably did
https://ntrs.nasa.gov/search.jsp?R=19940011900 2020-06-16T20:34:08+00:00Z
914 LPSC XXIV
TRITON'S VOLATILE BUDGET: Lunine J.I.
not involve clathrate hydrate formation [1, 7]. Rather than put together a more complicated model here, we include an
additional column in the table which assumes a volatile inventory for Triton based on comets [8]. While cometary and
proto-Triton material might have undergone modestly different histories, the inventory is inslructive in its differences
from the first two columns. The methane abundance in Comet Halley could be zero [10]; however, the observation of
interstellar methane [5] strongly argues for this molecule being introduced into outer solar system bodies, and we
adopt a finite lower value here. Likewise, the amount of formaldehyde in comets is controversial; we use a non-zero
lower value here based on [9].
Compm'ison of initial and present-day (surface) inventories: Sufficient amounts of nitrogen, carbon monoxide and
methane exist in the solar nebula and cometary models to account for the surface abundances, but the proto-satellite
inventory falls short on nitrogen. The nilrogen abundance is not fatal to the proto-satellite model since the present-
day surface amounts could have been derived from ammonia. Because the surface CO abundance is only an upper
limit, it is not inconsistent with the proto-satellite model.
What is striking about the solar nebula and cometary models is that the surface ratio of carbon monoxide to nitrogen,
required by the data to be <0.1, is very poorly matched in either case. Carbon monoxide is much more abundant than
nitrogen in either model volatile inventory. Selective outgassing could be invoked to achieve the surface ratio, but the
chemical differences between CO and N2 are not so large as to make this particularly appealing. The surface ratio of
CO/CH,= does not appear to cause problems for either model: One can choose an intermediate amount of grain catal-
ysis in the solar nebula model to yield the right surface ratio, and one is free to choose for the surface ratio CH4/CO =
0.1 which is consistent with the cometary model.
Perhaps the most profound process modifying Triton's volatile inventory is early escape of gases from an atmosphere
raised by tidal heating; [8] presents estimates for the total amount of escape during the lifetime of the atmosphere. If
the atmosphere survives for a time of order a gigayear, which is plausible based on the models in [8] and the cratering
record, then most of the initial volatile budget could be lost through escape. More detailed calculations are required to
quantify the relative amounts of each gas which is lost through early, rapid atmospheric escape.
Acknowledgment. This work was supported by NASA grant NAGW-1039.
TRITON'S VOLATILE BUDGET 1
Molecule Solar Nebula 2 Proto-satellite 2 Cometary model a Present Surface 4
N2 103, 108 10-11, 1017 104 106-103
NH3 106, 10 .4 10 1 10 3-10"2 ?
HCN .... 10 .3 ?
CH 4 10"7, 10 "1 10 -i 10-3.10 .2 10-6.10 -3
CO 10"1, 10 .7 10 -12, 1016 102-101 < 10-7-10'I
C02 10"3, 10 -a 10"8, 1012 10"1 *
H2CO .... 103-10 1 ?
CH3OH .... 102-101 ?
! Masses given, as order of magnitude only, in units of 1025 g.
2 Predictions of solar and proto-satellite nebula models. First number assumes gas phase reactions are not cata-
lyzed by grains; second number assumes efficient grain catalysis. Where only one number is given, results
for the two cases are the same. Symbol (--) indicates no prediction available from [12], or too uncertain.
3 Volatile budget based on abundances in comets. Cometary data from review by [10].
4 Estimated mass of surface frost deposits. The ranges for CO and N 2 are coupled, that is, CO/N 2 < 0.1 in any
case. Symbols:? = not yet observed ; * = observed but no abundance constraint available.
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246:1459. [3] Brown, R.H., etal. 1990. Science 250: 431. [41 Kirk, R.L. and Brown, R.H. 1992. BAA_ 23:1209. [5]
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Evolution of Triton's volatile budget

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