NASA Technical Reports Server (NTRS) 20000033157: The Formation Age of Comets: Predicted Physical and Chemical Trends

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The Formation Age of Comets: Predicted Physical and Chemical Trends
J. A. Nuth III and H. G. M. Hill*, Astrochemistry Branch, Code 691, NASA's Goddard Space
Flight Center, Greenbelt MD 20771 (*NAS/NRC Resident Research Associate)
The chemical composition of a comet has always at 950 K, and nearly 4 billion years at 850 K. Alter-
been considered to be a function of where it formed natively, if isolated presolar silicates become too hot
in the nebula. We suggest that the most important (e.g. T >1400 K)they will evaporate. Upon cooling,
factor in determining a comet's chemistry might this vapor will recondense to form amorphous sili-
actually be when it formed. We present specific cates, not crystalline grains. Therefore, the produc-
predictions of correlations between the dust and tion of significant quantities of crystalline olivine from
volatile components to test our hypothesis. initially amorphous interstellar materials requires dust
processing at moderate temperatures (T-1000 K),
It iswell established that the Falling Evaporative probably near 1AU or so3,followed by transport of
Bodies (FEBs) responsible for anomalous red-shifted the newly created crystalline olivine grains to beyond
UV extinction in Herbig Ae and Be stars are actually the nebular snowline for incorporation into the next
star-gazing comets 4.5undergoing rapid loss of both generation of embryonic cometesimals.
gas and dust close to the central star. FEBs have been
reported for Herbig Ae and Be systems of all ages, but One would expect comets and cometesimals to form
are most frequent in younger systems. Since the first throughout the history ofa protostellar disk. The
observations of the disk around Beta Pictoris 6( a rela- cometesimals that formed earliest must of necessity
tively old protostellar A star) it has been recognized contain nearly pure interstellar grains and ices. These
that such disks are unstable to dust loss via radiative comets would be dominated by amorphous silicates,
forces and must be replenished on a more-or-less con- CO, CO2, N2 and water ice, e.g., see Gerakines et
tinuous basis from larger bodies such as comets. al) 2.These materials would dominate the composition
Knacke et al.7demonstrated that the mid-infrared of the FEBs observed in very young Herbig Ae and Be
spectrum of the grains in the disk of Beta Pictoris is stars, as these comets replenish the dust disks around
consistent with the spectrum of dust grains observed such stars. Cometesimals formed later could contain
in Comet Halley s'9and attributed by the authors of the more highly processed dust, provided that some
latter papers to crystalline olivine. Sitko et al. 1°re- mechanism exists to transport this annealed dust com-
viewed the trends observed in the mid-IR spectra of ponent to beyond the snowline. Prinn t3and Steven-
Herbig Ae and Be stars as a function of age for stars son TMhave previously discussed the likelihood of out-
without detectable companions. They demonstrated ward mixing in a nebular environment due to non-
that this spectrum evolves smoothly from that of linear momentum terms often neglected in nebular
amorphous astronomical silicate l_to a spectrum con- models. The observation that the dust in disks around
sistent with crystalline olivine. Although these spec- older Herbig Ae and Be stars (without companions)
tral changes could potentially result from exposure of appears to have been annealed in a manner consistent
initially amorphous grains to x-rays from the young with the observed spectral evolution of the amorphous
star, a more likely explanation is that these changes magnesium silicates reported by Hallenbeck et al. _
result from thermal processing in the nebula. supports the hypothesis that such a transport mecha-
nism exists. Dust released by comets in older Herbig
The rate of spectral change in initially amorphous Ae and Be systems contains more highly processed
magnesium silicate smokes as a function of tempera- materials than the dust released just as such systems
ture has been measured by Hallenbeck et al. )who de- begin to break out of their dusty cocoons. These
termined that this rate was extremely sensitive to the grains most likely formed via thermal annealing of
annealing temperature. More recent studies allow one initially amorphous astronomical silicate grains. If
to predict the mid-infrared spectrum of annealing individual dust grains are transported from the inner
magnesium silicate dust if its time-temperature history nebula to beyond the snowline, then it makes sense to
can be hypothesized 2. Based on this work, the time assume that some portion of the gas in the inner neb-
required for initially amorphous astronomical silicate ula also makes this journey. Such transport would
grains to thermally anneal to crystalline olivine at have very interesting consequences for cometary
1000 K is on the order of a few hundred days. This chemistry.
same transition occurs in less than two hours at a
temperature of 1050 K, but requires at least 100 years
DeterminatioonfaCometarFyormation Age: J. A. Nuth & H. G. M. Hill
Models of cometary chemistry _s'16have always had on average - before a comet containing only moder-
difficulty explaining the observed ratio of interstellar ately annealed dust. Whereas the chemistry of the
molecules - such as CO, N2, etc. - to molecules pro- former comet would be rich in CO, CO2 and N2, the
duced in the solar nebula. Formation of these more latter comet should contain an abundance of hydro-
complex materials, e.g., hydrocarbons, ammonia, etc., carbons, ammonia and other essential prebiotic com-
requires higher pressures and temperatures than those pounds.
found in nebular models for regions beyond the orbit
of Jupiter or Saturn. A possible way around this di- We predict that the fraction of crystalline cometary
lemma has been to postulate the existence, and subse- dust is correlated to the ratios of hydrocarbons to
quent breakup, of Giant-Gaseous Protoplanets that CO/CO2 and ammonia/amines/amides to N2 and we
could provide the thermodynamic environment re- challenge the observational community to test this
quired to synthesize more complex molecules observed hypothesis. Such observations can simultaneously
in comets from interstellar ices that barely even va- corroborate the existence of a transport mechanism
porize in the outer regions of the Solar Nebula. Con- between the inner and outer regions of protoplanetary
tinuous cycling of just a small fraction of nebular gas nebulae based on our predictions concerning cometary
and grains from the higher pressure, higher tempera- chemistry and establish a method to determine the
ture inner nebula to beyond the nebular snowline ob- formation age of comets. An ability to sequence com-
viates the need for Giant-Gaseous Protoplanets as an ets according to their relative ages could be a useful
explanation for the more complex chemistry of com- tool in understanding a number of events in the
ets. However, operation of this transport mechanism primitive Solar Nebula and would provide an inter-
throughout the history of the nebula would predict esting discriminator in choosing targets for space
some very specific correlations between the chemistry flight missions.
of comets and the mid-infrared spectra of their dust.
References: 1.Hallenbeck, S. L., Nuth, J. A., Daukantas,
As noted previously, comets formed early in the his- P. L. (1998) Icarus, 131,198-209; 2. Hallenbeck S. L.,
tory of the Solar Nebula will consist almost exclu- Nuth J. A. and Nelson, R.N. (2000) Astrophys. J (in press);
sively of amorphous silicates and unaltered interstellar 3. Woolum, D. S. and Cassen, P. (1999), MAPS 34, 897 -
907; 4. Ferlet, R., Vidal-Madjar, A., and Hobbs, L. M.
ices. The probability that many such comets survive
(1987),Astron. Astrophys., 185, 267-270; 5. Hobbs, L.M.
to the present era cannot be adequately addressed
(1987) Astrophys. d..,308, 854-858; 6. Smith, B. A. and
here. However, we can conclude that cometesimals
Terrile, R. J. (1984) Science, 226, 1421; 7. Knacke, R. F.,
formed at later times will contain an increasing fi'ae- Fajardo.-Acosta, S.B., Telesco, C. M., Hackwell, J. A.,
tion of annealed silicate dust together with an in- Lynch, D. K., Russell, R. W., (1993), Astrophys. d., 418,
creased ratio of hydrocarbons to CO or CO2, and an 440-450; 8. Campins, H. and Ryan, E.V. (1989) Astro-
increased ratio of ammonia and more complex amines phys. J, 341, pp. 1059-1066; 9. Ryan E. V. and Campins
and amides to molecular N2. As these eometesimals H. (1991) AJ, 101,695-70; 10. Sitko, M., Grady, C.A.,
accrete into comets, the average comet formed late in Russell, R. W., Lynch, D. K., Harmer, M. S., Perez, M. R.,
Bjorkman, and K. S., DeWinter, W. (1999), Protostars and
nebular history will contain more hydrocarbons, am-
monia and annealed dust than one formed earlier. Planets V,eds. V. Mannings and M. S. Matthews (Univ.
Ariz. Press, Tucson), pp. ;I1. Draine B. T. and Lee H. M.
The time-dependent nature of the dust and gas ac-
(1984) Astrophys. d,,285, 89-108; 12.Gerakines, P. A.
creted into comets might easily obscure less signifi-
Whittet, D. C. B., Ehrenfreund, P., Boogert, A. C. A.,
cant differences in cometary chemistry - such as the Thielens, A. G. G. M., Schutte, W. A., Chiar, J.E., van
potential distinction between comets accreted in the Dishoeck, E. F., Prustl, T., Helmich, F. P. and de Graauw,
Jupiter-Saturn region from those accreted near Ura- Th. (1999) Astrophys. J. 522, 357 - 377; 13. Prinn, R. G.
nus-Neptune. One must also be cognizant of the sta- (1990) Astrophys. J, 34..__,87825-729; 14. Stevenson, D. J.
tistical nature of this indicator: dust annealed at a (1990), Astrophys. J. 348, 730- 737; 15. Prinn, R. G.,
wide variety of temperatures could find its way to the Fegley, B. (1989) inOrigin and Evolution of Planetary and
same comet. Thus, a minor component of very-well- Satellite Atmospheres., eds. S. K. Atreya, J. B. Pollock, and
M. S.Matthews (Univ. Ariz. Press, Tucson), pp. 78-136;
annealed dust in one comet would not necessarily im-
16. Fegley B. (1993) in TheChemistry ofLife's Origins,
ply the same chemistry found in a comet containing
eds. J. M. Greenberg, C.X. Mendoza-Gomez, and V. Pirro-
mostly moderately annealed dust. As a general rule,
nello (Kluwer, Dordrecht) pp. 75-147.
the chemistry of the comet would be correlated with
the bulk of the dust. For example, a comet containing
a minor component of very well annealed dust and a
major fraction of amorphous dust most likely formed -