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ACM '91 

N9 1 - 26 003 

"Infrared Remote Sensing of Cometary Parent Volatiles from the Ground, 

Air, and Space" 

Michael J. Mumma, Michael DiSanti, Susan Hoban, and Dennis C. Reuter 
Laboratory for Extraterrestrial Physics 
NASA Goddard Space Flight Center 
Greenbelt, Maryland 20771 

The last five years have seen an explosion in our ability to directly detect the parent species 
in comets, beginning with the first definitive detection of cometary water in December 
1985, from the Kuiper Airborne Observatory. In March 1986, infrared spectroscopy from 
the Vega- 1 spacecraft provided the first detections of CO 2 and of the carbonaceous feature 
(3.2-3.5 pm), a definite detection of H 2 CO, and a tentative detection of CO. Since then, 
the carbonaceous feature has been detected in every comet searched, and spectroscopy from 
the ground and air has produced tentative detections of CH 4 , CH 3 OH, and CO, and 
significant upper limits for CH 4 and H 2 CO. Meanwhile, advanced instruments promise 
routine detection of many species that at present are only marginally detectable in bright 
comets. Using these, we can expect to identify the volatile and refractory progenitors of 
the carbonaceous feature, and to provide routine study of the carbon chemistry (CH 3 OH, 
H 2 CO, CO, CH 4 ...), the nitrogen chemistry (NH 3 , HCN ...), and the sulphur chemistry 
(e.g. H 2 S). Airborne observations will provide studies of H 2 O and CS 2 , and spaceborne 
instruments (e.g. on ISO) will provide measurements of C02, and of strong terrestrial 
absorbers (H 2 O, CH 4 , CO, etc.) at arbitrary Doppler shift. Unambiguous deteiminations 
of the ortho-para ratios, and measurements of isotopic ratios in several key species should 
be possible. 

Difficulties lie ahead, however, for investigations that rely on small pixel sizes, such as 
certain advanced ground-based instruments and the HRS instrument on HST. The 
reduction in background needed to take full advantage of the 2-D array detectors in ground- 
based instruments leads to their use at high spectral resolution and small optical throughput. 
The spectral grasp is shortened, and the fraction of molecules sampled by a single pixel is 
also reduced, and this makes the retrieval of production rates increasingly sensitive to coma 
models of uncertain provenance. However, certain other aspects, such as co-registration of 
different spectral precursors (e.g. gases and refractories), can enhance the study of short 
term variability (therefore of nuclear heterogeneity), and of the morphology of the near 
nucleus region. 

In this paper, I will attempt to present a balanced view of the present generation of infrared 
instruments for cometary compositional studies. Ground-based instruments will be 
compared with airborne and spaceborne capabilities. I will attempt to give examples 01 the 
unique science achievable with each, and will place particular emphasis on the unique 
aspects of a dedicated Cometary Composition Telescope in Earth orbit for investigating the 
chemical and structural heterogeneity of the cometary nucleus. 

C- 3 

156 ACM ’91 


Tsuko NAKAMURA (NAO, Tokyo) & Makoto YOSHIKAVA (University of Tokyo) 

The orbits of more than 150 periodic coaets are integrated numerically in very 
high precision. The adopted integrator is a variable-step extraporation method 
in quadruple precision. This corresponds to a rounding-off error of 10" 28 at 
the start of integration, fe incorporate positions of 9 planets froa DE102 
epheaerides in the integration and the calculations are carried out for the full 
time span of DEI 02 (about 4400 years, frog BC1411 through AD3002). Error 
tolerance for a single integration step is set to 10* 22 . Accuracy of integration 
is checked by the round-trip error of closure test which covers about 3400 years, 

so that this allows us to estiaate the reliable time interval of our 
integration for each coaet. This can also be used to know, for a given number of 
significant digits of the observed orbital elements of a coaet, how the orbital 
error grows with the elapse of time. 

It is shown that the reliability interval is about 1000-1500 years for aost of 
low-inclination short-period comets whereas that for high-inclination and longer- 
period coaets is about 3000-4000 years or aore. It is also found that growth 
rate of the round-trip error has intimate correlation with the chaotic nature of 
coaetary orbits. 

In this poster session we graphically present the tine variations for 4400 
years of all the coaets in orbital elements, perihelion and aphelion distances, 
Tisserand’s invariant, mutual distances between planets and a coaet, the round- 
trip errors for orbital elements and position and velocity vectors. We are 
preparing a MT of about 50MB (nearly in double precision) in the standard FITS 
format to distribute to the interested researchers, which contains 64-day 
interval positions and velocities of all the coaets. le also have a plan to 
develop a program which enables us to present and compare easily, in an 
interactive aode on a graphic terminal, orbital behavior of coaets and other 

related dynamical quantities such as Tisserand’s invariant, libration arguments 
and so on. 

ACM ’91 



Tsuko NAKAMURA (NAO, Tokyo) & Makoto YOSHIKA»A (University of Tokyo) 

fe have performed systematic numerical integrations of more than 150 periodic 
comets for 4400 years (3400 years toward the past and 1000 years to the future), 
based on the JPL planetary ephemerides DE102. Details of this project are 
descirbed in our another paper which will be presented at the poster session. 
One of the most remarkable results of our project is that comets entering the 
capture region by Jupiter are proved to evolve to short-period (SP) comets in 
the framework of realistic dynamical model. It is found that more than 90X of 
the observed comets whose Tisserand’s invariant (J) is between 2.8 and 3.1 
actually take this evolutionary path within the past 3400 years. This evolution 
is much more rapid than that expected from Monte Carlo simulations for 
simplified dynamical models based on symmetric distribution of perturbations. 
This suggests that asymmetry of perturbation distribution plays an important 
role in cometary evolution. 

Some of SP comets are shown to evolve from the orbits of which perihelion 
distance is located near Saturn’ orbit and then is handed over under the control 
of Jupiter. This seems to support the multiple stage capture mechanism first 
proposed by Everhart (1977). We also found a comet which is ejected out of the 
solar system by Jupiter in a fairly strong hyperbolic orbit around 2330 AD. 

It is confirmed that captured low-inclination SP comets with the J in the 
range given above show more or less strong chaotic behavior of orbital evolution 
On the other hand, comets with longer orbital period and/or of high inclination 
reveal slow and quasi -per iodic nature of orbital evolution. 

158 ACM ’91 

of the 


ronooical Observator 

The sufficiently hi Lh space 1 and) s 
Comet Halley spectra obtained by 
find luminescent dust} partiklfts ijn 
othesis about the 

' b 

resertce, ofi 
ent Widths jof 

. _ Was 
the sbecbroi 
he solar lrgh' 
by sfrecftra 
» to the lurni 



deficiency of the equiva 
iffinHnuue of Cadet Halle,.. 

It eeans that eightner there 
scattered light inside: 
continuum which was not jl 
supposition was checkmli 
partition of the continue 
(following results. i 


from the nucleus 

arcsec j 



0 , 

20 i 
36 i 


Nifttus corresponds to the! 

ctral resolution (4 arcsec and 2 
he 6-meter telescope gave the possi 
the atmosphere of this comet t 1,2 3 
these particles is based on the ob 
'the Fraunhofer lines in the sp 

unusually high level of par 
or there wae the coeponent ( 
scattered by coeetary grains. The 
Jof reference stars and was rejected, 
ntescent and the scattered ones gavi 


v» w LMr 1 vaUUIIM* LW k It Br BUM met W Wit V 

Thus , tne luminescent particles (n 

Tlfcy di tauuvai ml thee elundu ft *. * i iW 

consideration that a spectrum of] tn 
arjcseconds with the rfesolvdd profile 





20000 i . 
sunward dinecqi 

•n seen previously. 

1. Nazarchuk, B.K. 

2. Nazarchuk; B.K. 




i r 

CONET HALLEYj B.K. Nazarchuk. The Main 
iinian Academy of Sciences, 232127, Kiev, 

) of 

t erved 

asi te 
if the 
> the 

jFraction of luminescence 
in the P/Halley continuum 
i n the range 3300-6000 A 
5.6 X 



»on. i 

robably, CHON-grains) are short-living. 

An ft as i- fa an ft n frem ft h m nu#l mum . TeUim ft i "« ■ 

e circumnuclear region within several 

s of the Fraunhofer lines is necessary to 

detect the luminescence,! one can easy understand why the luminescence has never 


N. 372. 1987, P. 2-3 <in russian). 
N. 377, 1987, P. 2-4 (in russian). 



ACM ’91 


Delivery of Meteorites from the Asteroid Belt. 

M. Nolan and R. Greenberg / University of Arizona 

The study of asteroid formation and composition is of keen interest, since the processes 
that formed our own Earth and the other planets may have been similar in some important 
ways. Also, the numerous objects in the main asteroid belt and elsewhere help us avoid 
the “sample of one” problem so common in planetary science. Unfortunately, asteroids are 
very difficult to study directly: we have relatively noisy, low resolution optical spectra of 
their disk-averaged surfaces in reflected sunlight or thermal emission, and even then we see 
only their “dirty” surfaces. Meteorites, on the other hand, can be studied in great detail at 
high resolution by a wide array of techniques with much lower noise. Thus it would aid our 
understanding to know how asteroids and meteorites are connected, even if only statistically. 

Transport processes for bringing asteroids from the asteroid belt to the Earth have been 
critically reviewed by Greenberg and Nolan [1989]. Wisdom [1983] and Froeschle and Scholl 
[1986] have shown that asteroidal material may be transported to the Earth by way of Jovian 
and secular resonances. We do not know for certain how asteroids get into the resonances, 
which are now fairly clear of asteroids, probably due to the same processes that bring material 
to the Earth. We probably understand in general the dynamical delivery mechanisms, but 
not their relative efficacy, or what regions of space they sample. 

The main belt size distribution is known for sizes 2; 30 km in diameter by direct telescopic 
observation, with some extrapolation and bias corrections for albedo at the smaller sizes. 
However, collisions are most likely to occur with smaller bodies. Thus our estimates for the 
collisional lifetimes of the bodies we can see are very uncertain. The collisional lifetimes 
affect in turn the expected steady-state population of bodies at all sizes. 

As an alternative to using a variety of poorly understood processes to analyze the 
meteorite delivery process from the main belt, we can look at the process from the other 
end: meteorites arriving at the Earth. Networks of cameras operating since the early 
1950s (cf. Jacchia and Whipple [1961]) photographed several thousand meteor trails. From 
these photographs, it was possible to determine the orbits of the asteroids which fell as 
meteors. Wetherill and ReVelle [1981] chose 27 meteors which they believed to be of ordinary 
chondritic composition (including Lost City, a recovered meteorite). Their orbital elements 
in a, e space show clusters near several Jovian resonances zones. We have similarly examined 
the orbits of 42 496 meteors from the IAU Meteor Data Center. Clustering persists only 
weakly in the vast data. The low accuracy of many of the orbits (D. Steele, pers. comm.) 
is a critical factor. There is a strong clustering toward orbits with perihelia near IAU. 
The Opik two-body treatment of the the gravitational attraction of the Earth may not be 
sufficient for these orbits. We are numerically integrating the orbits of these meteors, to 
determine how large a correction is required. These results will help constrain how many 
came to Earth-crossing by each of the possible routes.