A balanced view of the present generation of infrared instruments for cometary compositional studies is presented. Ground-based instruments are compared with airborne and spaceborne capabilities. An attempt to give examples of the unique science achievable with each is made, and particular emphasis is on the unique aspects of a dedicated Cometary Composition Telescope in earth orbit for investigating the chemical and structural heterogeneity of the cometary nucleus

Disanti, Michael A.

Hoban, Susan

Mumma, Michael J.

Reuter, Dennis C.

English

NASA Technical Reports Server

"Infrared Remote
-\ -ACH '91
N9 1 "26 003
Sensing of Cometary Parent Volatiles from the Ground,
Air, and Space"
155
Michael J. Mumma, Michael DiSanti, Susan Hobart, and Dennis C. Reuter
Laboratory for Extraterrestrial Physics
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
The last f'lve 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 CO2 and of the carbonaceous feature
(3.2-3.5 l.tm), a definite detection of H2CO, 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 CH4, CH3OH, and CO, and
significant upper limits for CH4 and H2CO. 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 (CH3OH,
H2CO, CO, CH4 ...), the nitrogen chemistry (NH 3, HCN ...), and the sulphur chemistry
(e.g. H2S). Airborne observations will provide studies of H20 and CS2, and spaceborne
instruments (e.g. on ISO) will provide measurements of CO2, and of strong terrestrial
absorbers (H20, CH4, CO, etc.) at arbitrary Doppler shift. Unambiguous determinations
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 of 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.
https://ntrs.nasa.gov/search.jsp?R=19910016689 2020-03-19T16:51:37+00:00Z
156 ACM '91
COSMO-DICE: PROJECT OF DYNAMICALINVESTIGATION OF COMETARYEVOLUTION
Tsuko NAKAMURA (NAO,Tokyo) & Makoto YOSHII(A|A(University of Tokyo)
The orbits of more than 150 periodic comets 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"=8 at
the start of integration. We incorporate positions of 9 planets from DEI02
ephemerides in the integration and the calculations are carried out for the full
time span of DEI02 (about 4400 years, from BCI4II through A03002). Error
tolerance for a single integration step is set to 10"=. Accuracy of integration
is checked by the round-trip error of closure test which covers about 3400 years,
so that this allows us to estimate the reliable time interval of our
integration for each comet. This can also be used to know, for a given number of
significant digits of the observed orbital elements of a comet, how the orbital
error grows with the elapse of time.
[t is shown that the reliability interval is about 1000-1500 years for most of
low-inclination short-period comets whereas that for high-inclination and longer-
period comets is about 3000-4000 years or more. it is also found that growth
rate of the round-trip error has intimate correlation with the chaotic nature of
cometary orbits.
In this poster session we graphically present the time variations for 4400
years of all the comets in orbital elements, perihelion and aphelion distances,
Tisserand's invariant, mutual distances between planets and a comet, the round-
trip errors for orbital elements and position and velocity vectors. We are
preparing a MT of about 501_ (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 comets. |e also have a plan to
develop a program which enables us to present and compare easily, in an
interactive mode on a graphic terminal, orbital behavior of comets and other
related dynamical quantities such as Tisserand's invariant, libration arguments
and so on.
ACM '91 157
LONG-TERM ORBITAL BEHAVIOR OF SHORT-PERIOD COMETS FOUND IN PROJECT COSWO-DICE
Tsuko NAKANURA (NAO, Tokyo) & Nakoto YOSHIKA|A (University of Tokyo)
We 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 DEI02. 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 thmt 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 sore than 90% 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 Nonte 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 sta=e 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 sore 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-periodic nature of orbital evolution.
158 ACH '91
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RENCES
N_ 372_ 1987_ P. 2-3 (in russian).377 t 1987_ P. 2-4 (in russian).
*,
OF POOR QUALITY
ACH 191 159
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 FroeschM 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 _> 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 1 AU.
The ()pik 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.


Infrared remote sensing of cometary parent volatiles from the ground, air, and space

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