Pergamon Adv. Space Re.s. Vol. 14, No. 6, pp. (6)143-(6)158, 1994

1994 COSPAR Printed in Great Britain. All rights reserved.

0273-1177/94 $6.00 + 0.00

SOLAR SYSTEM EXPLORATION FROM THE MOON: SYNOPTIC AND COMPARATIVE STUDY OF BODIES IN OUR PLANETARY SYSTEM

P. Bruston* and M. J. Mumma**

* EPCOS-LPCE, URA CNRS 1404, Universit( Paris Xll-Val de Marne, Avenue du G(n~ral de GauUe, 94010 Cr~teil Cidex, France; and lnstitut d'Astrophysique Spatiale, CNRS/Universit( Paris XI, France ** Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Code 690, Greenbelt, MD 20771, U.S.A.

ABSTRACT

An observational approach to Planetary Sciences and exploration from Earth applies to a quite limited number of targets, but most of these are spatially complex, and exhibit variability and evolution on a number of temporal scales which lie within the scope of possible observations. Advancing our understanding of the underlying physics requires the study of interactions between the various elements of such systems, and also requires study of the comparative response of both a given object to various conditions and of comparable objects to similar conditions. These studies are best conducted in "campaigns", i.e. comprehensive programmes combining simultaneous coherent observations of every interacting piece of the puzzle. The requirements include both imaging and spectroscopy over a wide spectral range, from UV t~ IR. While temporal simultaneity of operation in various modes is a key feature, these observations are also conducted over extended periods of time. The moon is a prime site offering long unbroken observation times and high positional stability, observations at small angular separation from the sun, comparative studies of planet Earth, and valuable technical advantages. A lunar observatory should become a central piece of any coherent set of planetary missions, supplying in-situ explorations with the synoptic and comparative data necessary for proper advance planning, correlative observations during the active exploratory phase, and follow-up studies of the target body or of related objects.

INTRODUCTION

Fly-by and in-situ missions are the preferred mode for exploring bodies within our planetary system. Every object visited has differed greatly from our preconceptions, and with hindsight we see that it should previously have been regarded as being almost unknown. However, such missions must be augmented with, and often preceded or followed by, astronomical observations fromnear-Earth space. A wealth of unexpected discoveries and significant contributions to Planetary Sciences were attained by astronomical means, even after in situ investigations were conducted.

It is clear that future developments of astronomical facilities in Earth orbit will benefit Planetary Sciences. All major planets (except Pluto), together with their satellites and rings, have already been visited, but in most cases the examination has spanned only a "snapshot" in time and some of those bodies will not be explored by a longqasting in-situ mission for decades to come. Among the minor

(6)143

(6)144 P. Brus~n and M. J. Mumma

planets, we can consider visiting only those very tew comets and asteroids having special combinations of parameters such as periodicity, orbital parameters and diameter.

Furthermore, while detailed geological mapping or measurement of precise atmospheric profiles need orbiters or descent probes, other important classes of investigations need complementary studies from Earth orbit, or are better done from that vantage point. These include global synoptic studies of phenomena which vary on various temporal and spatial scales, the study of a diverse range of interactive and time-variable phenomena, and the comparative study of phenomena associated with a set of related objects. Conducted over decadal or longer time scales, astronomical studies can provide the historical perspective needed to place phenomena revealed by local, shorter term measurements in their proper context for interpretation. A familiar example of this is provided by historical observations of periodic comet P/Halley. Observations during its two most recent apparitions (1910 and 1986) provided a font of knowledge against which the comet will be compared in 2062. Each apparition reveals a different layer of the nucleus as sublimation 'peels away' successive layers, and comparison then permits a kind of tomography revealing aspects of accretion in the solar nebula.

Considerations of the crucial role of planetary astronomy in the future advancement of Planetary Sciences, and of the scientific programmes that should be carried out in this way, lead however to the conclusion that astronomical Earth-orbiting observatories are quite limited in answering the measurement needs. The special characteristics of remote observations within our planetary system require specific dedicated instruments, and a dedicated mode of operation. For scientific and technical reasons, a lunar-based planetary observatory appears to be the fight answer.

Such an observatory should then become a central piece of any coherent set of planetary missions, both by giving access to investigative programmes of outstanding importance and by supplying in- situ explorations with the synoptic and comparative data necessary for proper advance planning, correlative observations during the active exploratory phase, and follow-up studies of the target body or of related objects.

CHARACTERISTICS OF REMOTE OBSERVATIONS OF BODIES IN OUR PLANETARY SYSTEM

Astronomical studies of objects outside our planetary system apply to a tremendous number of sources. Among the numberless observable sources, some belong to the same class of object and represent various phases of evolution. With modern instruments, these objects can be studied over an immense distance range thus an extended range of epochs can be explored. Although most are seen as unresolved point sources, various spatial resolutions are possible on some extended sources, depending on the type of object and method of study. The best angular resolution with current filled aperture telescopes (e.g. HST and NTT) is about 0.1-0.25 arc see, while with intefferometers (VLBI, VLA, VLT, Keck) it ranges from about 1-100 milliarcseconds, depending on wavelength and baseline. One milliarcsecond corresponds to a spatial resolution of one milli-AU at the nearest star, 0.02 AU at beta Hctoris, 0.14 AU at the nearest star-forming region (Taurus-Auriga), 10 AU at the galactic center, 2000 AU at the nearest galaxy (2 Mpc), and 5 pc at the distance of early galaxies (1 Gpc). The light travel time over 5 pc distance is about 16 years, so angular resolutions of one milliarcsec can potentially reveal light-speed interactive phenomena at all distance ranges, within a human lifetime. However, at much larger angular resolutions (say 1 arc-second), interactive phenomena are resolvable only over time scales -1600 years at 1Gpc, or -1.6 year at 1 MPC. Since propagation velocities are usually far slower than light-speed (typically 100 kin/see Or less), the observable distance range shrinks by another factor of 3000, to only 200 pc for 1 yr variability. For this reason, most astrophysical sources appear to be stable and observed variabilities are most often periodic, such as those within a stellar system. Otherwise, the entire data set that can be obtained on a given source can be considered as a single 'snapshot' in time.

On the other hand, studies of bodies in the solar system apply to relatively few observable targets: - 50 major bodies (planets and their principal satellites), hundreds of comets, thousands of asteroids. These individual bodies are parts of a much smaller number of interactive systems: planet-ring-

Study of Bodies in our Planetary System (6)145

satellite systems, asteroidal belts and families, comet belts and clouds, comet trails and meteor streams. Exploration has so far emphasized the targeted: bodies as individual entities rather than as related members of a single class, and they are obviously only observed at the present stage of their evolution. But when viewed at high spatial resolution, the bodies are seen to be complex systems made up of interacting elements on scales from meters to thousands of kilometers. Variability and evolution appear on a number of temporal scales (minutes to years), and investigation of these aspects are quite feasible.

Variabilities in solar system bodies occur for various reasons. They may be influenced by either internal or external forcing, or may be related to the sun-target-observer geometry. Examples include:

- internal activity: internal heat source, volcanism, conductivity,

- tidal effects, - diurnal or seasonal variations, secular changes, climate, - variations in external energy deposition - far UV or solar wind,

solar activity, heliocentric distance, - plasma and electromagnetic environment, fields and currents, - evolutionary processes, - change in viewing geometry (a survey of time variations leads to a stereoscopic data set).

Each process leads to characteristc sequential variations of observable quantifies through coupling processes. The forcing terms can be separately identified from the responses by making observations at different wavelengths, clarifying the coupling processes.

Those different characteristics lead to different scientific processes in Astrophysics and Planetary Sciences. In general astrophysics, the "snapshots" of observed sources are organized in evolutionary sequences for each class of object, and access to fainter sources is often required to extend the samples toward earlier stages. Physical processes at work are then mainly deduced from evolutionary curves. In Planetary Sciences, beyond the pioneer discoveries, physical processes are identified from the detailed exploration of coupling processes, time constants, and comparative responses to changing conditions. Of prime importance is the study of interactions between the various elements of planetary systems, and of the comparative response of a given object to various conditions and of comparable objects to similar conditions. The origin and evolution of bodies, and of the solar system as a whole, will then hopefully be deduced from the present stage considering the physical processes at work.

Different measurement needs then follow. Astronomical studies usually ask first for the highest sensitivity at every wavelength, without any special requirement on instrumental stability, pointing, or temporal consistency. In Planetary Astronomy, simultaneous access to the whole electromagnetic spectrum is of prime importance since non-simuitaneous observations may reflect different geometry and/or physical conditions. The historical and geographical context is required together with local studies, leading to the need for simultaneous imaging and spectroscopy and/or spectral imaging over a large field of view with high spatial resolution. High consistency is required in terms of pointing, field of view, and instrument geometry. Serial observations are needed over long terms, with time resolutions consistent with the various time constants of variable interactive phenomena. Most observing programmes must be repeated and applied to comparative bodies. Also, monitoring of external energy sources - solar UV flux and solar wind - is highly desirable.

Since many solar system objects fie near the plane of the ecliptic, the need for observations over an entire orbital period requires that observations be made at small solar elongations. This is illustrated in Figure 1, where it is seen that many objects would be observable less than half the time, or during the least interesting portion of their orbits, were planetary observations restricted to solar elongations greater than 90°/1/. This is also the case for high inclination objects (mainly the dynamically new

( 6 ) 1 4 6 P . B r u s t o n a n d M . J . M u m m a

180

150

.~120

~ 9o

~ 6o

3 O

-e-Mercur)" --G-Venus

0 200 400 600 800 Day Number

180

150

= 120

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~ 6 0

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\

200 400 600 800 Day Number

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/~ -e-Brorsen Comets =150 \ / i_e_Hal ley /~ / , . ~

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Fig. 1. Visibifity of Hanets and Comets. Solar elongation for planets and comets over a two year perio(L Solar-avoidance angles of 20 ° and 90 ° are shown, illustrating the interval for which observations of a given object are precluded. A lunar-based planetary observatory could have solar-avoidance angles as small as a few degrees, thereby enabling observations at virtually all times. After Mumma/1/.

TABLE 1 Frontiers of Planetary Astronomy in the 21st Century

MAJOR PLANETS: Atmospheric Composition and Slruclnre Hot Spots and Aurorae Meteorology and Dynamics Transport of Volatiles and Dust (MARS) Plasma environment and toil

Requires Large Aperture Telescope with Simultaneous Ultraviolet, Optical, and Infrared Spectral Imaging Capability.

MOONS AND RINGS: Structure and Spokes Mineralogy of Surface Units Volcanic Eruptions (IO, TRITON, EUROPA?) Atmospheric Chemistry (TITAN, TRITON, CHARON)

COMETS: Complete Characterization of the Nucleus for a Significant Sample of Comets Dynamically New and Old Comets

Nuclear Morphology Size, Shape, Albedo, Rotation Rate, Surface Temperature Chemical Heterogeneity (Both intra - - and inter--nuclear). Directly Sublimed Volatiles Condensed Phase Matter - - Refractories and Volatiles

Coma aeronomy; plasma physics of coma and tail

Requires Simultaneous UV/OpticafflR Spectroscopy and Synoptic Observing. Observation at Small Solar Elongations and Continuous Temporal Coverage are Required

for Many Comets.

ASTEROIDS: Morphology: Light Curves; Imaging Mineralogy of Surface Units

Main Advantage of Moon is for Observation of Asteroids at Small Solar Elongation

ORGANIC CHEMISTRY AND RELATION TO THE ORIGIN OF LIFE

SEARCH FOR EXTRA-SOLAR PLANETS AND PROTOPLANETARY SYSTEMS

Study of Bodies in our Planetary System (6)147

comets) because they are most active when closest to the sun, and this often corresponds to elongations less than 90 ° .

This need for observing at small solar elongation imposes severe operational constraints on most classes of telescopes, and presents perhaps the most striking operational difference between an observatory optimized for Planetary Sciences and one optimized for Astrophysics. The desired solar elongation limit for a Planetary Observatory is certainly no greater than a few degrees. The need for adequate solar rejection requires operations in space in order to eliminate atmospheric scatter, and thermal emission, and the lunar surface presents an attractive possibility for emplacing distant (and movable) sun screens.

The above discussion is obviously schematic. Extended (e.g. Young Planetary Systems) and variable (e.g. variable stars, supernovae) astronomical sources, are increasingly accessible to study because of improving experimental techniques. Studies of such sources are of increasing interest and call for new instrumental techniques and operational modes similar to those needed for Planetary Sciences.

FRONTIERS OF PLANETARY ASTRONOMY IN THE 21ST CENTURY

The frontiers of planetary astronomy in the 21st century have already been discussed by several authors, along with the lunar-based observatories needed to address them/I,2/. They are addressed in this symposium by several papers devoted to various fields such as solid surfaces, atmospheres, and plasma environments, and are summarized in Table 1 (after/1/). In the present paper, we highlight the measurement needs and instrumental requirements, and illustrate the advantages of a lunar-based planetary observatory, by discussing a few examples of complex interacting systems. The elements and some selected applications of a Lunar-based Planetary Observatory are given in Table 2.

A FIRST EXAMPLE OF A COMPLEX INTERACTING SYSTEM: THE JOVIAN SYSTEM

The Jovian system provides a good example of a highly dynamic ensemble. It is characterized by interactions between the volcanic activity, atmosphere, ionosphere and toms of Io; the magnetosphere, ionosphere and atmosphere of Jupiter; the solar activity; and the internal structure of the planet. The study of phenomena occurring in this environment strongly requires a synoptic, and spatially resolved, survey over the relevant time scales. The global processes together with the wavelength range concerned are summarized in Figure 2.

Io exhibits intense and highly variable volcanic activity, however the nature of this volcanism is poorly understood. An infrared image of thermal emission from two volcanic hot spots on Io are shown in Figure 3a (after/3/). One of these was active during the Voyager flyby, but the other became activated only recently. Volcanic plumes can be detected at ultraviolet (UV) wavelengths, and their frequency of occurrence, temporal evolution, and chemical composition can be determined by multi spectral observations. Their properties can be correlated with the hot spots observed in the thermal infrared (IR). This will improve our understanding of the nature of Io's volcanism.

The suggestion that Io's atmosphere is maintained by gas ejected from these vents is subject to large uncertainties, with implications for the modeling of toms loading mechanisms. Although the neutral environment of Io has been mapped in the Ca and Mg lines 141, the assumed main constituent, SO2, has only recently been detected 151, and the detailed atmospheric composition, structure and density remain almost unknown/6/. Such atmospheric properties and their relation to Io's activity, to toms characteristics, and to the structure and brightness of the Jovian anror~e, are needed if we are to understand this complex interacting system. Also poorly understood are the effects of the distortion of magnetic field lines on plasma flow in the toms. The modulation of Jovian non-thermal radio emissions by Io implies the existence in its vicinity of electric fields and currents whose presence might be revealed if aurorae are detected on Io.

(6)148 P. Bruston and M. J. Mumma

TABLE 2 Elements of a Lunar-based Planetary Observatory

Spectral Region Application

XUV - FUV

VUV- Mid UV

Optical

Infrared

Sub-millimeter - Radio

He II, He I, At, Ne, etc.

H, C, N, O, S, etc. CO, CO2 +, CS, OH, etc c o n t i n u u m

NH 2, NH, C2, C3, continuum

CO2, NH3, CI-I4, etc Organics, Silicates, Ices

(33 con~uum

Comets, Magnetospheres, Aurorae

Comets, Aurorae, Dayglow Photochemistry, Aerosols

Atmospheres, Clouds, Surfaces

Temperature profiles, Abundances Winds

Atmospheric structure, winds Radiation belts Sub-surface temperatures

Fig. 2. The Jovian System: Global Processes.

Io Solar Wind Volcanic Activity

~, satellites Atmosphere / Ionosphere ~ , - other than Io

of Io J ( U V - Vis - IR) fVis- uv) /

~. Jupiter Magnetic field Io toms , ~ --------*'and Magnetospheric Plasma

(Vis - V U V ) ~ - ~ - radio waves - ) / s (XUV)

f Jupiter Aurorae and Dayglow

• I Solar UV • Atmospheric Dynamics

Atmosph o (Vis- IR) a n d composition ~ [

(UV - Vis - IR - Submm) Intemal Structure

(oscillations)

Study of Bodies in our Planetary System (6)149

a) b)

c) d)

Fig. 3. Global-scale processes and time variable phenomena in Planetary Systems. ~ Jovian System as paradigm, a) Image of hot spots (volcanoes) on Io (3.8 IJ1n, Spencer et al./3/, h) Image of the Io plasma toms in the light of S + (visible, Schneider and Trauger/11/). c) Infrared aurorae seen in H3 + emission (3.4 tun, Baron et al./12/), d) Processes in the Jovian polar regions (Connerney/15/).

(6)150 P. Bruston and M. J. Mumma

The I t torus has been extensively observed ~om the ground/7,8/, by the Voyager UV spectrometer /9/and with IUE/101. Ground-based serial images reveal the dynamic nature of the toms, e.g. as seen in images of one form of ionized sulphur (SII) in Figure 3b/11/. However, our understanding of the torus remains uncertain because of the low spectral resolution and "snapshot" nature of the Voyager UVS; the low spatial resolution, limited spectral range and long integration time with IUE; and the limited spectral window accessible from the ground. Studies of the energy balance, structure, and composition of the torus need images in several - - mainly extreme ultraviolet (EUV) - - spectral lines (or multiplets) from each of the various ionization stages of both oxygen and sulphur. Observations are needed over a rotation period (10 hours) with adequate time resolution, and must be repeated for many rotations - ultimately spanning an entire solar cycle.

The plasma composition and temperature of the inner torus have been shown to vary through the Pioneer and Voyager encounters and ground-based observations /11/ Monitoring the temporal variabilities of the whole system should provide information about the magnetospheric processes at work. These results should explain in turn the remarkable persistence and stability of the toms over more than a decade of observations.

The Io torus is the main source of plasma in Jupiter's environment. Other likely sources however, from the Jovian atmosphere and the solar wind, are poorly known, limiting our ability to model the energy balance and transport mechanisms within the Jovian magnetosphere, and its complex interaction with the atmosphere of the planet. Mapping the magnetosphere in the He II XUV line over times intervals spanning diurnal and seasonal periods of both Io and Jupiter is a major challenge for future investigations.

Like Earth, Jupiter exhibits intense optical aurorae and non-thermal radio emission in its polar regions. Plasma electrons and/or ions precipitate along the magnetic field lines and interact with the Jovian atmosphere, giving rise to UV (H Lyman a and H2 Werner and Lyman bands) and IR emissions. An image of auroral emission in the light of H3 +, near 3.4 pan, is shown in Figure 3c/12/. The UV-H2 aurorae, seen with Voyager UVS, have been extensively studied with IUE/13,14/, and their rough behaviour is now relatively well understood. However, because of the low spatial resolution of IUE, their detailed structure and location in magnetic L coordinate, and their exact relationship to It's activity, to plasma interactions in the vicinity of Io, and/or to the Io toms, remain quite uncertain. Figure 3d presents a schematic view of Jovian high latitude phenomena/15/.

The UV aurorae are clearly caused by low-energy secondary electron excitation, but the nature of the primary particles is controversial and induces two-fold uncertainty in the inferred input power in the Jovian auroral atmosphere. Modeling of the structure and chemistry of the ionosphere also depends on the input flux of oxygen and sulfur ions. IR emissions have been observed in acetylene, methane, ethane, and H3 + bands, and show different behaviors/16,17,18,19L Their relationship to each other and to the UV emissions is not yet clear, since they originate at different altitudes in the atmosphere. Do they solely reflect differing excitation conditions, or also local modifications in atmospheric chemistry, induced by energetic particles and/or related to atmospheric dynamics? High spatial resolution monitoring in the various bands and simultaneous studies of the atmospheric behaviour are clearly needed. Another highly controversial question concerns the excitation process giving rise to spatially diffuse UV emission on the dayside of the giant planets.

Atmospheric structure, composition and dynamics are obviously strongly coupled, and related to the internal and external energy sources. The Jovian atmosphere exhibits dynamical structures over a wide range of spatial and temporal scales. High resolution global surveys of cloud features and variabilities, but also simultaneous mapping at wavelengths originating from various altitude levels including the thermal IR and submillimetric range, are needed in order to build up the basic data set needed to address the processes involved. Vertical profiles of temperature and abundances are needed to separate photochemical and dynamical effects. Of special interest are the abundance profiles of transient species, such as CO, AsH 3, and PH3, transported upward from the deep atmosphere in vertical streams, and photochemically dissociated in the stratosphere. Understanding

Study of Bodies in our Planetary System (6)151

the vertical distributions of these species will clarify the competitive roles of internal and external forcing terms. Comparative observations will have to be performed on Saturn, Uranus and Neptune.

One of the most promising applications of lunar-based planetary astronomy concerns the area of seismology of the outer planets. Analogous to helioseismology, inferences on the internal structure of the giant gaseous planets may be drawn from atmospheric manifestations of planet-scale wave motions, forced by internal gravity waves from the deep interior. Recent attempts have been made at infrared and optical wavelengths, with encouraging results. Denting et al. /20/discovered a set of planetary scale waves on Jupiter, which are stable over many rotations and which have wavenumbers 4-20. However, the data are not yet of sufficient quality to show rotational splitting or to determine whether the waves are locked to the interior rotation period or to the cloud-tops. Mosser et al. 1211 made optical measurements in sunlight scattered from the (integrated) disk, and found frequency signatures that seemed to correspond well to forcing by internal eigenmodes. However, both techniques suffer greatly from limitations imposed by earth-atmospheric conditions and from interruptions in the data stream imposed by Earth rotation, which introduces unwanted splittings in the modal frequencies. A lunar-based planetary observatory could provide uninterrupted data streams of two weeks or more, largely eliminating these and other problems associated with ground- based observations.

So, although existing observational capabilities (e.g. ground-based and HST), and mostly the Galileo and Cassini missions, will clearly contribute to our understanding of fundamental questions, a lunar- based planetary observatory would provide a unique opportunity to advance our understanding of the present state and evolutionary history of the atmospheres of the giant planets.

COMETS

Comets are among the most important targets for telescopic observations from earth-orbit, for three reasons. First, they provide a crucial link between processes now occurring in regions of active star- and planetary system-formation and those that did occur during the formation of our own planetary system (Figure 4a/22/). Second, the acquisition of information on certain key aspects of cometary composition and structure requires multi-spectral observations from space. Third, comparative observations of many comets and intensive measurements of individual comets are needed, surpassing the ability, of ground-based observatories to provide the necessary temporal coverage. Serial measurements of individual comets (spanning many cometary "days") can be used to reveal the degree of nuclear heterogeneity by measuring the change in relative and absolute production rates over times short compared to the "day". A recent review of cometary properties as they relate to their origin and evolution is given in 123/.

In the Goldreich-Ward model, an individual cometary nucleus formed from a single volume element of the solar nebula by gravitational collapse. Because of earlier dynamical mixing this volume element would have contained material with varied processing histories in the nebula (e.g. icy- mantled interstellar grains, 'stripped' grains, chondrules). Following collapse, these materials would be homogeneously mixed within the resulting cometary nucleus. However, processes that affected nucleus formation are poorly understood. Turbulence may have delayed the collapse, permitting collisional accretion of smaller cometesimals that only later accreted into the final nucleus. Weidenschilling /24/ predicts accumulation of icy planetesimals to sizes of order 50-100 meters before gravitational instabilities set in and agglomeration of the final cometary nucleus occurs.

Radial mixing of cometesimals formed in dissimilar regions of the solar nebula could have preceded agglomeration of the final nucleus, leading to a structure similar to Weissman's 'rubble pile' model (Figure 4b/25/). Such mixing is often invoked to explain the simultaneous presence of chondrules and more primitive matrix material in carbonaceous chondrites, for example. The presence of chemical heterogeneity in cometary nuclei is the expected signature of this cosmogony, much as chemical homogeneity may result from the Goldreich-Ward cosmogony. Measurements of the nuclear composition and heterogeneity are needed to clarify the processes that actually occurred in the nebular region(s) where comets formed. The degree to which cosmogonic signatures were

(6)152

Table 3.

P. Bruston and M. J. Mumma

Cometary Volatiles: Their Sublimation Temperatures, Distances in the Solar Nebula, and a Comparison with Interstellar Abundances. Mumma et al. 1231.

Species Temperature Distance RelativeAbundance (K) (AU) Comets ISM-gas ISM-iee

H.,O 152 3.4-5.4 100 < 100 100

CH3OH 99 1-5 0.01-0.1 7-40

HCN 95 0.02-0.1 0.01-0.1 4

SO., 83 < 0.002 0.01-0.1 "~ < 0.01

NH3 78 0.1-0.3 0.2-2 < 5

CO., 72 13.3-14.6 3 < 10 ?

H2CO 64 0-5 0.1-0.3 < 0.2 0.1-0.04

H.,S 57 0.2 <0.01 0.3

(CO) (50)

CH4 31

CO 25

N2 22

• $2 20

22-25 (If co-deposited with water)

60-50 0.2-1.2 1 < 1 < 0.2

84-63 ~7 500-2000 0-5 0-20

112-79 0.02 I00 ?

0.025 ? ?

Inf£1in$ : ~ mautri~d from siam

C'hcmicaUy Ncbu~ radiation active :m~ ~a ~ / '

Outer ,$ol at blcb~a: "$nowli~" Pro~stm Mix~ of ~u~ially proc~r, cd GMC

solar acbular la.s. |ovi~ planct.

b)

Fig. 4. a) Schematic representation of processes in the solar nebula. Cometary nuclei are thought to have formed in the Uranus-Neptune region and beyond, and their compositions should preserve a record of the processes, conditions, and mechanisms of their formation (After Lunine/22/). b) One concept of the structure of cometary nuclei: the rubble pile model of Weissman 1251. Individual eometesimals may have different compositions if they formed at different temperatures and at different distances from the proto-stm. See Mumma et al./23/for a detailed discussion.

Study of Bodies in our Planetary System (6)153

modified by cosmic ray processing in the to r t cloud and thermal processing after re-injection into the planetary region may also be revealed by making internuclear comparisons, e.g. of dynamically new and periodic comets.

In 1950, Whipple formulated the icy conglomerate model for comets in which water ice was dominant among other frozen volatiles and dust, and proposed that these ices preserve a record of processes from the earliest epoch of the solar system. The direct detection of these "parent volatiles" proved elusive (except for CO) until 1985, when the first clear detection of cometary water was achieved by infrared spectroscopy from NASA's Kuiper Airborne Observatory (KAO) 1261. Since then, infrared spectroscopy from space-borne (IKS on Vega-l), ground-based, and airborne observatories has provided quantitative detections of H20, CO2, CO, H2CO, and CH3OH 127-30/. For CH4, a provisional detection was achieved in a dynamically new comet (Wilson 1987 VII) 1311 and important upper limits were obtained for comet Levy (1990 XX)/32/. In addition, the nature of the 3.4 Ixm "organic" feature was explored 1331, and it was found that a significant portion of it actually originates from methanol 1341. It was also found that comets may fall into two groups depending on their methanol abundance. Millimeter observations provided clear detections of CH3OH, H2S, and H2CO and set upper limits for many other candidate molecules/35,36,37/. It is now amply clear that vibrational and rotational spectroscopy will provide profound advances in our understanding of the volatile fraction of comets, and thus will clarify aspects of the formation and the evolution of these bodies. A recent review of cometary properties and their relation to cometary origins is given by Mumma, Weissman, and Stern 1231.

High resolution infrared spectra of comet Halley showed individual lines of cometary water vapor, from which key information on kinetic, rotational, and nuclear spin temperatures was obtained/26/. However, additional spectral lines were obscured by a band of atmospheric CO2, and this lead to uncertainties in retrieved values for several key parameters. For example, the spin temperature for cometary water may have cosmogonic significance/38,39/, and it is desirable to obtain intensity measurements for the complete set of spectral lines of ortho- and para- water. This can best be achieved from space.

Other examples of key abundances and ratios include CO/CH4 and CO/CO2, and the abundances of the noble gases: At, Ne, He, all regarded as strong indicators of formation processes. The ratio CO/CH4 tests the oxidation state of volatile carbon in pre-cometary ices, while the ratio CO/CO2 may test the temperature at which the cometary ices condensed. CO and CI-I3OH both vary strongly from comet to comet, but CO2 + shows little (but not zero!) variability 1231. The abundance of the noble gases is strongly dependent on formation temperature. Ne and Ar may be most diagnostic since they sublime at 16K and 31 K, respectively, temperatures which are characteristic of the 100 and 30 AU ranges in models of the solar nebula. Helium is not expected to be present in cometary nuclei for physical reasons, but Ar and Ne can be retained in cometary nuclei at tor t cloud temperatures/23/. Direct detection of Ar and Ne require FUV spectroscopy from space, CO2 requires infrared observations from space, and CO is easily detected at arbitrary geocentric velocity from space.

In addition to the search for structural and compositional differences in the nuclei of short period comets, we also wish to study dynamically new comets. New comets may reveal variable chemical ratios, as sublimation proceeds from radiation-processed material in the outer layer to less processed material deeper down, on their first apparition. Sequential measurements of this type may provide information on their exposure time to cosmic rays, and may reveal the influence of other kinds of processing (e.g. eollisional gardening, regolith formation) on cometary evolution during residence in the tor t cloud.

New and old comets may also differ in other ways. The nuclear spin temperatures retrieved pre- and post-peribelion for water in comet Halley cluster near 29K, but the results for comet Wilson (1977 VII) are consistent with a spin temperature greater than 50K. According to current models/40/, cometary nuclei are processed by cosmic rays to depths of order 100-1000 gm cm -2 while in the t o r t cloud. Dynamically new comets (e.g. Wilson) should show the effects of this (and other) processing, but the currently active regions of short period comets (e.g. Halley) will have lost these JA~ ]4:6-K

(6)154 P. Bruston and M. J. Mamma

damaged layers during previous apparitions and their spin temperatures may be characteristic of the temperature at which the water ice was last processed prior to incorporation into the cometary nucleus. Comets in the to r t cloud also may experience various other processes including mild warming, and collisional gardening and erosion, but their relative importance is unknown at present /41,42/. We hope to constrain these models by observing the change in spin temperature and chemical composition with depth in new comets.

Abundances found for volatiles in recent comets are compared with those found in interstellar gas and ice in Table 3, and their condensation temperature and radius in the solar nebula are also shown 1231. The listed cometary abundances should be taken as only a guide, however, since comets are known to be variable. One powerful aspect of a lunar-based planetary observatory is that it could provide simultaneous coverage of a spectrally complete interval from far ultraviolet to infrared or even millimeter wavelengths. The near infrared (2-5 Ixm) includes at least one vibrational fundamental for virtually every infrared active molecule, the millimeter and sub-millimeter molecular lines provide very powerful probes of coma kinematics and rotational temperatures, and the thermal infrared (3-40 pan) is key for studies of the dust. Thus, whatever infrared active material might be produced in sufficient quantity from the nucleus will be seen. Ultraviolet spectroscopy provides an inventory of the atomic fraction and of certain key molecular dissociation products. If extended to far ultraviolet wavelengths, it would be possible to obtain sensitive abundance measures of noble gases (e.g. At, Ne) and of N2, which are sensitive probes of the formation temperature of cometary nuclei. The connection between the organic refractory grains discovered in comet Halley and the volatile carbon inventory could be investigated systematically. Optical imaging could record the smallest dust particles for comparison with the larger particles imaged at thermal wavelengths, and each of these could be compared with images in specific volatiles, such as CN, H2CO, and CO, to clarify the connection among these species.

Large scale intra-nuclear heterogeneity (e.g. H2CO, C02) has now been observed in comets P/Halley and Levy (1990 XX), and inter-nuclear variability has been found for several other chemicals (e.g. methanol, carbon monoxide, methane) in recent comets. The light curves for comet Halley revealed three major active regions during March and April 1986/43/. Although they appeared identical in CN, C2, NH2 and other species, the production of formaldehyde was significant for only one of these regions, suggesting that the nucleus was chemically heterogeneous on a macroscopic scale/29/. Hemispherical asymmetry was also observed for CO2 in comet Levy (1990 XX)144/. Possible explanations are: 1) the observed heterogeneity is cosmogonic, i.e. the active regions contain material (cometesimals) processed in different regions of the solar nebula, gravitationally scattered by the proto-planets, and only later accreted into the present cometary nucleus. 2) the observed heterogeneity is derived, e.g. a result of thermal processing which resulted in zonal distillation of the more volatile fraction. Comparison with other comets would help to resolve these possibilities.

The long range objectives of cometary science with a lunar-based planetary observatory thus have two key aspects:

1) We wish to investigate the chemical heterogeneity of the nucleus for short period comets. We wish to measure their serial production rates at time intervals which are short compared with a nuclear rotation period, and then to continue this process for many nuclear rotation periods. Since the shortest rotation periods known are ~6 hours, and the longest ~ 7.4 days (comet Halley), serial observations at hourly intervals and continuing for several months are needed, while the comet is in its active phase. We hope to see correlations with rotational phase similar to those found for H2CO in comet Halley. By examining the ratios CH4/CO, CO/CO2, CH3OH/H2CO, etc, and all of these relative to water, we can determine 1) the degree of processing experienced by this material prior to being incorporated into the nucleus, 2) the oxidation state of the region in which it was last processed, 3) the temperature at which it condensed, and 4) whether accretion occurred homogeneously or heterogeneously. It may be possible to identify the origins of short period comets ( Kuiper belt region vs, Uranus-Neptune region) on the basis of their compositions and spin temperatures, and thus to test current models for the origin of short period comets.

Study of Bodies in our Planeta~ System (6)155

2) We wish to make similar measurements on dynamically new comets, emphasizing the change in chemical composition with depth. Serial measurements of the production rates, beginning as early as possible after discovery, could reveal the processing experienced by cometary nuclei while in the Oort cloud (cosmic ray processing, collisional gardening, regolith formation, etc.). Observations of the first few 100 gra cm -2 are crucial. Serial observations can test the ideas that: 1) a regolith exists, 2) that radiation processing decreases with depth, 3) that erosion by interstellar grains has removed the outer layer of radiation-processed material, 4) that the spin temperature for cometary water varies with depth, and other related questions. As the processed layer is depleted serial measurements may reveal the presence of macroscopic heterogeneity and comparison with short-period comets could reveal whether the two populations formed in similar ways, and in the same or different regions of the solar nebula.

These aspects are of fundamental interest to questions concerning the origin of the solar system.

PLANETARY CAMPAIGNS

In order to advance our understanding of the underlying physics, we really need to study interactions between the various elements of planetary systems, and to study the comparative response of both a given object to various conditions and of comparable objects to similar conditions.

These studies are best conducted in "campaigns", i.e. comprehensive programmes combining simultaneous coherent observations of every interacting piece of the puzzle. While temporal simultaneity is a key feature, these observations must also be conducted over extended periods of time. The "campaigns" would be defined and carried out in cooperation by a set of scientific teams responsible for individual aspects of the investigations and their respective interpretations. They must include both imaging and spectroscopy over a wide spectral range, from ultraviolet to infrared, and must feature simultaneous operation in various modes. Table 2 gives a summary of the elements needed for an appropriate observatory

ADVANTAGES OF THE MOON AS A SITE FOR PLANETARY ASTRONOMY

The Moon presents many advantages as a site for astronomical observations. As far as planetary astronomy is concerned, two of them are of outstanding interest: the ability to observe at small angular distances from the sun with a large aperture telescope, and the long unbroken observation time available on source.

A large aperture is required in order to obtain the desired spatial resolution. For example, with a 3 m telescope, Neptune's diameter will subtend 57 pixels at 500 nm and 3 pixels at 10 Itm, Io's over 28 pixels at 500 nm and 2 at 10 ttm, Titan's over 19 at 500 nm but only 1 at 10 ttm. A 10-meter telescope on the moon would provide ten times more pixels on the areal disk, with a corresponding increase in scientific capability. For example, it would be possible to investigate the temporal development of the Great Dark Spot on Neptune, discovered by Voyager, and to investigate changes in its thermal structure and composition with time.

Observations at small solar elongations are needed on almost every solar system body. Indeed it is desirable to study planets over most of their orbital period, and comets over the entire time they are active but also during their inactive phase. Figure 1 shows the visibility of planets and comets over a two year period. A sun avoidance even as low as 30 ° limits the observability of Ve~ius to short periods at a very peculiar sun-planet-observer geometry. It prevents observations of most comets during at least part of their most active phase, and this is all the worse on some small-perihelion dynamically new comets which are of special interest.

Another interesting possibility follows from the large stable area of available surface and the small sidereal rate of a Lunar station. Those advantages allow the use of occulting masks at long distance from the telescope, and the deployment of very long baseline interferometric arrays. Of great interest

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in the search for extra-solar planetary or proto-planetary systems, such instruments will also allow detailed thermal scanning of small bodies in our planetary system.

REFERENCES

1. M. J. Mumma, Planetary Astronomy in the 21st Century: The Study of Planetary Systems, in Astrophysics from the Moon, AIP Conference Proceedings 207, eds. M. J. Mumma and H. J. Smith, AIP Press, New York 1990, pp. 13-29.

2. See Chapter 1, in Astrophysics from the Moon, AIP Conference Proceedings 207, eds. M. J. Mumma and H. J. Smith, AIP Press, New York 1990, 656 p.

3. J .R. Spencer, M. A. Shure, M. E. Ressler, J. D. Goguen, W. M. Sinton, D. W. Toomey, A. Denault, and J. Westfall, Discovery of hotspots on Io using disk-resolved infrared imaging, Nature 348, 618-621 (1990).

4. W.H. Smyth, lo's Sodium Clouds: Explanation of the East-West Asymmetries, Ap. J. 234, 1148- 1153 (1979).

5. E. Lellouch, M. Belton, I. De Pater, G. Paubert, S. Gulkis and T. Encrenaz, The structure, stability, and global distribution of Io's atmosphere, Icarus 98, 271-295 (1992). For a search at ultraviolet wavelengths, see: G. E. Ballester, D. F. Strobel, H. W. Moos, and P. D. Feldman, The atmospheric abundance of SO2 on Io, Icarus 88, 1-23 (1990).

6. G. E. Ballester, H.W. Moos, P. D. Feldman, D. F. Strobel, M. E. Summers, J.-L. Bertaux, T. E. Skinner, M. C. Festou and J. H. Lieske, Detection of neutral Oxygen and Sulfur emissions near Io using IUE, Ap. J. 319, L33-L38 (1987).

7. J. T. Trauger, The Jovian nebula: A post-Voyager perspective, Science 226, 337-341 (1984).

8. J. S. Morgan, Temporal and spatial variations in the Io torus, Icarus 62, 389-414 (1985).

9. A.L. Broadfoot, B. R. Sandel, D.E. Shemansky, J. C. McConnell, G. R. Smith, J. B. Holberg, S. K. Atreya, T. M. Donahue, D. F. Strobel and J. L. ,Bertaux, Overview of the Voyager ultraviolet spectrometry results through the Jupiter encounter, J. Geophys. Res. 86, 8259-8284 (1981).

10. H. W. Moos, T.E. Skinner, S. T. Durrance, P. D. Feldman, M. C. Festou, and J.-L. Bertaux, Long- Term stability of the Io high-temperature plasma toms, Ap. J. 294, 369-382 (1985).

11. N. M. Schneider and J. T. Trauger (unpublished data, personal communication). Also see: D.F. Strobel, Energetics, luminosity, and spectroscopy of Io's toms, in Time-Variable Phenomena in the Jovian System, eds. M. J. S. Belton, R. A. West, and J. Rahe, NASA SP-294, Washington D. C. 1989, 183-195.

12. R Baron, R. D. Joseph, T. Owen, J. Tennyson, S. Miller, and G. E. Ballester, Imaging Jupiter's aurorae from H3 + emissions in the 3-4 ixm band, Nature 353, L539-542 (1991).

13. T. A. Livengood and H. W. Moos, Jupiter's north and south polar aurora~ with IUE data, Geophys. Res. Lett. 17, 2265-2268 (1990).

14. T. A. Livengood, H. W. Moos and R. M. Prang6, Jovian ultraviolet auroral activity, 1981-1991, Icarus 97, 26-45 (1992); J. Clarke, J. Caldwell, T. Skinner, and R. Yelle, The aorora and airglow of Jupiter, in Time-Variable Phenomena in the Jovian System, eds. M. J. S. Belton, R. A. West, and J. Rahe, NASA SP-294, Washington D. C. 1989, 1211-220.

Study of Bodies in our Planetm'y System (6)157

15. J.E.P. Connerney, Doing more with Jupiter's magnetic field, in Planetary Radio Emissions 111, Proe. 3rd International Workshop (Sept 2-4, 1991 Graz, Austria), eds. H. O. Rucker, S. J. Bauer, and M. L. Kaiser, Austrian Academy of Sciences (Wien, Austria 1992), pp. 13-33. Also see: F. Bagenal, Torus-Magnetosphere coupling, in Time-Variable Phenomena in the Jovian System, eds. M. J. S, Belton, R. A. West, and J. Rahe, NASA SP-294, Washington D. C. 1989, 196-210.

16. P. Drossart, B. B6zard, S. Atreya, J. Lacy, E. Serabyn, A. Tokunaga and T. Encrenaz, Enhanced acetylene emission near the north pole of Jupiter, Icarus 66, 610-618 (1986).

17. J. Caldwell, R. Halthore, G. Orton and J. Bergstrahl, Infrared polar brightening on Jupiter. IV. Spatial properties of methane emission, Icarus 74, 331-339 (1988).

• 18. T. Kostiuk, F. Espenak, M. J. Mumma, D. Deming, and D. Zipoy, Variability of ethane on Jupiter, Icarus 72, 394-410 (1987); T. Kostiuk, F. Espenak, M. J. Mumma, and P. Romani, Infrared studies of hydrocarbons on Jupiter, Infrared Physics 29, 199-204 (1989).

19. P. Drossart, R. Prang6, and J.-P. Maillard, Morphology of infrared H3 + emissions in the auroral regions of Jupiter, Icarus 97, 10-25 (1992).

20. D. Deming, M. J. Mumma, F. Espenak, D. E. Jennings, T. Kostiuk, G. Wiedemann, R. Loewenstein, and J. Piscitelli, A search for p-mode oscillations of Jupiter: Serendipitous observations of nonaeoustic thermal wave structure, Ap. J. 343, 456-467 (1989).

21. F. X. Schmider, B. Mosser, and E. Fossat, Possible detection of Jovian global oscillations, Astron. and Astrophys. 248, 281-291 (1991); B. Mosser, F. X. Schmider, Ph. DeLache, and D. Gautier, A tentative identification of Jovian global oscillations, Astron. and Astrophys. 251,356-364 (1991).

22. J. I. Lunine, The Urey prize lecture. Volatile processes in the outer solar system, Icarus 81, 1-13 (1989).

23. M. J. Mumma, P. R. Weissman, and S. A. Stern, Comets and the origin of the Solar System: Reading the Rosetta Stone, in Protostars and Planets III, eds. E. H. Levy and J. I. Lunine, Univ. Adz Press., Tucson 1993, pp. 1177-1252.

24. S. J. Weidenschilling, Evolution of grains in a turbulent solar nebula, Icarus 60, 555-567 (1984).

25. P. R. Weissman, Are cometary nuclei primordial rubble piles?, Nature 320, 242-244 (1986).

26. M.J. Mumma, H. A. Weaver, H. P. Larson, D. S. Davis, and M. Williams, Detection of water vapor in Halley's comet. Science 232, 1523-1528 (1986).

27. M. Combes, et al. (17 authors), The 2.5-12 lxm spectrum of comet Halley from the IKS-Vega experiment, Icarus 76, 404-436 (1988).

28. M. DiSanti, M. J. Mumma, J. Lacy, and R. Parmar, A possible detection of infrared emission from CO in comet Austin (1989cl), Icarus 96, 151-160 (1992).

29. M. J. Mumma, and D.C. Reuter, On the identification of formaldehyde in Halley's comet, Ap. J. 344, 940-948 (1989).

30. S. Hoban, M. J. Mumma, D. C. Reuter, M. DiSanti, R. R. Joyce, and A. Storrs, A tentative identification of methanol as the progenitor of the 3.52 Ixm feature in several comets, Icarus 93, 122- 134 (1991).

31. H. P. Larson, H. A. Weaver, M. J. Mumma, and S. Drapatz, Airborne infrared spectroscopy of comet Wilson (19861) and comparisons with comet Halley, Ap. J. 338, 1106-1114 (1986).

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32. T. Y. Brooke, A. T. Toktmaga, H. A. Weaver, G. Chin, and T. R. Geballe, A sensitive upper limit on the methane abundance in comet Levy (1990c), Ap. J. 372, Ll13-116 (1991).

33. C. F. Chyba, C. Sagan, and M. J. Mumma, On the nature of the organic grain feature in comets, Icarus 79, 362-381 (1989).

34. D.C. Reuter, The contribution of methanol to the 3.41xm emission feature in comets, Ap. J. 386, 330-335 (1991).

35. D. Bockel6e-Morvan, P. Colom, J. Crovisier, D. Despois, and G. Paubert, Microwave detection of hydrogen sulphide and methanol in comet Austin (1989cl), Nature 350, 318-320 (1991).

36. P. Colom, J. Crovisier, D. Boekel6e-Morvan, D. Despois, and G. Paubert, Formaldehyde in comets: I. Microwave observations of P/Brorsen-Metcalf (1989 X), Austin (1990 V), and Levy (1990 XX), Astron. Astrophys. 264, 270-281 (1992).

37. J. Crovisier, D. Bockel~-Morvan, P. Colom, D. Despois, and G. Paubert, A search for parent molecules at millimetre wavelengths in comets Austin 1990 V and Levy 1990 XX: upper limits for undetected species, Astron. Astrophys. in press (1993).

38. M. J. Mumma, W. E. Blass, H. A. Weaver, and H. P. Larson, Measurements of the ortho-para ratio and nuclear spin temperature of water vapor in comets Halley and Wilson(19861) and implications for their origin and evolution, Proc. STScl Conference on Origins and Evolution of Planetary Systems, STScI Publication (1988).

39. M. J. Mumma, W. E. Blass, H. A. Weaver, and H. P. Larson, Measurements of the ortho-para ratio and nuclear spin temperature of water vapor in comets Halley and Wilson(19861) and implications for their origin and evolution, Bull. Am. Astron. Soc. 20, 826 (1988).

40. R. E. Johnson, Irradiation effects in a comet's outer layer, J. Geophys. Res. 96, 17,553-17,557 (1991).

41. S. A. Stern, Collisions in the tor t cloud, Icarus 73, 499-507 (1988).

42. S. A. Stern, and J. M. ShuU, The influence of supernovae and passing stars on comets in the tor t cloud, Nature 332, 407-411 (1988).

43. R. L. Millis, and D. G. Schleicher, Rotational period of comet Halley, Nature 324, 646-649 (1986).

44. P. D. Feldman, S. A. Budzien, M. C. Festou, M. F. A'Hearn, and G. P. Tozzi, Ultraviolet and visible variability of the coma of comet Levy, Icarus 95, 65-72 (1992).