Status Report

NRC Decadal Study: Planetary Atmospheres: 4th Draft White Paper

By SpaceRef Editor
October 22, 2001
Filed under , ,


D. L. Huestis (SRI), S. K. Atreya (U. Michigan), S. J. Bolton (JPL),
S. W. Bougher (U. Arizona), A. Coustenis (Paris-Meudon Obs.),
S. G. Edgington, A. J. Friedson (JPL), C. A. Griffith (NAU),
S. L. Guberman (ISR), H. B. Hammel (SSI), J. I. Lunine (U. Arizona),
M. Mendillo (Boston U.), J. Moses (LPI), I. Mueller-Wodarg (U. Col. London),
G. S. Orton (JPL), K. A. Rages (NASA Ames), T. G. Slanger (SRI),
D. V. Titov (MPI Aeronomy), R. Yelle (NAU)


Observing, characterizing, and understanding planetary atmospheres are key
components of solar system exploration. A planet’s atmosphere is the
interface between the surface and external energy and mass sources.
Understanding how atmospheres are formed, evolve, and respond to
perturbations is essential for addressing the long-range science objectives
of identifying the conditions that are favorable for producing and
supporting biological activity, managing the effects of human activity
on the Earth’s atmosphere, and planning and evaluating observations of
extra-solar planets.

Our current knowledge, based on very few observations, indicates that
the planets and moons in the solar system have diverse atmospheres with
a number of shared characteristics. Comparing and contrasting solar
system atmospheres provides the best near-term
means of addressing the broad
scientific goals. Additional space missions with specific atmospheric
objectives are required. At the same time, investment of additional
resources is needed in the infrastructure of observation and
interpretation of planetary atmospheres.

The State of Knowledge Today

The current observational characterization of planetary atmospheres is
roughly comparable to what had been learned about the Earth’s atmosphere
after the first rocket and satellite measurements in the 1950s and 1960s.
From telescope observations and planetary missions we have determined the
principal atmospheric constituents and the altitude profiles of pressure
and temperature. We are able to classify the atmospheres of many of the
larger solar system planets and moons into four groups:

  1. Nitrogen atmospheres (Earth, Titan, Triton, Pluto)
  2. Carbon dioxide atmospheres (Venus, Mars)
  3. Hydrogen gas giants (Jupiter, Saturn, Uranus, Neptune)
  4. Thin atmospheres, with three subgroups:
    • Rocky surfaces (Mercury, Moon)
    • Volcanic (Io)
    • Icy surfaces (Europa, Ganymede, Callisto)

Interpretative studies of radiative transport and collisional processes
in the atmospheres of Venus and Mars have helped us understand the
“greenhouse effect” and the impact of continued release of
carbon dioxide into the Earth’s atmosphere. Characterization of the
composition of the atmospheres of the gas giants provides guidance about
how planets and their atmospheres originate and how to interpret
observations of extrasolar planets. Exploration of the current and
historical abundance and state of water in the atmospheres, surfaces, and
subsurfaces of Mars, Europa, Venus, and the Moon will provide important
clues about photochemical stability of planetary atmospheres and the
production of prebiotic chemistry.

Unfortunately, even with an increasing volume of observational data,
planetary atmospheres are still grossly undersampled. For example,
at the relevant altitudes in the atmospheres of Mars and Venus
we have no observations of the minor chemical species (HOx,
ClOx, SOx) that models suggest are responsible
for the stability of CO2 atmospheres as a result of
catalytic recombination of CO and O2 and for catalytic
depletion of ozone in the Earth’s atmosphere.
Thus far we have
sampled only the upper atmosphere of Jupiter. Without knowledge of
the abundance of the heavier elements C, N, and O, in the deep
atmosphere, little can be said about whether the gas giant planets
reflect the initial elemental composition of the solar system.
The nitrogen/hydrocarbon atmospheres of Titan, Triton, and Pluto
can provide important clues about photochemical formation of complex
organic molecules in the early atmosphere of the Earth. From

In addition, investigations of the Earth’s atmosphere show
that significant unpredictable variations occur on time scales of
hours, vertical scales of a few kilometers, and horizontal scales of
hundreds of kilometers. The atmospheres of many planets reveal
structure and variation with respect to latitude, longitude, and season.
Everything changes with solar cycle. Atmospheric models are very
complicated. Many of the underlying chemical and physical processes
are still poorly characterized. We think that we can produce useful
explanations, but we do not have the data needed to ensure confidence
that models can make quantitative predictions.

Key Questions and Science Themes

  • Understanding Atmospheres

    The historical attempts to understand planetary atmospheres
    have emphasized identification
    of the underlying chemical and physical processes responsible for the
    many fascinating observations.
    It is
    appropriate that the focus should now shift toward comparative
    interpretation of what the atmospheric observations and discoveries
    on multiple planets can teach us about broader scientific goals.

  • Learning by Exploring Planets and Moons

    Atmospheres are different each time we look at them.

    All future planetary mission campaigns should include
    explicit atmospheric components. Increased availability
    for planetary astronomy
    of observing time on ground-based and near-Earth space-based
    telescopes is essential.

  • Providing the Required Research Infrastructure

    Visiting planets is only one of the objectives. Lasting value
    comes from analyzing, interpreting, and using the data to
    establish broader implications, supported by independent
    programs for laboratory experiments, fundamental theory,
    modeling, and reanalysis of historical observations.

  • Assimilating Space and Planetary Science with Earth Science

    From our near neighbors in the solar system we hope to acquire
    additional hints about our origins and the steps we should take
    to preserve our life-supporting environment. Better coordination
    between Earth science and space and planetary science can
    to shared science goals, and justification and mobilization of
    additional funding resources for both disciplines.

Summary of Recommendations

The recommendations of the Planetary Atmospheres Community Panel
fall into two broad categories. Recommendations
that apply to multiple planets include creation of a new Comparative
Planetary Atmospheres program, establishing a mechanism for secure funding
for analysis and interpretation of mission data, creation of a new
“Super-Discovery” program for more ambitious planetary missions,
enhancement of laboratory and theory research, and deployment of
space- or ground-based telescopes dedicated to planetary observations.
Recommendations for specific planetary missions with atmospheric
goals include deep-penetration multiprobes to determine elemental
compositions of giant planet atmospheres, Venus and Mars atmospheric
explorer missions, and a Post-Cassini atmospheric/surface mission to Titan.

Other Issues

  • Public Fascination with Planets

    Planetary observations make good news and well-watched television.
    Unfortunately, atmospheres look too much like chemistry and plasma
    physics. Other than the Jupiter impact of Shoemaker-Levy 9, neat
    colorful pictures are rare. U.S. citizens are better educated
    and intelligent than we suppose. Atmospheric scientists can do
    much more to explain why what we do is interesting, understandable,
    and important.


Introduction to Planets and Atmospheres

This document focuses on Planets, defined as
the large objects orbiting the Sun: including Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto,
along with their associated moons and companions.

The specific emphasis is on Atmospheres, defined as
the interface between the planetary interior and the
interplanetary medium: beginning with the top cm of the planetary
surface; including atoms, molecules, ions, electrons, and cloud
particles bound by the planet’s gravitational field; also
including planetary magnetic fields; and extending to the limit
of the planet’s non-gravitational influence on the interplanetary

It is generally believed that the atmospheres of the small “rocky”
planets and moons (e.g. Earth, Venus, Titan, etc.) are relatively young,
having been created largely by outgassing as the surface cooled
following planetary accretion; supplemented by later additions from
impacting meteoroids, asteroids, and comets; and depleted by gradual
escape of light elements to space. The “giant” or “gaseous” planets
(Jupiter, Saturn, etc.) consist mostly of an atmosphere that is
thought to roughly reflect the initial condensation of
interplanetary atoms, molecules, and dust. Significantly, we know of
only one planetary atmosphere with a large fraction of
molecular oxygen, which is believed to have been formed on Earth by

The surfaces and atmospheres of most planets in the solar system receive
much more energy from external sources (usually sunlight and solar wind,
supplemented on moons by tidal forces) than from upwelling from the planetary
interior (original accretion energy and decay of radioactive elements).
Jupiter and Neptune are exceptions in that their strong gravity
facilitates significant hydrogen
fusion, a process thought to be common on planetary objects called “brown
dwarfs” (i.e. almost or failed stars). In general, the temperature of the
planetary surface, and the altitude profile of temperature in the atmosphere,
are controlled by the absorption of energy from the sun, reflection of visible
radiation back to space, infrared emission by the surface (and clouds, if any),
which is partially absorbed by the atmosphere, and eventually reemitted to
space. This radiative transport problem defines the “greenhouse effect,” the
understanding of which is essential to predicting the impact of the increasing
carbon dioxide abundance in the Earth’s atmosphere resulting from combustion of
carbon as an energy source for human activity.

Observational Status

Direct observation of terrestrial and planetary atmospheres consists
largely of remote sensing. Only rarely can rockets and space probes be sent to
record in situ atmospheric composition and characteristics as functions of
altitude. Much of our knowledge comes from ground-based telescope
observations of optical absorptions and emissions.
Data from Earth-orbiting satellites are providing
important supplements to ground-based observations.

Some of the key discoveries and observations are listed below, sorted
by object and date of the first observation, along with the platform or
instrument used. [telescope] indicates use of a ground-based
telescope, otherwise the notation [Spacecraft instrument]
indicates an observation near the planet.

  • Venus
    • 1643 “Ashen Light” (night airglow, aurora, or lightning?) [telescope]
    • 1823 Fraunhofer lines in reflected sunlight [telescope]
    • 1932 CO2 absorption in reflected sunlight [telescope]

    • 1975 O2(c) visible night airglow [Venera 9,10 spectrometers]
    • 1978 Nitric oxide (NO) ultraviolet night airglow [Pioneer-Venus UV spectrometer]
    • 1978 Neutral atmospheric composition at 24, 44, and 54 km altitudes
      [Pioneer-Venus gas chromatograph]

    • 1978 142-250 km altitude profiles of CO2, CO, N2,
      O, N, and He [Pioneer-Venus mass spectrometer]

    • 1979 O2(a) infrared day and night airglow [telescope]
    • 1983 Lack of O2 absorption in reflected sunlight

    • 1989 O(1S-3P) ultraviolet day airglow
      [Pioneer-Venus UV spectrometer]

    • 1993 Meteor trail [Pioneer-Venus UV spectrometer]
    • 1999 O(1S-1D) Green Line in night airglow [telescope]
  • Mars
    • 1784 Seasonal variation of polar caps [telescope]
    • 1800 Dust storms [telescope]
    • 1823 Fraunhofer lines in reflected sunlight [telescope]
    • 1947 CO2 absorption in reflected sunlight [telescope]
    • 1963 H2O absorption in reflected sunlight [telescope]
    • 1969 CO absorption in reflected sunlight [telescope]

    • 1969 Ultraviolet day airglow [Mariner 6,7,9 spectrometers]
    • 1969 O3 detection and mapping [Mariner 7,9 spectrometers]
    • 1972 O2 absorption in reflected sunlight [telescope]

    • 1974 “No visible night airglow?” [Mars 5 spectrometer]
    • 1976 Atmospheric pressure and temperature profiles from 0 to 100 km
      [Viking 1,2]

    • 1976 Annual variation of surface temperature and H2O
      [Viking 1,2]

    • 1976 117-200 km altitude profiles of CO2, N2,
      CO, O2, NO, and Ar [Viking 1,2]

    • 1976 110-290 km altitude profiles of O2+,
      CO2+, and O+
      [Viking 1,2]

    • 1979 O2(a) infrared day airglow [telescope]
    • 1989 CO and H2O mapping [Phobos 2 spectrometer]
  • Jupiter
    • 1989 H3+ infrared aurora [telescope]
    • 1994 Shoemaker-Levy Comet impact
  • Saturn
    • 1993 H3+ infrared aurora [telescope]
  • Titan
    • 1944 A satellite with an atmosphere [telescope]
    • 1980 Major gaseous composition (N2, CH4,
      H2) and thermal structure [Voyager 1]

    • 1980 Spatial distributions of trace atmospheric constituents, haze
      and temperature [Voyager 1]

    • 1981-1983 Detection of CO and CH3D [telescope]
    • 1984 Organic tholins produced in a simulated Titan atmosphere [laboratory]
    • 1993 Surface heterogeneity observed [telescope]
    • 1996 First resolved images of Titan’s surface [HST]
    • 1998 Detection of water vapor [ISO]
  • Uranus
    • 1993 H3+ infrared aurora [telescope]

Status of Understanding of Atmospheric Composition, Origin,
Evolution, and Dynamics

the mechanisms responsible for the stability of the CO2
atmospheres of Venus and Mars is still a significant research subject.
There are only a handful of mass spectrometric measurements of the
altitude dependence of the chemical composition of the Venus and Mars
middle/upper atmospheres and ionospheres, and even these sample limited
ranges of altitude, high in the heterospheres. This lack makes it
difficult to be confident of the mechanisms for catalytic reformation
of CO2 from CO plus O/O2, and inhibits
inferences that might be drawn from the observed O2, O, and
NO nightglow emissions on Venus.


Status of Atmospheric Models

The compositional and thermal structure on Titan are well
modeled based on the observations from Voyager and ISO, but also from
ground-based measurements. Photochemical models have been produced to
satisfy the observational constraints. General circulation models are
currently developed. The dynamical aspects have been investigated. A
future heuristic combination of all these models, with the support of
new observations from Cassini/Huygens and from the ground for instance,
should produce a fully comprehensive model of the satellite’s

Status of Supporting Laboratory Investigations

The weak sister of atmospheric science has always been
determination of numerical values for modeling of microscopic
processes. Modelers speak of observations placing “constraints”
on microscopic processes, and adjust any available “free” parameters
to “explain” the observations. In previous decades there was
little choice, given the large number of processes that had not
received previous laboratory investigation. In fact, adjustment of
parameters in complicated models is not an accurate means for
determining collisional rates, cross sections, and branching ratios.
NASA and NSF need to strengthen their programs of laboratory


Laboratory measurements and simulations on the various
components of Titan’s atmosphere have been conducted: tholin material,
aerosols, various condensates, complex gases which could be present on
Titan have been investigated in the laboratory and are we are currently
still receiving information on optical constants, spectral features,
reaction rates, etc.


Planetary Atmospheres Goals and Objectives

The principal goals and objectives of the investigation of
planetary atmospheres are briefly outlined below.

  • Understanding the origin, history, composition, motion,
    and stability of planetary atmospheres:
    including formation during planetary accretion, by surface
    outgassing, or post-accretion deposition;
    modification of top-surface and atmospheric composition by
    external energy and mass sources; vertical and horizontal
    transport; clouds, winds, and storms; and loss of mass to
    space by surface ejection or exospheric escape.

  • Characterization of the chemical and physical
    processes responsible for import of energy and mass from the sun
    and the interplanetary medium, response of atmospheres
    to external inputs, and release of energy and mass back to space.

  • Identification of key observables for future planetary
    space missions and near-earth telescopic observations.

  • Comparing the diverse planetary atmospheres in the solar system
    to learn what each can teach us about the others; to better
    understand the potential impact of human modifications of
    the Earth’s atmosphere; to characterize the processes in the
    atmospheres or top surfaces that could generate molecules of
    prebiotic significance; and to identify what signatures might
    be useful for characterizing the atmospheres of extra-solar

Who Studies Planetary Atmospheres

Three communities of scientists collaborate in the
investigation, understanding, and interpretation of planetary
atmospheres. “Observers” record atmospheric “data” using direct-
sampling instruments on planetary probes, remote sensing instruments on
Earth-orbiting satellites and planetary orbiters, and ground-based
spectrometers, radar facilities, and telescopes. “Modelers” attempt to
explain atmospheric observations by simulations based on microscopic
processes that hopefully are well known from laboratory investigations,
but if necessary, plausible numerical parameters are inferred by
reproducing field observations. “Laboratory investigators”
quantitatively characterize the underlying microscopic processes.

How We Understand Planetary Atmospheres

Below we list a sequence of questions that illustrates the roughly
historical progression of observation, inference, and understanding in
the study of planetary atmospheres.

  • What is the Nature of the Observables?
    • Identification of the absorbers, emitters, and
      scatterers (O2, O,
      N2, CO2, O3,
      clouds, stratospheric hazes, etc)

    • Spatial and temporal variations
    • Radio reflection and ionospheres
    • Mass spectrometry by atmospheric probes
    • Satellite drag
  • What are the Energy Sources?
    • Sunlight
    • Internal heat and tidal forces
    • Solar wind
    • The role of planetary magnetic fields
  • What are the Underlying Chemical and Physical Processes?
    • Spectroscopy: radiative emission, absorption, and transport
    • Photon impact excitation, dissociation, and ionization
    • Electron impact excitation, dissociation, and ionization
    • Heavy-particle impact excitation, dissociation, and ionization
    • Momentum transfer collisions
    • Charge transfer reactions
    • Ion-molecule chemical reactions
    • Electron-ion recombination
    • Three-body atom/molecule recombination
    • Excited state quenching and energy transfer
    • Neutral atom and molecule chemical reactions
    • Coagulation and condsensation
    • Sedimentation and molecular and eddy diffusion
    • Mechanisms generating vertical and horizontal winds
  • What are the Numbers?
    • Atmospheric composition and thermal structure
    • Intensities of atmospheric ultraviolet, visible, and
      infrared emitted and scattered light

    • Solar spectrum
    • Composition of the solar wind
    • Rates and cross sections
  • Why do Observables Vary in Space and Time?
    • Solar cycle and coronal mass ejections
    • Winds, waves, and transport
    • Seasons and global patterns
    • Storms and lightning
    • Atmospheric regions: The “spheres”
  • What can be Learned from Systematic Observations?
    • Expose vulnerabilities and uncertainties in atmospheric models
    • Learn about possible signatures of “interesting” atmospheres on
      extra-solar planets

    • Long-term changes due to human perturbation (ozone depletion,
      global climate change)

    • Predict sporadic short-term interference with human technology
      (space weather)

In the earliest stage we attempt to explain the macroscopic
observables, such as colors of aurorae being due to atomic and molecular
emissions and that an ionized atmosphere can reflect radio waves. In the second
stage we infer what sources of energy could produce the observed perturbations
of the atmosphere. Next we attempt a microscopic description of the specific
processes that could be used to construct a quantitative model. But the model
will not work unless we have accurate numerical values for the starting
conditions, energy inputs, and the rate parameters for energy deposition and
chemical transformation. As this microscopic local model begins to be trusted
we back off from a local or point description and attempt to understand how
variations in energy sources generate atmospheric dynamics. Finally we come to
what atmospheric scientists tell the general public are the reasons why their field is

Atoms, molecules, electrons, ions, and photons; their internal energy
levels and translational energy content; and their collisional
interactions; are clearly central to atmospheric science.
Historically, observations in the
atmosphere have often provided the first information about atomic and molecular
structure and collisional processes. In these cases, the modeler is “free” to
infer the numerical values of microscopic parameters by “fitting” or adjusting
the model to match or explain the atmospheric observations. Sometimes this
works beautifully in deriving values that are subsequently confirmed by
laboratory measurements. In any case, the atmosphere supplies a continuing list
of microscopic processes that appear to be important enough to justify devising
approaches to characterize them quantitatively. What should not be done is to
assume that the modeling inference alone is the final answer.

All of this presents the modeler with daunting prospects. First, the
list of microscopic processes is very long. Second, it is hard to know in
advance which ones will have a significant effect on the observables being
modeled. Third, not all of the important microscopic processes will have been
already examined in the laboratory. Fourth, it is a big job to survey the
primary literature to find the rates and cross sections that have been
measured. Finally, it is difficult for the modeler to assess the accuracy of
laboratory measurements, whose details and vulnerabilities are outside his/her
primary areas of expertise.

Specific Needs by Category and Object

  • Comparative Understanding Needs
    • Formation, evolution, stability, and structure of
      All: Non-thermal exospheric escape
      Venus/Mars: Stability of CO2 atmospheres
      Giants: Planetary elemental composition

    • Atmospheric motion: vertical and horizontal transport, mixing,
      and diffusion

    • Planetary magnetic fields:
      differences between and implications of interactions of the
      solar wind with the atmospheres of planets with magnetic fields
      (e.g. Earth and Jupiter) compared to those without (e.g. Venus,
      Mars, Titan)
  • Observational Needs
    • All: Repeated systematic observations
    • All: Signatures of winds and transport
    • All: Direct measurements of exospheric escape
    • Venus/Mars: Minor species composition below 120 km
    • Venus: Is “ashen light” real?
    • Mars: Does Mars have nightglow emission?
    • Giants: Homospheric elemental abundance of N and O
    • Titan: more data on Titan’s lower
      atmosphere (troposphere) and surface, the CH4 cycle, higher precision on
      the D/H and other isotopic ratios, etc.
  • Modeling Needs
    • Venus/Earth/Mars/Titan: Are general circulation models
      evolving toward a unified description that explains how
      planetary parameters control energy and momentum budgets?

    • Venus: What can be learned from nightglow variability?
    • Earth/Jupiter: What can be learned from auroral emissions?
    • Titan: a unified description combining GCM, dynamics and
      photochemical models.
  • Laboratory and Theory Needs
    • Mercury/Moons: Trapping of volatiles in the top surface,
      radiation and impact induced chemistry and desorption

    • Venus/Earth: Relaxation of O2 excited states
    • Venus/Earth/Mars: Rate of CO2(000)
      + O(3P) <-> CO2(010)
      + O(3P)

    • Venus/Mars: Yields of O(1S,1D)
      from e + O2+(v>0)

    • Giants: Equations of state, solubility, and molecular diffusion
      in H2/He at low temperature and high density

    • Giants: CH4/CH3/CH2/CH

    • Jupiter: Ammonia isotopic fractionation
    • Titan/Triton: CH4 condensation and
      polyacetylene/nitrile photochemstry.
      More supporting laboratory measurements
      on aerosols, polymers, tholins and organic material.
  • Maintaining Future Capabilities
    • Justifying space missions
    • NGST capabilities for planetary atmospheres
    • Assets on ISS
    • Competition for time on large telescopes
    • Political support for NASA and NSF
    • Communicating with the public and congress
    • Enhancing laboratory research
    • Career prospects in planetary atmospheres


The discussion above illustrates that planetary atmospheres
are important, interesting, and complicated. We have learned
quite a bit, but our partial understanding leads to many new
questions. The following themes may facilitate prioritizing
the numeous science needs and mission possiblities.


  • Broad Science Goals

    Understanding how atmospheres are formed, evolve, and respond to
    perturbations is essential for addressing the long-range science objectives
    of identifying the conditions that are favorable for producing and
    supporting biological activity, managing the effects of human activity
    on the Earth’s atmosphere, and planning and evaluating observations of
    extra-solar planets.

  • Understanding Atmospheres

    The historical attempts to understand the atmospheres
    of the Earth and other
    solar system objects have emphasized identification
    of the underlying chemical and physical processes responsible for the
    many fascinating observations. After decades of exciting discoveries,
    and with anticipated future discoveries of no less interest, it is
    appropriate that the focus should shift toward comparative
    interpretation of what the atmospheric observations and discoveries
    on multiple planets can teach us about broader scientific goals.

  • Learning by Exploring Planets and Moons

    Atmospheres evolve, move, change, and vary from place to place.
    A single observation is never enough.

    Planning of all future planetary mission campaigns should include
    explicit atmospheric components with specific scientific objectives
    emphasizing the need to fill gaps in our understanding. For example,
    the current strong Mars program is weak in atmospheric observations.
    The resource of ground-based and near-Earth space-based telescopes
    has been very productive historically, but observations of solar
    system atmospheres are currently not rated highly by time allocation
    committees nor are current priorities for future space telescopes.

  • Providing the Required Research Infrastructure

    Visiting planets is only one of the objectives. Lasting value
    comes from analyzing, interpreting, and using the data to
    establish broader implications. Funding for post-flight
    analysis and interpretation of mission data needs to be reserved
    in advance and secured against escalation of hardware costs.
    In addition, well-funded independent Research and Analysis (R&A)
    programs, including laboratory experiments, fundamental theory,
    modeling, and reanalysis of historical observations, are essential
    contributors to the impact of what can be learned from the study
    of planetary atmospheres.

  • Assimilating Space and Planetary Science with Earth Science

    The broad science goals for planetary science are actually inward
    looking. From our near neighbors we hope to acquire additional hints
    about our origins and the steps we should take to preserve our
    life-supporting environment. In contrast, the current organization
    of research programs at NASA and NSF suggest a strong distinction
    between Earth science and space and planetary science, demarked
    by a boundary about 50 km above the Earth’s surface. While this
    may reflect a perception of separate communities of researchers,
    it presents a barrier for effective communication, contribution
    to shared science goals, and justification and mobilization of
    funding resources for both disciplines.

Specific Recommendations Applying to Multiple Planets

  1. Secure Funding for Mission Data Analysis and

  2. Dedicated Telescopes for Planetary Astronomy
  3. Comparative Planetary Atmospheres Program
  4. “Super Discovery” Program for $500-700 Missions
  5. Enhancement of Laboratory and Theory Programs

Specific Mission Recommendations

  1. Deep Penetration Probes to Determine Elemental Composition
    of Giant Planets

    The Galileo Probe data on the elemental abundances of Jupiter has
    challenged our views of the formation of the giant planets and the
    subsequent evolution of their atmospheres. Contrary to expectations, the
    Probe measurements revealed, for the fist time, that in the deep
    well-mixed regions of Jupiters atmosphere, the heavy elements,
    C, N, S, Ar, Kr, and Xe are enriched relative to their solar proportions by a
    factor of 2-3. A plausible explanation is that the heavy elements were
    delivered to Jupiter largely by cold icy planetesimals whose temperature
    must be 30 K or lower. Perhaps direct infall of such planetesimals
    from the interstellar cloud was prevalent during and after
    Jupiter’s accretion.

    Unfortunately, the elemental abundances measured by the “single” Galileo
    Probe may or may not be representative of the entire planet. To complicate
    matters, the Probe also entered a meteorologically anomalous region known
    as a (5 micron) hot spot, where downwelling is expected to alter the
    distribution of volatiles, especially the condensible volatiles. The
    biggest mysrery is, however, the abundance of the carrier of the heavy
    elements, water, whose mixing ratio in the deep well-mixed part of the
    atmosphere– where it should be uniformly mixed–could not be measured.
    This is due to the fact that in the hot spot where the Probe made its
    measurements, the region of the uniformly mixed water must be well below
    the deepest level probed, 21 bars, where the water vapor mixing ratio was
    found to be still increasing.

    If the above hypothesis of the heavy element enhancement on Jupiter is
    correct, the water abundance, hence the oxygen elemental ratio, must show
    at least as much enrichment as the other heavy elements. In fact, water
    being the carrier of the heavy elements, the oxygen elemental enrichment
    could be even greater than that of the other elements.

    In order to understand the formation of Jupiter and the evolution of its
    atmosphere, it is thus imperative that all heavy elements, including
    oxygen, be measured accurately in the deep well-mixed regions of Jupiter
    at several different latitude/longitude locations, and for comparison on
    Saturn. A comprehensive understanding of the formation of the giant
    planets and their atmosphere would be crucial also for modeling the
    formation of the extrasolar planets and the origin of their atmospheres.

    In summary, cleverly instrumented deep multiprobes into Jupiter, followed
    by Saturn, and eventually, Uranus and Neptune are recommended. In
    addition, it would be highly desirable to explore the possibility of
    precursor missions that could determine by remote sensing at least the N
    and O elemental abundances, as they would help with much more intelligent
    and sophisticated instrumentation and planning for full-up elemental
    abundance measurements with the multiprobes.

  2. Mars Atmospheric Explorer Mission
  3. Post-Cassini Atmospheric/Surface Mission to Titan

  4. Venus Atmospheric Explorer Mission


  • How do we convince “real” astronomers that the solar system is
    still interesting?

  • How should the planetary atmospheres community help NSF and NASA
    compete for resources to retain and enlarge strong research

  • Planets are interesting to the general public. How do we
    explain the importance of atmospheres? How do we translate
    this into research support?

  • The planetary atmospheres community of laboratory investigators
    is very small. How do we attract the interest of the much
    larger community of chemists and physicists? How do we teach
    them what is important? How will they be funded?

  • What are the career prospects in planetary atmospheres? What
    should we do about it?


The plan is to publish a reference book containing the
“White Papers” being prepared by approximately 25 Community Panels
of planetary science volunteers. The NRC Discipline Panels
will use this material in preparing the NAS Report.

Above is the current (October 22, 2001) draft version of the White
Paper from the Planetary Atmospheres Community Panel. Future versions
will be made available. The target date for
completion is November 1, 2001. The current version has lots of
holes and needs input and paragraphs in many areas. Please join the
panel and make suggestions and contributions.


SpaceRef staff editor.