From: National Research Council
Posted: Monday, October 22, 2001
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:
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 Voyager observations we LEARNED WHAT? WHAT WILL BE LEARNED FROM CASSINI?
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
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.
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.
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.
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 contribute to shared science goals, and justification and mobilization of additional funding resources for both disciplines.
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
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 medium.
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 photosynthesis.
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.
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.
Understanding 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 atmosphere.
Status of Supporting Laboratory Investigations
The weak sister of atmospheric science has always been laboratory 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 investigations.
SPECIFIC EXAMPLES AND RECOMMENDATIONS...
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. KEY SCIENCE QUESTIONS
Planetary Atmospheres Goals and Objectives
The principal goals and objectives of the investigation of planetary atmospheres are briefly outlined below.
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.
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 important.
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
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.
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.
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.
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.
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
Specific Mission Recommendations
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.
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.
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