ATMOSPHERIC PROBES: NEEDS AND PROSPECTS Tobias Owen University of Hawaii, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA, Email: [email protected] ABSTRACT There is only one Rosetta Stone in the Solar System; it’s in the British Museum. We cannot understand the inner planets by simply studying the Earth, nor can we apprehend the giants by examining only Jupiter. Despite the stunning successes of previous probes to Venus and the Galileo probe to Jupiter, our knowledge of the atmospheres of even these two planets remains tantalizingly incomplete. We must therefore return to Venus and consider the challenge of exploring all of the outer planets with a family of identical probes, a project that could commemorate the vision of multiple worlds championed by Giordano Bruno. 1. INTRODUCTION Knowledge of atmospheric composition is essential to an understanding of atmospheric origin and evolution. For the giant planets, we may also hope to gain an understanding of the interior structure and the origin of the planets themselves. Remote sensing observations, whether from Earth or from spacecraft, can only take us so far. In particular, we cannot learn the abundances and isotope ratios of noble gases, nor can we assess abundances of species that condense under local atmospheric conditions. For example, we were all certain that sulfur must be present on Jupiter as H2S, but it was not until the Galileo Probe dove deep into the atmosphere that we were able to detect this gas. Atmospheric probes can do much more than determine atmospheric composition. They can obtain the local altitude profile of temperature, pressure and wind speed; search for and identify cloud layers; detect lightning discharges, etc. In this essay, I will concentrate on composition as illustrative of the accomplishments and future promise of probes. In situ exploration of Mars is currently proceeding by means of landers, so we will begin with a consideration of Venus. 2. VENUS Beautiful Venus illustrates perfectly the danger of extrapolating the properties of one planet to another. With our deeply ingrained Ptolemaic bias, we commonly refer to Mercury, Venus, Earth and Mars as “terrestrial planets,” as if they all somehow copy the Earth. But they don’t! The atmosphere of Venus consists mainly of a mixture of CO2 and N2 that is

remarkably similar to the amounts of these two gases that have been outgassed by the Earth. At first this seems to affirm our Earth-centered bias. However, when we look at the noble gases on Venus, the picture immediately becomes more complex. The relative abundances of noble gases in the atmospheres of Mars and Earth (normalized at argon) are virtually identical, but Venus is distinctly different. On Venus, the ratio of Ar to Kr looks more like that in the Sun than that in Earth’s atmosphere, and the amount of Ar per gm of planet is far higher than on Earth [1]. Is it possible that Venus has somehow acquired noble gases in the proportions that exist in the solar wind? Uncertainties at present are too great to allow us to tell; xenon is not yet even detected. But a suitably instrumented atmospheric probe could easily answer this question. Turning to nitrogen, we would love to know whether the N2 in the atmosphere of Venus shows the same solar influence we appear to find in the noble gases. It seems unlikely, because N/36Ar on Venus is orders of magnitude larger than N/36Ar in the Sun. The solar influence, if it exists, would be revealed in the ratio 15 N/14N , which would be distinctly lower than the value our own atmosphere [2]. However, we are frustrated once again by the quality of the data at our disposal. The current uncertainty on the N isotope measurement in the atmosphere of Venus does not allow us to make this determination, although it was a fine first measurement [1]. And again, the solution is obvious: the same entry probe that gives us the noble gases could measure nitrogen, while producing much additional information on trace gases, other isotopic ratios, and a vertical profile of local pressure, temperature, winds and cloud layers. 3. THE OUTER PLANETS 3.1. Jupiter: Galileo Probe Results Considering the outer planets, we have the splendid suite of results from the Galileo Probe into Jupiter’s atmosphere to guide our analysis [3]. We might start with the determination of the relative abundances of hydrogen and helium, the two major constituents of the planet. The measurements by two instruments on the Galileo Probe gave the same result: the mass fraction of helium

on Jupiter is Y = 0.234 [4],[5]. This is slightly less than the protosolar value of 0.26, indicating that some He has precipitated out of solution in Jupiter’s metallic hydrogen, as predicted long ago [6]. Equally significant for this conference, however, the Galileo Probe determination of Y is significantly different from the determination made by analyses of the planet’s infrared spectrum recorded by the Voyager IRIS instrument, together with data from Voyager radio occultations [7]. Evidently there is a systematic error in the occultation data that has presumably affected Voyager determinations of Y on the other outer planets as well. Conrad and Gautier [8] have corrected the Saturn value of Y, which will be further tested by the Cassini-Huygens mission [9], but without an in situ measurement by a probe, we will be left with unknown uncertainties. Our knowledge about the relative abundances of hydrogen and helium in the atmospheres of Uranus and Neptune is in an even more primitive state. In the case of the helium mass fraction, we expect that a measurement made by a probe anywhere in a planet’s atmosphere will give the same result. For gases that condense, the problem is more difficult. The role of condensation in limiting abundances in Jupiter’s upper troposphere was vividly illustrated by the Galileo Probe Mass Spectrometer, which entered a so-called “5micron hot spot” [3],[4]. Such hot spots are relatively cloud-free regions in the atmosphere where cold, driedout air is apparently being recycled downwards. The probe indeed found an absence of thick clouds [10] and a strong depletion of condensible gases in the upper part of its trajectory [4]. As the probe descended it found that the mixing ratios of H2S and NH3 steadily increased until they reached constant values, signifying that the probe had gotten below the condensation levels for both gases. But the mixing ratio of water vapor was still increasing with the last measurement, indicating that the probe had not penetrated below the H2O condensation level in this part of the atmosphere. This experience demonstrates the necessity of multiple probes to determine the composition of a planetary atmosphere, preferably after a remote survey by microwave radiometry that is capable of mapping compositional anomalies below the cloud layers. 3.2. A New Constraint on Giant Planet Formation Knowledge of the global abundances of the major elements helps to constrain models for the interiors of these planets and to develop theories of formation that can be applied to extrasolar giants as well. Recent observations of the metallicity (proportion of heavy elements) in a survey of Sun-like stars by Fischer and Valenti [11] has demonstrated a striking correlation between the presence of planets and the abundances of

heavy elements in the star that anchors the system This result favors models for giant planet formation in which a core of icy planetesimals accretes first, ultimately reaching a mass sufficient to cause surrounding nebular gas to collapse around it, as planetesimals continue to impact the growing planet. The planet is then expected to have an atmosphere consisting of a mixture of nebula gas and gases produced from the core and from impacting icy planetesimals dissolving in the gaseous envelope [12]. What is the nature of the icy planetesimals that built the core and enriched the envelope? The tradition has been to assume they were essentially identical to comets. Despite the lack of knowledge about the global oxygen abundance on Jupiter, the Galileo probe provided some exciting evidence that this assumption is not correct. Prior to the mission, it was known that C/H was enriched on this giant planet by about a factor 3, relative to solar abundances [13]. But it was widely assumed that N/H would be closer to the solar value, given the depletion of nitrogen in comets [14],[15]. Laboratory experiments had demonstrated that both N2 and Ar were difficult to trap in forming amorphous ice; hence argon was also predicted to be deficient on Jupiter [15]. However, reality proved to be different. All the elements whose abundances could be determined by the Galileo Probe Mass Spectrometer were found to be enriched by the same factor of 3 ± 1, including both argon and nitrogen, compared with solar abundances [16]. This led to the suggestion that Jupiter was enriched by a new class of icy planetesimals called SCIPs that had been formed at temperatures low enough to allow all gases to be equally well-trapped [17]. This suggestion was supported by the low value of 15N/14N = 2.3 ± 0.3 × 10–3 in Jupiter’s atmosphere, indicating that Jupiter’s nitrogen had been supplied in the form of N2 [2]. In principle N2 and Ar could be trapped either in amorphous ice or in crystalline ice that would form clathrate hydrates [18]. The distinction is important, because the amorphous ice would probably have to form in the interstellar medium at T < 20 K, whereas crystalline ice is expected to form in the solar nebula at T > 130 K but it would have to cool to T < 38 K to allow the formation of argon hydrate, a process that would take approximately 2 million years [18]. If all the heavy elements on Jupiter were enriched by the same factor 3 ± 1, as amorphous ice trapping would predict, the planet would have 18 ME of heavy elements, well below the current upper limit of 42 ME set by models for the interior. But if clathrate hydrates were formed, Jupiter would need at least 9 × the solar O abundance to make a sufficient amount of water to form

the cages that trapped the molecules [18]. If formation of clathrates was not 100% efficient, the total mass of heavy elements would be much greater. It might even exceed the theoretical limit. In any case, it is worth noting that if the other outer planets got their heavy elements in the same way, SCIPs must have been the most abundant solid material in the original solar nebula. 3.3. Saturn, Uranus and Neptune But did any of the other giant planets form from SCIPs? To find out, we will need multiple probes into the atmospheres of each of them. Probes, because remote sensing cannot give us the noble gases and the isotopes we need for our assessment, and multiple probes to avoid a single set of measurements in a meteorologically anomalous region, as happened to the Galileo Probe. Obviously the discovery of the 3 ± 1 × solar enrichment of heavy elements on Jupiter was a surprise. The apparent requirement for icy planetesimals formed at low temperature raises a number of issues that are not yet resolved. Where did these objects form? How did they get from their place of origin to the present orbit of Jupiter? So we naturally ask: What other observable consequences of the postulated existence of SCIPs could be used to test this scenario? In principle, the puzzling noble gas abundances in the atmosphere of Venus could have been delivered by SCIPs, but we seek more direct evidence for their existence. If these objects were so abundant in the early solar system, we might well expect to find remnants among the present population of comets. To identify them, we need to look for comets in which nitrogen and argon exhibit solar abundances relative to oxygen or carbon. This can only happen if the comets contain a high proportion of N2, and N2+ can be observed from the ground in spectra of comets’ ion tails. To date, no such N2-rich comets have been reported, and recent highresolution observations have failed to find any N2+ at all in three different comets [19], [20]. What about the other giant planets? If SCIPs brought the heavy elements to them, there would certainly be observable consequences, but it appears that only Saturn may provide an opportunity for remote sensing to test this idea. If there is sufficient NH3 above Saturn’s clouds to allow detection of 15NH3 in the IR spectrum, the ratio 15N/14N would be diagnostic for delivery of nitrogen as N2. If we find the same value as on Jupiter, we would conclude that SCIPs were again at work. But we would not know if the volatiles were trapped in amorphous ice or in clathrate hydrates.

To find out, we will need probes. For example, Hersant et al. [21] have suggested that if clathrates were the carriers, Saturn should exhibit an enrichment of Xe/H that is 17× solar, while C and N would be enhanced by factors of 2.5 and 2.0, respectively. Uranus and Neptune would have Xe in the range of 35 to 170 times the solar abundance relative to hydrogen, with O in the same range and N very much less.. Hersant et al. [21] have calculated the corresponding enrichments of all the other elements detected by the Galileo Mass Spectrometer for clathrate hydrate formation. Trapping in amorphous ice would yield uniform enrichments for all these planets, as on Jupiter, but by different, unpredictable factors Thus measurements of noble gases plus methane would permit a distinction between amorphous ice and clathrate hydrates as the volatile carriers and would then permit the deduction of O/H on Saturn, Uranus and Neptune. All we need are the atmospheric probes that can accumulate the necessary data, plus the usual additional information on atmospheric structure and dynamics. 4. A MODEST PROPOSAL: THE GIORDANO BRUNO PROJECT Obviously the technical challenges to reach depths greater than a thousand bars in the atmospheres of Uranus and Neptune, where we can expect H2O to be well-mixed, are immense (Atreya, this volume). But we might start with a more modest enterprise that harks back to an idea that was popular some 30 years ago: a family of four identical spacecraft each equipped with at least two identical probes, that could sample all four of these atmospheres down to depths of 50-100 bars. Mass production would lower costs significantly. Such probes would yield the following information: 1. Basic atmospheric composition: He/H, C/H, S/H, Ne/H, Ar/H, Kr/H, Xe/H. 2. Diagnostic isotope ratios: D/H, 12C/13C, 15N/14N, 3 He/4He and ratios for all of the other noble gases. 3. Temperature-pressure profiles, wind profiles, the location, composition and extent of cloud layers and a search for lightning discharges. It would not be possible to obtain N/H, and O/H without going to pressures greater than 1000 bars on Uranus and Neptune, but a 50-100 bar probe on Saturn would yield these elements as well (Atreya, this volume). The nitrogen isotopes would permit a test for the presence of SCIPs as described above for remote observations of Saturn. The carbon and noble gas abundances would permit a clear distinction between amorphous ice and clathrate hydrates as the volatile

carriers. One could then deduce the value of O/H. The ratio of Ne/Ar would constrain the temperature at which the ices formed. The value of D/H would reveal how much mixing had occurred between hydrogen brought in by the ices and the hydrogen from the solar nebula. The NH3 mixing ratio below the NH3 cloud would constrain models for a deep ionic ocean on Uranus and Neptune. This is a rich harvest, which would allow comparative planetology studies in the outer solar system at an unprecedented level of detail, even without direct measurements of O/H. These data would also provide further constraints on models for interiors and for the origin of giant planets. This effort could be led by ESA and NASA as a joint project, following the highly successful precedent set by Cassini-Huygens. All other space-faring nations on the planet should be invited to participate. A working title for this enterprise could be the Giordano Bruno Project: Multiple Spacecraft to Multiple Worlds. While the worlds in question would still be those in our own solar system, the knowledge about giant planets that we would obtain from the project would have obvious application to the extrasolar planets whose existence Bruno correctly surmised. Acknowledgements: Support for this study was provided by the NASA Galileo Project. I am grateful for helpful discussions with Sushil Atreya and Daniel Gautier. 1. Donahue, T. M. and Pollack, J. B. Origin and Evolution of the Atmosphere of Venus. In Venus, ed. D. M. Hunten, L. Colin, T. M. Donahue, V. I. Moroz, U. Arizona Press, Tucson, AZ, 1003-1036, 1983. 2. Owen, T., Mahaffy, P. R., Niemann, H. B., Atreya, S. and Wong, M. Astrophys. J., Vol. 553, L77-L79, 2001. 3. Young, R. E. The Galileo Probe: How It Has Changed Our Understanding of Jupiter. New Astron. Rev., Vol. 47, 1-51, 2003. 4. Niemann, H. B., et al. The Composition of the Jovian Atmosphere as Determined by the Galileo Probe Mass Spectrometer. J. Geophys. Res., Vol. 103, 22831-22845, 1998. 5. von Zahn, U., Hunten, D. M. and Lehmacher, G. Helium in Jupiter’s Atmosphere: Results from the Galileo Probe Helium Interferometer Experiment. J. Geophys. Res., Vol. 103, 2281-22829, 1998. 6. Stevenson, D. J. and Salpeter, E. E. The Phase Diagram and Transport Properties for Hydrogen-Helium Fluid Planets. Astrophys. J. Suppl., Vol. 35, 221-237, 1977.

7. Gautier, D., et al. The Helium Abundance of Jupiter from Voyager. J. Geophys. Res., Vol. 86, 8713-8720, 1981. 8. Conrath, B. J. and Gautier, D. Saturn Helium Abundance: A Reanalysis of Voyager Measurements. Icarus, Vol. 144, 124-134, 2000. 9. Flasar, F. M., et al. Exploring the Saturn System in the Thermal Infrared: The Composite Infrared Spectrometer. Space Sci. Rev. (in press), 2004. 10. Ragent, B., et al. The Clouds of Jupiter: Results from the Galileo Jupiter Mission Probe Nephelometer Experiment. J. Geophys. Res., Vol. 103, 22891-22909, 1998. 11. Fischer, D. A. and Valenti, J. A. Metallicities of Stars with Extrasolar Planets. In Scientific Frontiers in Research on Extrasolar Planets, ed. D. Deiming and S. Seager, ASP Conference Series, vol. 294, Astronomical Society of the Pacific, San Francisco, 117-128, 2003. 12. Pollack, J. B., et al. Formation of the Giant Planets by Concurrent Accretion of Solids and Gases. Icarus, Vol. 124, 62-85, 1996. 13. Gautier, D. and Owen, T. The Composition of Outer Planet Atmospheres. In Origin and Evolution of Planetary and Satellite Atmospheres, ed. S. K. Atreya, J. B. Pollack, and M. S. Matthews, U. of Arizona Press, Tucson, AZ, 487-512, 1989. 14. Pollack, J. B. and Bondenheimer, P. Theories of the Origin and Evolution of the Giant Planets. In Origin and Evolution of Planetary and Satellite Atmospheres, ed. S. K. Atreya, J. B. Pollack, and M. S. Matthews, U. of Arizona Press, Tucson, AZ, 464-405, 1989. 15. Owen, T. and Bar-Nun, A. Comets, Impacts and Atmospheres. Icarus, Vol. 116, 215-226, 1995. 16. Owen, T., et al. A Low Temperature Origin for the Planetesimals that Formed Jupiter. Nature, Vol. 402, 269-270, 1999. 17. Owen, T. and Encrenaz, Th. Element Abundances and Isotope Ratios in the Giant Planets and Titan. Space Sci. Rev., Vol. 106, 121-138, 2003. 18. Gautier, D., et al. Enrichments in Volatiles in Jupiter: A New Interpretation of the Galileo Measurements. Astrophys. J., Vol. 550, L227-L230, 2001. Erratum, ibid., Vol. 559, L183-L183, 2001.

19. Cochran, A. L., Cochran, W. D. and Barker, E. S. N2+ and CO+ in Comets 122P/1995 S1 (de Vico) and C/1995 O1 (Hale-Bopp). Icarus, Vol. 146, 583-593, 2000. 20. Cochran, A. L. A Search for N2+ in Spectra of Comet C/2002 C1 (Ikeya-Zhang). Astrophys. J., Vol. 576, L165-L168, 2002. 21. Hersant, F., Gautier, D. and Lunine, J. I. Enrichments in Volatiles in the Giant Planets of the Solar System. Planet. Space Sci. (in press), 2003.