Dr. Julianne I. Moses
Recent Research

Exchange of Material in the Outer Solar System

The outer planets are not completely isolated bodies — they continually encounter material that has originated from other parts of the solar system. In the early days of solar-system formation, objects ranging from small dust grains to large planetesimals participated in a complicated dynamical dance, with bodies being passed from the gravitational control of one giant planet to another. Eventually Jupiter and the other giant planets helped clear the solar system of such bodies by ejecting the objects entirely (where some, like the Oort Cloud comets, are still gravitationally bound to the solar system), or by swallowing the objects into the giant planets' massive atmospheres, or by sending the objects into the inner solar system where they might have a chance of impacting the terrestrial planets on their way to colliding with the Sun. As was dramatically demonstrated by the impact of Comet Shoemaker-Levy 9 with Jupiter in 1994 (see Figure 1), this exchange of solar system material is still occurring today. Dust and larger objects from the Kuiper Belt beyond the orbit of Neptune can migrate inward, encountering the giant planets along the way. Dust and larger objects from the main asteroid belt can be nudged into Jupiter-crossing orbits. Dust released from comets can also end up on orbits that have semimajor axes within the outer solar system, despite the fact that the parent comet may not have been on such an orbit. Material can be ejected from satellite and ring systems through interaction with magnetospheric particles and interplanetary/interstellar dust grains or through larger impacts, and this material can find its way into planetary atmospheres throughout the solar system. Comets that originate from either the Oort Cloud or Kuiper Belt can pass through the solar system, potentially impacting the planets as they travel. Such encounters are interesting because they allow for the exchange of volatiles and other materials (including potential life-forming elements) among solar system bodies.

My Research

One focus of my research has been to try and characterize this exchange of material in the outer solar system. What is the flux of external material currently impacting the giant planets? Does this exogenic material derive mainly from large cometary impacts, small interplanetary dust particles, or local ring/satellite systems, and how can we distinguish between these sources? How does the external material affect the physics and chemistry of giant-planet atmospheres?

Large cometary impacts such as those from the collision of the Shoemaker-Levy 9 cometary fragments with Jupiter have immediate and conspicuous effects on the atmosphere through the sudden deposition of large amounts of vaporized and solid debris and through the perturbations caused by the bolide's passage through the atmosphere. The newly deposited vapor can initiate interesting oxygen, nitrogen, and sulfur chemistry in giant-planet stratospheres (e.g., Moses et al. 1995a, 1995b, Geophys. Res. Lett. 22, p. 1597 and p. 1601), while the solid debris can cause localized heating and can affect atmospheric dynamics (see The Collision of Comet Shoemaker-Levy 9 and Jupiter, edited by K. S. Noll, H. A. Weaver, and P. D. Feldman, Cambridge University Press, 1996). Strong shocks created during the bolide and plume-splashback phases of the cometary impacts profoundly affect the thermal structure and chemistry of localized areas. Interplanetary dust particles that ablate high in the planets' atmospheres will also affect atmospheric chemistry and thermal structure (see Moses 1992, Icarus 99, p. 368 and Moses 1997, J. Geophys. Res. 102, p. 21,619), but less sporadically and more globally than comets. For example, the ablated meteoric material can affect ionospheric structure and chemistry — metal ions can replace hydrocarbon ions as the dominate species in the lower ionosphere (facilitating layered structures), and recondensed refractory material might affect the structure and diurnal variability of the lower ionosphere (e.g., Moses and Bass 2000, J. Geophys. Res. 105, p. 7013). Micrometeoroid ablation or satellite and ring-particle diffusion can also introduce oxygen compounds and instigate neutral oxygen photochemistry in the stratospheres of the outer planets, leading to the presence of unexpected stratospheric species such as H₂O, CO₂, and CO (e.g., Feuchtgruber et al. 1997, Nature 389, p. 159; Moses et al. 2000, Icarus 145, p. 166). Water and refractory material from meteoric or ring/satellite sources can condense in outer-planetary stratospheres, contributing to global haze layers, potentially affecting stratospheric temperatures, and providing surfaces upon which other molecules can condense or react chemically.

Emmanuel Lellouch, Bruno Bézard, Helmut Feuchtgruber, Randy Gladstone, Mark Allen, and I have attempted to constrain the influx of external material to Saturn through comparisons of photochemical models with Infrared Space Observatory observations of stratospheric CO₂ and H₂O (Moses et al. 2000, Icarus 145, p. 166). From the inferred total influx rate of (4 ± 2) × 10⁶ O atoms cm^–2 s^–1, from the importance of H₂O as an initial component, and from arguments related to the impact rate of large comets with Saturn and to the diffusion of material through Saturn's stratosphere, we conclude that micrometeoroid ablation or ring-particle diffusion are the main sources of the external oxygen currently observed in Saturn's atmosphere. The results for Saturn differ markedly from those at Jupiter, where the observed stratospheric H₂O and CO₂ seem to relate to the Shoemaker-Levy 9 impacts (Lellouch et al. 2002, Icarus, 159, p. 112; see also Lellouch et al. (2006, Icarus 184, p. 478). Interestingly, observations of stratospheric CO (Bezard et al. 2002, Icarus 159, p. 95) require large stratospheric CO production rates of (2–12) × 10⁶ cm^–2 s^–1 that cannot result from the SL9 plume splashback because any SL9-derived CO would take ~300 years to diffuse from the splashback region (~0.1 mbar) to the tropopause bottleneck (~300 mbar), where it is observed today. Because the inferred CO/H₂O influx ratio is representative of large cometary impacts and because the inferred influx rate of CO is roughly consistent with estimates of the impact rate of sub-kilometer to kilometer-sized comets with Jupiter, Bézard et al. conclude that cometary impacts rather than meteoritic or ring/satellite sources supply CO to Jupiter.

Recent observations of CO on Neptune (Lellouch et al. 2005, Astron. Astrophys. 430, p. L37) suggest that a ~2 km comet may have impacted Neptune ~200 years ago. These observations have many interesting implications with regard to outer-solar-system impact rates and atmospheric chemistry.

Note that although the total "background" (non-SL9) influx rate for external oxygen on Jupiter is of the same order of magnitude as that on Saturn, the form that oxygen takes within the stratosphere (CO vs. H₂O) appears quite different for the two planets. Although this difference suggests different external sources, other possibilities cannot be ruled out at this time. Further observations and modeling are warranted.

Last updated
April 4, 2008