P51B-1423 0800h
Atmospheric Circulation of the Extrasolar Giant Planet HD209458b Under Diababtic Heating
A large fraction of the more than 120 extrasolar giant planets currently known has orbits that are very close to their host stars. These close-in planets are likely to be tidally locked and thus continuously heated on the same side. The atmospheric circulation and temperature distribution resulting from such a state is a key issue in the study of extrasolar planets. For one planet, HD209458b, some crucial physical properties (e.g., radius and mass) have been directly measured. Past studies have focused on flows driven by simple, ad-hoc models of radiative heating and cooling. In this work, we drive the flow with heating rates derived from a full radiative transfer model, which has been used in the past to successfully predict Na absorption in HD209458b's atmosphere. The flow model is a high-resolution equivalent barotropic model, capable of resolving small-scale eddies and waves. From our extensive set of simulations, we find that, although the planet rotates slowly (once per ~84 hours), the effects of rotation cannot be ignored: a strong zonal asymmetry is induced by the rotation on a very short timescale (~hours), leading to a complex heat distribution at early evolution times. At long times (~10 rotation periods), the flow evolves to a state marked by a broad, well-homogenized equatorial zone and a coherent circumpolar vortex at each poles, as in the recent adiabatic calculations of Cho et al. (2003). The stability of the 2 to 3 zonal jet circulation pattern found in the adiabatic calculation is also a robust feature of the present diabatic calculation. Wave breaking activity in the equatorial zone and propagation across the zone is enhanced by the strength of the heating. In the near future, some of these findings will be directly tested by observations, such as measurements of day-night temperature difference and temporal variations in the IR flux.
P51B-1424 0800h
Equation of State and Electrical Conductivity of Helium at High Pressures and Temperatures
Helium, the second-most abundant element in the universe and giant planets, is expected to metallize at much higher pressures and temperatures than the most abundant element, hydrogen. The difference in chemical-bonding character, between insulator and metal, is expected to make hydrogen-helium mixtures immiscible throughout large fractions of planetary interiors, and therefore subject to gravitational separation contributing significantly to the internal dynamics of giant planets. Using laser-driven shock waves on samples pre-compressed in high-pressure cells, we have obtained the first measurements of optical reflectivity from the shock front in helium to pressures of 146 GPa. The reflectivity exceeds 5% above \ensuremath{\sim} 100 GPa, indicating high electrical conductivity. By varying the initial pressure (hence density) of the sample, we can access a much wider range of final pressure-temperature conditions than is possible in conventional Hugoniot experiments. Our work increases by nine-fold the pressure range of single-shock measurements, in comparison with gas-gun experiments, and yields results in agreement with the Saumon, Chabrier and Van Horn (1994) equation of state for helium. This changes the internal structures inferred for Jupiter-size planets, relative to models based on earlier equations of state (e. g., SESAME).
P51B-1425 0800h
Giant Planet Cores: Solid vs. Stable
Planets with stable (non-convecting) fluid cores are usually modeled using a solid impermeable core. An alternative is to model the central region as a stable fluid with a subadiabatic entropy gradient. The energy in this region is transported via radiation rather than convection. This is a more realistic representation for a giant gaseous planet as it allows for some convective penetration into the stable region and eliminates the artificial rigid boundary condition at the interface. We use a 2D finite volume hydrodynamic code to compare the fluid behavior in two density stratified cases, one with a solid core and one with a stable subadiabatic fluid core. Our goal is to identify the differences between the convective patterns in these two cases and the resulting kinetic energy distribution and entropy profiles. Using these results, and subsequent 3D simulations, it may be possible to determine the core structure in giant planets based on surface observations such as the zonal wind patterns and strengths.
P51B-1426 0800h
Tangent Cylinder Effects in Jovian Convection
Two properties of Jupiter's dynamics that are still not well understood are the extent and driving force of the zonal winds and the meridionally-independent heat loss. These may be linked by the presence of a tangent cylinder effect in the interior of the planet. A tangent cylinder could be formed by rotational constraints at the molecular to metallic hydrogen transition at ~0.8 jovian radii. Evidence for the existence of a tangent cylinder can be seen in pictures of Jupiter's northern hemisphere taken by the Cassini spacecraft en route to Saturn. At the latitude corresponding to the surface projection of the tangent cylinder, cloud motions are separated into a equatorial region with large scale zonal motions and a polar region with smaller scale, chaotic motions. To investigate tangent cylinder effects in Jupiter, we study a 3-D rotating thermal convection numerical simulation in a thin shell ($r_{inner}/r_{outer}=0.75$). Thermal convection solutions are obtained over a range of Rayleigh numbers $Ra$, for both rigid and free lower stress boundary conditions. The zonal winds and heat loss at the top of the shell and meridional flow throughout the shell are compared with observations. The numerical zonal winds recover some characteristics of observed winds on Jupiter, such as the large prograde equatorial zonal jet. Furthermore, at $Ra > 4E6$, alternating jets appear poleward of the tangent cylinder, with wavelength similar to those seen in the Cassini image. Equatorial winds outside of the tangent cylinder are driven by the Reynold stresses from sloping fluid columns. Winds inside the tangent cylinder are non-geostrophic and require a different mechanism for their generation. Also at $Ra > 4E6$, heat loss at the top of the shell is greater inside the tangent cylinder than outside. The tangent cylinder acts as a barrier to fluid motions, preventing meridional heat transfer and enhancing interior heat loss at the poles. This offers an explanation for why the total (interior + solar) heat loss from Jupiter is observed to be nearly independent of latitude.
P51B-1427 0800h
Zonal Winds on Giant Planets
The deep fluid interiors of giant planets have angular velocities that depend on radius and latitude. Years ago it was proposed that the observed differential rotation on the surfaces of Jupiter and Saturn, i.e., the pattern of alternating zonal winds, is maintained by vortex stretching of convective fluid columns within the deep interiors due to the curvature of the spherical surfaces. This now classic mechanism certainly maintains differential rotation in laboratory experiments of rotating laminar convection of incompressible fluids and in many computer simulations of rotating laminar convection. However, because of the low (eddy) viscosities and high rotation rates of giant planets, this theory predicts extremely thin convective columns (with aspect ratio of order the Ekman number to the 1/3 power). It is highly unlikely that these convective columns exist and remain intact without buckling in the compressible turbulent interiors of giant planets. Here we propose a much more robust mechanism for maintaining differential rotation in such environments based on the local expansion of rising fluid and contraction of sinking fluid. Two dimensional computer simulations of rotating turbulent convection illustrate how the resulting pattern of differential rotation depends on the radial profile of density.
P51B-1428 0800h
Interaction of Moist Convection With Jupiter's Zonal Jets
Since Voyager times, observations have suggested that Jupiter's zonal jets violate the barotropic stability criterion (BSTC) (Ingersoll et al., 1981; Limaye, 1986; Li et al., in press). Recently, images from the Cassini Imaging Science System (ISS) (Porco et al., 2003; Li et al., in press) and from the Galileo imaging system (Little et al., 1999; Gierasch et al., 2000) have revealed important features of moist convection on Jupiter and suggest that moist convection may be driving the zonal jets. Here we investigate the interaction of moist convection with the zonal jets in a reduced-gravity quasi-geostrophic model using a moist convection parameterization that is based on the new observations. Our study shows that moist convection can excite multiple jets when the velocity of the flow in the deep underlying layer is zero, but these jets never violate the BSTC. However, based on a model of the interaction between the magnetic field and the zonal flow, Liu and Stevenson (2003, DPS 35th meeting) predict that there are easterly flows in the deep underlying layer at middle latitudes. With easterly flows in the deep underlying layer we can get stable multiple jets that violate the BSTC. Furthermore, the modeled jets have almost same width and amplitude as the observed jets. An easterly flow in the lower layer provides a simple explanation for why the upper layer jets are stable even though they violate the BSTC. The model reproduces the tilted, chevron-shaped cloud features provided we assume that the clouds persist longer than the moist convective storms that produce them.
P51B-1429 0800h
The Relationship Between Eddies and Zonal Flow on Jupiter: Results From the Cassini Flyby
Using images of Jupiter from the Voyager missions, Ingersoll et al. (JGR 86, p. 8733-8743, 1981) found that longitudinally averaged eddy momentum flux, $\overline{u'v'}$, was positively correlated with the variation of zonal velocity with latitude, $\frac{d\bar{u}}{dy}$, implying that Jupiter's eddies are supplying energy to the zonal wind flow. Using a conservative estimate of the mass being transported in this process, they derive a power per unit area of between 1.5 and 3 $ W m^{-2}$. The amount of energy transferred would thus be more than 10% of Jupiter's thermal radiation; in comparison, Earth's eddies transfer an amount of energy that is about 0.1% of its emitted thermal energy. The analysis of Ingersoll et al., however, was challenged by Sromovsky et al. (JAtS 39, p. 1433-1445, 1982) who demonstrated that sampling biases could skew the results of such a study. Among their concerns were the possibility that a few, well-sampled areas of the planet were contributing disproportionately to the observed correlation, that the longitudinally averaged $\overline{u'v'}$ was strongly affected by values of $u'v'$ far from the median of the sample, and that the human eye, which was used for feature correlation, may have introduced unknown biases. After more than 20 years, we revisit these issues. We report results of an analysis of eddy momentum flux using images from the recent Cassini flyby of Jupiter analyzed with an automatic feature-tracker, which provided the advantages of more even planetary coverage, a greater number of tracked features and the absence of human bias. We find a similar correlation between $\overline{u'v'}$ and $\frac{d\overline{u}}{dy}$. We will present this result, our estimate of the eddy energy transfer rate, a discussion of the statistics of our sample and results of a second look at Voyager images.
P51B-1430 0800h
Laboratory Studies of Ammonia Ices Relevant to the Jovian Atmosphere
Ammonia ice condensation and cloud formation microphysics are topics of relevance for understanding the atmospheres of the
giant planets. Ammonia ices are also considered important components of the icy satellites found in the outer solar system,
and are thought to play an important role in their geological activity.
Although observational evidence and thermochemical models suggest ammonia clouds in the Jovian atmosphere should be
ubiquitous, less than only 1% of Jupiter's atmosphere appears covered by spectrally identifiable ammonia clouds, with a
clear preference in turbulent regions.1,2 The paradox of the rather scarce spectroscopic signatures of ammonia clouds
and their appearance in turbulent regions suggests that the nascent ammonia clouds may undergo processing that modifies their
spectroscopic properties. No relevant laboratory experimental results are available to resolve this problem. Two possible
sources of processing that have been suggested in the literature include photochemical solid-state modification
("tanning") and coating of ammonia particles by other substances present in the stratospheric haze.2,3
We are performing laboratory investigations with the objective to provide information on the photophysical and chemical
processes that control the optical properties of the Jovian ammonia clouds. In the experiments, thin ice films of ammonia are
coated with organic molecules, such as saturated and aromatic hydrocarbons, and characterized by infrared spectroscopy.
Preliminary results indicate suppression of the ammonia absorption feature at 2.97 μm by a thin layer of hydrocarbons. The
implications for the spectral signatures of ammonia clouds in the atmospheres of Jupiter and Saturn will be discussed.
Funding from the NSF Planetary Astronomy Program under grant AST-0206270 is gratefully acknowledged. The participation of
Rhiannon Meharchand and Christina Baer was made possible by the NSF Research Experiences for Undergraduates Program under
grant PHY-0353745.
1. S. K. Atreya and A.-S. Wong, Eos. Trans. 84(46), Fall. Meet. Suppl., Abstract A12A-0072 (2003), and references therein.
2. K. H. Baines, R. W. Carlson, and L. W. Kamp, Icarus 159, 74 (2002).
3. A.-S. Wong, Y. L. Yung, and A. J. Friedson, Geophys. Res. Lett. 30, 1447 (2003).
P51B-1431 0800h
Jovian Radio Burst Remote Sensing of Solar Wind Triggered Jovian Magnetospheric Activity: Ulysses Observations
In 2003 and early 2004, the Ulysses spacecraft descended from high heliographic latitudes towards perihelion, bringing it relatively close to Jupiter. The geometry of this distant flyby (0.8 AU closest approach) caused Ulysses to spend more than 6 months above a jovicentric latitude of 50 deg at a range of less than 2 AU, while the spacecraft traversed a considerable range of Jovian local time (9 hrs to 17 hrs). During much of this time interval, Jupiter was intercepted each solar rotation by two corotating high density structures and sector boundaries. From the perspective of Ulysses, the radio response of the magnetosphere to a given corotating structure was the intensification of either Jovian broad-band kilometric (bKOM) emission or of a combination of emissions, including bKOM and Jovian narrow-band kilometric (nKOM) emission. Such enhancements have been studied previously with Voyager, Ulysses, Galileo, and Cassini radio data. For Ulysses observations in 1991 and 1992, in particular, the typical scenario was brightening in the Jovian bKOM emission, followed by a sudden cessation of the bKOM emission and an onset of an nKOM "event" that lasted for some 120 hours (Reiner et al., 2000). For the sequences of events in 2003-2004, the two episodes per solar rotation; driven by high speed streams and/or current sheet crossing, clearly have different morphologies. In this presentation, variations in solar wind kinetic pressure and magnetic field direction (measured by Ulysses) are analyzed as triggers of Jovian magnetospheric activity, indicated by the intense radio emissions. Several intervals when magnetic clouds intercept Jupiter and produce Jovian radio events are also examined.
P51B-1432 0800h
Auroral and Non-auroral X-ray Emissions from Jupiter: A Comparative View
Jovian X-rays can be broadly classified into two categories: (1) "auroral" emission, which is confined to high-latitudes ($\sim > 60\deg$) at both polar regions, and (2) "dayglow" emission, which originates from the sunlit low-latitude ($\sim < 50\deg$) regions of the disk (hereafter called "disk" emissions). Recent X-ray observations of Jupiter by Chandra and XMM-Newton have shown that these two types of X-ray emission from Jupiter have different morphological, temporal, and spectral characteristics. In particular: 1) contrary to the auroral X-rays, which are concentrated in a spot in the north and in a band that runs half-way across the planet in the south, the low-latitude X-ray disk is almost uniform; 2) unlike the $\sim40\pm20$-min periodic oscillations seen in the auroral X-ray emissions, the disk emissions do not show any periodic oscillations; 3) the disk emission is harder and extends to higher energies than the auroral spectrum; and 4) the disk X-ray emission show time variability similar to that seen in solar X-rays. These differences and features imply that the processes producing X-rays are different at these two latitude regions on Jupiter. We will present the details of these and other features that suggest the differences between these two classes of X-ray emissions from Jupiter, and discuss the current scenario of the production mechanism of them.
P51B-1433 0800h
Investigating Structures in Thick, Complex Atmospheres Using Advanced Multiple Phase Screen Simulation of Radio Occultation
Advances in forward simulation of planetary radio occultation for thick atmospheres have allowed for a more accurate representation of the effects of atmospheric structure on radio occultation experiments. As a result, the effectiveness of inversion methods for recovering atmospheric profiles (e.g., refractivity) from occultation data can be determined more exactly. This research presents 1) advances in the multiple phase screen (MPS) technique for forward simulation of thick atmosphere radio occultation, and 2) results of inversion of simulated data for occultation of thick, complex atmospheres. While the traditional MPS method has proven reliable for thin atmosphere simulations, it fairs less favorably in simulations for occultation of highly refractive, thick atmospheres. This research attempts to produce more accurate forward simulations for thick atmosphere radio occultation by accounting for ray curvature in the standard MPS technique. Using these results, this research also probes the effect of complex atmospheric structures, such as ducts and critical refraction, on radio occultation as well as the effectiveness of Abel inversion in recovering atmospheric profiles for refractivity. Diffraction from a planet limb and sub-Fresnel-scale structure in thick atmospheres is also investigated, along with the effectiveness of wave-optic inversion techniques, i.e., back propagation.
P51B-1434 0800h
Exploration of the Jovian System Tapping Jupiter's Rotational Energy
Tours of the great outer planets and their moons can be accomplished by utilizing an electrodynamic tether attached to the spacecraft as both power system and propulsion device. Through interaction with the planetary magnetic field and inner plasmasphere, the tether could get electrical power, and either thrust or drag, out of the rotational motion of the planet; the relatively low altitude of the stationary orbit makes it possible to produce drag and power in portions of elliptical orbits inside the stationary orbit, and thrust and power outside. A Hollow Cathode and the (bare) tether itself would establish cathodic and anodic contact, respectively. By switching on and off the electrodynamic system at periapse and apoapse in specifically designed sequences, the orbit can be made to evolve to allow spacecraft capture, navigation through the moon system, and gravitational escape, without recourse to propellant and on-board power sources. For a tether tape in a Jovian tour, results are presented on a detailed orbital calculation of capture; on electrical power that can be drawn; on tether heating; and on expellant consumed at the Hollow Cathode throughout the tour. Orbital windows to get a secondary probe released from the spacecraft or the spacecraft itself into low orbit around Jupiter or one of its big moons, for scientific explorations, are discussed.
P51B-1435 0800h
The Juno New Frontiers Jupiter Polar Orbiter Mission
The Juno mission is currently in Phase A Concept Study as a candidate for the next NASA New Frontiers program investigation. The overarching scientific goal of the Juno mission is to understand the origin and evolution of Jupiter. As the archetype of giant planets, Jupiter can provide the knowledge we need to understand the origin of our own solar system and the planetary systems being discovered around other stars. Juno's investigation of Jupiter focuses on four themes: Origin, Interior Structure, Atmospheric Composition and Dynamics, and the Polar Magnetosphere. The mission is a Jupiter polar orbiter which uses a spinning spacecraft to make global maps of the gravity, magnetic fields, and atmospheric composition of Jupiter from a unique polar orbit with a close perijove. Juno's 32 orbits extensively sample Jupiter's full range of latitudes and longitudes. High sensitivity radiometric measurements thus yield a 3-dimensional view of Jupiter's deep atmosphere (down to ~100bars) to infer the bulk abundance of water, which could not be derived from Galileo data, and understand its complex meteorology. Determining water abundance permits discrimination between various scenarios of the formation of Jupiter. The radiometry will also confirm the global ammonia abundance. The gravity data constrain the planet's interior rotation and structure. The precise magnetic field measurements are used to infer how the interior dynamo works and generates the most powerful magnetic field of planets in the Solar System. From its polar perspective Juno combines in situ and remote sensing observations to explore the polar magnetosphere and determine what drives Jupiter's remarkable auroras. Without landers, probes, or returned samples, the mission has extremely low risk. The JPL contribution to this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
P51B-1436 0800h
An Updated View of Saturn's Upper Atmosphere From a Reanalysis of the Voyager 1 and 2 UVS Occultations
The Voyager 1 and 2 Ultraviolet Spectrometer (UVS) solar and stellar occultation data set represents one of the primary, pre-Cassini sources of information that we have on the neutral upper atmosphere of Saturn. Despite its importance, however, the full set of occultations has never received a consistent analysis, and the results derived from the initial analyses over twenty years ago left questions about the temperature and density profiles unanswered. We have reanalyzed all six of the UVS occultations (three solar and three stellar) to provide an up-to-date, pre-Cassini view of Saturn's upper atmosphere. This analysis has resulted in altitude profiles for H$_2$, H, CH$_4$, C$_2$H$_2$, C$_2$H$_4$, and C$_2$H$_6$ as well as information on the temperature structure. In addition, our analysis provides an explanation for the two different thermospheric temperatures derived in earlier analyses (400-450 K versus 800 K) as well as the unusual shape of the total density profile noted by Hubbard et al. (Icarus, 130, 404-425, 1997). The results of our reanalysis will provide a useful baseline for interpreting new data from Cassini, particularly in the context of change over the past two decades.
P51B-1437 0800h
Clouds of Neptune and Uranus: Implications for Entry Probes
Abundances of heavy elements below cloud levels provide critical constraints to the models of formation of giant planets and the origin of their atmospheres. In this study we present the results of an equilibrium cloud condensation model which calculates the bases and concentrations of methane ice, ammonia ice, ammonium hydrosulfide-solid, water ice, and aqueous-ammonia solution (``droplet'') clouds. Due to their similar p-T structures, the Neptune and Uranus results are similar. Based on the measured CH$_4$ mixing ratio, the C/H at Neptune is 30-50$\times$ solar, and 20-30$\times$ solar at Uranus. Assuming similar enhancement for the other condensibles, as expected from formation models, we find that the base of the droplet cloud is at the 370 bars for 30$\times$ solar, and at 500 bar for 50$\times$ solar cases. On the other hand, noble gases and H$_2$S can be accessed at much shallower levels, and still permit the retrieval of information critical to the formation of these planets and their atmospheres.
P51B-1438 0800h
Outstanding Science at Neptune: Aerocapture Implementation of NASA's "Neptune Orbiter With Probes" Vision Mission
After successfully proposing to NASA's "Vision Missions Studies" NRA (NRA-03-OSS-01-VM) the authors are studying an implementation option for the "Neptune Orbiter With Probes" mission that performs Cassini-level science without fission-based electric power or propulsion. Our Study Team includes a Science Team composed of experienced planetary scientists, many of whom helped draft the Neptune discussions in the Solar System Exploration Decadal Survey, and an Implementation Team with experienced engineers and technologists from multiple NASA Centers and JPL. The key characteristics of our mission concept are a mix of Solar Electric Propulsion and gravity assists to reach Neptune in 12 years or less, aerocapture into an eccentric Neptune orbit for a Triton-driven orbital tour, and a well designed set of science objectives guiding a very capable science payload including multiple Neptune entry probes. Significant pathfinding work done in 2002-03 by NASA's Aerocapture Systems Analysis Team allowed focusing quickly on principal issues. By the end of May 2004 the Science Team had drafted a thorough and coherent set of science objectives, leading to our first series of design sessions with JPL's "Team X" in early June. The initial design options studied in those sessions produced flight system designs that all fit easily on soon-to-be-operational launch vehicles. Since then, students working in Caltech's Laboratory for Space Mission Design have studied the potential benefits of incorporating new technologies into those initial designs. Later this year another series of Team X sessions will incorporate the most beneficial technologies into the best design options. The poster will discuss the mission's science objectives, and summarize study results to date. This work was performed at the Jet Propulsion Laboratory and its parent institution, the California Institute of Technology, under contract to NASA's Office of Space Science.
P51B-1439 0800h
A Neptune/Triton Vision Mission Using Nuclear Electric Propulsion
The giant planets of the outer solar system divide into two distinct classes: the `gas giants' Jupiter and Saturn, primarily comprising hydrogen and helium; and the `ice giants' Uranus and Neptune that are believed to contain significant amounts of the heavier elements including oxygen, nitrogen, carbon, and sulfur. Detailed comparisons of the internal structures and compositions of the gas giants with those of the ice giants will yield valuable insights into the processes that formed the solar system and, perhaps, extrasolar systems. By 2012, Pioneer, Voyager, Galileo, Cassini, and possibly a New Frontiers Jupiter mission will have yielded significant information on the chemical and physical properties of Jupiter and Saturn. A Neptune mission would deliver the corresponding key data for an ice giant planet. A Neptune Orbiter with Probes mission utilizing nuclear electric propulsion (NEP) to study Triton, Nereid, the other icy satellites of Neptune, Neptune's system of rings, and the deep Neptune atmosphere to pressures ranging from several hundred bars to possibly several kilobars is being examined. Power and propulsion would be provided using nuclear electric technologies. Such an ambitious mission requires a number of technical issues be investigated and resolved, including: (1) developing a realizable mission design that allows proper targeting and timing of the entry probe(s) while offering adequate opportunities for detailed measurements of Triton, the other icy satellites and ring science, (2) giant-planet atmospheric probe thermal protection system (TPS) design, (3) descent probe design including seals, windows, penetrations and inlets, and pressure vessel, (4) probe telecommunications through the dense and absorbing Neptunian atmosphere, and (5) within NEP mass and power constraints, defining an appropriate suite of science instruments to explore the depths of the Neptune atmosphere, magnetic field, Triton, and the icy satellites. Another driving factor in the design of the Orbiter and Probes is the necessity to maintain a fully operational flight system during the lengthy transit time from launch through Neptune encounter, and beyond. Following our response to the recent NASA Research Announcement (NRA) for Space Science Vision Missions for mission studies by NASA for implementation in the 2013 or later time frame, our team has been selected to explore the feasibility of such a Neptune mission.