I am broadly interested in the dynamics and evolution of planetary atmospheres and interiors. My approach includes theoretical modeling and direct interpretation of spacecraft and groundbased data. Currently, my research efforts focus on understanding (1) atmospheric dynamics on giant planets inside and outside the solar system, and (2) the evolution and geophysics of the Galilean satellites. My major contributions include advances in our understanding of jet streams on gas giant planets; the first nonlinear simulations of Jupiter's 5-micron hot spots (which explain the perplexing dryness measured by the Galileo probe inside a hot spot); the first 3D numerical simulations of close-in extrasolar giant planets; and an elucidation of the processes that caused disrupted terrains on Europa, Ganymede, and Saturn's icy satellites.
Over the next few years, my research will encompass four major projects. First, questions exist concerning the dynamics of jets, vortices, and other features on all four giant planets, and I believe fundamental progress can be made with new circulation models that include clouds. Second, I will continue to study the atmospheres of extrasolar giant planets, where the circulation can affect the long-term evolution (hence present-day radius) as well as observables such as albedo and day-night temperature differences. Third, I will study the formation of ridges, grooves, and other disrupted terrains on Ganymede, Europa, and Saturn's satellites; this will help constrain the satellites' thermal histories and deepen our understanding of geological activity in a non-terrestrial environment. Upcoming groundbased and spacecraft observations ensure that these will be robust research arenas in the coming decade. Finally, in the near future I plan to launch a research program involving Mars atmospheric circulation and climate. The President's Moon-Mars initiative and NASA's continuing interest in "follow the water" indicate that this will be a major growth area.
Jovian atmospheric dynamics and chemistry
Over the past three years, I have pursued a range of investigations that shed light on the dynamics of the numerous high-speed east-west jet streams that dominate the circulations of Jupiter, Saturn, Uranus, and Neptune -- a key unsolved problem. Using full 3D numerical simulations, graduate student Yuan Lian and I showed that shallow cloud-level forcing can produce numerous Jupiter-like jets, including the strongly superrotating equatorial jet that is difficult to reproduce in two-dimensional models (Lian et al. 2006). This is a major success and supports the idea that cloud-layer forcing plays a key role in pumping jets on giant planets. Furthermore, we showed that interaction between the cloud-layer and deeper flows can drive deep jets that extend to essentially arbitrary depths even when the forcing is confined to the the top of the system (Showman et al. 2006; Lian et al. 2006). This disproves the assumption -- common in the literature -- that shallow (cloud-layer) forcing can only produce shallow jets and that deep jets can only result from deep forcing. It also throws into doubt the claim, made by numerous authors, that Galileo Probe observations of strong wind at 20 bars (150 km below the clouds) provides evidence for deep forcing. This led to a press release, which was picked up by USA Today, space.com, spacedaily.com, spaceref.com, Astrobiology Magazine, and saturntoday.com, among others.
I used EPIC, a global-scale numerical atmospheric circulation model written by Timothy Dowling, to explain the puzzling dryness measured by the Galileo probe. According to probe measurements, the abundances of ammonia, H2S, and water -- which condense and form clouds near 0.5, 2, and 5 bars, respectively -- were low at the condensation altitudes and rose toward expected values only 80 km deeper. The water abundance at 20 bars was only one-tenth that expected from formation models and meteorological studies. Several authors suggested that the probe entered a localized region of dry, descending air, which is consistent with the fact that it entered an anomalously cloud-free feature called a 5-micron "hot spot." However, until recently, the downdraft hypothesis remained untested.
|My simulations (performed in collaboration with Timothy Dowling) demonstrate that massive subsidence can occur in local regions, explaining the low cloudiness and the dryness measured by the probe (Showman and Dowling 2000, Science 289, 1737-1740; Showman and Ingersoll 1998, Icarus 132, 205-220). The downdrafts also match hot spot behavior quite well. My results imply that Jupiter may be "wet" after all; the observed dryness is probably local. Furthermore, at the simulated probe-entry site, the simulations match the wind shear observed by Doppler tracking of the probe signal. But the simulated winds have different patterns elsewhere. This implies that the observed winds result from the eddy structure of hot spots (rather than representing Jupiter as a whole). Future work will test the idea that moist convection occurring between hot spots provide the energy that drives these wave-like features.|
My plan for the future is motivated by fundamental, unanswered questions that exist regarding the global circulations of all four giant planets. Although one-layer (i.e., two-dimensional) models reproduce gross features of the observed zonal jets and large vortices, we poorly understand the flow's energetics and vertical structure. It is not known what pumps up the zonal jets, how energy cascades among atmospheric features, or whether interaction with the interior is important. The most promising jet-pumping mechanism, moist convection, has never been included in a dynamical model, and the relative roles of solar energy absorption and the intrinsic heat flux at Jupiter, Saturn, and Neptune remain obscure. In collaboration with Peter Gierasch, Timothy Dowling, Richard Young, and graduate students here at LPL, I am attacking these questions using new, three-dimensional models that include simple representations of moist convection. Additional projects that I am currently pursuing include the study of cloud formation on Uranus and Neptune; tidal dissipation in the interior of giant planets; the dynamics of waves in Jupiter's stratosphere; deep convection as a mechanism for pumping giant-planet jets; and the development of a new hybrid-coordinate (pressure-entropy) general circulation model applicable to both gas-giant and terrestrial planet atmospheres.
Click here for some results on shallow-atmosphere turbulence for Jupiter.
Atmospheres of extrasolar giant planets
In collaboration with Tristan Guillot and graduate student Curtis Cooper, I am investigating the atmospheric circulation and evolution of "hot Jupiters," which are extrasolar giant planets only ~0.03-0.2 AU from their stars. The evolutionary histories and present-day planetary radii of hot Jupiters depend on atmospheric dynamics. For example, clouds affect atmospheric temperatures (hence planetary radius) by altering the albedo and depth to which stellar light penetrates; whether clouds form on the dayside depends on whether sufficient upward motion occurs there. Further, because hot Jupiters are expected to be tidally locked, the day-night temperature difference --- which likely affects the intrinsic heat flux and radius --- depends on wind speed. Using the EPIC model, I conducted three-dimensional numerical simulations to constrain the winds, temperatures, and locations of cloud formation and understand the implications for the evolution (Showman and Guillot 2002, Astron. & Astrophys. 385, 166-180; Guillot and Showman 2002, Astron. & Astrophys. 385, 156-165). Under my guidance, Mr. Cooper has adapted a new general-circulation model for giant-planet use, and we have been obtaining new results, soon to be published, for extrasolar planet circulation. Stellar transits allow the radius to be measured and provide observational tests (already successful for one object, HD209458b, with more transit detections sure to follow). Direct measurement of albedo and day-night temperature differences may also be possible in the near future, providing additional tests.
Third, I plan to investigate the dynamics of martian dust storms. Viking and Mars Global Surveyor observations suggest that hundreds of local dust storms, typically 102-103 km across, occur every martian year. Once every few years, a local storm violently explodes in horizontal extent, encircling the planet in an opaque pall of airborne dust. Some workers hypothesize that the storms grow by a positive feedback that occurs when the circulation (hence dust lifting) strengthens as a result of heating by the airborne dust. However, this feedback has not been adequately explored for local storms. The fact that only ~0.1% of local storms become great storms suggests that this feedback does not usually occur. I would like to analyze data from Viking and Mars Global Surveyor to characterize local storm behavior. In collaboration with Mark Richardson and others, I will also use a Mars general circulation model to explore dust storm evolution. My aim is to determine whether the hypothesized feedback can account for the violent growth of great storms, and if so, why it occurs in so few local storms.
|(Left): Regional Galileo image of Ganymede bright and dark terrain.|
Resurfacing and tectonics of icy satellites
Another research effort focuses on the internal and orbital evolution of icy satellites in the outer solar system. The dichotomy between Callisto's old, cratered surface and Ganymede's younger, grooved surface (see image above) has long puzzled researchers. While in graduate school, I explored the hypothesis that Ganymede was resurfaced as a result of tidal heating in an ancient orbital resonance; Renu Malhotra and I showed that Io, Europa, and Ganymede could have passed through either of two previously unexplored Laplace-like resonances that cause heating in Ganymede, and we determined the effects of such heating on Ganymede's interior (Showman and Malhotra 1997, Icarus 127, 93-111, Showman et al. 1997, Icarus 129, 367-383). Ganymede appears to have been cryovolcanically resurfaced, but the volcanic melt --- liquid water --- is denser than ice, so the melt percolates down and cannot reach the surface. This problem has been a fundamental stumbling block to understanding Ganymede for over 20 years. Recently, I showed that topography induces subsurface pressure gradients that can "suck" liquid water upward to the surface despite its negative buoyancy (Showman, Mosqueira, and Head 2004, Icarus, 172, 625-640). This model provides the first viable mechanism for explaining Ganymede's bright terrain through cryovolcanism. The figure to the left illustrates this effect. For the topography in (a), representing rift zone 2-km deep, the pressure-gradient forces that drive the flow are shown in (b). The forces are upward underneath the rift zone. (c) shows the net force acting on pore-space liquid water (produced by tidal heating, for example). In most places, the forces point downward, which represent the natural tendency of water, which is dense, to percolate downward through ice. Under the rift zone, however, the "topographic" forces dominate over the negative buoyancy, and liquid water can be pumped upward into the rift zone despite its negative buoyancy. This might help explain the existence of bright terrain lanes such as shown in the figure of Ganymede above.
Europa has also been an exciting topic. In collaboration with Lijie Han, I have investigated whether convection in Europa's ice shell can produce Europa's numerous pits, uplifts, and disrupted regions (Showman and Han, J. Geophys. Res., 2004). Our recent models that include the effects of plastic rheology, which provides a simple continuum representation for brittle/semibrittle deformation along discrete fractures, suggest that chaos terrains can result from convection in the ice shell yet demonstrate that producing pits and domes with convection is quite difficult (Showman and Han, Icarus, in press). Under my guidance, Giuseppe Mitri has performed numerical simulations of convection in Europa's ice shell demonstrating that, for a range of conditions, two equilibrium states --- corresponding to thin, conductive and thick, convective ice shells --- exist for a given heat flux. We showed how Europa can switch between these states, with important consequences for the surface (Mitri and Showman 2004, Icarus, in press).
In the future, I will construct new coupled models of crustal deformation and heat transport to investigate the formation of ridges, grooves, and chaotic terrains on Ganymede, Europa, and Saturn's icy satellites. These studies will help constrain the satellites' evolutionary histories and subsurface structure. New data from Cassini and a future Jupiter icy-moons orbiter guarantee that this will continue to be an exciting area.
The projects described on this webpage have in part been supported by NSF grants AST-0206269 and AST-0307664 and NASA grants NAG5-13329 and NNG04GI46G to Adam Showman.