Xylar Asay-Davis

Forming Planetesimals by Gravitational Instability . I . the Role of the Richardson Number in Triggering the Kelvin – Helmholtz Instability

Gravitational instability (GI) of a dust-rich layer at the midplane of a gaseous circumstellar disk is one proposed mechanism to form planetesimals, the building blocks of rocky planets and gas giant cores. Self-gravity competes against the Kelvin-Helmholtz instability (KHI): gradients in dust content drive a vertical shear which risks overturning the dusty subdisk and forestalling GI. To understand the conditions under which the disk can resist the KHI, we perform 3D simulations of stratified subdisks in the limit that dust particles are small and aerodynamically well coupled to gas. This limit screens out the streaming instability and isolates the KHI. Each subdisk is assumed to have a vertical density profile given by a spatially constant Richardson number Ri. We vary Ri and the midplane dust-to-gas ratio mu and find that the critical Richardson number dividing KH-unstable from KH-stable flows is not unique; rather Ri_crit grows nearly linearly with mu for mu=0.3-10. Only for disks of bulk solar metallicity is Ri_crit ~ 0.2, close to the classical value. Our results suggest that a dusty sublayer can gravitationally fragment and presumably spawn planetesimals if embedded within a solar metallicity gas disk ~4x more massive than the minimum-mass solar nebula; or a minimum-mass disk having ~3x solar metallicity; or some intermediate combination of these two possibilities. Gravitational instability seems possible without resorting to the streaming instability or to turbulent concentration of particles.

FORMING PLANETESIMALS BY GRAVITATIONAL INSTABILITY. II. HOW DUST SETTLES TO ITS MARGINALLY STABLE STATE

Dust at the midplane of a circumstellar disk can become gravitationally unstable and fragment into planetesimals if the local dust-to-gas ratio μ0 ≡ ρd/ρg is sufficiently high. We simulate how dust settles in passive disks and ask how high μ0 can become. We implement a hybrid scheme that alternates between a one-dimensional code to settle dust and a three-dimensional shearing box code to test for dynamical stability. This scheme allows us to explore the behavior of small particles having short but non-zero stopping times in gas: 0 < t stop the orbital period. The streaming instability is thereby filtered out. Dust settles until Kelvin-Helmholtz-type instabilities at the top and bottom faces of the dust layer threaten to overturn the entire layer. In this state of marginal stability, μ0 = 2.9 for a disk whose bulk (height-integrated) metallicity Σd/Σg is solar—thus μ0 increases by more than two orders of magnitude from its well-mixed initial value of μ0,init = Σd/Σg = 0.015. For a disk whose bulk metallicity is 4× solar (μ0,init = Σd/Σg = 0.06), the marginally stable state has μ0 = 26.4. These maximum values of μ0, which depend on the background radial pressure gradient, are so large that gravitational instability of small particles is viable in disks whose bulk metallicities are just a few (4) times solar. Our result supports earlier studies that assumed that dust settles until the Richardson number Ri is spatially constant. Our simulations are free of this assumption but provide evidence for it within the boundaries of the dust layer, with the proviso that Ri increases with Σd/Σg in the same way that we found in Paper I. Because increasing the dust content decreases the vertical shear and increases stability, the midplane μ0 increases with Σd/Σg in a faster than linear way, so fast that modest enhancements in Σd/Σg can spawn planetesimals directly from small particles.

Dramatic Change in Jupiter's Great Red Spot from Spacecraft Observations

Jupiters Great Red Spot (GRS) is one of its most distinct and enduring features. Since the advent of modern telescopes, keen observers have noted its appearance and documented a change in shape from very oblong to oval, confirmed in measurements from spacecraft data. It currently spans the smallest latitude and longitude size ever recorded. Here we show that this change has been accompanied by an increase in cloud/haze reflectance as sensed in methane gas absorption bands, increased absorption at wavelengths shorter than 500 nm, and increased spectral slope between 500 and 630 nm. These changes occurred between 2012 and 2014, without a significant change in internal tangential wind speeds; the decreased size results in a 3.2 day horizontal cloud circulation period, shorter than previously observed. As the GRS has narrowed in latitude, it interacts less with the jets flanking its north and south edges, perhaps allowing for less cloud mixing and longer UV irradiation of cloud and aerosol particles. Given its long life and observational record, we expect that future modeling of the GRSs changes, in concert with laboratory flow experiments, will drive our understanding of vortex evolution and stability in a confined flow field crucial for comparison with other planetary atmospheres.

Developments in Simulating and Parameterizing Interactions Between the Southern Ocean and the Antarctic Ice Sheet

Recent advances in both ocean modeling and melt parameterization in ice-sheet models point the way toward coupled ice sheet–ocean modeling, which is needed to quantify Antarctic mass loss and the resulting sea-level rise. The latest Antarctic ocean modeling shows that complex interactions between the atmosphere, sea ice, icebergs, bathymetric features, and ocean circulation on many scales determine which water masses reach ice-shelf cavities and how much heat is available to melt ice. Meanwhile, parameterizations of basal melting in standalone ice-sheet models have evolved from simplified, depth-dependent functions to more sophisticated models, accounting for ice-shelf basal topography, and the evolution of the sub-ice-shelf buoyant flow. The focus of recent work has been on better understanding processes or adding new model capabilities, but a broader community effort is needed in validating models against observations and producing melt-rate projections. Given time, community efforts in coupled ice sheet–ocean modeling, already underway, will tackle the considerable challenges involved in building, initializing, constraining, and performing projections with coupled models, leading to reduced uncertainties in Antarctica’s contribution to future sea-level rise.

Jupiter’s Red Oval BA: Dynamics, Color, and Relationship to Jovian Climate Change

Jupiter now has a second red spot, the Oval BA. The first red spot, the Great Red Spot (GRS), is at least 180 yr old. The Oval BA formed in 2000 was originally white, but part turned red in 2005. Unlike the Great Red Spot, the red color of the Oval BA is confined to an annulus. The Oval’s horizontal velocity and shape and the elevation of the haze layer above it were unchanged between 2000 and 2006. These observations, coupled with Jupiter’s rapid rotation and stratification, are shown to imply that the Oval BA’s 3D properties, such as its vertical thickness, were also unchanged. Therefore, neither a change in size nor velocity caused the Oval BA to turn partially red. An atmospheric warming can account for both the timing of the color change of the Oval BA as well as the persistent confinement of its red color to an annulus.