Nitrogen Frost Research Articles

An atmospheric general circulation model for Pluto with predictions for New Horizons temperature profiles

Results are presented from a 3-D Pluto general circulation model (GCM) that includes conductive heating and cooling, non-local thermodynamic equilibrium (non-LTE) heating by methane at 2.3 and 3.3 microns, non-LTE cooling by cooling by methane at 7.6 microns, and LTE CO rotational line cooling. The GCM also includes a treatment of the subsurface temperature and surface-atmosphere mass exchange. An initially 1 m thick layer of surface nitrogen frost was assumed such that it was large enough to act as a large heat sink (compared with the solar heating term) but small enough that the water ice subsurface properties were also significant. Structure was found in all three directions of the 3-D wind field (with a maximum magnitude of order 10 m/s in the horizontal directions and 10$^-5$ microbar/s in the vertical direction). Prograde jets were found at several altitudes. The direction of flow over the poles was found to very with altitude. Broad regions of up-welling and down-welling were also found. Predictions of vertical temperature profiles are provided for the Alice and REX instruments on New Horizons, while predictions of light curves are provided for ground-based stellar occultation observations. With this model methane concentrations of 0.2% and 1.0% and 8 and 24 microbar surface pressures are distinguishable. For ground-based stellar occultations, a detectable difference exists between light curves with the different methane concentrations, but not for different initial global mean surface pressures.

Probing Pluto’s underworld: Ice temperatures from microwave radiometry decoupled from surface conditions

Present models admit a wide range of 2015 surface conditions at Pluto and Charon, where the atmospheric pressure may undergo dramatic seasonal variation and for which measurements are imminent from the New Horizons mission. One anticipated observation is the microwave brightness temperature, heretofore anticipated as indicating surface conditions relevant to surface–atmosphere equilibrium. However, drawing on recent experience with Cassini observations at Iapetus and Titan, we call attention to the large electrical skin depth of outer Solar System materials such as methane, nitrogen or water ice, such that this observation may indicate temperatures averaged over depths of several or tens of meters beneath the surface. Using a seasonally-forced thermal model to determine microwave emission we predict that the southern hemisphere observations (in polar night) of New Horizons in July 2015 will suggest effective temperatures of ∼40K, reflecting deep heat buried over the last century of summer, even if the atmospheric pressure suggests that the surface nitrogen frost point may be much lower.

Photometry of Triton 1992–2004: Surface volatile transport and discovery of a remarkable opposition surge

Triton, the large satellite of Neptune, was imaged by the Voyager 2 spacecraft in 1989 with dark plumes originating in its volatile-rich south polar region. Southern summer solstice, a time when seasonal volatile transport should be at a maximum, occurred in 2001. Ground-based observations of Triton’s rotational light curve obtained from Table Mountain Observatory in 2000–2004 reveal volatile transport on its surface. When compared with a static frost model constructed from Voyager images, the light curve shows an increase in total amplitude. An earlier light curve obtained in 1992 from Mauna Kea Observatory is consistent with the static frost model. This movement of volatiles on the surface agrees with recent imaging results from the Hubble Space Telescope (Bauer, J.M., Buratti, B.J., Li, J.-Y., Mosher, J.A., Hicks, M.D., Schmidt, B.E., Goguen, J.D. [2010]. Astrophys. J. 723, L49–L52). The changes in the light curve can be explained by the transport of nitrogen frost on the surface or by the uncovering of bedrock of less volatile methane. We also find that Triton exhibits a large opposition surge at solar phase angles less than 0.1°. This surge cannot be entirely explained by the effects of coherent backscatter.

VERTICAL STRUCTURE IN PLUTO'S ATMOSPHERE FROM THE 2006 JUNE 12 STELLAR OCCULTATION

Pluto occultations are historically rare events, having been observed in 1988, 2002, 2006, and, as Pluto moves into the crowded Galactic plane, on several occasions in 2007. Here we present six results from our observations of the 2006 June 12 event from several sites in Australia and New Zealand. First, we show that Pluto's 2006 bulk atmospheric column abundance, as in 2002, is over twice the value measured in 1988, implying that nitrogen frost on Pluto's surface is 1.2-1.7 K warmer in 2006 than 1988 despite a 9% drop in incident solar flux. We measure a half-light shadow radius of 1216 ± 8.6 km in 2006, nominally larger than published values of 1213 ± 16 km measured in 2002. Given the current error bars, this latest half-light radius cannot discriminate between continued atmospheric growth or shrinkage, but it rules out several of the volatile transport scenarios modeled by Hansen & Paige. Second, we resolve spikes in the occultation light curve that are similar to those seen in 2002 and model the vertical temperature fluctuations that cause them. Third, we show that Pluto's upper atmosphere appears to hold a steady temperature of ~100 K, as predicted from the methane thermostat model, even at latitudes where the methane thermostat is inoperative. This implies that energy transport rates are faster than radiational cooling rates. Fourth, this occultation has provided the first significant detection of a non-isothermal temperature gradient in Pluto's upper atmosphere also reported by Elliot et al., possibly the result of CO gas in Pluto's upper atmosphere. Fifth, we show that a haze-only explanation for Pluto's light curve is extremely unlikely; a thermal inversion is necessary to explain the observed light curve. And sixth, we derive an upper limit for the haze optical depth of 0.0023 in the zenith direction at average CCD wavelengths.

Pluto's Spectrum from 1.0 to 4.2 μm: Implications for Surface Properties

We present spectra of Pluto's anti-Charon hemisphere obtained from the Keck and Subaru telescopes from 2.8 to 4.2 μm. Combined with 1-2.5 μm spectra from the Infrared Telescope Facility, this collective data set lets us constrain several surface frost properties. The surface area of pure nitrogen frost (as opposed to nitrogen with dissolved methane) is constrained to be 6% or less. The areal fractions of pure methane and methane dissolved in nitrogen are almost equal. The grain size of pure methane is constrained to be near 200 μm. An additional surface component with spectral properties similar to Titan tholin was necessary to fit the entire 1-4.2 μm spectrum; our best-fit model requires 21% of Pluto's anti-Charon hemisphere (by area) to be this Titan tholin component. Contrary to Sasaki et al.'s spectra of Pluto's sub-Charon hemisphere, we find no evidence for other hydrocarbons on this face of Pluto from data in the 3-3.3 μm region. We were not able to constrain the temperature of pure methane.

Photometric Diversity of Terrains on Triton

Voyager disk-resolved images of Triton in the violet (0.41 μm) and green (0.56 μm) wavelengths have been analyzed to derive the photometric characteristics of terrains on Triton. Similar conclusions are found using two distinct but related definitions of photometric units, one based on color ratio and albedo properties (A. S. McEwen, 1990, Geophys. Res. Lett. 17, 1765-1768), the other on albedo and brightness ratios at different phase angles (cf. P. Lee et al.; 1992, Icarus 99, 82-97). A significant diversity of photometric behavior, much broader than that discovered so far on any other icy satellite, occurs among Triton's terrains. Remarkably, differences in photometric behavior do not correlate well with geologic terrain boundaries defined on the basis of surface morphology. This suggests that in most cases photometric properties on Triton are controlled by thin deposits superposed on underlying geologic units.Single scattering albedus are 0.98 or higher and asymmetry factors range from -0.35 to -0.45 for most units. The most distinct scattering behavior is exhibited by the reddish northern units already identified as the Anomalously Scattering Region (ASR) by Lee et al., which scatters light almost isotropically with g = -0.04. In part due to the effects of Triton's clouds and haze, it is difficult to constrain the value of θ̄ , Hapke's macroscopic roughness parameter, precisely for Triton or to map differences in θ̄ among the different photometric terrains. However, our study shows that Triton must be relatively smooth, with θ̄ less than 15-20°, and suggests that a value of 14° is appropriate. The differences in photometric characteristics lead to significantly different phase angle behavior for the various terrains. For example, a terrain (e.g., the ASR) that appears dark relative to another at low phase angles will reverse its contrast (become relatively brighter) at larger phase angles.The photometric parameters have been used to calculate hemispherical albedos for the units and to infer likely surface temperatures. Based on these results, we determine that all but the most southerly regions (≃south of the equator) of the reddish northern terrains are likely to have been covered with deposits of nitrogen frost at the time of the Voyager flyby, in agreement with the suggestion from the photometry that these units are overlain by a thin veneer of material.

A thermal model for the seasonal nitrogen cycle on Triton

A Mars thermal model has been adapted to study the seasonal nitrogen cycle on Triton. Unlike other models published to date, it incorporates diurnal and seasonal subsurface heat conduction, and accounts for the heat capacity of N 2 frost deposits. Key observables that this model attempts to reproduce include Triton's atmospheric pressure and surface albedo features at the time of the Voyager encounter, as well as changes in Triton's disk-integrated spectral properties between 1977 and 1989. Subsurface heat conduction is found to have an important effect on the behavior of Triton's polar caps and atmospheric pressure fluctuations. The results of this model differ from those of previous studies in that they generally do not predict the catastrophic freeze-out of Triton's atmosphere on a seasonal basis. The model results indicate that even for a wide range of possible input parameters, it is unlikely that Triton's bright southern polar cap is a seasonal nitrogen deposit, because it would have largely sublimated before the Voyager encounter. However, these results do not rule out the possibility of a large bright permanent polar cap in Triton's southern hemisphere. The best agreement between the model results and the available observations is obtained when Triton's seasonal nitrogen frost deposits are assumed to be darker than the undelying substrate. Assuming a constant nitrogen frost albedo of 0.62, a frost emissivity of 0.5, a substrate albedo of 0.8, and a substrate thermal inertia of 7 × 10 −3 cal cm −2 K −1 sec −1 2 yields good agreement with the surface pressure and global albedo patterns observed during the Voyager encounter, as well as a rapid change in disk-integrated albedo from 1977 to 1989 due to the retreat and disappearance of a dark south seasonal polar cap. These results lend support to microphysical arguments for the presence of dark, or smooth, translucent nitrogen frost on the surface of Triton.

Triton's Plumes: The Dust Devil Hypothesis

Triton's plumes are narrow columns 10 kilometers in height, with tails extending horizontally for distances over 100 kilometers. This structure suggests that the plumes are an atmospheric rather than a surface phenomenon. The closest terrestrial analogs may be dust devils, which are atmospheric vortices originating in the unstable layer close to the ground. Since Triton has such a low surface pressure, extremely unstable layers could develop during the day. Patches of unfrosted ground near the subsolar point could act as sites for dust devil formation because they heat up relative to the surrounding nitrogen frost. The resulting convection would warm the atmosphere to temperatures of 48 kelvin or higher, as observed by the Voyager radio science team. Assuming that velocity scales as the square root of temperature difference times the height of the mixed layer, a velocity of 20 meters per second is derived for the strongest dust devils on Triton. Winds of this speed could raise particles provided they are a factor of 103 to 104 less cohesive than those on Earth.

KOYAANISMUUYAW: THE HYPOTHESIS OF A PERENNIALLY DICHOTOMOUS TRITON

The Voyager 2 observation that, at the height of a “major” southern summer, the southern “cap” of Triton extends nearly to the equator, and the northern temperate regions (where they can be seen) are relatively dark even though nitrogen frost should be accumulating at those latitudes, has prompted us to propose a hypothesis that there has been a net non‐reversable flux of surficial volatiles on Triton from the northern hemisphere to the southern hemisphere over much of the satellite's history and up to the present. The north‐south hemispherical albedo difference is permanent and overwhelms the seasonal effect on volatile transport. This hypothesis assumes that volatile erosion, transport, and deposition has been in the form of sublimation, advection, and precipitation.

NITROGEN FROST MIGRATION ON TRITON: A historical model

I present the results of numerical simulation of the seasonal migration of nitrogen frost on Triton constrained by Voyager observations of atmospheric pressure, temperature, and albedo distribution. Most of the exposed nitrogen is probably seasonal frost, whose migration can produce major variations in atmospheric pressure. For instance, models explored here predict a tenfold pressure drop in the coming decade. The observed patterns can be understood if fresh nitrogen frost is relatively dark but brightens with increasing insolation in a manner analogous to the Martian southern CO2 cap.

GLOBAL COLOR AND ALBEDO VARIATIONS ON TRITON

Global multispectral mosaics of Triton have been produced from Voyager approach images; six spectral units are defined and mapped. The margin of the south polar cap (SPC) is scalloped and ranges in latitude from + 10° to −30°. A bright fringe is closely associated with the cap's margin; from it, diffuse bright rays extend north‐northeast for hundreds of kilometers. Thus, the rays may consist of fringe materials that were redistributed by northward‐going Coriolis‐deflected winds. From 1977 to 1989, Triton's foil‐disk spectrum changed from markedly red and uv‐dark to nearly neutral white and uv‐bright. This spectral change can be explained by new deposition of nitrogen frost over both the northern hemisphere and parts of a formerly redder SPC. Frost deposition in the southern hemisphere during southern summer is possible over relatively high albedo areas of the cap (Stansbeny et al, 1990), which helps to explain the apparent stability of the unexpectedly large SPC and the presence of the bright fringe.

Voyager 2 at Neptune: Imaging Science Results

Voyager 2 images of Neptune reveal a windy planet characterized by bright clouds of methane ice suspended in an exceptionally clear atmosphere above a lower deck of hydrogen sulfide or ammonia ices. Neptune's atmosphere is dominated by a large anticyclonic storm system that has been named the Great Dark Spot (GDS). About the same size as Earth in extent, the GDS bears both many similarities and some differences to the Great Red Spot of Jupiter. Neptune's zonal wind profile is remarkably similar to that of Uranus. Neptune has three major rings at radii of 42,000, 53,000, and 63,000 kilometers. The outer ring contains three higher density arc-like segments that were apparently responsible for most of the ground-based occultation events observed during the current decade. Like the rings of Uranus, the Neptune rings are composed of very dark material; unlike that of Uranus, the Neptune system is very dusty. Six new regular satellites were found, with dark surfaces and radii ranging from 200 to 25 kilometers. All lie inside the orbit of Triton and the inner four are located within the ring system. Triton is seen to be a differentiated body, with a radius of 1350 kilometers and a density of 2.1 grams per cubic centimeter; it exhibits clear evidence of early episodes of surface melting. A now rigid crust of what is probably water ice is overlain with a brilliant coating of nitrogen frost, slightly darkened and reddened with organic polymer material. Streaks of organic polymer suggest seasonal winds strong enough to move particles of micrometer size or larger, once they become airborne. At least two active plumes were seen, carrying dark material 8 kilometers above the surface before being transported downstream by high level winds. The plumes may be driven by solar heating and the subsequent violent vaporization of subsurface nitrogen.