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1.
We present the results of a three-year observational program of long-slit spectroscopy and UBVRI photometry of Triton. We find evidence for short-lived albedo variations at wavelengths less than 0.5 μm with the most notable reddening event measured in October 1997 and “normal” colors returning by May 1998. We report a possible variation in the relative 0.73/0.89 μm methane absorption band strengths, suggesting that the transport or denudation of this species may play a major role in the reddening events. 相似文献
2.
Hubble Space Telescope (HST) and ground-based observations of Neptune from 1991 to 2000 show that Neptune's northern Great Dark Spots (NGDS) remained remarkably stable in latitude and longitudinal drift rate, in marked contrast to the 1989 southern Great Dark Spot (GDS), which moved continuously equatorward during 1989 and dissipated unseen during 1990. NGDS-32, discovered in October 1994 HST images, (H. B. Hammel et al., 1995, Science268, 1740-1742), stayed at ∼32°N from 1994 through at least 1996, and possibly through 2000. The second northern GDS (NGDS-15), discovered in August 1996 HST images, (L. A. Sromovsky et al. 2001, Icarus146, 459-488), appears to have existed as early as 8 March 1996 and remained near 15°N for the 16 months over which it was observed. NGDS-32 had a very uniform longitudinal drift rate averaging −36.28±0.04°/day from 10 October 1994 to 2 November 1995, and −35.84±0.02°/day from 1 September 1995 through 24 November 1995. A single circulation feature certainly exists during each of the first two periods, though it is not certain that it is the same feature. It is probable, but less certain, that only a single circulation feature was tracked during the 1996-1998 period, during which positions are consistent with a modulated drift rate averaging −35.401±0.001°/day, but with a peak-to-peak modulation of 1.5°/day with an ∼760-day period. If NDS-32 varied its drift rate in accord with the local latitudinal shear in the zonal wind, then all its drift-rate changes might be due to only ∼0.4° of latitudinal motion. The movement of NGDS-15 is also not consistent with a uniform longitudinal drift rate, but the nature of its variation cannot be estimated from the limited set of observations. The relatively stable latitudinal positions of both northern dark spots are not consistent with current numerical model calculations treating them as anticyclonic vortices in a region of uniform potential vorticity gradient (R. P. Lebeau and T. E. Dowling 1998, Icarus132, 239-265). Possible explanations include unresolved latitudinal structure in the zonal wind background or unaccounted-for variations in vertical stability structure. 相似文献
3.
G.W. LockwoodD.T. Thompson 《Icarus》2002,156(1):37-51
We present Strömgren b (472-nm) and y (551-nm) photometry of Neptune based on photoelectric measurements obtained at every apparition from 1972 to 2000. Neptune has brightened by 11% in b and 10% in y since 1980 with most of the increase occurring after 1990. By appending b data to published B magnitudes measured at Lowell from 1950 to 1966 and transformed to b, we show that Neptune is now brighter than at any time during the past half century. The nature of the year-to-year variations changed around 1990 when a steady rising trend overshadowed what appeared to be an inverse correlation with cyclic solar activity. By matching observations in b and y with near-infrared J (1.2-μm) and K (2.2-μm) photometry before, during, and after Neptune's 1976 infrared outburst, we show that the pattern of visible albedo variation parallels the infrared variation but with an amplitude 20-50 times smaller. A detailed comparison of photometry with ground-based and Voyager images at visible and red wavelengths during the 1989 Voyager encounter shows that small brightness variations occur when large discrete features rotate across Neptune's disk. This provides a rough association between visible features and photometric effects that we use to infer the state of Neptune's atmosphere for years when only photometry was available. A year-by-year analysis of variance of the photometry suggests that the 1976 and 1986-1989 infrared outbursts were isolated episodes of unusually vigorous atmospheric activity. Detrended magnitudes of Neptune are correlated with solar activity over the entire 29-year interval as well as 22-year subintervals, with solar UV now being favored as a causative mechanism rather than solar modulated galactic cosmic rays. 相似文献
4.
Stephen R. Kane 《Icarus》2011,214(1):327-333
With more than 15 years since the first radial velocity discovery of a planet orbiting a Sun-like star, the time baseline for radial velocity surveys is now extending out beyond the orbit of Jupiter analogs. The sensitivity to exoplanet orbital periods beyond that of Saturn orbital radii however is still beyond our reach such that very few clues regarding the prevalence of ice giants orbiting solar analogs are available to us. Here we simulate the radial velocity, transit, and photometric phase amplitude signatures of the Solar System giant planets, in particular Uranus and Neptune, and assess their detectability. We scale these results for application to monitoring low-mass stars and compare the relative detection prospects with other potential methods, such as astrometry and imaging. These results quantitatively show how many of the existing techniques are suitable for the detection of ice giants beyond the snow line for late-type stars and the challenges that lie ahead for the detection true Uranus/Neptune analogs around solar-type stars. 相似文献
5.
We investigate the dynamical evolution of trans-neptunian objects (TNOs) in typical scattered disk orbits (scattered TNOs) by performing simulations using several thousand particles lying initially on Neptune-encountering orbits. We explore the role of resonance sticking in the scattered disk, a phenomenon characterized by multiple temporary resonance captures (‘resonances’ refers to external mean motion resonances with Neptune, which can be described in the form r:s, where the arguments r and s are integers). First, all scattered TNOs evolve through intermittent temporary resonance capture events and gravitational scattering by Neptune. Each scattered TNO experiences tens to hundreds of resonance captures over a period of 4 Gyr, which represents about 38% of the object's lifetime (mean value). Second, resonance sticking plays an important role at semimajor axes , where the great majority of such captures occurred. It is noteworthy that the stickiest (i.e., dominant) resonances in the scattered disk are located within this distance range and are those possessing the lowest argument s. This was evinced by r:1, r:2 and r:3 resonances, which played the greatest role during resonance sticking evolution, often leading to captures in several of their neighboring resonances. Finally, the timescales and likelihood of temporary resonance captures are roughly proportional to resonance strength. The dominance of low s resonances is also related to the latter. In sum, resonance sticking has an important impact on the evolution of scattered TNOs, contributing significantly to the longevity of these objects. 相似文献
6.
Previous studies have used models of three-dimensional (3D) Boussinesq convection in a rotating spherical shell to explain the zonal flows on the gas giants, Jupiter and Saturn. In this paper we demonstrate that this approach can also generate flow patterns similar to those observed on the ice giants, Uranus and Neptune. The equatorial jets of Uranus and Neptune are often assumed to result from baroclinic cloud layer processes and have been simulated with shallow layer models. Here we show that vigorous, 3D convection in a spherical shell can produce the retrograde (westward) equatorial flows that occur on the ice giants as well as the prograde (eastward) equatorial flows of the gas giants. In our models, the direction of the equatorial jet depends on the ratio of buoyancy to Coriolis forces in the system. In cases where Coriolis forces dominate buoyancy, cylindrical Reynolds stresses drive prograde equatorial jets. However, as buoyancy forces approach and exceed Coriolis forces, the cylindrical nature of the flow is lost and 3D mixing homogenizes the fluid's angular momentum; the equatorial jet reverses direction, while strong prograde jets form in the polar regions. Although the results suggest that conditions involving strong atmospheric mixing are responsible for generating the zonal flows on the ice giants, our present models require roughly 100 and 10 times the internal heat fluxes observed on Uranus and Neptune, respectively. 相似文献
7.
We present eight new 0.8 to 2.4 μm spectral observations of Neptune's satellite Triton, obtained at IRTF/SpeX during 2002 July 15-22 UT. Our objective was to determine how Triton's near-infrared spectrum varies as Triton rotates, and to establish an accurate baseline for comparison with past and future observations. The most striking spectral change detected was in Triton's nitrogen ice absorption band at 2.15 μm; its strength varies by about a factor of two as Triton rotates. Maximum N2 absorption approximately coincides with Triton's Neptune-facing hemisphere, which is also the longitude where the polar cap extends nearest Triton's equator. More subtle rotational variations are reported for Triton's CH4 and H2O ice absorption bands. Unlike the other ices, Triton's CO2 ice absorption bands remain nearly constant as Triton rotates. Triton's H2O ice is shown to be crystalline, rather than amorphous. Triton's N2 ice is confirmed to be the warmer, hexagonal, β N2 phase, and its CH4 is confirmed to be highly diluted in N2 ice. 相似文献
8.
The existence of Uranus and Neptune presents severe difficulties for the core accretion model for the formation of ice giant planets. We suggest an alternative mechanism, namely disk instability leading to the formation of gas giant protoplanets, coagulation and settling of dust grains to form ice-rock cores at their centers, and photoevaporation of their gaseous envelopes by a nearby OB star, as a possible means of forming ice giant planets. 相似文献
9.
Connecting atmospheric science and atmospheric models for aerocapture at Titan and the outer planets
Many atmospheric measurement systems, such as the sounding instruments on Voyager, gather atmospheric information in the form of temperature versus pressure level. In these terms, there is considerable consistency among the mean atmospheric profiles of the outer planets Jupiter through Neptune, including Titan. On a given planet or on Titan, the range of variability of temperature versus pressure level due to seasonal, latitudinal, and diurnal variations is also not large. However, many engineering needs for atmospheric models relate not to temperature versus pressure level but atmospheric density versus geometric altitude. This need is especially true for design and analysis of aerocapture systems. Drag force available for aerocapture is directly proportional to atmospheric density. Available aerocapture “corridor width” (allowable range of atmospheric entry angle) also depends on height rate of change of atmospheric density, as characterized by density scale height. Characteristics of hydrostatics and the gas law equation mean that relatively small systematic differences in temperature versus pressure profiles can integrate at high altitudes to very large differences in density versus altitude profiles. Thus, a given periapsis density required to accomplish successful aerocapture can occur at substantially different altitudes (∼150-300 km) on the various outer planets, and significantly different density scale heights (∼20-50 km) can occur at these periapsis altitudes. This paper will illustrate these effects and discuss implications for improvements in atmospheric measurements to yield significant impact on design of aerocapture systems for future missions to Titan and the outer planets. Relatively small-scale atmospheric perturbations, such as gravity waves, tides, and other atmospheric variations can also have significant effect on design details for aerocapture guidance and control systems. This paper will discuss benefits that would result from improved understanding of Titan and outer planetary atmospheric perturbation characteristics. Details of recent engineering-level atmospheric models for Titan and Neptune will be presented, and effects of present and future levels of atmospheric uncertainty and variability characteristics will be examined. 相似文献
10.
We present the first spectra of Neptune taken with the Spitzer Space Telescope, highlighting the high-sensitivity, moderate-resolution 10-20 μm (500-1000 cm−1) spectra. We report the discovery of methylacetylene (CH3C2H) and diacetylene (C4H2) with derived 0.1-mbar volume mixing ratios of (1.2±0.1)×10−10 and (3±1)×10−12 respectively. 相似文献