Limitations to growth of microorganisms on Uranus,Neptune, and Titan

We have examined a wide range of physical, chemical, and thermal models of the atmosphere of Uranus. In that model, which we believe maximizes favorable conditions for the support of life [Weidenschilling and Lewis, Icarus20, 465–476 (1973)], we find the probability of growth of a contaminant terrestrial microorganisms to be nil. If, as is likely, conditions are even more extreme on Neptune, the probability of contamination of both of the outer planets Uranus and Neptune is nil. The Wiedenschilling and Lewis model guarantees the presence of water droplets through the temperature range 0 to 100°C; other published models add water liquid at higher temperatures or fail to provide liquid water at all within this temperature range. In this model the heavy elements (C, N, O, etc.) are enhanced in Uranus by a factor sufficient to form a deep massive cloud layer of aqueous ammonia solution droplets. We can estimate the probability of growth with respect to the following factors: the presence of stable liquid water, convection of parcels of atmosphere to lethally hot depths, solar energy sources reaching the water layer, organic molecular production by solar ultraviolet light, ammonia concentration at the water cloud level, ionic species distribution, and concentrations at the water cloud level. The evaluation of these factors suggests that most terrestrial life as we know it would be excluded on the basis of any one of them. We know of no organism that would be adapted to all the stringent Uranus conditions simultaneously. The discovery of even a single species of Earth organism that can survive or grow under allowable outer planetary conditions would establish new principles in biology.Titan, the methane-rich moon of Saturn, may be more hospitable for terrestrial organisms than any of the other objects of the outer solar system. Even there we see formidable barriers to the growth of an Earth organism in Titan's atmosphere. We recognize that revision of our views concerning Titan must occur as more is learned about this satellite.We advocate the abandonment, in principle, of the probabilistic approach to the estimation of growth of terrestrial organisms on spacecraft, planets, and satellites in the solar system. We do not support an approach which estimates probabilities of qualitatively unknown phenomena. We recommend a strategy which involves identification and intensive study of those organisms most likely to thrive under known conditions for each of the planets respectively. (Unknown environmental conditions may be allowed to vary optimally.) Some explicit areas for Earth-based experimentation are indicated.     

Uranus and Neptune interior models

This paper is concerned with the interior structure of Uranus and Neptune. Our approach is three-fold. First, a set of three-layer models for both Uranus and Neptune are constructed using a method similar to that used in the study of the terrestrial planets. The variations of the mass density (s) and flattening e(s) with fractional mean radius s for two representative models of Uranus and Neptune are calculated. The results are tabulated. A comparison of these models shows that these two planets are probably very similar to each other in their basic dynamical features. Such similarity is very seldom seen in our solar system. Secondly, we check the conformance between the theoretical results and observational data for the two planets. And thirdly, the 6th degree Stokes zonal parameters for Uranus and for Neptune are predicted, based on the interior models put forward in this paper.     

Ultraviolet observations of Uranus and Neptune

Far and extreme ultraviolet observations of Uranus and Neptune, principally by the ultraviolet spectrometer (UVS) on Voyager 2, are reviewed. Occultation observations have characterized the temperature, energy deposition, and major-constituent compositional profiles of these atmospheres above the 0.1–1 mbar level. Observations of airglow, light emitted by these atmospheres, are more complex to interpret but yield insight into atmospheric energy balance and chemistry.     

Ionospheric models of Saturn,Uranus, and Neptune

Model ionospheres are calculated for Saturn, Uranus, and Neptune. Protons are the major ions above 150 km altitude measured from a reference level where the hydrogen density is 1 × 10¹⁶ molecules cm^?3, while below 150 km quick conversion of protons to H₃⁺ ions by a three-body association mechanism leads to a rapid removal of ionization in dissociative recombination of H₃⁺. Electron density maxima are found at about 260 km for Saturn and Uranus and 200 km for Neptune. Present knowledge of the physical and chemical processes in the atmospheres of these planets suggests that their ionospheres probably will not be Jupiter-like.     

Five-micrometer measurements of Uranus and Neptune

We have obtained 5-μm brightness temperatures and brightness temperature upper limits for Uranus and Neptune which are substantially lower than those of Jupiter and Saturn and which correspond to a geometric albedo of approximately 0.01, in agreement with results reported by F. C. Gillet and G. H. Rieke (1977, Astrophys. J.218, L141–L144). Phospine and CH₃D, which are observed at 5 μm on Jupiter and Saturn, are discussed as possible sources of opacity at 5 μm in the atmospheres of Uranus and Neptune.     

Comparative models of Uranus and Neptune

Models of Uranus and Neptune are computed based on the assumption that these planets consist of three layers: a rock core, an ice shell, and an atmosphere. Uranus models require that the ice shell have a density some 10% lower than the canonical density for an ice mixture. Two Neptune models are found, one with the canonical density in the ice shell, and one with a density 20% lower. The implications of these models are discussed.     

Comparative thermal evolution of Uranus and Neptune

We extend a Jovian convective-cooling model to Uranus and Neptune. The model assumes that efficient interior convection prevails, so that escape of interior heat is governed by the radiative properties of the atmosphere. A comparison of the thermal evolution of Uranus and Neptune indicates that the larger amount of solar radiation absorbed in Uranus' atmosphere tends to differentially suppress the escape of interior heat. The model is shown to be consistent with recent infrared observations of the thermal balance of Uranus and Neptune, and with the presumed age of these planets.     

The rotational periods of Uranus and Neptune

Observations of tilts of spectral lines in the spectrum of Uranus and Neptune yield the following rotational periods: “Uranus,” 24 ± 3 hr; “Neptune,” 22 ± 4 hr. Neptune is confirmed to rotate in a direct sense. The position angle of the pole of Uranus, projected onto the plane of the sky, is found to be 283 ± 4°. The value for Neptune is 32 ± 11°. These results agree with the direction of the pole of Uranus inferred from the common plane of its four brightest satellites and with the direction of the pole of Neptune as inferred from the precession of Triton's orbit. The rotational period of Uranus is found to be consistent with modern values of its optical and dynamical oblateness and the theory of solid-body rotation with hydrostatic equilibrium. This is barely the case for the period derived for Neptune and we suspect that future observations made under better seeing conditions may lead to a shorter rotation period between 15 and 18 hr. Because of a substantial difference between our results and those of earlier spectroscopic and photometric investigations we include an assessment of several previously published photometric studies and a new reduction of the original Lowell and Slipher spectroscopic plates of Uranus [Lowell Obs. Bull. 2, 17–18, 19–20 (1912)]. The early visual photometry of Campbell (Uranus) and Hall (Neptune) is found to be more satisfactorily accounted for by periods of 21.6 and 23.1 hr, respectively, than by the periods originally suggested by the observers. Our reduction of the Lowell and Slipher Uranus plates yields a period near 33 hr uncorrected for seeing. This value is consistent with the results based on the 4-m echelle date.     

Detecting the signatures of Uranus and Neptune

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.     

Uranus and Neptune: Shape and rotation

Both Uranus and Neptune are thought to have strong zonal winds with velocities of several 100 m s⁻¹. These wind velocities, however, assume solid-body rotation periods based on Voyager 2 measurements of periodic variations in the planets’ radio signals and of fits to the planets’ magnetic fields; 17.24 h and 16.11 h for Uranus and Neptune, respectively. The realization that the radio period of Saturn does not represent the planet’s deep interior rotation and the complexity of the magnetic fields of Uranus and Neptune raise the possibility that the Voyager 2 radio and magnetic periods might not represent the deep interior rotation periods of the ice giants. Moreover, if there is deep differential rotation within Uranus and Neptune no single solid-body rotation period could characterize the bulk rotation of the planets. We use wind and shape data to investigate the rotation of Uranus and Neptune. The shapes (flattening) of the ice giants are not measured, but only inferred from atmospheric wind speeds and radio occultation measurements at a single latitude. The inferred oblateness values of Uranus and Neptune do not correspond to bodies rotating with the Voyager rotation periods. Minimization of wind velocities or dynamic heights of the 1 bar isosurfaces, constrained by the single occultation radii and gravitational coefficients of the planets, leads to solid-body rotation periods of ∼16.58 h for Uranus and ∼17.46 h for Neptune. Uranus might be rotating faster and Neptune slower than Voyager rotation speeds. We derive shapes for the planets based on these rotation rates. Wind velocities with respect to these rotation periods are essentially identical on Uranus and Neptune and wind speeds are slower than previously thought. Alternatively, if we interpret wind measurements in terms of differential rotation on cylinders there are essentially no residual atmospheric winds.     

Long-term atmospheric variability on Uranus and Neptune

Long-term photometric measurements of Uranus and Neptune through 2005 show variations in brightness. For Uranus, much of the variation can be interpreted as seasonal, i.e., caused by viewing angle changes of an oblate planet. The photometry suggests that if seasonal variations on Uranus are north-south symmetric, then the northern pole should begin to brighten in 2006. However, seasonal “aspect” changes cannot explain all the variation; the Uranus observations require intrinsic atmospheric change. Furthermore, Uranus observations spanning many scale heights in the atmosphere may show similar change. For Neptune, variations in sub-solar latitude may explain the general shape of the long-term light curve, but significant deviations occur that have no explanation at present. Observations are needed over a longer temporal baseline than currently exists to fully characterize both atmospheres.     

Restored methane band images of Uranus and Neptune

Charge-coupled device images of Uranus and Neptune taken in the 8900-Å absorption band of methane are presented. The images have been digitally processed by means of nonlinear deconvolution techniques to partially remove the effects of atmospheric seeing. The restored Uranus images show strong limb brightening consistent with previous observations and theoretical models of the planet's atmosphere. The computer-processed images of Neptune show discreted cloud features similar to those reported previously by B. A. Smith, H. J. Reitsema and S. M. Larson (1979 Bull. Amer. Astron. Soc.11, 570). A time series of the restored Neptune images shows a continuous variation which may be due to the planet's rotation.     

Photometric variability of Uranus and Neptune, 1950-2004

Photoelectric intermediate-band b and y photometry of Uranus and Neptune obtained at each apparition since 1972, combined with broadband B and V photometry from 1950 to 1966, provide a record of planetary variability covering 2/3 of Uranus' 84-year orbital period and 1/3 of Neptune's 165-year orbital period. Almost all of the data were obtained with a dedicated 21-inch photometric telescope at Lowell Observatory. The data are quite homogeneous, with yearly uncertainties typically smaller than 0.01 mag (1%). The lightcurve of Uranus is sinusoidal with peaks at the solstices. The b amplitude slightly exceeds the expected 0.025 mag purely geometrical variation caused by oblateness as the planetary aspect changes, seen from Earth. The y amplitude is several times larger, indicating a strong equator-to-pole albedo gradient. The lightcurve is asymmetrical with respect to southern solstice, evidence of a temporal albedo variation. Neptune's post-1972 lightcurve exhibits a generally rising trend since 1972 interpreted by Sromovsky et al. [Sromovsky, L.A., Fry, P.M., Limaye, S.S., Baines, K.H., 2003. Icarus 163, 256-261] as a lagged sinusoidal seasonal variation. However, the 1950-1966 lightcurve segments are much fainter than expected, missing the proposed seasonal sinusoid by 0.1-0.2 mag. A major unknown component is therefore needed to explain Neptune's long-term variation. The apparent relationship between Neptune's brightness variation and the 11-year solar cycle seen in cycles 21-22 (1972-1996) has apparently now faded away. Further interpretation of the data in this paper will be found in a companion paper by Hammel and Lockwood [Hammel, H.B., Lockwood, G.W., 2005. Icarus. Submitted for publication].     

Synchrotron radio emission from Uranus and Neptune

A number of charged-particle radiation belt models for Uranus and Neptune are postulated, and the synchrotron emission spectrum for each is calculated over the frequency range 18.75 to 2400 MHz. Although no radio observations of these planets exist at frequencies below 1400MHz, available observations are used in conjunction with the synchrotron calculations to establish a rough upper limit to the size and strength of the planets' magnetic field strength and radiation belt intensity. The magnitude of synchrotron radio emission expected in many of the models is within the detection range of several existing ground-based telescopes, at decimetric and metric wavelengths.     

Submillimeter and millimeter observations of Uranus and Neptune

We have made narrowband photometric measurements of Uranus and Neptune covering the wavelength range from 0.35 to 3.3 mm. The observations provide accurate comparative radiometry of these planets. Absolute calibration was referenced to Mars, and to Jupiter as a secondary standard. The results establish Uranus and Neptune as reliable secondary calibrators in their own right. We have combined our observations with other measurements made in the period 1978 through 1984 in the spectral range of 17 μm through 3 mm to form models for atmospheric temperature structure in the vertical range from 100 mbar to 8 bar. The simplest models imply that the tropospheres of both planets are consistent with “frozen” equilibrium H₂ and a mixing ratio of CH₄ of about 2% by volume in the deep atmosphere. There is some evidence in the Uranus data which implies the presence of discrete spectral lines. These could be due to CH₄ pure rotational or dimer transitions or to minor constituents such as CO, which remain uncondensed even at the cold temperatures in the atmosphere of Uranus.     

On the structure and composition of Uranus and Neptune

We present a series of models of Uranus and Neptune in which the relative amounts of (1) rock, (2) ices, and (3) hydrogen and helium are allowed to vary. By fitting the density and the gravitational quadruppole moment, the model composition can be determined. Because of the ambiguity in the rotation periods of these planets, several possible models are presented and discussed.     

The atmospheres of Uranus and Neptune: A review

Plausible models for the atmospheres of Uranus and Neptune are reviewed. Current ideas favor the presence of massive atmospheres above solid cores. Observations of Uranus imply the presence of a visible cloud layer (probably composed of solid methane) beneath about 100 km amagats of hydrogen. A number of other cloud layers far below this upper layer are also possible. However, before any of these conclusions can be considered firm, a number of further crucial observations are required.     

Analysis of photometric variations of Uranus and Neptune since 1953

Photoelectric photometry of Uranus and Neptune, obtained between 1953 and 1976, has been analyzed for evidence of planetary variability on various time scales. There is no evidence for either periodic or secular variability at a level of 0.003 mag or greater over intervals of a few months or less, but slow variations of several hundredths of a magnitude have occured for both planets over many years. Variations at the 0.001 mag level may be present on shorter time scales. Solar phase coefficients for both planets are much larger than predicted for clear molecular atmospheres, and the coefficient for Neptune is significantly variable. The lightcurve of Uranus is consistent with a geometrical oblateness of about 0.03, or polar brightening. While the annual mean magnitudes of the planets are highly correlated with one another, no direct correlation with cyclical solar activity is demonstrated.     

Far-infrared photometry of Titan and Iapetus

Titan was observed in four broad passbands between 35 and 150 μm. The brightness temperature in this interval is roughly constant at 76 ± 3°K. Integrating Titan's spectrum from 5 to 150 μm yields an effective temperature of 86 ± 3°K. Both the bright and dark hemispheres of Iapetus were observed in one broadband filter with λ_e ～ 66 μm. The brightness temperatures for these two sides of Iapetus are 96 ± 9°K and 114 ± 10°K, respectively. The bright-side Bond albedo is calculated to be 0.61_?0.22^+0.16.     

Observations of Ganymede,Callisto, Ceres,Uranus, and Neptune at 3.33 mm wavelength

We have measured the 3.33 mm wavelength disk brightness temperatures of Ganymede (136 ± 21°K), Callisto (95 ± 17°K), Ceres (137 ± 25°K), Uranus (125 ± 9°K), and Neptune (126 ± 9°K). Our observations of Ganymede are consistent with the radiation from a blackbody in solar equilibrium, whereas Callisto's microwave spectrum indicates a surface similar to that of the Moon. The disk temperature for Ceres agrees with that expected from a rapidly rotating blackbody. The millimeter temperatures of Uranus and Neptune greatly exceed solar equilibrium values, implying atmospheres with large temperature gradients.