Thermal tides on Pluto

Stellar occultations have shown that vertical profiles of density fluctuations in the atmosphere of Pluto typically show wave-like structure with an amplitude of a few percent and vertical wavelengths of a few kilometers. Here we calculate the tidal response of Pluto’s atmosphere to solar-induced sublimation “breathing” from N₂ frost patches. Solutions show global-scale wave-like density structure capable of explaining the observations. The atmospheric response is a combination of eastward and westward migrating tides, together with a zonally symmetric mode. Calculated vertical wavelengths and amplitudes are similar to observations.     

Polarimetry of Pluto

The polarization of Pluto has been measured for a range of solar phase angles from 0.8 to 1.8°. A mean linear polarization of 0.29 ± 0.01% (error of the mean) was found. No dependence of both the amount of polarization and position angles with rotational phase or solar phase angle could be detected. The positional angles of polarization agree with calculated position angles of the defect of illumination and are therefore parallel to the scattering plane. The observed polarization cannot be explained as resulting purely from a surface material which is similar to asteroidal surfaces. A hypothesis of polarization from a thin atmosphere, in addition to the surface polarization, is advanced.     

Ephemeris of Pluto

The limited period of observational data and the long orbital period have limited the accuracy of the Pluto ephemerides. This condition will continue for some time. Ephemerides of Pluto are compared with the observational data.     

The obliquity of Pluto

Pluto's obliquity (the angle between its spin axis and orbit normal) varies between ～102 and ～126° over a period of about 3 million years. These oscillations are nearly sinusoidal and quite stable, leading to only modest changes in the insolation regime. Thus, Pluto's rotation has been slightly retrograde ever since its current orbit and rotation rate were established.     

The insolation at Pluto

Calculations of the daily solar radiation incident at the top of Pluto's atmosphere and its variability with latitude and season and of the latitudinal variation of the mean annual daily insolation are presented. The large eccentricity of Pluto produces significant north-south seasonal asymmetries in the daily insolation. As for Uranus, having a similarly large obliquity, the equator receives less annual average energy than the poles.     

Surface composition of Pluto

Infrared photometric observations (1–4 μm) of Pluto have been made using broadband (Δλ/λ ～ 0.3) and narrowband (Δλ/λ ～ 0.05–0.1) filters. We confirm the probable presence of methane in some form, though, in detail the match between the spectrum of Pluto and the laboratory spectrum of methane frost is poor.     

Pluto: Dwarf planet 134340

In recent decades, investigations of Pluto with up-to-date astronomical instruments yielded results that have been generally confirmed by the New Horizons mission. In 2006, in Prague, the General Assembly of the International Astronomical Union (IAU) reclassified Pluto as a member of the dwarf planet category according to the criteria defined by the IAU for the term “planet”. At the same time, interest in studies of Pluto was increasing, while the space investigations of Pluto were delayed. In 2006, the New Horizons Pluto spacecraft started its journey to Pluto. On July 14, 2015, the spacecraft, being in fly-by mode, made its closest approach to Pluto. The heterogeneities and properties of the surface and rarified atmosphere were investigated thoroughly. Due to the extreme remoteness of the spacecraft and the energy limitations, it will take 18 months to transmit the whole data volume. Along with the preliminary results of the New Horizons Pluto mission, this paper reviews the basics on Pluto and its moons acquired from the ground-based observations and with the Hubble Space Telescope (HST). There are only a few meteorite craters on the surfaces of Pluto and Charon, which distinctly marks them apart from such satellites of the giant planets as Ganymede and Callisto. The explanation is that the surface of Pluto is young: its age is estimated at less than 100 Myr. Ice glaciers of apparently a nitrogen nature were found. Nitrogen is also the main component of the atmosphere of Pluto. The planet demonstrates the signs of strong geologic activity, though the energy sources of these processes are unknown.     

A possible atmosphere for Pluto

At the temperature of Pluto (～43°K) the only gas which would neither condense nor escape is neon. Since neon is cosmically abundant it is suggested that Pluto may have a fairly extensive atmosphere consisting of almost pure neon. The possibility that such an atmosphere exists is analyzed, along with the possibility that oceans of liquid neon may exist at the surface.A neon atmosphere would not produce any observable absorption lines. However, if it were very thick, then Rayleigh scattering would result in Pluto having a much higher albedo in the ultraviolet than in the visual, which is not observed to be the case. This enables us to set an upper limit on the mass of Pluto's atmosphere.     

A search for ethane on Pluto and Triton

We present here a search for solid ethane, C₂H₆, on the surfaces of Pluto and Triton, based on near-infrared spectral observations in the H and K bands (1.4-2.45 μm) using the Very Large Telescope (VLT) and the United Kingdom Infrared Telescope (UKIRT). We model each surface using a radiative transfer model based on Hapke theory (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press, Cambridge, UK) with three basic models: without ethane, with pure ethane, and with ethane diluted in nitrogen. On Pluto we detect weak features near 2.27, 2.405, 2.457, and 2.461 μm that match the strongest features of pure ethane. An additional feature seen at 2.317 μm is shifted to longer wavelengths than ethane by at least 0.002 μm. The strength of the features seen in the models suggests that pure ethane is limited to no more than a few percent of the surface of Pluto. On Triton, features in the H band could potentially be explained by ethane diluted in N₂, however, the lack of corresponding features in the K band makes this unlikely (also noted by Quirico et al. (Quirico, E., Doute, S., Schmitt, B., de Bergh, C., Cruikshank, D.P., Owen, T.C., Geballe, T.R., Roush, T.L. [1999]. Icarus 139, 159-178)). While Cruikshank et al. (Cruikshank, D.P., Mason, R.E., Dalle Ore, C.M., Bernstein, M.P., Quirico, E., Mastrapa, R.M., Emery, J.P., Owen, T.C. [2006]. Bull. Am. Astron. Soc. 38, 518) find that the 2.406-μm feature on Triton could not be completely due to ¹³CO, our models show that it could not be accounted for entirely by ethane either. The multiple origin of this feature complicates constraints on the contribution of ethane for both bodies.     

On the long-term motion of Pluto

A modified periodic orbit of the third kind is introduced that is closely related to periodic orbits of the third kind as defined by Poincaré. It is shown that Pluto librates about the periodic orbit with apparent stability. This further explains the librational motion of the resonant argument of Pluto and the avoidance of a Pluto-Neptune close approach as found by Cohen and Hubbard and the long-term motion of Pluto and the librational motion of the perihelion as found by Williams and Benson. With libration about a periodic orbit, the numerical solution of Williams and Benson can be extrapolated to longer times in the past and future.     

Satellite searches at Pluto and Mars

We report the results of searches for outer satellites of Pluto and Mars, carried out with the Hale 5-m telescope in 1999 and 2001, respectively. No new satellites were found down to limiting magnitudes of mR=25.0 at Pluto and mV∼22 at Mars, corresponding to diameters of 35 and 1 km, respectively, for an assumed geometric albedo of 0.07. A faint trans-neptunian object, 1999 LB₃₇, was discovered in the Pluto fields; given the depth of our survey, discovery of one such object in the background Kuiper belt is in reasonable agreement with expectations.     

Speckle interferometric observations of Pluto and Charon

We report speckle interferometric observations of Pluto and its moon (1978 P1) Charon obtained on 5 June 1980 with a single 1.8-m mirror of the Multiple Mirror Telescope. Our observations yield a separation of 0″.31 (±0″.05) between Pluto and Charon at position angle 285° (±7°) for JD 2444395.75. This result and other direct observations indicate an adjustment of +4.0 hr to the orbital epoch of R. S. Harrington and J. W. Christy [Astron.J.86, 442–443 (1981)]. Our observation, which represents the first resolution of the system near minimum separation, also suggests that the inclination of the orbit to the plane of the sky should be increased by 3°; this will delay the onset of the predicted eclipsee season by one apparition to 1984 or 1985. Our data are consistent with Pluto diameter 0″.14 (±0″.02) = 3000 (±400) km and Charon diameter 0″..05 (±0″.03) = 1100 (±600) km.     

Polar wander on Triton and Pluto due to volatile migration

Polar wander may occur on Triton and Pluto because of volatile migration. Triton, with its low obliquity, can theoretically sublimate volatiles (mostly nitrogen) at the rate of ∼10¹³ kg year⁻¹ from the equatorial regions and deposit them at the poles. Assuming Triton to be rigid on the sublimation timescale, after ∼10⁵ years the polar caps would become large enough to cancel the rotational flattening, with a total mass equivalent to a global layer ∼120-250 m in depth. At this point the pole wanders about the tidal bulge axis, which is the line joining Triton and Neptune. Rotation about the bulge axis might be expected to disturb the leading side/trailing side cratering statistics. Because no such disturbance is observed, it may be that Triton’s surface volatile inventory is too low to permit wander. On the other hand, its mantle viscosity might be low, so that any uncompensated cap load might be expected to wander toward the tidal bulge axis. In this case, the axis of wander passes through the equator from the leading side to the trailing side; rotation about this wander axis would not disturb the cratering statistics. Low-viscosity polar wander may explain the bright southern hemisphere: this is the pole which is wandering toward the sub-Neptune point. In any case the “permanent” polar caps may be geologically very young. Polar wander may possibly take place on Pluto, due to its obliquity oscillations and perihelion-pole geometry. However, Pluto is probably not experiencing any wander at present. The Sun has been shining strongly on the poles over the last half of the obliquity cycle, so that volatiles should migrate to the equator, stabilizing the planet against wander. Spacecraft missions to Triton and Pluto which measure the dynamical flattening could give information about the accumulation of volatiles at the poles. Such information is best obtained by measuring gravity and topography from orbiters, as was done for Mars with the highly successful Mars Global Surveyor.     

Mass-radius relationships and constraints on the composition of Pluto,II

A new estimate of Pluto's mass within the range of possible masses considered in an earlier work has enabled us to refine our model of Pluto's interior.     

Accurate analytical representation of Pluto modern ephemeris

An accurate development of the latest JPL’s numerical ephemeris of Pluto, DE421, to compact analytical series is done. Rectangular barycentric ICRF coordinates of Pluto from DE421 are approximated by compact Fourier series with a maximum error of 1.3 km over 1900–2050 (the entire time interval covered by the ephemeris). To calculate Pluto positions relative to the Sun, a development of rectangular heliocentric ICRF coordinates of the Solar System barycenter to Poisson series is additionally made. As a result, DE421 Pluto heliocentric positions by the new analytical series are represented to an accuracy of better than 5 km over 1900–2050.     

A review of the motion of Pluto

A review is given of the determination of the long-term motion of Pluto. In particular, the discovery of the librational character of the two critical arguments is discussed. The stability of the motion of Pluto is shown to have been established when allknown gravitational forces are considered.Presented at the Symposium Star Catalogues, Positional Astronomy and Celestial Mechanics, held in honor of Paul Herget at the U.S. Naval Observatory, Washington, November 30, 1978.     

Study of the Kinetics of Metastable Molecular Nitrogen in the Atmospheres of the Earth,Triton, Titan,and Pluto

Solar System Research - In the interaction of high-energy electrons with gases of planetary atmospheres where the primary component is molecular nitrogen, a significant fraction of particle energy...     

A dust cloud around Pluto and Charon

We suggest that Pluto and Charon are immersed in a tenuous dust cloud. The cloud consists of ejecta from Pluto and—especially—Charon, released from their surfaces by impacts of micrometeoroids originating from Edgeworth-Kuiper belt objects. The motion of the ejected grains is dominated by the gravity of Pluto and Charon, which determines a pear-shape of the densest part of the cloud. While the production rates of escaping particles from both sides are comparable, the lifetimes of the Charon particles inside the Hill sphere of Pluto-Charon with respect to the Sun are much longer than of the Pluto ejecta, so that the cloud is composed predominantly of Charon grains. The dust cloud is dense enough to be detected with an in situ dust detector onboard a future space mission to Pluto. The cloud's maximum optical depth of τ≈3×10⁻¹¹ is, however, too low to allow remote sensing observations.     

Detection of a CH4 atmosphere on Pluto

Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of $\text{～25}\text{A}\text{?}$ and a signal-to-noise ratio of ～300. It showed CH₄ absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH₄ band model parameters to our data, a total CH₄ abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH₄ surface pressure of 1.5 × 10^?4 atm. An upper limit to the total pressure of ～0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH₄ atmosphere would aid its stability. The relatively large amount of gaseous CH₄ observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH₄ absorptions rather than surface frost as interpreted earlier.