Pluto's Atmosphere Research Articles

Emissivity and the Fate of Pluto's Atmosphere

We present a simplified model for seasonal changes in Pluto's surface–atmosphere system. The model demonstrates the potential importance of the solid-state phase transition between α-N 2 and β-N 2, and the accompanying change in emissivity, for predicting the seasonal bulk of Pluto's (and Triton's) atmosphere. Specifically, the model shows that under simplified but not unreasonable assumptions Pluto may have nearly the same atmospheric pressure at aphelion as it does now, near perihelion. The emissivity change which accompanies the α–β phase change should be included in the next generation of Pluto and Triton seasonal models for the purposes of understanding the evolution of their atmospheres over seasonal and climatic timescales.

Photochemistry of Pluto's atmosphere and ionosphere near perihelion

We consider Pluto's photochemistry using a background model for a hydrodynamically escaping atmosphere byKrasnopolsky[1999]. Some adjustments are made in the basic continuity equation and in the boundary conditions to account for hydrodynamic flow in the atmosphere. We model the photochemistry for 44 neutral and 23 ion species. Because of the high methane mixing ratio, Pluto's photochemistry is more similar to that of Titan than that of Triton. Charge exchange between N2+and CH4significantly reduces the production of atomic nitrogen. The most abundant photochemical products are C2H2(3×1017), C4H2(1017), HCN (6×1016), H2(4×1016), C2H4(4×1016), HC3N (3.4×1016), C2H6(2×1016), C3H2(9×1015), and C3H4(8×1015, all in cm−2). In addition to the parent N2, CH4, and CO molecules which absorb photons with λ<145 nm, these products absorb almost completely photons with λ<185 nm, therefore significantly increasing the number of dissociation events. Photochemical losses of the parent species are much smaller than their escape. Precipitation rates are the highest for C2H2, C4H2, HC3N, HCN, C2H6, and C2H4(65, 58, 23, 14, 9, and 6 g cm−2Byr−1, respectively, reduced by a factor of 3 to account for seasonal variations). Escape of photochemical products is highest for H2, H, C2H2, C2H4, HCN, and N (2×1026, 1.4×1026, 6×1024, 3.6×1024, 2.3×1024, and 1.8×1024s−1, respectively). The electron density reaches a maximum of 800 cm−3at 2250 km. The most abundant ions are HCNH+, C3H3+, and C3H5+. Some of the photochemical products might be detected using the technique of UV solar occultation spectroscopy from a spacecraft flyby.

Hydrodynamic flow of N2 from Pluto

A new method of analytic solution for the equation of hydrodynamic flow from an atmosphere has been developed. This method conserves a term neglected in the equation for the McNutt [1989] approximation and significantly improves uncertainty of the solution. Structure of Pluto's upper atmosphere near perihelion was calculated by this method at various solar activity and at various methane mixing ratios. Both the UV absorption by CH4 at 1000–1400 Å and the EUV absorption by N2 heat the upper atmosphere with the global‐mean rates of 1.4 × 10−3 and 2.9 × 10−4 ergs cm−2 s−1, respectively, at mean solar activity. Hydrodynamic outflow of N2 from Pluto at perihelion is equal to (2.0–2.6)×1027 s−1 at mean solar activity and varies by a factor of 3–4 from solar minimum to solar maximum. The exobase lies below the sonic level. Pluto's atmosphere does not completely fit both the conditions of static and hydrodynamically escaping atmospheres and is an intermediate case of a slow hydrodynamic escape. The total loss for the age of the solar system corresponds to an ice layer that is 500 m thick and 5 × 10−4 of Pluto's mass. Pluto's atmosphere is restricted to 3000–4500 km, which makes possible a close flyby of future spacecraft.

Photochemistry of Pluto’s atmosphere and ionosphere near perihelion

Photochemistry of Pluto’s atmosphere and ionosphere near perihelion

Photochemical Models of Pluto's Atmosphere

A one-dimensional photochemical steady state model is used to study the vertical distribution of different gas species in the neutral atmosphere of Pluto. The major species is supposed to be N2with CH4as the leading minor species with a mixing ratio between 3.7 × 10−3and 7.4 × 10−3, depending on the pressure and temperature on the surface. As a result of photolytic reactions, a number of hydrocarbons and nitriles are mainly produced in the 50–300 km region. Because of the uncertainty in the atmospheric thermal structure of Pluto, photochemical models with three differentT(p) models have been developed. The turbulent processes have been parameterized, as in previous atmospheric models, with an eddy diffusion coefficientK=Ko(no/n)0.5placing the homopause at an assumed altitude of ∼150 km. For the case we call nominal (Tsurf= 37 K andpsurf= 10 μbar) parent molecules such as N2, CH4, and CO must be supplied from the surface with rates of 1.85 × 107, 2.87 × 108, and 6.83 × 103cm−2sec−1, respectively. As a consequence of photolysis, H and H2escaping fluxes, scaled to the surface, are 3.50 × 108and 2.12 × 108cm−2sec−1, respectively. Condensation of light hydrocarbons and nitriles occur between 3 and 5 km, depending on the compound and the surface temperature. Scaling the downward flux of these compounds to the amount of ice that would be deposited on the surface, we have deduced that the most abundant surface ices would be C2H6, C2H2, HCN, and C2H4, in that order.

Solar wind-Pluto interaction revised

The interaction of the solar wind (SW) with Pluto's atmosphere at about 30 AU is studied by means of both 2D bi-ion fluid and hybrid code simulations taking into account that the characteristic size of the interaction region is much smaller than the pick-up radius of the planetary ions. This is due to the weak magnetic field (B ≈ 0.1 nT) and the strongly reduced ionization rate of the planetary molecules ( ν ph ≈ 10 −9s −1) at these large distances from the Sun. The results are the following: For weak production rates (q ≈ 10 27s −1) the planetary ions are forced to move on cyclodial orbits, like in the solar wind produced test particles. Higher productivity leads to a structuring of all parameters due to bi-ion discontinuities which arise in the source region by the relative drift between both ion fluids. Further increase of the heavy ion production leads to the generation of a comet-like tail. Only for the largest production rates found in the literature a detached bow shock as predicted from one-fluid massloading models (Bagenal and McNutt, 1989) is formed. For moderate conditions, the fast pick-up of the newly generated heavy ions by the solar wind prevents the formation of a well developed ionosphere.

A model for the overabundance of methane in the atmospheres of Pluto and Triton

A model for producing atmospheric CH 4 mixing ratios larger than would be expected from simple vapor pressure equilibrium over a solid solution of N 2 and CH 4 is described. Laboratory experiments show that rapid sublimation of a dilute (0.2% mole fraction) solid solution of CH 4 in α-N 2 produces a residue of nearly pure CH 4 grains. The CH 4 grains begin to form very quickly and most of the CH 4 originally in solid solution with the N 2 is taken up by the grainy residue rather than sublimating. If the same is true for the much slower sublimation rates on Pluto, patches of nearly pure CH 4 ice grains will be built up on subseasonal timescales. Such CH 4 patches will be in contact with Pluto's predommantly N 2 atmosphere. Further sublimation of these patches will be controlled by molecular and turbulent diffusion, as will be the condensation of CH 4 from the atmosphere in other areas. It is shown that the balance between diffusive sublimation and condensation can easily produce 1% mixing ratios of CH 4 in the atmosphere, generally consistent with requirements for explaining Pluto's 100 K upper-atmospheric temperature and producing a steep positive temperature gradient in the 2–3 μb region. The same mechanism can explain Triton's less elevated atmospheric CH 4 mixing ratio.

The emissivity of volatile ices on Triton and Pluto

The Hapke theory is used to calculate the emissivity of a semi-infinite layer of granular N 2 ice with CH 4 and CO as contaminants. It is assumed that the layer is composed of grains which can be characterized as having a single size, and that temperature gradients in the emitting layers of the surface are negligible. The emission spectrum for β-N 2, stable above 35.6 K, results from a very broad peak in the absorption spectrum centered at 154 μm, while two absorption peaks, at 143 and 204 μm, produce the emission spectrum of the lower temperature α-N 2 phase. For a grain size of 1 cm the Planck-mean bolometric emissivity calculated for the pure β-N 2 ice is 0.85. If the effective N 2 grain size is 1 mm the emissivity is 0.40. Both are low enough to significantly affect surface energy balance calculations. The very narrow absorption features of α-N 2 result in even smaller bolometric emissivities of only 0.11 and 0.30 for 1 mm and 1 cm grain sizes at 34 K. The effect of CH 4 and CO, in solid solution with N 2 or as separate, intimately mixed grains, on the emissivity is also estimated. It is found that the presence of either or both of these two molecules in solid solution with the N 2 ice on Triton and Pluto only slightly increases the β-N 2 emissivity. The emissivity of intimate mixtures of grains of CH 4 and CO with N 2 is much less certain, and probably much less applicable to Triton and Pluto. CH 4 and CO in solid solution with α-N 2 increase the emissivity by about 50%. For an α-N 2 grain size of 1 cm, the addition of 2% each CH 4 and CO in solid solution with the N 2 increases the emissivity from 0.30 to 0.48 at 34 K. For a 1 mm grain size the emissivity of such a solid solution changes to 0.16 from 0.11. However, the emissivity of α-N 2 even with CH 4 and/or CO in solution is still considerably lower than for β-N 2. Seasonal variations on Triton and Pluto could be strongly influenced by this emissivity contrast between the α and β phases. In the extreme case of pure N 2 ice, Pluto's atmosphere could be prevented from freezing out, even at aphelion.

On the Vertical Thermal Structure of Pluto's Atmosphere

A radiative–conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the CH43.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH47.6 μm band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the CH4molecule. Partial thermalization of absorbed solar radiation in the CH43.3 and 2.3 μm bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 μm bands exceeds heating in 3.3 μm bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients ∼10–20 K km−1forp> 2 μbar when the surface or tropopause pressure is ∼3 μbar and the CH4mixing ratio is a constant 3%. This calculated structure may account for the “knee” in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and N2individually as the major constituent. If a steep temperature gradient ∼20 K km−1is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% CH4mixing ratio to attain a calculated ratio ofT/M(= 3.5 K amu−1) in agreement with inferred values from stellar occultation data. However, pure Ar and N2ices at the same temperature yield an Ar vapor pressure of only ∼0.04 times the N2vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N2as the dominant constituent. One possibility is a 3 μbar N2atmosphere with 0.3% CH4that has 106 K isothermal region (T/M= 3.8 K amu−1) and ∼8 K km−1surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ∼10 μbar with a scattering haze forp> 2 μbar. Our model with appropriate adjustments in the CH4density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992.J. Geophys. Res.98, 3065–3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993.Bull. Am. Astron. Soc.25, 1106.).

Mirages and the Nature of Pluto's Atmosphere

We present model occultation lightcurves demonstrating that a strong thermal inversion layer at the base of Pluto's stratosphere can reproduce the minimum flux measured by the Kuiper Airborne Observatory (KAO) during the 1988 occultation of a star by Pluto. The inversion layer also forms the occultation equivalent of a mirage at a radius of 1198 km, which is capable of hiding tropospheres of significant depth. Pluto's surface lies below 1198 km, its radius depending on the depth of the troposphere. We begin by computing plausible temperature structures for Pluto's lower atmosphere, constrained by a calculation of the temperature of the atmosphere near the surface. We then trace rays from the occulted star through the model atmosphere, computing the resultant bending of the ray. Model light curves are obtained by summing the contribution of individual rays within the shadow of Pluto on Earth. We find that we can reproduce the KAO lightcurve using model atmospheres with a temperature inversion and no haze. We have explored models with tropospheres as deep as 40 km (implying a Pluto radius of 1158 km) that reproduce the suite of occultation data. Deeper tropospheres can be fitted to the data, but the mutual event radius of 1150 km probably provides a lower bound. If Pluto has a shallow or nonexistent troposphere, its density is consistent with formation in the solar nebula with modest water loss due to impact ejection. If the troposphere is relatively deep, implying a smaller radius and larger density, signficant amounts of water loss are required.

The Thermal Structure of Pluto's Atmosphere: Clear vs Hazy Models

The problem of Pluto's atmosphere thermal structure, as determined from the June 9, 1988 occultation data, is reassessed in the light of the detection of N 2 and CO ices, along with CH 4, on Pluto's surface and of recent surface temperature measurements. Firm inferences from these observations are that N 2 must be the major gas of Pluto's atmosphere and that the temperature at the microbar level is about 105 K. We estimate that CH 4 and CO are present at the 0.1% level of N 2 in the atmosphere and we develop simple 1-D aeronomical models (solving the heat equation) for various atmospheric cases. It is found in particular that (i) if CO cooling is omitted, heating by CH 4 can explain the temperature rise between the surface and the μbar level, but not the drop-off observed in several occultation lightcurves below 1215 km, unless the CH 4 mixing ratio is larger than 10%; (ii) if, as expected, CO is present at the ≈0.1% level, and CH 4 is lower than 10%, then cooling by CO rotational lines is so large that CH 4 heating cannot explain a 105 K temperature at 1-2 μbar. A possible way to explain the temperature rise between the surface and 1 μbar and the lightcurve drop-off is to invoke heating by a Titan-like haze layer with extinction optical depth ≈(0.4-4) × 10 -3 for particle sizes in the range 0.01-2 μm. Such a haze can plausibly result from methane photolysis and subsequent chemistry, provided that the CH 4 mixing ratio is significantly larger than 10 -4. A similar haze is not expected to be present on Triton (where the methane mixing ratio is about (2-20) × 10 -5), which is consistent with the fact that Triton's atmosphere near the μbar level is much colder than Pluto's. Estimating production and fallout time constants suggests that haze particles in Pluto's atmosphere must be small (≤0.1 μm).

A search for CO emission from the Pluto-Charon system

view Abstract Citations (6) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Search for CO Emission From the Pluto-Charon System Barnes, Peter J. Abstract Recent observations and theoretical models have indicated the presence of a thin but extended atmosphere around Pluto. Various models predict that, besides the generally accepted methane, the atmosphere may contain significant quantities of a heavier component, with nitrogen and/or carbon monoxide being the most likely candidates. I report here a search for CO emission from such an atmospheric component in the Pluto-Charon system. A simple model is also presented that predicts the CO visibility is a distended atmosphere around Pluto. Publication: The Astronomical Journal Pub Date: December 1993 DOI: 10.1086/116823 Bibcode: 1993AJ....106.2540B Keywords: Astronomical Models; Atmospheric Composition; Carbon Monoxide; Charon; Pluto (Planet); Pluto Atmosphere; Spectrum Analysis; Astronomical Spectroscopy; Methane; Nitrogen; Occultation; Thermodynamics; Astronomy; PLANETS AND SATELLITES: INDIVIDUAL: PLUTO; PLANETS AND SATELLITES: INDIVIDUAL: CHARON full text sources ADS |

Pluto's Radius and Atmosphere: Results from the Entire 9 June 1988 Occultation Data Set

We have analyzed all photometric observations of the 9 June 1988 occultation of the star P8 by Pluto in order to derive the radius of Pluto and certain parameters of its atmosphere. Our analysis is based on a "haze" model; but where relevant, we also discuss the thermal gradient model. With either model, the occultation observations yield a diameter for the limb of Pluto that is significantly larger than that found from mutual event observations by D. J. Tholen and M. W. Buie (1989, Bull. Am. Astron. Soc. 21, 981-982), but is in good agreement with the value from E. F. Young (1992, Ph.D. thesis, Massachusetts Institute of Technology). For a clear atmosphere (i.e., the thermal gradient model), we find the radius of the solid surface of Pluto and, obviously, also of the limb, to be 1195 ± 5 km. If the haze model is correct, the solid surface of the planet falls below 1180 ± 5 km, but the radius of the visible limb would be in the haze layer between 1185 and 1200 km. The degree to which the structure of Pluto's atmosphere is globally homogeneous is also discussed.

Evidence for a Low Surface Temperature on Pluto from Millimeter-Wave Thermal Emission Measurements

Thermal continuum emission from the Pluto-Charon system has been detected at wavelents of 800 and 1300 micrometers, and significant upper limits have been obtained at 450 and 1100 micrometers. After the subtraction of emission from Charon, the deduced surface temperature of much of Pluto is between 30 and 44 kein, probably near 35 to 37 kelvin. This range is significantly cooler than what radiative equilibrium models have suged and cooler than the surface temperature derived by the Infrared Astronomy Satellite. The low temperature indicates that methane cannot be present at the microbar pressure levels indicated by the 1988 stellar occultation measurements and that the methane features in Pluto's spectrum are from solid, not gas-phase, absorptions. This result is evidence that Pluto's atmosphere is dominated by nitrogen or carbon monoxide rather than methane.

Exploration of Pluto

Pluto is the last known planet in our Solar System awaiting spacecraft reconnaissance. In its eccentric orbit taking it 50 AU from the Sun, Pluto presently has a thin atmosphere containing methane, which is projected to “collapse” back to the icy planet's surface in about three decades, following Pluto's 1989 perihelion pass at 30 AU. Based on ground and Earth-orbit-based observing capabilities limited by Pluto's small size and extreme distance, present top-priority scientific questions for the first mission concern Pluto and Charon's surface geology, morphology and composition, and Pluto's neutral atmosphere composition. Budgetary realities preclude a large, many-instrument flyby spacecraft, while distance and launch energy requirements preclude any but the smallest orbiter using presently available launch vehicles and propulsion techniques. A NASA-sponsored Pluto Mission Development activity began this year. Two alternative cost-constrained mission implementations are described, based on which a primary implementation will be chosen. The Pluto Fast Flyby (PFF) mission utilizes an 83 kg (dry) spacecraft launched in 1998 aboard a Titan IV(SRMU)/Centaur for an ∼7 year direct trajectory to Pluto. Instruments described are an integrated CCD-imaging/ultraviolet spectrometer, with a possible integrated infrared spectrometer. The larger Pluto-350 spacecraft, ∼316 kg, carries a broader instrument set, greater redundancy, and requires > 11 year flight time launching in 2001 aboard a Delta or Atlas, toward Earth and Jupiter swingbys to provide the energy to reach Pluto. Launch by Proton is under consideration. Both mission implementations store data during the brief encounter, to be played back over several months. Cost is the primary design driver of both alternatives, with major tradeoffs between spacecraft development, launch services, radioisotope thermoelectric generator procurement and launch approval, and mission operations. Significant benefits are apparent from incorporating “microspacecraft” technologies from Earth orbiters.

Pick‐up ions at Pluto

Methane molecules escaping from Pluto's atmosphere are ionized, and the resulting ions are picked up by the solar wind. The mass loading associated with this ion pick‐up can produce a comet‐like interaction of the solar wind with Pluto. Heavy ion gyroradii are as large as a half million km in the weak interplanetary magnetic field that exists at 30 AU, which is about an order of magnitude larger than the size of the “interaction region.” We have calculated velocity space distributions of pick‐up ions using numerically determined ion trajectories. The predicted pick‐up ion fluxes are high enough to be detectable by standard charged particle detectors as far upstream of Pluto as 106 km.

Analysis of stellar occultation data for planetary atmospheres. I - Model fitting, with application to Pluto

Consideration is given to an analytic model for a stellar-occultation light curve developed for a small, spherically symmetric planetary atmosphere that includes thermal and molecular weight gradients in a region that overlies an extinction layer. The model incorporates two equivalent sets of parameters. One set specifies the occultation light curve in terms of signal levels, times, and time intervals. The other set specifies physical parameters of the planetary atmosphere. Equations are given for the transforming between the sets of parameters, including their errors and correlation coefficients. Detailed numerical calculations are presented for a benchmark case. The results obtained are consistent with the isothermal prediction of the 'methane-thermostat' model of Pluto's atmosphere.

Pluto's extended atmosphere: An escape model and initial observations

We have calculated the rates of production and hydrodynamic outflow of atomic hydrogen resulting from the photodissociation of methane in the upper atmosphere of Pluto. Under the present near-perihelion conditions this yields an extended cloud of H around Pluto which is likely to be the most easily observable signature of Pluto's extended atmosphere, and thereby provide information on the extent, escape rate, and composition of Pluto's upper atmosphere. We have also performed initial observations with the IUE attempting to detect the H Ly α emission from the extended H cloud, which we use to derive upper limits to the cloud properties as a function of the cloud extent.

A two-component volatile atmosphere for Pluto. I - The bulk hydrodynamic escape regime

The seasonal effects on Pluto's atmosphere of a simplified system of CH4 and N2 saturated over a solid solution are investigated, and the results are compared with previous CH4 models. It is found that bulk escape occurs for CH4 mole fractions less than 0.7 of Pluto's volatile reservoir. Greater CH4 abundance leads to diffusive separation during the escape of both species and an atmospheric mixing ratio of about Xc(0). If Xc(0) is in the range 0.02-0.10, Pluto's atmosphere remains largely intact at aphelion rather than virtually freezing out as it does for Xc(0) greater than 0.3 or less than 0.001, or form an atmosphere with only a single volatile gas. An upper limit for the CH4 mixing ratio is about 0.07 if N2 is the second gas. On the other hand, CH4 is the dominant surface constituent of the volatile deposit if Xc(0) is greater than 0.0001.

Nonisothermal Pluto atmosphere models

We calculate the thermal profile of a model Pluto atmosphere containing a substantial number fraction of methane molecules, taking into account the heating of the atmosphere by absorption of solar infrared flux and the cooling by conductive transport to the planet's surface. We assume an equilibrium temperature of the high atmosphere equal to 106°K and an atmospheric composition with an appreciable fraction of molecules heavier than methane (such as, perhaps, CO or N 2). For surface pressures of a few microbars, we find that an appreciable temperature gradient must exist close to the surface. The predicted stellar occultation lightcurve for such a Pluto atmosphere agrees with observations, suggesting that an inferred Pluto haze layer may not exist. Instead, similar effects on the lightcurve could be produced by the strong temperature gradient near the surface. Our alternative model implies that Pluto's solid surface probably lies in the vicinity of 1180 km from the planet's center and that the normal and tangential optical depth of the atmosphere is quite negligible. We calculate an upper limit to the escape rate for the model atmosphere deduced here and find that the minimum time for depletion of the atmospheric methane inventory is about 30 Earth years.