Atmosphere of Pluto
The atmosphere of Pluto is the tenuous layer of gases surrounding Pluto. It consists mainly of nitrogen (N2), with minor amounts of methane (CH4) and carbon monoxide (CO), all of which are vaporized from their ices on Pluto's surface. It contains layered haze, probably consisting of heavier compounds which form from these gases due to high-energy radiation. The atmosphere of Pluto is notable for its strong and not completely clear seasonal changes caused by peculiarities of the orbital and axial rotation of Pluto, and relatively quick escape into outer space.
Surface pressure of the atmosphere of Pluto, measured by New Horizons in 2015, is about 1 Pa (10 μbar), roughly 100,000 times less than Earth's atmospheric pressure. Temperature on the surface is 40–60 K, but it quickly grows with altitude due to methane-generated greenhouse effect. Near the altitude 30 km it reaches 110 K, and then slowly decreases.
The atmosphere of Pluto has been studied since 1980s by way of earth-based observation of occultations of stars by Pluto and spectroscopy. In 2015, it was studied from a close distance by the spacecraft New Horizons.
The main component of the atmosphere of Pluto is nitrogen. Content of methane, according to measurements by New Horizons, is 0.25% (Earth-based observations suggested about 0.4–0.6% in 2008 and 0.3–0.4% in 2012). For carbon monoxide, there are Earth-based estimates 0.025–0.15% (2010) and 0.05–0.075% (2015). Under influence of high-energy cosmic radiation, these gases react to form more complex compounds (not volatile at temperatures of surface of Pluto), including ethane (C2H6), ethylene (C2H4), acetylene (C2H2), more heavy hydrocarbons and nitriles and hydrogen cyanide (HCN) (amount of ethylene is about 0.0001%, and amount of acetylene is about 0.0003%). These compounds slowly precipitate on the surface. Probably, they also include tholins, which are responsible for the brown color of Pluto (like some other bodies in the outer solar system).
The most volatile compound of the atmosphere of Pluto is nitrogen, the second is carbon monoxide and the third is methane. The indicator of volatility is saturated vapor pressure (sublimation pressure). At temperature 40 K (close to minimum value for surface of Pluto) it is about 10 Pa for nitrogen, 1 Pa for carbon monoxide and 0.001 Pa for methane. It quickly increases with temperature, and at 60 K (close to the maximum value) approaches to 10 000 Pa, 3000 Pa and 10 Pa respectively. For heavier-than-methane hydrocarbons, water, ammonia, carbon dioxide and hydrogen cyanide, this pressure remains negligibly low (about 10–5 Pa or still lower), which indicates absence of volatility at Pluto's conditions (at least in cold lower atmosphere).
Methane and carbon monoxide, due to their lower abundance and volatility, could be expected to demonstrate stronger deviations from pressure equilibrium with surface ices and bigger temporal and spatial variations of concentration. But actually concentration of, at least, methane, does not depends noticeably of height (at least, in the lower 20–30 km), longitude and time. But temperature dependence of volatility of methane and nitrogen suggest that concentration of methane will decrease during moving of Pluto further from the Sun. It's notable that observed concentration of methane is 2 orders of magnitude higher than expected from Raoult's law on the basis of its concentration in surface ice and ratio of sublimation pressures of methane and nitrogen. Reasons of this discrepancy are unknown. It can be due to existence of separate patches of relatively clean methane ice, or due to increased content of methane in the uppermost layer of usual mixed ice.
Seasonal and orbital changes of insolation results in migration of surface ices: they sublimate in some places and condensate in another. According to some estimates, it gives meter-sized changes of their thickness. This (but also changes in viewing geometry) results in appreciable changes of brightness and color of Pluto.
Methane and carbon monoxide, despite their low abundance, are significant for thermal structure of the atmosphere: methane is a strong heating agent and carbon monoxide is a cooling one (although amount of this cooling is not completely clear).
New Horizons discovered in the atmosphere of Pluto a multi-layered haze, which covers all the dwarf planet and reaches altitude 150 km. Probably, it consists of particles of non-volatile compounds, which are synthesized from atmospheric gases under influence of cosmic high-energy radiation. The layers can be formed due to gravity waves (presence of which is suggested also by observations of occultations).
Despite very low density of the atmosphere, the haze is rather appreciable: it even scatters enough light to give possibility to photograph some details of Pluto's night side. Its normal optical depth is about 0.004 (so, it diminishes intensity of vertical beam of light by 1–e–0.004=0.4%, and for grazing light this quantity would be much higher). Scale height of the haze is 45–55 km; it approximately coincides with scale height of pressure in the middle atmosphere.
Probably, the haze is the reason of a kink on the curve of light intensity vs time obtained by New Horizons during flight across Pluto's shadow (on the image): below altitude 150 km the atmosphere attenuates light much stronger than above. A similar kink was observed during stellar occultation in 1988. Firstly it was also interpreted as weakening of light by haze. Besides that, it could be explained as a result of temperature inversion, and some researchers favoured this version. Height of the layer, which created the kink, relative to surface was impossible to determine at these times due to uncertainty of Pluto's radius. But the distance from Pluto's center was calculated, and after subtraction of modern radius of Pluto it gives height 2±24 km, so, this "haze" is indistinguishable from the surface of Pluto. During next occultations (when the atmosphere of Pluto was already ≥2 times denser) this kink was absent.
Another evidence of the haze was obtained in 2002 due to new occultation: the part of stellar light, which even in the middle of the event reached Earth (due to refraction in Pluto's atmosphere), demonstrated increase of intensity with wavelength.[Note 1] This was interpreted as a reliable evidence of scattering of the light by aerosol (similar to reddening of the rising Sun). But this feature was absent during next eclipses (including 29 June 2015), and 14 July 2015 New Horizons revealed blue color of the haze.
Temperature and thermal structure
Pluto has no or almost no troposphere; observations by New Horizons suggest a thin tropospheric boundary layer consistent with those models which predicted its thickness ≤1 km. Above it lays a layer with fast increase of temperature with height: stratosphere. The temperature gradient is estimated to be 2.2, 3–15 or 5.5 degrees per km. It is a result of greenhouse effect, caused by methane. Mean temperature of the surface is 42±4 K (measured in 2006), and the mean value for all the atmosphere is 90+25
−18 K (2008).
On the height 20–40 km temperature reaches its maximum (100–110 K; stratopause) and then slowly decreases (about 0.2 K/km; mesosphere). Causes of this decrease are unclear; it can be related to cooling action of carbon monoxide, hydrogen cyanide or other reasons. Above 200 km temperature reaches approximately 80 K and then remains constant.
Temperature of upper layers of the atmosphere does not show noticeable temporal changes. In 1988, 2002 and 2006 it was approximately constant and equal to 100 K (with uncertainty about 10 K), despite twofold increase of the pressure. Dependence on latitude or morning/evening conditions is also absent: temperature is the same above all the surface. It is consistent with theoretical data, which predict fast mixing of the atmosphere. But there are evidence for a small vertical heterogeneities of temperature. They reveal themselves in sharp and brief spikes of brightness during stellar occultations. Amplitude of these heterogeneities is estimated to be 0.5–0.8 K on the scale of a few km. They can be caused by gravity waves or turbulence, which can be related to convection or wind.
Interaction with the atmosphere significantly influences temperature of the surface. Calculations show that the atmosphere, despite very low pressure, can significantly diminish diurnal variations of this temperature. But there still remain temperature variations about 20 K – partly because of cooling of the surface due to sublimation of ices.
Pressure of the atmosphere of Pluto is very low and strongly time-dependent. Observations of stellar occultations by Pluto show that it increased about 3 times between 1988 and 2013, despite that Pluto is moving away from the Sun since 1989. Probably, it is caused by entrance of the north pole into polar day in 1987, which intensified evaporation of nitrogen from the northern hemisphere,[Note 2] while the southern is still too warm for its condensation. Absolute values of surface pressure are difficult to obtain from occultation data, because usually these data do not reach the lowest layers of the atmosphere. So, surface pressure has to be extrapolated, and this is somewhat ambiguous due to not completely clear height dependence of temperature and, consequently, pressure. Radius of Pluto must also be known, but it was poorly constrained before 2015. So, precise values of surface pressure were impossible to calculate in previous times. For some occultations since 1988, pressure was calculated for reference level of 1275 km from the center of Pluto (which later turned out to be 88±4 km from the surface).
Curves "pressure vs distance from the center", obtained for occultations of 1988 and 2002, in combination with modern radius of Pluto (1187±4 km) give values about 0.4 Pa for 1988 and 1.0 Pa for 2002. Spectral data provided values 0.94 Pa in 2008 and 1.23 Pa in 2012 for the distance from the center 1188 km (1±4 km from the surface). Occultation of 4 May 2013 gave data almost precisely for the surface level (1190 km from the center, or 3±4 km from the surface): 1.13±0.007 Pa. Occultation of 29/30 June 2015, just 2 weeks before New Horizons encounter, provided surface value 1.8±0.3 Pa.
First direct and reliable data about lowermost layers of atmosphere of Pluto were obtained by New Horizons 14 July 2015 due to radio occultation measurements. The surface pressure was estimated to be 1 Pa. Some of the previous estimates agree with this value, but some others were about 2 times higher. Maybe, the pressure really abruptly decreased, or, maybe, the discrepancy is a result of mentioned problems with calibration of occultation data.
Scale height of pressure in Plutonian atmosphere significantly varies with height (in other words, height dependence of pressure deviates from exponential one). It is caused by strong height variations of temperature. For the lowermost layer of the atmosphere scale height is about 17–19 km, and for heights 30–100 km — 50–60 km.
Due to orbital eccentricity, in the aphelion Pluto receives 2.8 times less heat than in perihelion.[Note 3] It should cause strong changes in its atmosphere, although details of these processes are not clear. Firstly it was thought that in aphelion the atmosphere must largely freeze out and fall on the surface (this is suggested by strong temperature dependence of sublimation pressure of its compounds), but more elaborated models predict that Pluto has significant atmosphere during all its year.
As for 2015, Pluto is moving away from the Sun (last passage of perihelion was 5 September 1989), and overall illumination of its surface is decreasing. But the situation is complicated by its big axial tilt (122.5°), which results in long polar days and nights on large part of its surface. Shortly before the perihelion, 16 December 1987, Pluto underwent equinox, and its north pole[Note 4] came out of the polar night, which lasted 124 Earth years.
Data, existing as for 2014, permitted to built such model of seasonal changes in Pluto's atmosphere. During passage of aphelion (in 1865 for the last time) significant quantity of volatile ices was present in both northern and southern hemispheres. Approximately in the same time equinox occurred, and the southern hemisphere became tilted towards the Sun. Local ices began to migrate to the northern hemisphere, and near 1900 the southern became largely devoid of them. After next equinox (1987) southern hemisphere turned away from the Sun. But its surface was already substantially heated, and its big thermal inertia (provided by non-volatile water ice) greatly slowed down its cooling. That's why gases, which now intensively evaporate from northern hemisphere, can't quickly condense in the southern, and they accumulate in the atmosphere, increasing its pressure. In 2035–2050 the southern hemisphere will cool enough to permit intensive condensation of the gases, and they will migrate there from the northern hemisphere, where is polar day. It will last till equinox near aphelion (about 2113). The northern hemisphere will not lose its volatile ices completely, and their evaporation will supply the atmosphere even at the aphelion. Overall change of atmospheric pressure in this model is about 4 times; the minimum was reached near 1970–1980, and the maximum near 2030. Full temperature range is only several degrees.
Weakness of Pluto's gravitation and high temperature of upper layers of its atmosphere causes quick atmospheric escape into outer space. The escape rate is estimated to be 1027–1028 molecules (50–500 kg) of nitrogen per second. It corresponds to loss of surface layer several hundred meters or several kilometers thick during lifetime of solar system. Relative rate of the escape is much greater than for any big planets, and even for Triton. In this way, Pluto resembles a comet. It doesn't have sources of replenishment of volatiles: impacts of minor objects can't give necessary amounts.
Molecules with high enough velocity, which escape into outer space, are ionized by solar ultraviolet radiation. As the solar wind encounters the obstacle formed by the ions, it is slowed and diverted, possibly forming a shock wave upstream of Pluto. The ions are "picked up" by the solar wind and carried in its flow past the dwarf planet to form an ion or plasma tail. The Solar Wind around Pluto (SWAP) instrument on the New Horizons spacecraft made the first measurements of this region of low-energy atmospheric ions shortly after its closest approach on 14 July 2015. Such measurements will enable the SWAP team to determine the rate at which Pluto loses its atmosphere and, in turn, will yield insight into the evolution of the Pluto’s atmosphere and surface.
History of study
As early as in 1940s, Gerard Kuiper searched evidence for the atmosphere in the spectrum of Pluto, but unsuccessfully. In 1970s, some astronomers considered the hypothesis about thick atmosphere and even oceans of neon: according to some views of those times, all other abundant in the solar system gases would either freeze or escape. But this hypothesis was based on heavily overestimated mass of Pluto. No observational data about its atmosphere and chemical composition existed in these days.
First strong, though indirect evidence of the atmosphere appeared in 1976. Infrared photometry on 4-meter Nicholas U. Mayall Telescope revealed on the surface of Pluto methane ice, which must evaporate significantly at Plutonian temperatures.
Existence of the atmosphere of Pluto was proved due to occultations of stars. If the star is occulted by a body without atmosphere, its light disappears sharply, but occultations by Pluto give gradual decrease. This is mainly due to atmospheric refraction (not absorption or scattering). First such observations were made 19 August 1985 by Noah Brosch and Haim Mendelson of the Wise Observatory in Israel. But quality of the data was rather low due to unfavourable observational conditions (in addition, the detailed description was published only 10 years later). 9 June 1988 existence of the atmosphere was convincingly proved by observations of a new occultation from 8 sites (the best data were obtained by the Kuiper Airborne Observatory). Scale height of the atmosphere was measured, and it gave possibility to calculate ratio of the temperature to mean molecular mass. The temperature and pressure themselves were impossible to calculate in that time due to absence of data about chemical composition of the atmosphere and large uncertainty in radius and mass of Pluto.
The question about composition was solved in 1992 due to exploring of infrared spectrum of Pluto on 3.8-meter United Kingdom Infrared Telescope. The surface of Pluto turned out to be covered mainly by nitrogen ice. Since nitrogen is, in addition, more volatile than methane, this observation implied prevalence of nitrogen also in the atmosphere (although gaseous nitrogen wasn't seen in the spectrum). Beside this, a small admixture of frozen carbon monoxide was discovered. The same year observations on 3.0-meter NASA Infrared Telescope Facility revealed first conclusive evidence of gaseous methane. For understanding of the state of the atmosphere, knowing of the surface temperature is critically important. Best its estimates are derived from measurements of thermal emission of Pluto. The first values, calculated in 1987 from observations of IRAS, were about 55–60 K, and next works resulted in 30–40 K. In 2006, observations on Submillimeter Array succeeded in separating emission of Pluto and Charon, and average temperature of Pluto's surface was measured to be 42±4 K (−231±4°C). It was approximately 10 K colder than expected; maybe, the difference is caused by cooling from sublimation of nitrogen ice. The other works revealed that the temperature is strongly different in different regions: from 40 to 55–60 K.
Near 2000, Pluto entered star-rich fields of Milky Way, and will reside there till 2020s. First stellar occultations after 1988 were observed 20 July and 21 August 2002 by teams led by Bruno Sicardy of the Paris Observatory and James L. Elliot of MIT. Atmospheric pressure turned out to be about 2 times higher than in 1988. Next occultation was observed 12 June 2006, and later they happened more often. Processing of this data shows that the pressure continues to increase. An occultation of exceptionally bright star, about 10 times brighter than Pluto itself, was observed 29/30 June 2015 – only 2 weeks before New Horizons encounter. The surface pressure was estimated to be 1.8±0.3 Pa.
14 July 2015 the spacecraft New Horizons made first explorations of the atmosphere of Pluto from close distance, including radio occultation measurements and observations of weakening of solar radiation during flight through Pluto's shadow. It provided the first direct measurements of parameters of lower atmosphere. The preliminary estimate of surface pressure is about 1.0 Pa. It is nearly 2 times lower than some of the preceding estimates from stellar occultations; causes of this discrepancy are not completely clear yet. Presence of atmospheric gases was traced up to altitude of 1670 km.
- At least, in infrared range — from 0.75 to 2 µm.
- In these sources this pole is called "south" according to nomenclature of that time.
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