Pluto's atmosphere. Léa E. Bonnefoy. (Dated: April 25, 2017)

Pluto's atmosphere Léa E. Bonnefoy (Dated: April 25, 2017) I. INTRODUCTION The dwarf planet Pluto has a high eccentricity, of 0.25, and a period of 248 years. At perihelion it can get as close as 29.7 AU to the Sun (within the orbit of Neptune), while its aphelion is at 49.3 AU, causing a solar ux 2.8 times larger at perihelion than aphelion. The existence of an atmosphere of Pluto was rst conrmed in 1988 (Hubbard et al. (1988) and Elliot et al. (1989)) using Earth-based stellar occultation proles. Studies have since continued to use this method to characterize the atmosphere's composition, mass, temperatures, and seasonal variations. The New Horizons 2015 yby, however, provided exceptional new observations of the atmosphere, its hazes, and its interactions with the surface. II. TEMPERATURE PROFILES The pressure and temperature proles shown in Fig. 1 and Fig. 2 were obtained using the Radio Science Experiment (REX) on the New Horizons spacecraft, through uplink X-band radio occultations, on 14 July 2015. The atmospheric proles extracted from this dataset were the rst to reach the surface. At lower altitudes, the temperature proles at ingress and egress show a signicant dierence (Fig. 1). Indeed, the temperature inversion is much stronger at entry (larger temperature gradient). The surface temperature is 38.9± 2.1 K at entry and 57.0± 3.7 K at exit (an 18 K dierence!). The entry prole, unlike the exit prole, shows a cold boundary layer. This is primarily because of the geographic location of the entry point: it is located within Sputnik Planitia, near its southeastern edge. Sputnik Planitia contains very bright, km-deep N 2 glaciers and its surface is 2-3 km below the surrounding terrain. A Pluto Global Climate Model (GCM) by Forget et al. (2016) shows that a cold boundary layer can be produced by daytime sublimation of N 2 in Sputnik Planum. This GCM includes atmospheric dynamics and transport, turbulence, radiative transfer, molecular conduction, organic hazes, and phase changes for N 2, CH 4, and CO. Sublimation occurs when the surface is illuminated by the Sun; then the N 2 is condensated back to the surface during the night. The cold dense air released by sublimation is conned with the low topography of the Sputnik Planitia basin. The REX entry pro- le was taken near sunset, after about three Earth days (the Pluto period is 6.3 Earth days) of N 2 sublimation, when the cold boundary layer was at its maximum. The boundary layer is not expected in the exit prole, both because it is taken in Pluto morning and because it is in a location at a lower elevation and containing less or no N 2. Hinson et al. (2017) point out that the REX entry point is located at the southern tip of Sputnik Plantitia, where there is less frozen N 2 and where less

2 Figure 1: REX proles of temperature versus (A) radius and (B) local altitude. In both panels the orange line is the entry prole, the blue line is the exit prole, and the black line is the saturation temperature of N 2. The base of each prole in (A) is 1 km above the local surface. The proles are plotted versus altitude in (B) to emphasize the dierence in conditions near the ground. Figure from Hinson et al. (2017). sunlight reaches the surface. They argue that, if it were due only to local sublimation, the boundary layer would be much smaller. Instead, there is a southward atmospheric ow within Sputnik Planitia, due to stronger sublimation in its northern part These proles agree well with the REX prole at higher altitude but show the temperature inversion at a slightly dierent radius, which could arise from uncertainties in the radius during the stellar occultation measurements. and guided by katabatic winds at its edges. This southward transport of cold N 2 would then be the primary cause for the observed boundary layer. III. DRY LAPSE RATE The dry adiabatic lapse rate is given by: Temperature proles had previously been derived, with less accuracy, from stellar occultations before the New Horizons 2015 yby (Sicardy et al. (2003), Elliot et al. (2007), Dias-Oliveira et al. (2015), Lellouch et al. (2016)); however, these pro- les were unable to observe the lower atmosphere Γ = dt dz = g c p (1) where the gravitational acceleration is g = GM R 2. For Pluto, using M = 1.3 10 22 kg, R = 1187 km, down to the surface. The New Horizons proles and c p = 1.04 10 3 J/K/kg for N 2 (from Gladstone et al. (2016)), we obtain a dry lapse rate of (Fig. 1) and the proles from Earth-based stellar occultation are consistent: both show a temperature inversion around a 1210 km radius. Sicardy et al. (2016) took stellar occulation measurements during the New Horizons yby, as shown in Fig. 2. Γ = 0.59 K/km at the surface. Fig. 3 shows the observed temperature gradient compared to the dry lapse rate. At entry, over the nitrogen glaciers in Sputnik Planitia, the observed temperature gradient at the surface ( 0.5±0.7 K) corresponds well

3 A C B Figure 2: Pressure-temperature prole derived from REX measurements at (A) entry and (B) exit. The base of each prole is 1 km above the local surface. The blue line is the saturation temperature of N 2. (C) Comparison of the REX entry temperature prole with a model derived from contemporaneous stellar occulation measurements in red (Sicardy et al. (2016)). Gray shading denotes the standard deviation of temperature. Figure from Hinson et al. (2017). with either the dry adiabat or an isothermal atmosphere. This is not the case at exit, where there is no boundary layer, as discussed above. The dry adiabatic lapse rate is only applicable where the air is well mixed. This only happens very close to the surface and within the boundary layer, which is why it corresponds well to the entry measurements. We can also calculate the blackbody radiation of the Sun (5777 K) received at Pluto's aphelion. This are plotted in Fig. 4. We can see that the emission from Pluto dominates at wavelengths >10 µm. It would only be necessary to consider both the ux from the Sun and from Pluto very close to that wavelengths, as they are several orders of magnitude apart otherwise. IV. ENERGY BUDGET V. ATMOSPHERIC COMPOSITION The average surface temperature of Pluto is 44 K, which we use to nd its blackbody spectrum. The Alice instrument aboard the New Horizons spacecraft was able to provide line-of-sight abun-

4 Figure 3: The temperature gradient dt dz obtained from the REX proles at entry (orange) and exit (blue). The black line is the dry adiabat (Γ = 0.59 K/km). Figure from Hinson et al. (2017) Figure 5: Line-of-sight column density proles retrieved from the observed transmission data using known absorption cross sections for the indicated species. Figure from Gladstone et al. (2016). Figure 4: The blackbody radiation of Pluto and of the Sun at Pluto's aphelion. dance of the molecules contributing most to the opacity, as shown in Fig 5. Alice obtained ultraviolet spectra along the line of sight during entry and exit of the solar occultation. By comparing these observation with modeled transmission using known molecular cross-section, it is then possible to extract lin-of-sight molecular abundances. N 2 is by far the dominant component of the at- mosphere, while CH 4, C 2 H 2, C 2 H 4, and C 2 H 6 signicantly contribute to its opacity. The estimated mixing ratio for CH 4 is 4 10 3 at the surface (Wong et al. (2016), Lellouch et al. (2016)). Using Earth-based observations, CO and HCN have also been detected in Pluto's atmosphere, with mixing ratios of the order of 5 10 4 for CO and 10 5 for HCN (Lellouch et al. (2016)). These measurements were made by combining ALMA observations and a radiative transfer model. The GCM by Forget et al. (2016) predicts mixing ratios close to those observed for CO and CH 4.

5 VI. CLOUDS AND HAZE While no clouds have been observed on Pluto, which does have a rather tenuous atmosphere (surface pressure of 11.5±0.7 microbars), beautiful hazes have been seen by New horizons, as shown in Fig. 6 and Fig. 7. These hazes extend to altitudes of >200 km, and are nely structured into about 20 layers, continuous over hundreds of km. Their blue color and phase variations are consistent with fractal aggregate particles: randomly shaped particles of a fraction of a micron in radius, composed of 10 nm spheres (Gao et al. (2016), Gladstone et al. (2016), Bertrand & Forget (2017)). The haze is fainter and bluer at higher altitudes due to fewer and smaller haze particles. The haze is consistent with tholin-like particles photochemically produced in a way similar to Titan's hazes. The main components of the atmosphere (N 2, CH 4, CO, and other hydrocarbons) are also similar to Titan's. The photolysis of CH 4 in the upper atmosphere by UV radiation, which react to form of hydrocarbons such as C 2 H 2, C 2 H 4, and C 2 H 6, all observed in Pluto's atmosphere. These hydrocarbons then turn into organic aerosols and eventually into heavier haze particles. VII. ATMOSPHERIC DYNAMICS AND VARIATIONS There are little to no pressure variations in the current atmosphere on a global scale, so horizontal winds are unlikely to form. The steep positive temperature gradient seen in the lower atmosphere by both REX and ground-based occultation measurements (see Fig. 2) makes convection unlikely as well. However, due to signicant topography, orographic winds are likely to occur. Pluto's unusual orbital characteristics (high eccentricity, orbital period, and inclination as well as cycles of obliquity change and polar precession) have had a large impact on the solar ux reaching dierent regions of the dwarf planet. It was rst believed that the atmosphere would freeze out onto the surface at aphelion, but later models suggest than Pluto can retain its atmosphere throughout its orbit (Olkin et al. (2015)). The atmospheric pressure has been observed to increase from 1988 to 2013. Note that perihelion was in September 1989 and equinox in December 1987 (the North pole then coming out of polar winter). Stern et al. (2016) argue that there is geomorphological evidence of higher surface pressures in the past. Dendritic channels (if formed by owing liquids), cryovolcanism, and a frozen lake-like feature (see Fig. 8) all suggest a higher pressure, required for liquids (whether N 2 or CH 4 ) to ow on the surface. Bertrand, T., & Forget, F. 2017, Icarus, doi:10.1016/j.icarus.2017.01.016 Bosh, A., Person, M., Levine, S., et al. 2015, Icarus, 246, 237, special Issue: The Pluto System

6 Figure 6: Multispectral Visible Imaging Camera (MVIS) image of haze layers above Pluto's limb. About 20 haze layers are seen from a phase angle of 147. The layers typically extend horizontally over hundreds of kilometers but are not exactly horizontal. For example, white arrows on the left indicate a layer about 5 km above the surface, which has descended to the surface at the right. Figure from Gladstone et al. (2016). Figure 7: Pluto hazes. LORRI two-image stack at 0.95 km pixel resolution, showing many haze layers up to an altitude of 200 km, as well as night-side surface illumination. Acquired at a range from Pluto of 196 586 km and a phase angle of 169. (Inset) The orientation of the image, with Pluto's south pole (SP) indicated, along with the direction to the Sun (11 from Pluto), and the latitude and longitude of the sub-anti-sun (AS) position.figure from Gladstone et al. (2016).

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