The New Horizons spacecraft encountered the Kuiper Belt object (KBO) Arrokoth (486958), originally designated as 2014 MU 69 and formerly called “Ultima Thule,” on 2019 January 01. At 43.3 au from the Sun and 44.4 au from Earth, this was the most distant spacecraft reconnaissance of a solar system body to date. The Radio Science Experiment (REX) on New Horizons performed radiometry measurements of the KBO's thermal emission at λ = 4.2 cm in two observation slots, one before (dayside) and one after (nightside) the point of closest approach. Owing to the small size of the target, the intensity of the thermal emission was expected to be only marginally detectable. The KBO was not detected on approach because of unexpectedly large variations in the REX system temperature. A brightness temperature T b = 29 ± 5 K was derived for the nightside observation, considerably less than the predicted equilibrium temperature of ∼50 K derived for Arrokoth on the dayside. A model explaining this day−night contrast is used to constrain the global values of emissivity, thermal inertia, and electrical skin depth of the KBO. In particular, models with small values of thermal inertia and small values of electrical skin depth are excluded. Relatively high values of the effective radio emissivity ( E eff > 0.8) provide better agreement with the REX observation.

The radio thermal emission from Pluto was observed from the New Horizons spacecraft at a wavelength of 4.2 cm along two scans across the planetary disk shortly after closest approach to Pluto on 14 July 2015. The measurements were performed as part of the New Horizons Radio Science Experiment (REX) using the 2.1 m High Gain Antenna (HGA) and the spacecraft's X-Band receiver. The HGA boresight first scanned along a diametric chord across the Pluto disk and then reversed direction to traverse a chord that crossed close to Pluto's winter pole. The diametric scan reveals a "hot spot" on the Pluto nightside associated with an optically bright region centered roughly at the planetocentric coordinates 280 degrees E, 55 degrees S, imaged in 2002-03 with the Hubble Space Telescope. The nightside was also found to be warmer than the dayside during the polar scan. The highest emission was not observed at the maximum southern latitude, however, but rather near the outbound Pluto limb at lower latitude. The REX emission profile from the polar scan is qualitatively consistent with a bright U-shaped polar cap observed on Pluto's Charon-facing hemisphere during the recurring Pluto/Charon mutual events in the late 1980's. The REX radiometer measurements show distinct variations in microwave brightness that constrain volatile transport models and provide unique information on the thermal structure and composition on the regions in winter night during the New Horizons encounter at Pluto.

We investigate the behavior of the Martian daytime convective boundary layer (CBL) through a combination of data analysis and modeling. This study relies on two subsets of Mars Express radio occultation (RO) measurements that sounded the atmosphere in northern spring of successive Mars years. Only the first year of observations has been examined previously (Hinson et al., 2008); the second year provides complementary spatial coverage and greatly increases the total number of observations. Analysis of the RO profiles yields basic characteristics of the CBL, such as its depth D and the average potential temperature of the mixed layer theta(m). We also combine RO retrievals of surface pressure with surface temperatures from infrared sounding to characterize the surface forcing, expressing the result as a potential temperature O. These observations are at local times in early afternoon for e s and late afternoon for theta(m) and D, when each parameter is near its diurnal maximum. We use measurements at mid-to-low latitudes, which sample a wide range of theta(s) (227-294 K), to determine the response of the lower atmosphere to spatial variations in surface forcing. The depth of the CBL ranges from less than 3 km in the midlatitude topographic basins to more than 9 km above elevated terrain in the tropics. The dependence of theta(m) on theta(s) is linear, with a characteristic slope of about 0.7 in both years. We gain further insight by performing a simulation with the Oregon State University Mars Mesoscale Model in a region centered on Isidis Planitia, which includes two potential landing sites for the Mars 2020 Rover. As expected from previous modeling of much smaller craters, the arc of steep topography along the western and southern margins of Isidis produces a distinctive, diurnally varying, mesoscale circulation. The simulation captures key features of the observations, such as the wide variations in theta(m) and D - by 34 K and 9 km, respectively - that occur within this region. The model also accounts for peculiar features of RO profiles on the rim of Isidis, where the wind field strongly influences the depth and diurnal evolution of the CBL. Detailed comparisons with the observations validate the general performance of the model and confirm several aspects of the simulated wind field.

One component of the REX instrument on NASA's New Horizons spacecraft was an investigation of the radio continuum radiation from Pluto and Charon during the flyby on 14 July 2015. The planetary thermal emission was recorded at a wavelength of 4.17 cm (7.18 GHz) during approach, departure, and specifically on the non illuminated hemispheres of Pluto and Charon during the respective intervals between occultation ingress and egress. We derive the brightness temperatures for these disk-resolved and unresolved observations. The mean values and la deviations of brightness temperature for the unresolved sunlit disk are 33.2 +/- 1.4 K and 47.2 +/- 5.3 K for Pluto and Charon, respectively, consistent with the global albedos of the two bodies as well as with previous ground-based estimates at smaller wavelengths. A slightly colder temperature of 29.0 +/- 2.5 K was determined for the disk-integrated nightside of Pluto and a larger drop in temperature was observed for Charon (40.9 +/- 0.9 K), implying a smaller thermal inertia for Charon than Pluto. The measured brightness temperature of Pluto across the nightside diametric scan reached a maximum of 29.0 +/- 1.5 K in the center of the disk. The profile shape is attributed to an emissivity effect, which favors thermal emission toward higher elevation angles. As a first approximation, the effective emissivity for thermal emission is calculated for the case when Pluto and Charon are uniformly smooth homogenous spheres. Under this assumption, the effective emissivity for these observations is close to unity for all probable surface constituents, implying that the effective temperature of the Pluto subsurface is only a few percent higher than the observed brightness temperature. A considerably lower subsurface emissivity is implied, however, if the higher atmospheric temperatures near the surface determined from the REX occultation measurements are also valid for the subsurface.

On 14 July 2015 New Horizons performed a radio occultation (RO) that sounded Pluto's atmosphere down to the surface. The sensitivity of the measurements was enhanced by a unique configuration of ground equipment and spacecraft instrumentation. Signals were transmitted simultaneously by four antennas of the NASA Deep Space Network, each radiating 20 kW at a wavelength of 4.2 cm. The polarization was right circular for one pair of signals and left circular for the other pair. New Horizons received the four signals and separated them by polarization for processing by two independent receivers, each referenced to a different ultra-stable oscillator. The two data streams were digitized, filtered, and stored on the spacecraft for later transmission to Earth. The results reported here are the first to utilize the complete set of observations. We calibrated each signal to remove effects not associated with Pluto's atmosphere, including the limb diffraction pattern. We then applied a specialized method of analysis to retrieve profiles of number density, pressure, and temperature from the combined phase measurements. Occultation entry sounded the atmosphere at sunset at 193.5 E, 17.0 S on the southeast margin of an ice-filled basin known informally as Sputnik Planitia (SP); occultation exit occurred at sunrise at 15.7 E, 15.1 degrees N - near the center of the Charon-facing hemisphere. Above 1215 km radius (similar to 25 km altitude) there is no discernible difference between the measurements at entry and exit, and the RO profiles are consistent with results derived from ground-based stellar occultation measurements. At lower altitudes the RO measurements reveal horizontal variations in atmospheric structure that had not been observed previously, and they are the first to reach the ground. The entry profile has a strong temperature inversion that ends 3.5 km above the surface, and the temperature in the cold boundary layer beneath the inversion is nearly constant, 38.9 +/- 2.1 K, and close to the saturation temperature of N-2. The exit profile has a much weaker inversion that extends all the way to the ground, where the air temperature is 51.6 +/- 3.8 K. Three factors appear to be responsible for the presence of a cold boundary layer in the entry profile (Forget et al., 2017): a substantial diurnal cycle of sublimation and condensation of N-2 ice in SP, the local time of the RO observation, and confinement within SP by the surrounding topography and katabatic winds. We have also determined the surface pressure and the local radius at both entry and exit. The best pressure reference is the mean value: 11.5 +/- 0.7 microbar at 1189.9 +/- 0.2 km. (C) 2017 Elsevier Inc. All rights reserved.

In their recent paper, Grandin et al. (2014) claim to have developed a novel approach, principally a ray tracing method, to analyze radio sounding data from occulted spacecraft signals by planetary atmospheres without the usual assumptions of the radio occultation inversion method of a stratified, layered, symmetric atmosphere. They apply their “new approach” to observations of the Mars Express Radio Science (MaRS) experiment and compare their resulting temperature, neutral number density, and electron density profiles with those from MaRS, claiming that there is good agreement with the observations. The fact is, however, that there are serious disagreements in the most important altitude ranges. Their temperature profile shows a 30 K shift or a 300σ (1σ standard deviation = 0.1 K for the MaRS profile near the surface) difference toward warmer temperatures at the surface when compared with MaRS, while the MaRS profile is in best agreement with the profile from the Mars Climate Data Base V5.0 (MCD V5.0). Their full temperature profile from the surface to 250 km altitude deviates significantly from the MCD V5.0 profile. Their ionospheric electron density profile is considerably different from that derived from the MaRs observations. Although Grandin et al. (2014) claim to derive the neutral number density and temperature profiles above 200 km, including the asymptotic exosphere temperature, it is simply not possible to derive this information from what is essentially noise.

The Mars Express spacecraft is operating in Mars orbit since early 2004. The Mars Express Radio Science Experiment (MaRS) employs the spacecraft and ground station radio systems (i) to conduct radio occultations of the atmosphere and ionosphere to obtain vertical profiles of temperature, pressure, neutral number densities and electron density, (ii) to conduct bistatic radar experiments to obtain information on the dielectric and scattering properties of the surface, (iii) to investigate the structure and variation of the crust and lithosphere in selected target areas, (iv) to determine the mass, bulk and internal structure of the moon Phobos, and (v) to track the MEX radio signals during superior solar conjunction to study the morphology of coronal mass ejections (CMEs). Here we report observations, results and discoveries made in the Mars environment between 2004 and 2014 over almost an entire solar cycle.

Gravity waves are a ubiquitous feature in all stably stratified planetary atmospheres. They are known to play a significant role in the energy and momentum budget of the Earth and they are assumed to be of importance for the redistribution of energy. This high vertical resolution of the radio occultation profiles from the MaRS experiment on Mars Express provides the unique opportunity to study small scale vertical wave structures in the Martian lower atmosphere. These small scale temperature perturbations are most probably caused by gravity waves (buoyancy waves) produced by the displacement of air masses flowing over elevated topographical features or other atmospheric sources like convection in the surface boundary layer or wind shear. A study of the global distribution of gravity waves provides insight into possible source mechanisms, local time dependencies, seasonal dependencies and/or topographical dependencies.

MATION. S.J. Robbins1, K. Runyon, K.N. Singer, V.J. Bray, P. Schenk, O.L. White, W.B. McKinnon, J.M. Moore, R.P. Binzel, M.W. Buie, B.J. Buratti, A.F. Cheng, W.M. Grundy, I.R. Linscott, H.J. Reitsema, M.R. Showalter, J.R. Spencer, G.L. Tyler, H.A. Weaver, L.A. Young, C.B. Olkin, K. Ennico, S.A. Stern, the New Horizons GGI Theme Team, New Horizons Pluto Encounter Team, New Horizons LORRI Instrument Team, and New Horizons MVIC Instrument Team. Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302. The Johns Hopkins University, Baltimore, MD. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ. Lunar and Planetary Institute, Houston, TX. NASA Ames Research Center, Moffett Field, CA 84043. Washington University in St. Louis, St. Louis, MO. Massachusetts Institute of Technology, Cambridge, MA. NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA. Lowell Observatory, Flagstaff, AZ. Stanford University, Stanford, CA. Ball Aerospace [retired], Boulder, CO. Sagan Center at the SETI Institute. stuart@boulder.swri.edu