Ground-based Stellar Occultation Research Articles

New constraints on Triton’s atmosphere from the 6 October 2022 stellar occultation

The atmosphere of Triton was probed directly by observing a ground-based stellar occultation on 6 October 2022. This rare event yielded 23 positive light curves collected from 13 separate observation stations contributing to our campaign. The significance of this event lies in its potential to directly validate the modest pressure fluctuation on Triton, a phenomenon not definitively verified by previous observations, including only five stellar occultations, and the Voyager 2 radio occultation in 1989. Using an approach consistent with a comparable study, we precisely determined a surface pressure of 14.07−0.13+0.21 μbar in 2022. This new pressure rules out any significant monotonic variation in pressure between 2017 and 2022 through direct observations, as it is in alignment with the 2017 value. Additionally, both the pressures in 2017 and 2022 align with the 1989 value. This provides further support for the conclusion drawn from the previous volatile transport model simulation, which is consistent with the observed alignment between the pressures in 1989 and 2017; that is to say, the pressure fluctuation is modest. Moreover, this conclusion suggests the existence of a northern polar cap extended down to at least 45°N–60°N and the presence of nitrogen between 30°S and 0°.

Constraints on the evolution of the Triton atmosphere from occultations: 1989–2022

Context. In about 2000, the south pole of Triton experienced an extreme summer solstice that occurs every ∼650 years, when the subsolar latitude reached about 50°S. Bracketing this epoch, a few occultations probed the Triton atmosphere in 1989, 1995, 1997, 2008, and 2017. A recent ground-based stellar occultation observed on 6 October 2022 provides a new measurement of the atmospheric pressure on Triton. This is presented here. Aims. The goal is to constrain the volatile transport models (VTMs) of the Triton atmosphere. The atmosphere is basically in vapor pressure equilibrium with the nitrogen ice at its surface. Methods. Fits to the occultation light curves yield the atmospheric pressure of Triton at the reference radius 1400 km, from which the surface pressure is deduced. Results. The fits provide a pressure p1400 = 1.211 ± 0.039 μbar at radius 1400 km (47 km altitude), from which a surface pressure of psurf = 14.54 ± 0.47 μbar is deduced (1σ error bars). To within the error bars, this is identical to the pressure derived from the previous occultation of 5 October 2017, p1400 = 1.18 ± 0.03 μbar and psurf = 14.1 ± 0.4 μbar, respectively. Based on recent models of the volatile cycles of Triton, the overall evolution of the surface pressure over the last 30 years is consistent with N2 condensation taking place in the northern hemisphere. However, models typically predict a steady decrease in the surface pressure for the period 2005-2060, which is not confirmed by this observation. Complex surface-atmosphere interactions, such as ice albedo runaway and formation of local N2 frosts in the equatorial regions of Triton, could explain the relatively constant pressure between 2017 and 2022.

Observations of Gravity Waves in the Middle Atmosphere of Mars

Abstract Gravity waves are ubiquitous throughout the atmosphere of Mars. Their propagation and dissipation influence the circulation and thermal structure of the middle and upper atmosphere. Yet there have been few studies of gravity wave characteristics in the middle atmosphere, a region that is critical for their propagation from generation in the lower atmosphere to dissipation and associated exchanges of momentum and energy in the upper atmosphere. There have been few studies because few atmospheric profiles span the middle atmosphere with the kilometer-scale or smaller vertical resolution that is Required to characterize gravity waves. Here we report the characterization of gravity waves in the middle atmosphere using 12 high-resolution atmospheric density profiles. Four of these were acquired from a ground-based stellar occultation from 1976 that yielded immersion and emersion profiles on opposite sides of the planet, and the remaining eight were measured during atmospheric entry by landers and rovers. Predominant wavelengths were 3–14 km, and amplitudes were generally 0.8%–2.5%. Where static stability is large and positive, gravity waves grow efficiently. In other instances, static stability is not large and positive over a wide altitude range, and gravity wave amplitudes do not behave as neatly. These observations of gravity waves in the middle atmosphere of Mars can be used to test gravity wave parameterizations in large-scale general circulation models and to investigate predictions for how gravity wave propagation and dissipation influence the circulation and thermal structure of the middle and upper atmosphere.

Stellar Occultation by the Resonant Trans-Neptunian Object (523764) 2014 WC510 Reveals a Close Binary TNO

Abstract We report on the stellar occultation by (523764) 2014 WC510 observed on 2018 December 1 UT. This occultation campaign was part of the Research and Education Collaborative Occultation Network (RECON), a network of small telescopes spread over 2000 km in western USA and Canada. Light curves from six stations revealed three groups of two or more consecutive flux drops correlated in time between adjacent stations. A Bayesian model comparison reveals that a model with a double object occulting a double star is favored over alternative models considered. For the statistically favored model, we determined that the primary component of the object has a diameter d p = 181 ± 16 km and the secondary d s = 138 ± 32 km, assuming identical geometric albedo between the two components. The two components have a projected separation of 349 ± 26 km. Adopting an absolute magnitude for the system of H V = 7.2 from the Minor Planet Center, we derive a geometric albedo of p V = 5.1% ± 1.7%. This is the smallest resonant object with an occultation size measurement and with a detected secondary from a ground-based stellar occultation, filling a region of the size versus separation parameter space of binary objects that is largely unexplored. The results show the capabilities of the unique design of the RECON experiment sensitive to small objects and close binaries. 2014 WC510 is presently at a low galactic latitude where the high surface density of stars will provide good occultation opportunities in the upcoming years.

Global climate model occultation lightcurves tested by August 2018 ground-based stellar occultation

Pluto's atmospheric profiles (temperature and pressure) have been studied for decades from stellar occultation lightcurves. In this paper, we look at recent Pluto Global Climate Model (GCM) results (3D temperature, pressure, and density fields) from Bertrand et al. (2020) and use the results to generate model observer's plane intensity fields (OPIF) and lightcurves by using a Fourier optics scheme to model light passing through Pluto's atmosphere (Young, 2012). This approach can accommodate arbitrary atmospheric structures and 3D distributions of haze. We compared the GCM model lightcurves with the lightcurves observed during the 15-AUG-2018 Pluto stellar occultation. We find that the climate scenario which best reproduces the observed data includes a N2 ice mid latitude band in the southern hemisphere. We have also studied different haze and P/T ratio profiles: the haze effectively reduces the central flash strength, and a lower P/T ratio both reduces the central flash strength and incurs anomalies in the shoulders of the central flash.

Lower atmosphere and pressure evolution on Pluto from ground-based stellar occultations, 1988–2016

Context. The tenuous nitrogen (N2) atmosphere on Pluto undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has recently (July 2015) been observed by the New Horizons spacecraft. Aims. The main goals of this study are (i) to construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) to constrain the structure of the lower atmosphere using a central flash observed in 2015. Methods. Eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between altitude levels of ~5 and ~380 km (i.e. pressures from ~ 10 μbar to 10 nbar). Results. (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived. (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia; and/or (b) hazes with tangential optical depth of ~0.3 are present at 4–7 km altitude levels; and/or (c) the nominal REX density values are overestimated by an implausibly large factor of ~20%; and/or (d) higher terrains block part of the flash in the Charon facing hemisphere.

Lower atmosphere and pressure evolution on Pluto from ground-based stellar occultations, 1988-2016

Lower atmosphere and pressure evolution on Pluto from ground-based stellar occultations, 1988-2016

A prediction method for ground-based stellar occultations by ellipsoidal solar system bodies and its application

A new programmable prediction method is developed to refine the occultation band by taking into consideration the triaxiality of an occulting body, as well as two more factors, namely, the barycenter offset of an occulting planet from the relevant planetary satellite system and the gravitational deflection of light rays due to an occulting planet. Although these factors can be neglected in most cases, it is shown that there are cases when these factors can cause a variation ranging from several tens to thousands of kilometers in the boundaries of occultation bands. Knowledge of analytic geometry simplifies the process of derivation and computation. This method is applied to long-term predictions of Jovian and Saturnian events.

Radio occultation measurements of Pluto’s neutral atmosphere with New Horizons

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°N — near the center of the Charon-facing hemisphere. Above 1215 km radius (∼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 N2. 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 N2 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.

An atmospheric general circulation model for Pluto with predictions for New Horizons temperature profiles

Results are presented from a 3-D Pluto general circulation model (GCM) that includes conductive heating and cooling, non-local thermodynamic equilibrium (non-LTE) heating by methane at 2.3 and 3.3 microns, non-LTE cooling by cooling by methane at 7.6 microns, and LTE CO rotational line cooling. The GCM also includes a treatment of the subsurface temperature and surface-atmosphere mass exchange. An initially 1 m thick layer of surface nitrogen frost was assumed such that it was large enough to act as a large heat sink (compared with the solar heating term) but small enough that the water ice subsurface properties were also significant. Structure was found in all three directions of the 3-D wind field (with a maximum magnitude of order 10 m/s in the horizontal directions and 10$^-5$ microbar/s in the vertical direction). Prograde jets were found at several altitudes. The direction of flow over the poles was found to very with altitude. Broad regions of up-welling and down-welling were also found. Predictions of vertical temperature profiles are provided for the Alice and REX instruments on New Horizons, while predictions of light curves are provided for ground-based stellar occultation observations. With this model methane concentrations of 0.2% and 1.0% and 8 and 24 microbar surface pressures are distinguishable. For ground-based stellar occultations, a detectable difference exists between light curves with the different methane concentrations, but not for different initial global mean surface pressures.

Structure of self-gravity wakes in Saturn's A ring as measured by Cassini CIRS

The CIRS infrared spectrometer onboard the Cassini spacecraft has scanned Saturn's A ring azimuthally from several viewing angles since its orbit insertion in 2004. A quadrupolar asymmetry has been detected in this ring at spacecraft elevations ranging between 16° to 37°. Its fractional amplitude decreases from 22% to 8% from 20° to 37° elevations. The patterns observed in two almost complete azimuthal scans at elevations 20° and 36° strongly favor the self-gravity wakes as the origin of the asymmetry. The elliptical, infinite cylinder model of Hedman et al. [Hedman, M.M., Nicholson, P.D., Salo, H., Wallis, B.D., Buratti, B.J., Baines, K.H., Brown, R.H., Clark, R.N., 2007. Astron. J. 133, 2624–2629] can reproduce the CIRS observations well. Such wakes are found to have an average height-to-spacing ratio H / λ = 0.1607 ± 0.0002 , a width-over-spacing W / λ = 0.3833 ± 0.0008 . Gaps between wakes, which are filled with particles, have an optical depth τ G = 0.1231 ± 0.0005 . The wakes mean pitch angle Φ W is 70.70 ° ± 0.07 ° , relative to the radial direction. The comparison of ground-based visible data with CIRS observations constrains the A ring to be a monolayer. For a surface mass density of 40 g cm −2 [Tiscarino, M.S., Burns, J.A., Nicholson, P.D., Hedman, M.M., Porco, C.C., 2007. Icarus 189, 14–34], the expected spacing of wakes is λ ≈ 60 m . Their height and width would then be H ≈ 10 m and W ≈ 24 m , values that match the maximum size of particles in this ring as determined from ground-based stellar occultations [French, R.G., Nicholson, P.D., 2000. Icarus 145, 502–523].

The Thermal Structure of Triton's Atmosphere: Results from the 1993 and 1995 Occultations

This paper presents new results about Triton's atmospheric structure from the analysis of all ground-based stellar occultation data recorded to date, including one single-chord occultation recorded on 1993 July 10 and nine occultation lightcurves from the double-star event on 1995 August 14. These stellar occultation observations made both in the visible and in the infrared have good spatial coverage of Triton, including the first Triton central-flash observations, and are the first data to probe the altitude level 20–100 km on Triton. The small-planet lightcurve model of J. L. Elliot and L. A. Young (1992,Astron. J.103,991–1015) was generalized to include stellar flux refracted by the far limb, and then fitted to the data. Values of the pressure, derived from separate immersion and emersion chords, show no significant trends with latitude, indicating that Triton's atmosphere is spherically symmetric at ∼50-km altitude to within the error of the measurements; however, asymmetry observed in the central flash indicates the atmosphere is not homogeneous at the lowest levels probed (∼20-km altitude). From the average of the 1995 occultation data, the equivalent-isothermal temperature of the atmosphere is 47 ± 1 K and the atmospheric pressure at 1400-km radius (∼50-km altitude) is 1.4 ± 0.1 μbar. Both of these are not consistent with a model based on Voyager UVS and RSS observations in 1989 (D. F. Strobel, X. Zhu, M. E. Summers, and M. H. Stevens, 1996,Icarus120,266–289). The atmospheric temperature from the occultation is 5 K colder than that predicted by the model and the observed pressure is a factor of 1.8 greater than the model. In our opinion, the disagreement in temperature and pressure is probably due to modeling problems at the microbar level, since measurements at this level have not previously been made. Alternatively, the difference could be due to seasonal change in Triton's atmospheric structure.

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.).

Neptune's rings, 1983–1989: Ground-based stellar occultation observations: I. Ring-like arc detections

A systematic campaign of stellar occultation observations by Neptune was conducted by our group between 1983 and 1989, and led to the initial discovery of Neptune's rings. These observations provide 24 independent scans across Neptune's equatorial plane. Two of them give evidence for ring-like arcs around the planet. The two detections, made respectively on July 22, 1984 and August 20, 1985, are the only ones ever observed simultaneously by two or more telescopes. The corresponding lightcurves are analyzed using Neptune's pole position recently determined after the Voyager 2 spacecraft observations. Assuming that the arcs lie in Neptune's equatorial plane, the 1984 detection corresponds to material orbiting 65,300 ± 3000 km from the center of the planet, and the 1985 detection corresponds to material which lies at 63,160 ± 200 km, so that both events are compatible with the Voyager 2 observations of the three arc structures observed near the 63,000-km radius (we propose the names Liberty, Equality, and Fraternity for these arcs). The arc detected in 1984 has a radial width of W r = 15.1 ± 0.1 km, an azimuthal extension greater than 100 km, and a normal optical depth of 0.074 ± 0.003 (these quantities are projected in Neptune's equatorial plane). The arc detected in 1985 has a radial width of W r = 15.3 ± 0.2 km and a normal optical depth of 0.058 ± 0.001. Our observations do not show evidence for any additional ring-like arc, nor for any perceptible diffuse, narrow, or broad rings with normal optical depth larger than about 4 × 10 −3 near the 63,000- and 53,000-km radii, where the Voyager 2 spacecraft observed continuous Neptunian rings. Finally, a preliminary search on our best lightcurves near these two radii does not show any short, correlated event with equivalent width larger than ∼75 m.

Avoiding the last resort

Life was easy for solar system ring dynamicists before 1977. The most difficult dynamical problem at the time was to explain how the resonant action of the satellite Mimas could produce an obvious gap, the Cassini division, which separated the broad A and B rings of the only ringed planet, Saturn. The accidental discovery of nine narrow rings around Uranus in March 1977 using ground-based stellar occultation changed things. Narrow rings posed a severe dynamical problem since the particle collisions and an effect called Poynting-Robertson light drag cause unconfined narrow rings to spread, giving them a maximum lifetime of only a few million years.