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Register a new userJWST observations of stellar occultations by solar system bodies and rings
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P. Santos-Sanz
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In this paper we investigate the opportunities provided by the James Webb Space Telescope (JWST) for significant scientific advances in the study of solar system bodies and rings using stellar occultations. The strengths and weaknesses of the stellar occultation technique are evaluated in light of JWSTs unique capabilities. We identify several possible JWST occultation events by minor bodies and rings, and evaluate their potential scientific value. These predictions depend critically on accurate a priori knowledge of the orbit of JWST near the Sun-Earth Lagrange-point 2 (L2). We also explore the possibility of serendipitous stellar occultations by very small minor bodies as a by-product of other JWST observing programs. Finally, to optimize the potential scientific return of stellar occultation observations, we identify several characteristics of JWSTs orbit and instrumentation that should be taken into account during JWSTs development.
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Two narrow and dense rings (called C1R and C2R) were discovered around the Centaur object (10199) Chariklo during a stellar occultation observed on 2013 June 3. Following this discovery, we planned observations of several occultations by Chariklos system in order to better characterize the physical properties of the ring and main body. Here, we use 12 successful occulations by Chariklo observed between 2014 and 2016. They provide ring profiles (physical width, opacity, edge structure) and constraints on the radii and pole position. Our new observations are currently consistent with the circular ring solution and pole position, to within the $pm 3.3$ km formal uncertainty for the ring radii derived by Braga-Ribas et al. The six resolved C1R profiles reveal significant width variations from $sim 5$ to 7.5 km. The width of the fainter ring C2R is less constrained, and may vary between 0.1 and 1 km. The inner and outer edges of C1R are consistent with infinitely sharp boundaries, with typical upper limits of one kilometer for the transition zone between the ring and empty space. No constraint on the sharpness of C2Rs edges is available. A 1$sigma$ upper limit of $sim 20$ m is derived for the equivalent width of narrow (physical width <4 km) rings up to distances of 12,000 km, counted in the ring plane.
The aim of the chapter is to summarize our understanding of the compositional distribution across the different reservoirs of small bodies (main belt asteroids, giant planet trojans, irregular satellites of the giant planets, TNOs, comets). We then use this information to i) discuss current dynamical models (Nice and Grand Tack models), ii) mention possible caveats in these models if any, and iii) draw a preliminary version of the primordial compositional gradient across the solar system before planetary migrations occured. Note that the composition of both planetary satellites (the regular ones) and that of the transient populations (NEOs, centaurs) is not discussed here. We strictly focus on the composition of the main reservoirs of small bodies. The manuscripts objective is to provide a global and synthetic view of small bodies compositions rather than a very detailed one, for specific reviews regarding the composition of small bodies, see papers by Burbine (2014) for asteroids, Emery et al. (2015) for Jupiter trojans, Mumma and Charnley (2011) for comets, and Brown (2012) for KBOs.
In terms of scientific output, the best way to study solar system bodies is sending spacecraft to make in-situ measurements or to observe at close distance. Probably, the second best means to learn about important physical properties of solar system objects is through stellar occultations. By combining occultation observations from several sites, size and shape can be derived with kilometric accuracy. Also, atmospheric properties can be derived if the body has an atmosphere. Furthermore, the technique can detect rings and even satellites (although rarely) around the main body. Except for the very special cases of Pluto and Charon, stellar occultations by Transneptunian Objects (TNOs) had never been observed until October 2009. This was because the ephemeris of the TNOs have much larger uncertainties than their angular diameters (typically of the order of ~10 milliarcsecond) and also because stellar catalogs were not accurate to the milliarcsecond level. Despite the difficulties, at the time of this writing, 43 occultations by 22 different Trans-Neptunian Objects, and 17 occultations by 5 Centaurs have been detected thanks to the efforts of several teams. Due to the complications of accurately predicting and observing these events, most of the successes have been achieved through wide international collaboration, which is a key issue to succeed in observing stellar occultations by TNOs. Multichord occultations are typically detected at a rate of ~3 per year on average, whereas the majority of the observed occultations are single-chord detections, which means that only one site detects the occultation. In these cases, no tight constraints on size and shape can be derived from those observations alone. Here we review most of the aspects involved in the complex process to successfully observe occultations, and present some of the lessons learned.
The Centaur (10199) Chariklo has the first rings system discovered around a small object. It was first observed using stellar occultation in 2013. Stellar occultations allow the determination of sizes and shapes with kilometre accuracy and obtain characteristics of the occulting object and its vicinity. Using stellar occultations observed between 2017 and 2020, we aim at constraining Chariklos and its rings physical parameters. We also determine the rings structure, and obtain precise astrometrical positions of Chariklo. We predicted and organised several observational campaigns of stellar occultations by Chariklo. Occultation light curves were measured from the data sets, from which ingress and egress times, and rings width and opacity were obtained. These measurements, combined with results from previous works, allow us to obtain significant constraints on Chariklos shape and rings structure. We characterise Chariklos ring system (C1R and C2R), and obtain radii and pole orientations that are consistent with, but more accurate than, results from previous occultations. We confirmed the detection of W-shaped structures within C1R and an evident variation of radial width. The observed width ranges between 4.8 and 9.1 km with a mean value of 6.5 km. One dual observation (visible and red) does not reveal any differences in the C1R opacity profiles, indicating ring particles size larger than a few microns. The C1R ring eccentricity is found to be smaller than 0.022 (3-sigma), and its width variations may indicate an eccentricity higher than 0.005. We fit a tri-axial shape to Chariklos detections over eleven occultations and determine that Chariklo is consistent with an ellipsoid with semi-axes of 143.8, 135.2 and 99.1 km. Ultimately, we provided seven astrometric positions at a milliarcseconds accuracy level, based on Gaia EDR3, and use it to improve Chariklos ephemeris.
This brief review will discuss the current knowledge on the origin and evolution of the nitrogen atmospheres of the icy bodies in the solar system, particularly of Titan, Triton and Pluto. An important tool to analyse and understand the origin and evolution of these atmospheres can be found in the different isotopic signatures of their atmospheric constituents. The $^{14}$N/$^{15}$N ratio of the N$_2$-dominated atmospheres of these bodies serve as a footprint of the building blocks from which Titan, Triton and Pluto originated and of the diverse fractionation processes that shaped these atmospheres over their entire evolution. Together with other measured isotopic and elemental ratios such as $^{12}$C/$^{13}$C or Ar/N these atmospheres can give important insights into the history of the icy bodies in the solar system, the diverse processes that affect their N$_2$-dominated atmospheres, and the therewith connected solar activity evolution. Titans gaseous envelope most likely originated from ammonia ices with possible contributions from refractory organics. Its isotopic signatures can yet be seen in the - compared to Earth - comparatively heavy $^{14}$N/$^{15}$N ratio of 167.7, even though this value slightly evolved over its history due to atmospheric escape and photodissociation of N$_2$. The origin and evolution of Plutos and Tritons tenuous nitrogen atmospheres remain unclear, even though it might be likely that their atmospheres originated from the protosolar nebula or from comets. An in-situ space mission to Triton such as the recently proposed Trident mission, and/or to the ice giants would be a crucial cornerstone for a better understanding of the origin and evolution of the icy bodies in the outer solar system and their atmospheres in general.
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