Periodicities in the Io plasma torus

We present a 6-month baseline of spatially resolved measurements of the Io plasma torus intensity and perpendicular ion temperature which we use to determine the periodicities of the torus during this time. We find large anticorrelated variations in the intensity and ion temperature which are periodic with the Jovian rotation rate (System III). The intensity variations are found to be a simple manifestation of the temperature variations, though no explanation for the temperature variations is apparent. Periodogram analysis shows an additional intensity periodicity which rotates 2.91±0.06% more slowly than System III. This period is found only in the intensity and not in the ion temperature. We conclude from these observations that the torus has a sinusoidal ion temperature variation locked into the rotation of Jupiter and that superimposed on this is a long-lived density pattern which rotates 2.91% more slowly than Jupiter. Based on the spatial structure and physical properties, we rule out all currently proposed mechanisms for the creation of these periodicities within the torus

The 3-4 μ\mum Spectra of Jupiter Trojan Asteroids

To date, reflectance spectra of Jupiter Trojan asteroids have revealed no distinctive absorption features. For this reason, the surface composition of these objects remains a subject of speculation. Spectra have revealed, however, that the Jupiter Trojan asteroids consist of two distinct sub-populations which differ in the optical to near-infrared colors. The origins and compositional differences between the two sub-populations remain unclear. Here we report the results from a 2.2-3.8 $\mu$m spectral survey of a collection of 16 Jupiter Trojan asteroids, divided equally between the two sub-populations. We find clear spectral absorption features centered around 3.1 $\mu$m in the less red population. Additional absorption consistent with expected from organic materials might also be present. No such features are see in the red population. A strong correlation exists between the strength of the 3.1 $\mu$m absorption feature and the optical to near-infrared color of the objects. While traditionally absorptions such as these in dark asteroids are modeled as being due to fine-grain water frost, we find it physically implausible that the special circumstances required to create such fine-grained frost would exist on a substantial fraction of the Jupiter Trojan asteroids. We suggest, instead, that the 3.1 $\mu$m absorption on Trojans and other dark asteroids could be due to N-H stretch features. Additionally, we point out that reflectivities derived from WISE observations show a strong absorption beyond 4$\mu$m for both populations. The continuum of 3.1 $\mu$m features and the common absorption beyond 4 $\mu$m might suggest that both sub-populations of Jupiter Trojan asteroids formed in the same general region of the early solar system.Comment: AJ, in pres

Multiband photometry of a Patroclus-Menoetius mutual event: Constraints on surface heterogeneity

We present the first complete multiband observations of a binary asteroid mutual event. We obtained high-cadence, high-signal-to-noise photometry of the UT 2018 April 9 inferior shadowing event in the Jupiter Trojan binary system Patroclus-Menoetius in four Sloan bands $-$ $g'$, $r'$, $i'$, and $z'$. We use an eclipse lightcurve model to fit for a precise mid-eclipse time and estimate the minimum separation of the two eclipsing components during the event. Our best-fit mid-eclipse time of $2458217.80943^{+0.00057}_{-0.00050}$ is 19 minutes later than the prediction of Grundy et al. (2018); the minimum separation between the center of Menoetius' shadow and the center of Patroclus is $72.5\pm0.7$ km $-$ slightly larger than the predicted 69.5 km. Using the derived lightcurves, we find no evidence for significant albedo variations or large-scale topographic features on the Earth-facing hemisphere and limb of Patroclus. We also apply the technique of eclipse mapping to place an upper bound of $\sim$0.15 mag on wide-scale surface color variability across Patroclus.Comment: 5 pages, 3 figures, accepted for publication in A

Observational constraints on the orbit and location of Planet Nine in the outer solar system

We use an extensive suite of numerical simulations to constrain the mass and orbit of Planet Nine, the recently proposed perturber in a distant eccentric orbit in the outer solar system. We compare our simulations to the observed population of aligned eccentric high semimajor axis Kuiper belt objects and determine which simulation parameters are statistically compatible with the observations. We find that only a narrow range of orbital elements can reproduce the observations. In particular, the combination of semimajor axis, eccentricity, and mass of Planet Nine strongly dictates the semimajor axis range of the orbital confinement of the distant eccentric Kuiper belt objects. Allowed orbits, which confine Kuiper belt objects with semimajor axis beyond 380~AU, have perihelia roughly between 150 and 350~AU, semimajor axes between 380 and 980~AU, and masses between 5 and 20 Earth masses. Orbitally confined objects also generally have orbital planes similar to that of the planet, suggesting that the planet is inclined approximately 30~degrees to the ecliptic. We compare the allowed orbital positions and estimated brightness of Planet Nine to previous and ongoing surveys which would be sensitive to the planet's detection and use these surveys to rule out approximately two-thirds of the planet's orbit. Planet Nine is likely near aphelion with an approximate brightness of $22<V<25$. At opposition, its motion, mainly due to parallax, can easily be detected within 24 hours.Comment: ApJL, in press. Added stronger semimajor axis, eccentricity, and mass constraints for Planet Nin

The Plane of the Kuiper Belt

We present a robust method for measuring the effective plane of the Kuiper belt. The derived plane has an inclination with respect to the ecliptic of 1º.86 and an ascending node of 81º.6, with a 1 σ error in pole position of the plane of 0º.37. The plane of the Kuiper belt is inconsistent with the invariable plane, the plane of Jupiter, and the plane of Neptune at the greater than 3 σ level. Using linear secular perturbation theory, we show that the plane of the Kuiper belt is expected to oscillate about the position of the invariable plane with a period of 1.9 million years and an amplitude of 1º.2. The present predicted position of the plane of the Kuiper belt has an inclination with respect to the ecliptic of 1º.74 and an ascending node of 86º.7, within 0º.20 of our measured position

Early Dynamical Evolution of the Solar System: Pinning Down the Initial Condition of the Nice Model

In the recent years, the "Nice" model of solar system formation has attained an unprecedented level of success in reproducing much of the observed orbital architecture of the solar system by evolving the planets to their current locations from a more compact configuration. Within the context of this model, the formation of the classical Kuiper belt requires a phase during which the ice giants have a high eccentricity. An outstanding question of this model is the initial configuration from which the Solar System started out. Recent work has shown that multi-resonant initial conditions can serve as good candidates, as they naturally prevent vigorous type-II migration. In this paper, we use analytical arguments, as well as self-consistent numerical N-body simulations to identify fully-resonant initial conditions, whose dynamical evolution is characterized by an eccentric phase of the ice-giants, as well as planetary scattering. We find a total of eight such initial conditions. Four of these primordial states are compatible with the canonical "Nice" model, while the others imply slightly different evolutions. The results presented here should prove useful in further development of a comprehensive model for solar system formation.Comment: 10 pages, 8 figures, 4 tables. Accepted to the Astrophysical Journal

Orbital clustering in the distant solar system

The most distant Kuiper belt objects appear to be clustered in longitude of perihelion and in orbital pole position. To date, the only two suggestions for the cause of these apparent clusterings have been either the effects of observational bias or the existence of the distant giant planet in an eccentric inclined orbit known as Planet Nine. To determine if observational bias can be the cause of these apparent clusterings, we develop a rigorous method of quantifying the observational biases in the observations of longitude of perihelion and orbital pole position. From this now more complete understanding of the biases we calculate that the probability that these distant Kuiper belt objects would be clustered as strongly as observed in both longitude of perihelion and in orbital pole position is only 0.2%. While explanations other than Planet Nine may someday be found, the statistical significance of this clustering is now difficult to discount

Planetesimals to brown dwarfs: What is a planet?

The past 15 years have brought about a revolution in our understanding of our Solar System and other planetary systems. During this time, discoveries include the first Kuiper belt objects (KBOs), the first brown dwarfs, and the first extrasolar planets. Although discoveries continue apace, they have called into question our previous perspectives on planets, both here and elsewhere. The result has been a debate about the meaning of the word "planet" itself. It is clear that scientists do not have a widely accepted or clear definition of what a planet is, and both scientists and the public are confused (and sometimes annoyed) by its use in various contexts. Because "planet" is a very widely used term, it seems worth the attempt to resolve this problem. In this essay, we try to cover all the issues that have come to the fore and bring clarity (if not resolution) to the debate

Europa's Optical Aurora

Auroral emissions provide opportunities to study the tenuous atmospheres of Solar System satellites, revealing the presence and abundance of molecular and atomic species as well as their spatial and temporal variability. Far-UV aurorae have been used for decades to study the atmospheres of the galilean satellites. Here we present the first detection of Europa's visible-wavelength atomic oxygen aurora at 6300/6364 \AA{} arising from the metastable O$(^1$D) state, observed with the Keck I and Hubble Space Telescopes while Europa was in eclipse by Jupiter on six occasions in February-April 2018. The disk-integrated O($^1$D) brightness varies from $<$500 R up to more than 2 kR between dates, a factor of 15 higher than the OI 1356 \AA{} brightness on average. The ratio of emission at 6300/5577 \AA{} is diagnostic of parent molecule; the 5577 \AA{} emission was not detected in our dataset, which favors O$_2$ as the dominant atmospheric constituent and rules out an O/O$_2$ mixing ratio above 0.35. For an O$_2$ atmosphere and typical plasma conditions at Europa's orbit, the measured surface brightness range corresponds to column densities of 1-9$\times$10$^{14}$ cm$^{-2}$

Observational bias and the clustering of distant eccentric Kuiper belt objects

The hypothesis that a massive Planet Nine exists in the outer solar system on a distant eccentric orbit was inspired by observations showing that the objects with the most distant eccentric orbits in the Kuiper belt have orbits which are physically aligned, that is, they are clustered in longitude of perihelion and have similar orbital planes. Questions have remained, however, about the effects of observational bias on these observations, particularly on the longitudes of perihelion. Specifically, distant eccentric Kuiper belt objects tend to be faint and only observable near their perihelia, suggesting that the longitudes of perihelion of the known distant objects could be strongly biased by the limited number of locations in the sky where deep surveys have been carried out. We have developed a method to rigorously estimate the longitude of perihelion bias for Kuiper belt observations. We find that the probability that the 10 known Kuiper belt objects with semimajor axis beyond 230 AU are drawn from a population with uniform longitude of perihelion is 1.2%. Combined with the observation that the orbital poles of these object are also clustered, the overall probability of detecting these two independent clusterings in a randomly distributed sample is 0.025%. While observational bias is clearly present in these observations, it is unlikely to explain the observed alignment of the distant eccentric Kuiper belt objects.Comment: AJ, in pres