Vincent Hue

Jupiter’s interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft

Juno swoops around giant Jupiter Jupiter is the largest and most massive planet in our solar system. NASAs Juno spacecraft arrived at Jupiter on 4 July 2016 and made its first close pass on 27 August 2016. Bolton et al. present results from Junos flight just above the cloud tops, including images of weather in the polar regions and measurements of the magnetic and gravitational fields. Juno also used microwaves to peer below the visible surface, spotting gas welling up from the deep interior. Connerney et al. measured Jupiters aurorae and plasma environment, both as Juno approached the planet and during its first close orbit. Science, this issue p. 821, p. 826 Juno’s first close pass over Jupiter provides answers and fresh questions about the giant planet. On 27 August 2016, the Juno spacecraft acquired science observations of Jupiter, passing less than 5000 kilometers above the equatorial cloud tops. Images of Jupiter’s poles show a chaotic scene, unlike Saturn’s poles. Microwave sounding reveals weather features at pressures deeper than 100 bars, dominated by an ammonia-rich, narrow low-latitude plume resembling a deeper, wider version of Earth’s Hadley cell. Near-infrared mapping reveals the relative humidity within prominent downwelling regions. Juno’s measured gravity field differs substantially from the last available estimate and is one order of magnitude more precise. This has implications for the distribution of heavy elements in the interior, including the existence and mass of Jupiter’s core. The observed magnetic field exhibits smaller spatial variations than expected, indicative of a rich harmonic content.

Juno‐UVS approach observations of Jupiter's auroras

Abstract Juno ultraviolet spectrograph (UVS) observations of Jupiters aurora obtained during approach are presented. Prior to the bow shock crossing on 24 June 2016, the Juno approach provided a rare opportunity to correlate local solar wind conditions with Jovian auroral emissions. Some of Jupiters auroral emissions are expected to be controlled or modified by local solar wind conditions. Here we compare synoptic Juno‐UVS observations of Jupiters auroral emissions, acquired during 3–29 June 2016, with in situ solar wind observations, and related Jupiter observations from Earth. Four large auroral brightening events are evident in the synoptic data, in which the total emitted auroral power increases by a factor of 3–4 for a few hours. Only one of these brightening events correlates well with large transient increases in solar wind ram pressure. The brightening events which are not associated with the solar wind generally have a risetime of ~2 h and a decay time of ~5 h.

Spatial Distribution and Properties of 0.1 – 100 keV Electrons in Jupiter's Polar Auroral Region

We present observations of 0.1 – 100 keV electrons from Junos Jovian Auroral Distributions Experiment Electron instrument (JADE-E) over Jupiters polar auroral region for periods around four Juno perijoves (PJ1, PJ3, PJ4, PJ5). The observations reveal regions containing magnetic field aligned beams of bi-directional electrons having broad energy distributions interspersed between beams of upward electrons with narrow, peaked energy distributions, regions void of these electrons, and regions dominated by penetrating radiation. The electrons show evidence of acceleration via parallel electric fields (inverted-V structures) and via stochastic processes (bi-directional distributions). The inverted-V structures shown here were observed from ~1.4 – 2.9 RJ and had spatial scales of 100s to 1000s of kilometers along Junos trajectory. The upward electron energy flux was typically greater than the downward flux, the latter ranging between ~0.01 – 5 mW m-2 for two cases shown here which we estimate could produce ~0.1 – 50 kilorayleighs of ultraviolet emission.

Scientific rationale for Uranus and Neptune in situ explorations

The ice giants Uranus and Neptune are the least understood class of planets in our solar system but the most frequently observed type of exoplanets. Presumed to have a small rocky core, a deep interior comprising ~70% heavy elements surrounded by a more dilute outer envelope of H2 and He, Uranus and Neptune are fundamentally different from the better-explored gas giants Jupiter and Saturn. Because of the lack of dedicated exploration missions, our knowledge of the composition and atmospheric processes of these distant worlds is primarily derived from remote sensing from Earth-based observatories and space telescopes. As a result, Uranuss and Neptunes physical and atmospheric properties remain poorly constrained and their roles in the evolution of the Solar System not well understood. Exploration of an ice giant system is therefore a high-priority science objective as these systems (including the magnetosphere, satellites, rings, atmosphere, and interior) challenge our understanding of planetary formation and evolution. Here we describe the main scientific goals to be addressed by a future in situ exploration of an ice giant. An atmospheric entry probe targeting the 10-bar level, about 5 scale heights beneath the tropopause, would yield insight into two broad themes: i) the formation history of the ice giants and, in a broader extent, that of the Solar System, and ii) the processes at play in planetary atmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, with data returned to Earth using a Carrier Relay Spacecraft as a relay station. In addition, possible mission concepts and partnerships are presented, and a strawman ice-giant probe payload is described. An ice-giant atmospheric probe could represent a significant ESA contribution to a future NASA ice-giant flagship mission.

Understanding the Origin of Jupiter's Diffuse Aurora Using Juno's First Perijove Observations

Juno observed the low-altitude polar region during perijove 1 on 27 August 2016 for the first time. Auroral intensity and false-color maps from the UVS instrument show extensive diffuse aurora observed equatorward of the main auroral oval. Juno passed over the diffuse auroral region near the System III longitude of 120°–150° (90°–120°) in the northern (southern) hemisphere. In the region where these diffuse auroral emissions were observed, the JEDI and JADE instruments measured nearly full loss cone distributions for the downward-going electrons over energies of 0.1–700 keV, but very few upward-going electrons. The false-color maps from UVS indicate more energetic electron precipitation at lower latitudes than less energetic electron precipitation, consistent with observations of precipitating electrons measured by JEDI and JADE. The comparison between particle and aurora measurements provide first direct evidence that these precipitating energetic electrons are mainly responsible for the diffuse auroral emissions at Jupiter.

Stray and scattered light properties of the Juno ultraviolet spectrograph

We describe the stray and scattered light properties of the Juno Ultraviolet Spectrograph (Juno-UVS). Juno-UVS is a modest-powered (9.0 W) instrument that is designed to characterize Jupiter’s auroral emissions and relate them to in situ measurements made by Juno’s particle and wave instruments. A notable scattered light feature has been discovered during UVS operations; a minor solar glint that reveals itself during specific spacecraft orientations when the spin axis is pointed a certain angle away from the sun. This scattered light feature has become more important now that the Juno mission has decided to stay in its 53-day parking orbit instead of transitioning to the planned 14-day science orbit. The impact of the scattered light feature on future instrument operations is discussed.

In-flight characterization and calibration of the Juno-Ultraviolet Spectrograph (Juno-UVS)

The Juno mission is a NASA New Frontiers mission, orbiting Jupiter since 4 July 2016 and placed on a 53-day period, highly elliptical, polar orbit. The Ultraviolet Spectrograph onboard Juno (Juno-UVS) is a photoncounting imaging spectrograph, designed to cover the 68-210 nm spectral range.1 This range includes the H2 bands and the Lyman series produced in Jupiter’s far-ultraviolet (FUV) auroras. The purpose of Juno-UVS is to study Jupiter’s auroras from the unique vantage point above both poles allowed by Juno’s orbit, and to provide a wider auroral context for the in-situ particle and field instruments on Juno. Because of the 2 rpm spin of Juno, UVS nominally observes 7.5°x360° swaths of the sky during each spin of the spacecraft. The spatial resolutions along the slit and across the slit, i.e. in the spin direction, are respectively 0.16° and 0.2° , while the filled-slit spectral resolution is ∼1.3 nm.2 UVS borrows heavily from previous instruments led by Southwest Research Institute (New-Horizons and Rosetta Alices, LRO-LAMP), major improvements are: (i) an extensive radiation shielding; (ii) a scan mirror which allows targeting specific auroral features; and (iii) an improved cross-delay line readout scheme of the microchannel plate (MCP) detector. The ability offered by the scan mirror combined with Juno’s spin allows UVS access to half of the sky during every spacecraft rotation. This pointing flexibility, combined with the changing spin-axis of the spacecraft since launch, has allowed UVS to map 99 % of the sky in the 68-210 nm range. This paper describes the substantial number of spectra that have been used to monitor the health of the instrument over the course of the mission. More than 5800 spectra of mainly O, A, and B spectral-type stars in the V-magnitude range of ∼0-7 have been extracted to date. Selected stars among this list are used to calibrate the UVS instrument. This paper describes how previous spectral databases from the International Ultraviolet Explorer have been refined and adapted for UVS’ calibration purposes, in combination with observations from the Hubble Space Telescope. The retrieved effective area of the instrument peaks around 0.28 at ∼125 nm, with uncertainties lower than 10%.

Planning operations in Jupiter's high-radiation environment: optimization strategies from Juno-UVS

The Juno Ultraviolet Spectrograph (Juno-UVS) is a remote-sensing science instrument onboard the Juno spacecraft that has been in polar orbit around Jupiter since July 2016. Juno-UVS measures photon events in the ultraviolet from about 68 to 210 nm. It is primarily used to observe emission from the Jovian aurorae, but is also sensitive to other sources such as UV-bright stars, sky background Lyman-alpha emission, and reflected sunlight. However, Juno-UVS is also sensitive to the effects of penetrating high-energy radiation, which results in elevated count rates as measured by the instrument detector array. This radiation presents a challenge for efficiently planning the acquisition of mission science data, as data volume is a valuable (and finite) resource that can quickly be filled when the spacecraft periodically passes through regions of high radiation. This background radiation has been found to vary significantly on both short (spacecraft spin-modulated) time scales, as well as longer timescales from minutes to hours during each close approach to Jupiter. This variability has required a multi-pronged approach in the operation planning of hardware (such as dynamic instrument voltage adjustment) as well as onboard software (such as utilizing data quality factors for the selective storage of science data). We present an overview of these current mitigation/optimization techniques and planning strategies used for this instrument, which will likely also be useful for the development and operations of future instruments within high radiation space environments (e.g., the ESA JUICE mission or NASA’s Europa Clipper).

Seasonal stratospheric photochemistry on Uranus and Neptune

A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planets very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C2H2 and C2H6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation - which was not included in these models - may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.