Kurt Retherford

Southwest Research Institute · Space Science and Engineering Division

NASA’s Europa Clipper mission is designed to provide a diversity of measurements to further our understanding of the potential habitability of this intriguing ocean world. The Europa mission’s Ultraviolet Spectrograph (Europa-UVS), built at the Southwest Research Institute (SwRI), is primarily a “plume finder” and tenuous atmosphere investigation....

The Lunar Reconnaissance Orbiter Lyman-Alpha Mapping Project (LAMP) has been mapping the Moon since its launch in 2009. Faint ultraviolet illumination of the lunar dark side includes light from stars and from hydrogen Ly α emissions, mostly attributed to sunlight scattered by hydrogen atoms near the Sun with a smaller contribution from the whole Ga...

Our understanding of solar system evolution is closely tied to interpretations of asteroid composition, particularly the M-class asteroids. These asteroids were initially thought to be the exposed cores of differentiated planetesimals, a hypothesis based on their spectral similarity to iron meteorites. However, recent astronomical observations have...

We present the state of the art on the study of surfaces and tenuous atmospheres of the icy Galilean satellites Ganymede, Europa and Callisto, from past and ongoing space exploration conducted with several spacecraft to recent telescopic observations, and we show how the ESA JUICE mission plans to explore these surfaces and atmospheres in detail wi...

Our understanding of Solar System evolution is closely tied to interpretations of asteroid composition, particularly the M-class asteroids. These asteroids were initially thought to be the exposed cores of differentiated planetesimals, a hypothesis based on their spectral similarity to iron meteorites. However, recent astronomical observations have...

The JUpiter ICy moons Explorer (JUICE) of ESA was launched on 14 April 2023 and will arrive at Jupiter and its moons in July 2031. In this review article, we describe how JUICE will investigate the interior of the three icy Galilean moons, Ganymede, Callisto and Europa, during its Jupiter orbital tour and the final orbital phase around Ganymede. De...

Jupiter’s icy moon, Europa, harbors a subsurface liquid water ocean; the prospect of this ocean being habitable motivates further exploration of the moon with the upcoming NASA Europa Clipper mission. Key among the mission goals is a comprehensive assessment of the moon’s composition, which is essential for assessing Europa’s habitability. Through...

The goal of NASA’s Europa Clipper mission is to assess the habitability of Jupiter’s moon Europa. After entering Jupiter orbit in 2030, the flight system will collect science data while flying past Europa 49 times at typical closest approach distances of 25–100 km. The mission’s objectives are to investigate Europa’s interior (ice shell and ocean),...

Ultraviolet spectroscopy is a powerful method to study planetary surface composition through reflectance measurements, atmospheric composition through stellar/solar occultations, transits of other planetary bodies, and direct imaging of airglow and auroral emissions. The next generation of ultraviolet spectrographs (UVS) on board ESA’s Jupiter Icy...

Geological investigations planned for the Europa Clipper mission will examine the formation, evolution, and expression of geomorphic structures found on the surface. Understanding geologic features, their formation, and any recent activity are key inputs in constraining Europa’s potential for habitability. In addition to providing information about...

Condensed volatiles within lunar permanently shadowed regions are of high scientific and resource utilization importance. Volatiles remain elusive and difficult to observe directly, due to low direct solar illumination. In this work, we investigate correlations between, as well as possible effects of, condensed volatiles and surface roughness. We a...

The Cassini Orbiter Ultraviolet Imaging Spectrograph (UVIS) obtained interplanetary hydrogen Ly α observations from 1999 to 2017, with mid-2004 to 2017 observations obtained from Saturn orbit. During its Saturn orbital phase, the spacecraft moved from mostly downwind and sidewind in the heliosphere to upwind. We analyze the full set of observations...

We present an overview of the radiation environment monitoring program planned for the Europa Clipper mission. The harsh radiation environment of Jupiter will be measured by a dedicated Radiation Monitor (RadMon) subsystem, yielding mission accumulative Total Ionizing Dose (TID) and instantaneous electron flux measurements with a 1-Hz cadence. The...

ESA’s Jupiter Icy Moons Explorer (JUICE) will provide a detailed investigation of the Jovian system in the 2030s, combining a suite of state-of-the-art instruments with an orbital tour tailored to maximise observing opportunities. We review the Jupiter science enabled by the JUICE mission, building on the legacy of discoveries from the Galileo, Cas...

The Galileo mission to Jupiter revealed that Europa is an ocean world. The Galileo magnetometer experiment in particular provided strong evidence for a salty subsurface ocean beneath the ice shell, likely in contact with the rocky core. Within the ice shell and ocean, a number of tectonic and geodynamic processes may operate today or have operated...

Aristarchus crater is a Copernican-age impact crater well known for its high-albedo ejecta blanket and nearby flow features rich in pyroclastic materials. The Lyman Alpha Mapping Project (LAMP) is a far-ultraviolet (FUV) spectrograph (57–197 nm) on board the Lunar Reconnaissance Orbiter (LRO). LAMP data identified Aristarchus crater as having a hig...

Nighttime Lyman Alpha Mapping Project (LAMP) observations are used to investigate condensed volatiles at the south polar region of the Moon. This study incorporates LAMP data from the first ∼7 years of the mission and Diviner annual maximum temperatures to search for volatile signatures associated with H2O, NH3, and CO2. Other stable potential spec...

In situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter provide indirect evidence that an enhancement of neutral gas is present near the orbit of the moon Europa. Simulations suggest that such a neutral gas torus can be sustained by escape from Europa’s atmosphere and consists primarily of molecular hydrogen,...

ESA's Jupiter Icy Moons Explorer (JUICE) will provide a detailed investigation of the Jovian system in the 2030s, combining a suite of state-of-the-art instruments with an orbital tour tailored to maximise observing opportunities. We review the Jupiter science enabled by the JUICE mission, building on the legacy of discoveries from the Galileo, Cas...

In-situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter provide indirect evidence that an enhancement of neutral gas is present near the orbit of the moon Europa. Simulations suggest that such a neutral gas torus can be sustained by escape from Europa's atmosphere and consists primarily of molecular hydrogen,...

Ultraviolet spectroscopy is a powerful method to study planetary surface composition through reflectance measurements and atmospheric composition through stellar/solar occultations, transits of other planetary bodies, and direct imaging of airglow and auroral emissions. The next generation of ultraviolet spectrographs (UVS) on board ESA's JUICE (Ju...

We report results from far-ultraviolet observations by the Hubble Space Telescope of Jupiter’s largest moon, Ganymede, transiting across the planet’s dayside hemisphere. Within a targeted campaign on 2021 September 9 two exposures were taken during one transit passage to probe for attenuation of Jupiter's hydrogen Ly α dayglow above the moon limb....

We report results from far-ultraviolet observations by the Hubble Space Telescope of Jupiter's largest moon Ganymede transiting across the planet's dayside hemisphere. {Within} a targeted campaign on 9 September 2021 two exposures were taken during one transit passage to probe for attenuation of Jupiter's hydrogen Lyman-$\alpha$ dayglow above the m...

We report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one....

We have characterized the far‐ultraviolet (FUV) spectro‐photometric response of lunar soil simulants JSC‐1A and LMS‐1, reporting notable differences from our previous results for Apollo soil 10084 (Raut et al., 2018, https://doi.org/10.1029/2018JE005567). While JSC‐1A and LMS‐1 were designed to emulate the geotechnical and compositional properties...

We employ nighttime observations from the Lyman Alpha Mapping Project (LAMP) ultraviolet (UV) spectrograph onboard the Lunar Reconnaissance Orbiter to investigate the presence of condensed water ice within lunar south pole cold traps. This study incorporates LAMP observations between 2009 and 2016, which more than doubles the number of observations...

With the wealth of missions selected to visit the lunar surface in the decade ahead, preparatory investigations into surface conditions are underway to explore potential challenges and science returns during these missions. One such mission, Lunar Vertex, is slated to explore a much-anticipated region–the lunar swirl and magnetic anomaly known as R...

We analyze Hubble Space Telescope observations of Ganymede made with the Space Telescope Imaging Spectrograph between 1998 and 2017 to generate a brightness map of Ganymede's oxygen emission at 1,356 Å. Our Mercator projected map demonstrates that the brightness along Ganymede's northern and southern auroral ovals strongly varies with longitude. To...

We analyze Hubble Space Telescope (HST) observations of Ganymede made with the Space Telescope Imaging Spectrograph (STIS) between 1998 and 2017 to generate a brightness map of Ganymede's oxygen emission at 1356 A. Our Mercator projected map demonstrates that the brightness along Ganymede's northern and southern auroral ovals strongly varies with l...

Returning humans to the Moon presents an unprecedented opportunity to determine the origin of volatiles stored in the permanently shaded regions (PSRs), which trace the history of lunar volcanic activity, solar wind surface chemistry, and volatile delivery to the Earth and Moon through impacts of comets, asteroids, and micrometeoroids. So far, the...

We present reflectance spectra of Ganymede's leading and trailing hemispheres in the wavelength range 138–215 nm, obtained by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014. The most notable feature of both spectra is the absence of a sharp water absorption edge at ~165 nm, seen in laboratory measurements of ice reflectivi...

The Alice UV spectrograph aboard NASA’s New Horizons mission is sensitive to MeV electrons that penetrate the instrument’s thin aluminum housing and interact with its microchannel plate detector. We have searched for penetrating electrons at heliocentric distance of 2–45 au, finding no evidence of discrete events outside of the Jovian magnetosphere...

The Alice UV spectrograph aboard NASA's New Horizons mission is sensitive to MeV electrons that penetrate the instrument's thin aluminum housing and interact with its microchannel plate detector. We have searched for penetrating electrons at heliocentric distance of 2-45 AU, finding no evidence of discrete events outside of the Jovian magnetosphere...

Several recent studies derived the existence of plumes on Jupiter’s moon Europa. The only technique that provided multiple detections is the far-ultraviolet imaging observations of Europa in transit of Jupiter taken by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope ( HST ). In this study, we reanalyze the three HST /S...

Using data from the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP), we investigate the spectral properties of rayed craters in the far‐ultraviolet (FUV). Because LAMP is sensitive to the uppermost layer of the lunar surface and regolith grains, it is ideal for characterizing regolith maturity and space weathering products suc...

Previous investigations proved the existence of local density enhancements in Europa’s atmosphere, advancing the idea of a possible origination from water plumes. These measurement strategies, however, were sensitive either to total absorption or atomic emissions, which limited the ability to assess the water content. Here we present direct searche...

A few hours after its encounter with Pluto, the Alice UV instrument on the New Horizons spacecraft observed the simultaneous stellar occultation and appulse of two UV-bright stars, HD 43153 and HD 42545, respectively. As during the solar occultation that occurred immediately prior, the transmission of starlight through Pluto’s atmosphere was sensit...

We observed the 2018 August 4 stellar occultation by the Kuiper Belt object (486958) 2014 MU 69 , the first close flyby target of the extended New Horizons mission. Rather than capture a solid-body occultation by the KBO itself, our program aimed to constrain the opacity of rings, moons, or other debris in the nearby environment. We used the Hubble...

We report results from a new technique for mapping Io’s SO 2 vapor distribution. The Space Telescope Imaging Spectrograph (STIS) instrument on the Hubble Space Telescope observed Io during four Jupiter transit events to obtain medium resolution far-UV spectral images near the Ly α wavelength of 121.6 nm. Jupiter’s bright Ly α dayglow provides a bri...

Unexpected dynamic phenomena have surprised solar system observers in the past and have led to important discoveries about solar system workings. Observations at the initial stages of these events provide crucial information on the physical processes at work. We advocate for long-term/permanent programs on ground-based and space-based telescopes of...

Unexpected dynamic phenomena have surprised solar system observers in the past and have led to important discoveries about solar system workings. Observations at the initial stages of these events provide crucial information on the physical processes at work. We advocate for long-term/permanent programs on ground-based and space-based telescopes of...

The lunar South Pole crater Amundsen is a prime location to study the effects of space weathering in the far ultraviolet. Amundsen's equator‐facing terrace walls are highly illuminated while the northern side of the crater has permanently shaded regions (PSRs). Using data from the Lunar Reconnaissance Orbiter Lyman Alpha Mapping Project, we investi...

Data from the Lunar Reconnaissance Orbiter Lyman Alpha Mapping Project and Diviner are consistent with surface water on the Moon varying in abundance with both terrain type and local time/temperature. A thermal desorption model including latitudinally varying desorption activation energy reproduces the observations. We interpret the observed variab...

The Lyman Alpha Mapping Project has detected five discrete low‐albedo anomalies in Lyman‐α (Ly‐α; 121.6 nm) nighttime reflectance maps. These anomalies reside on the nearside of the Moon within the southeastern Oceanus Procellarum and northwestern Mare Nubium, coincident with regions that have been observed to be photometrically anomalous at visibl...

Micro-Channel Plate (MCP) detectors can suffer from a form of degradation known as gain sag in regions with significant fluence. We have developed a method to recover the total Lyman-Alpha (Ly-α) emission line (121.6nm) flux for the Lyman-Alpha Mapping Project (LAMP) UV imaging spectrograph onboard of the Lunar Reconnaissance Orbiter (LRO), where g...

We present detailed photometric properties of the Moon at far ultraviolet wavelengths. The far ultraviolet data from the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) instrument were used to derive two Hapke photometric parameters, the single-scattering albedo, w, and the asymmetry factor, b, in the single-particle phase fun...

Recent observations of interplanetary medium atomic hydrogen Lyman-α emission in the outer solar system, made with the Alice ultraviolet spectrograph on New Horizons, are presented. The observations include regularly spaced great circle scans of the sky and pointed observations near the downstream and upstream flow directions of interstellar H atom...

Recent observations of interplanetary medium (IPM) atomic hydrogen Lyman-{\alpha} (Ly{\alpha}) emission in the outer solar system, made with the Alice ultraviolet spectrograph on New Horizons (NH), are presented. The observations include regularly spaced great-circle scans of the sky and pointed observations near the downstream and upstream flow di...

We observed the 2017 July 17 stellar occultation of HD 168233 by the Kuiper Belt Object (486958) 2014 MU69, the close flyby target of the extended New Horizons mission. Rather than capture a solid body occultation by the KBO itself, our program aimed to constrain the opacity of rings, moons, or other debris in the nearby environment. We used the Hu...

We observed the July 17, 2017 stellar occultation of HD 168233 by the Kuiper Belt Object (486958) 2014 MU$_{69}$, the close flyby target of the extended New Horizons mission. Rather than capture a solid body occultation by the KBO itself, our program aimed to constrain the opacity of rings, moons, or other debris in the nearby environment. We used...

We present an analysis of Europa's far-UV spectral albedo using observations during the 1999-2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk-integrated observations show that the far-UV spectrum in the ~130 to 170-nm range is relatively flat or slightly blue (increasing albedo with decreasing wa...

Context. Recent measurements by IBEX and detailed modeling have changed our understanding of the flow of the interstellar medium through the solar system. In particular, a time dependence of the direction of the interstellar medium flow has been proposed, and a new population of helium atoms, called the “warm breeze”, has been discovered. Aims. We...