•LRO/LAMP UV spectrograph detected fluorescence of HeI 584Å in the lunar exosphere.•LAMP-derived He source rate is directly related to the solar wind α-particle flux.•LAMP-derived He surface density ...is consistent with LACE measurements in 1973.•These observations offer insight on He density on both latitude & local solar time.•These observations will help constraining models of lunar volatiles transport.
We present results from Lunar Reconnaissance Orbiter’s (LRO) UV spectrograph LAMP (Lyman-Alpha Mapping Project) campaign to study the lunar atmosphere. Several off-nadir maneuvers (lateral rolls) were performed to search for resonantly scattering species, increasing the illuminated line-of-sight (and hence the signal from atoms resonantly scattering the solar photons) compared to previously reported LAMP’s “twilight observations” (Cook, J.C., Stern, S.A. 2014. Icarus 236, 48–55). Helium was the only element distinguishable on a daily basis, and we present latitudinal profiles of its line-of-sight column density in December 2013. We compared the helium line-of-sight column densities with solar wind alpha particle fluxes measured from the ARTEMIS (Acceleration, Reconnection, Turbulence, & Electrodynamics of Moon’s Interaction with the Sun) twin spacecraft. Our data show a correlation with the solar wind alpha particle flux, confirming that the solar wind is the main source of the lunar helium. We also support the finding by Benna et al. (Benna, M. et al. 2015. Geophys. Res. Lett. 42, 3723–3729) and Hurley et al. (Hurley, D.M. et al. 2015. Icarus, this issue), that a non-zero contribution from endogenic helium, coming from radioactive decay of 232Th and 238U, is present. Moreover, our results suggest that not all of the incident alpha particles are converted to thermalized helium, allowing for a non-negligible fraction to escape as suprathermal helium or simply backscattered from the lunar surface. We compare LAMP-derived helium surface density with the one recorded by the mass spectrometer LACE (Lunar Atmospheric Composition Experiment) deployed on the lunar surface during the Apollo 17 mission, finding good agreement between the two measurements. The LRO/LAMP roll observations presented here are in agreement with the most recent lunar exospheric helium model (Hurley, D.M. et al. 2015. Icarus, this issue) around mid- to high-latitudes (50–70°) regardless of the local solar time, while there is an underestimation of the model around the low- to mid-latitudes (10–30°), especially around the dawn terminator. The LRO/LAMP roll observations presented here provide unique coverage of local solar time and latitude of the lunar exospheric helium, filling a gap in the knowledge of the structure of the lunar exosphere as a whole. These observations will inform future models of transport of volatiles, since at the terminator the analytic expressions for the surface temperature, essential to determine the energy distribution, the residence time, and the hop length of the particles, is least accurate.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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 reflectivity and in previous observations of Saturn's icy moons and rings. Rather than displaying a sharp change in the reflectivity at the wavelength of the water ice absorption edge, Ganymede's reflectance gradually increases with wavelength at λ > 165 nm. We show that the observed shape of Ganymede's UV reflectance is inconsistent with intimate mixture models of pure ice with UV‐dark materials including tholins, amorphous carbon, graphite, and silicates. However, we find that intraparticle models, in which a small proportion of a UV‐absorbing contaminant is trapped as inclusions within the ice matrix, are able to suppress the 165 nm feature at contaminant concentrations of <1%. We show that models of ice with inclusions of silicates, Triton‐type tholin, or H2O2 are able to produce the observed gradual increase in reflectivity at λ > 165 nm, but additional absorbing surface materials are required to produce a good fit to Ganymede's previously observed near‐UV and visible reflectance.
Plain Language Summary
We use the Hubble Space Telescope to study how Jupiter's moon Ganymede reflects sunlight at ultraviolet (UV) wavelengths. Ganymede's surface is known to contain significant amounts of water ice, which is very reflective at UV wavelengths longer than 165 nm, but reflects very little light at shorter wavelengths. We find that Ganymede does not show the same abrupt change in reflectivity at 165 nm as pure water ice. We use computer models that account for how light interacts with different materials to try and explain why we cannot see evidence of water ice in UV observations of Ganymede, even though we know ice is present there. We find that models that include pure water ice cannot explain our observations, but if we model ice containing a small fraction of impurities, we get a good match with Ganymede's UV reflectance. Models like the ones we produced for this study will be useful for interpreting observations of Ganymede and Jupiter's other icy moons, Europa and Callisto, by the ultraviolet instrument on the upcoming JUpiter ICy moons Explorer (JUICE) mission. Determining which impurities exist in icy moon surfaces helps us to understand the habitability of these worlds—JUICE's main scientific goal.
Key Points
Ganymede's UV reflectance spectrum lacks the sharp H2O feature at 165 nm observed at Saturn's icy moons
Adding small fractions (<1%) of contaminants within intraparticle models of H2O suppresses the 165 nm feature
The contaminant materials found to produce the best fit to Ganymede's UV spectrum were silicates, a Triton‐type tholin, and H2O2
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The LAMP far ultraviolet spectrograph aboard the NASA Lunar Reconnaissance Orbiter has been used in 2011 to search for helium, the lightest noble gas in the tenuous lunar atmosphere. Based on that ...search, we report here the first detection of lunar atmospheric He by remote sensing, and point to future observations that can address questions about its source; we also discuss a search for lunar atmospheric argon.
Key Points
Helium has been detected in the lunar atmosphere by remote sensing for the first
The amount of helium is close to what was detected in situ during Apollo
Future observations can be used to address questions about its source
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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 quantify this variation around Ganymede, we investigate the brightness averaged over 36°‐wide longitude corridors centered around the sub‐Jovian (0° W), leading (90° W), anti‐Jovian (180° W), and trailing (270° W) central longitudes. In the northern hemisphere, the brightness of the auroral oval is 3.7 ± 0.4 times lower in the sub‐Jovian and anti‐Jovian corridors compared to the trailing and leading corridors. The southern oval is overall brighter than the northern oval, and only 2.5 ± 0.2 times fainter on the sub‐ and anti‐Jovian corridors compared to the trailing and leading corridors. This demonstrates that Ganymede's auroral ovals are strongly structured in auroral crescents on the leading side (plasma downstream side) and on the trailing side (plasma upstream side). We also find that the brightness is not symmetric with respect to the 270° meridian, but shifted by ∼20° towards the Jovian‐facing hemisphere. Our map will be useful for subsequent studies to understand the processes that generate the aurora in Ganymede's non‐rotationally driven, sub‐Alfvénic magnetosphere.
Plain Language Summary
Northern lights often illuminate the night sky in a shimmering green or red tone at high geographic latitudes. This emission, scientifically referred to as aurora, is a result of electrically charged particles that move along Earth's magnetic field lines and interact with its atmosphere to produce auroral emission. Apart from the Earth, multiple other planets in our solar system also exhibit auroral emission. By characterizing the brightness and structure of these lights, we are therefore able to deduce insights about a planet's atmosphere, magnetic field and the physical processes occurring along the field lines from afar. In this work, we used observations from the Hubble Space Telescope to analyze the auroral emission of Jupiter's largest moon Ganymede. We combined multiple images of Ganymede to create the first complete map that displays the auroral brightness. Our map revealed that the emission on Ganymede's auroral ovals varies strongly in brightness with divisions into two distinct bright and faint regions. They resemble two auroral crescents in the north and south respectively, and demonstrate the uniqueness of Ganymede's aurora in comparison with the auroral ovals of other planets in the solar system.
Key Points
Brightness map of Ganymede's ultraviolet auroral emission has been constructed based on Hubble Space Telescope observations from 1998 to 2017
Auroral ovals are structured in upstream and downstream “crescents”
Brightness on sub‐Jovian and anti‐Jovian side is strongly reduced by a factor of 3–4 compared to upstream and downstream side
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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 investigate signatures of space weathering in different regions of Amundsen. We find that regions of the surface that receive large amounts of solar illumination and solar wind flux (e.g., the southern terrace walls) display high Lyman‐α albedos and blue spectral slopes in the 175–190‐nm region, indicative of increased regolith maturity due to solar wind weathering and thermal cycling. Amundsen's PSRs, however, receive no direct solar illumination and very little solar wind flux and have a lower albedo across Lyman Alpha Mapping Project's entire band pass (57–197 nm) than illuminated regions of the crater. We conclude that the low PSR albedos correspond to high regolith porosity in the PSRs. These PSRs are extremely cold regions with very minor thermal cycling. Thermal cycling might be a process that reduces porosity in regions of the crater exposed to a wide range of temperatures, thus increasing their albedos across the entire wavelength range. However, our analysis of the present data set was unable to uniquely identify its role. The low albedos in the PSRs may also result from extreme charging effects inside the PSRs, causing lofting and redeposition of dust, as well as dielectric breakdown, which would act to increase regolith porosity.
Plain Language Summary
Space weathering on the Moon is caused by the interaction of solar wind particles and micrometeoroids with the surface. The effects of these interactions can be investigated by analyzing the reflectance of the surface using data from the Lyman Alpha Mapping Project, an instrument onboard the Lunar Reconnaissance Orbiter. In this study, we analyzed one particular crater (Amundsen) near the lunar South Pole because it contains regions of high and low amounts of space weathering in close proximities. We found that regions of the crater that receive large amounts of sunlight and solar wind flux display signatures of space weathering, while regions that receive no sunlight and very little solar wind flux do not. We also found that the regions of the crater that receive no sunlight have high porosity in the upper layer of the soil, and presented a number of possible explanations for this finding.
Key Points
Highly illuminated regions of Amundsen crater display signatures of space weathering in the far‐UV
Amundsen's permanently shaded regions have low albedos across the full LAMP band pass, consistent with high regolith porosity
Differences in porosity between illuminated and shaded regions may be a result of thermal cycling, dielectric breakdown, and/or dust lofting
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The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star χ Ophiucus by Jupiter’s atmosphere on February 22 and 23, 2007 during the approach ...phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32°N latitude near the dawn terminator, while egress probed 18°N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be
3.4
-
2.8
+
9.0
×
10
6
cm
2
s
−1, consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. 1981. J. Geophys. Res. 86, 5717–5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. 1995. Icarus 114, 163–173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. 1996. J. Geophys. Res. 101 (E1), 2149–2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by
Festou et al. (1981) and Yelle et al. (1996) from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32°N latitude, the methane concentration is
3.1
-
0.5
+
0.5
×
10
8
cm
−3 at 70,397
km, the methane concentration is
1.2
-
0.3
+
0.3
×
10
9
cm
−3 at 70,383
km, the acetylene concentration is
1.4
-
0.2
+
0.4
×
10
8
cm
−3 at 70,364
km, and the ethane concentration is
6.8
-
0.8
+
1.1
×
10
8
cm
−3 at 70,360
km. At 18°N latitude, the methane concentration is
3.2
-
0.7
+
0.7
×
10
8
cm
−3 at 71,345
km, the methane concentration is
1.2
-
0.2
+
0.6
×
10
9
cm
−3 at 71,332
km, the acetylene concentration is
1.6
-
0.6
+
0.3
×
10
8
cm
−3 at 71,318
km, and the ethane concentration is
7.0
-
2.5
+
2.4
×
10
8
cm
−3 at 71,315
km. We also find that the H
2 occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by
in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. 1998. J. Geophys. Res. 103 (E10), 22857–22889) – the Sieff et al. temperature profile leads to too much absorption from H
2 at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter’s atmosphere.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Despite the numerous modeling efforts of the past, our knowledge on the radiation-induced physical and chemical processes in Europa’s tenuous atmosphere and on the exchange of material between the ...moon’s surface and Jupiter’s magnetosphere remains limited. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios and considerations has resulted in a fragmentary understanding of the interactions of the magnetospheric ion population with both the moon’s icy surface and neutral gas envelope. Models show large discrepancy in the source and loss rates of the different constituents as well as in the determination of the spatial distribution of the atmosphere and its variation with time. The existence of several models based on very different approaches highlights the need of a detailed comparison among them with the final goal of developing a unified model of Europa’s tenuous atmosphere. The availability to the science community of such a model could be of particular interest in view of the planning of the future mission observations (e.g., ESA’s JUpiter ICy moons Explorer (JUICE) mission, and NASA’s Europa Clipper mission). We review the existing models of Europa’s tenuous atmosphere and discuss each of their derived characteristics of the neutral environment. We also discuss discrepancies among different models and the assumptions of the plasma environment in the vicinity of Europa. A summary of the existing observations of both the neutral and the plasma environments at Europa is also presented. The characteristics of a global unified model of the tenuous atmosphere are, then, discussed. Finally, we identify needed future experimental work in laboratories and propose some suitable observation strategies for upcoming missions.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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 species, for example, SO2 and H2S, are not identifiable with the ultraviolet ratio‐temperature techniques and are not directly addressed in this study. We confidently detect a ∼20% increase in normalized Off‐band (175–190 nm) to On‐band (148–162 nm) albedo ratios (consistent with condensed surface volatiles) at temperatures below ∼115 K. Elevated normalized ratios extend to temperatures capable of supporting pure aforementioned ices over geologically long time scales. Although ∼115 K is consistent with H2O lifetimes of ∼1‐Myr, the presence of CO2 and NH3 are not uniquely delineated by the data trends with temperature. Future spectral modeling to appropriately identify the composition and abundance of these condensed volatiles remains necessary. Normalized albedo ratios are further analyzed for candidate species via maximum temperatures to inform the likelihood of ice signatures: H2O (70 K < T ≤ 115 K), NH3 + H2O (60 K < T ≤ 70 K), and CO2 + NH3 + H2O (T ≤ 60 K). We compare normalized albedo ratios across seven regions of interest (ROI), including Faustini, Shoemaker, Haworth, Cabeus, Amundsen, Nobile, and an unnamed region. Such comparisons allow for characterization of relative abundances of volatiles across the ROI important for their utilization in future crewed and robotic missions to the Moon.
Plain Language Summary
Spectra taken by the Lyman Alpha Mapping Project ultraviolet spectrograph and surface temperature inferred from the Diviner radiometer (both onboard the Lunar Reconnaissance Orbiter) are used to investigate the presence of ices within craters at the Moon's south pole. We analyze spectral features at temperatures where H2O, NH3, and CO2 ice are stable. We find signatures consistent with condensed surface volatiles at temperatures below the H2O ice stability temperature, but are unable to confidently delineate NH3 and CO2 signatures. We further find that normalized signatures can be used to estimate the relative abundance of ice species across regions of interest. Volatile studies, such as the one presented here, are important for resource utilization in future crewed and robotic missions to the Moon.
Key Points
Cold traps may support volatile reserves, which may be useful for resource utilization and for determining the origin of lunar volatiles
An increase in volatile signatures where H2O is thermodynamically stable is found; CO2, and NH3 are not confidently delineated
Normalized albedo differences provide insight on relative volatile species abundances between cold traps of interest
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We report far ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived ...brightnesses at the oxygen lines at 1304 Å and 1356 Å are consistent with zero signals within the 1σ propagated statistical uncertainties. The OI 1304 Å brightness of 0.12 ± 0.20 Rayleighs can be explained by solar resonant scattering from an atomic oxygen column density of (8.2 ± 13.4) × 1010 cm−2. Assuming that O is produced by photodissociation of H2O, we derive an upper limit for H2O abundance and compare it to previous observations. Our upper limit is well above the expected O brightness for a tenuous sublimated H2O exosphere, but it suggests that H2O production with a rate higher than 4 × 1026 molecules s−1 was not present at the time of the COS observation. Additionally, we derive an extremely low geometric albedo of ∼1% in the 1300 Å to 1400 Å range.
Key Points
New unprecedented far ultraviolet observations at Ceres with the Hubble Space Telescope
Upper limits for oxygen emission are derived and related to upper limits for oxygen abundance
The found upper limit for O suggests that H2O production is not higher than during previous observations
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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