Abstract
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 with our existing hot hydrogen density model with a solar illumination model most recently used to study Solar and Heliospheric Observatory Solar Wind Anisotropy Experiment data and selected Cassini UVIS observations from 2003 to 2004. We find general agreement between data and model, but with evidence for a decline in UVIS Ly
α
sensitivity, with a significant decline in 2002 June during a starburn event and an overall roughly linear decline in sensitivity. While earlier work by Pryor et al. fit the UVIS Ly
α
data from 2003 to 2004 with a hydrogen density in the outer heliosphere (but after filtration at outer heliospheric boundaries) of 0.085 cm
−3
using the UVIS laboratory sensitivity calibration, including the sensitivity decline found here leads to a revised hydrogen density estimate of
n
H
= 0.14 ± 0.03 cm
−3
. This density estimate is consistent with a recent neutral hydrogen density estimate near the termination shock of 0.127 ± 0.015 cm
−3
based on models of observations of pick-up hydrogen ions from the New Horizons spacecraft.
Abstract
Cassini UVIS interplanetary hydrogen Ly
α
measurements from 2003 to 2004, obtained from a heliospheric downwind to sidewind location on approach to Saturn and during the first Saturn orbit, ...are presented and fit by a heliospheric hot density model with solar illumination. Adding the recently proposed 43 ± 3 Rayleigh (
R
) isotropic galactic hydrogen Ly
α
signal derived from New Horizons data improves our model’s ability to fit the observed Cassini “27 day” signal modulations from both upwind and downwind directions. Our modeling of the UVIS data favors a galactic Ly
α
background of ∼40–100
R
over a model with no significant galactic background.
Although of great interest for science and resource utilization, the Moon's permanently shadowed regions (PSRs) near each pole present difficult targets for remote sensing. The Lyman Alpha Mapping ...Project (LAMP) instrument on the Lunar Reconnaissance Orbiter (LRO) mission is able to map PSRs at far‐ultraviolet (FUV) wavelengths using two faint sources of illumination from the night sky: the all‐sky Ly α glow produced as interplanetary medium (IPM) H atoms scatter the Sun's Ly α emissions, and the much fainter source from UV‐bright stars. The reflected light from these two sources produces only a few hundred events per second in the photon‐counting LAMP instrument, so building maps with useful signal‐to‐noise (SNR) ratios requires the careful accumulation of the observations from thousands of individual LRO orbits. In this paper we present the first FUV albedo maps obtained by LAMP of the Moon's southern and northern polar regions. The results show that (1) most PSR regions are darker at all FUV wavelengths, consistent with their surface soils having much larger porosities than non‐PSR regions (e.g., ∼70% compared to ∼40% or so), and (2) most PSRs are somewhat “redder” (i.e., more reflective at the longer FUV wavelengths) than non‐PSR regions, consistent with the presence of ∼1–2% water frost at the surface.
Key Points
New FUV albedo maps of the Moon's poles are presented
Most permanently shadowed regions (PSRs) have low FUV albedos
Most PSRs are relatively red at long FUV wavelengths
LRO-LAMP Observations of the LCROSS Impact Plume Gladstone, G. Randall; Hurley, Dana M; Retherford, Kurt D ...
Science (American Association for the Advancement of Science),
10/2010, Letnik:
330, Številka:
6003
Journal Article
Recenzirano
On 9 October 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) sent a kinetic impactor to strike Cabeus crater, on a mission to search for water ice and other volatiles expected to be ...trapped in lunar polar soils. The Lyman Alpha Mapping Project (LAMP) ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO) observed the plume generated by the LCROSS impact as far-ultraviolet emissions from the fluorescence of sunlight by molecular hydrogen and carbon monoxide, plus resonantly scattered sunlight from atomic mercury, with contributions from calcium and magnesium. The observed light curve is well simulated by the expansion of a vapor cloud at a temperature of approximately 1000 kelvin, containing approximately 570 kilograms (kg) of carbon monoxide, approximately 140 kg of molecular hydrogen, approximately 160 kg of calcium, approximately 120 kg of mercury, and approximately 40 kg of magnesium.
ABSTRACT
We report a comprehensive study by the UV spectrograph LAMP (Lyman-Alpha Mapping Project) onboard the Lunar Reconnaissance Orbiter to map the spatial distribution and temporal evolution of ...helium atoms in the lunar exosphere, via spectroscopy of the He i emission line at 58.4 nm. Comparisons with several Monte Carlo models show that lunar exospheric helium is fully thermalized with the surface (accommodation coefficient of 1.0). LAMP-derived helium source rates are compared to the flux of solar wind alpha particles measured in situ by the ARTEMIS twin spacecraft. Our observations confirm that these alpha particles (He++) are the main source of lunar exospheric helium, representing 79 per cent of the total source rate, with the remaining 21 per cent presumed to be outgassing from the lunar interior. The endogenic source rate we derive, (1.49 ± 0.08) × 106 cm−2 s−1, is consistent with previous measurements but is now better constrained. LAMP-constrained exospheric surface densities present a dawn/dusk ratio of ∼1.8, within the value measured by the Apollo 17 surface mass spectrometer LACE (Lunar Atmosphere Composition Experiment). Finally, observations of lunar helium during three Earth’s magnetotail crossings, when the Moon is shielded from the solar wind, confirm previous observations of an exponential decay of helium with a time constant of 4.5 d
We present an analysis of Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) measurements of the dayside lunar surface at far‐ultraviolet wavelengths. We use the strong 165 nm H2O ...absorption edge to look for diurnal variations in hydration. We find that diurnal variations in spectral slope are indeed present; they are superimposed on latitudinal and spatial variations related to composition and weathering. We use two different spectral regions (164–173 nm and 175–190 nm) to separate out these effects. Highlands and mare regions have distinct reflectance spectra, with mare regions being spectrally bluer than highlands regions, a consequence of the greater abundance of opaque minerals in mare regions. Bright ray terrains and areas known to be young such as Giordano Bruno crater, are found to be relatively spectrally flat or red in the far‐UV; this is consistent with a lack of space weathering, which tends to make the far‐UV spectrum bluer due to the spectral behavior of nanophase iron. Large‐scale latitudinal variations in FUV slope are distinct and are likely due to a gradient in space weathering. The diurnal variation in hydration is consistent with a solar wind origin and with loss of H2O at temperatures above ∼320 K. Far‐UV spectroscopy is thus shown to represent a viable method for mapping aqueous alteration, even on the dayside of the Moon, and potentially elsewhere in the solar system.
Key Points
The FUV water spectral feature is used to look for hydration on the Moon
Diurnally variable amounts of water and effects of space weathering are found
We use dayside data from the LRO/LAMP instrument
Abstract
Since 2007 the Alice spectrograph on the New Horizons (NH) spacecraft has been used to periodically observe the Lyman-
α
(Ly
α
) emissions of the interplanetary medium (IPM), which mostly ...result from resonant scattering of solar Ly
α
emissions by interstellar hydrogen atoms passing through the solar system. Three observations of IPM Ly
α
along a single great circle were made during the NH cruise to Pluto, and these have been supplemented by observations along six great circles (spread over the sky at 30° intervals), acquired one month before and one day after the NH flyby of Pluto, and on a further five occasions since then, out to just over 47 au from the Sun. These data indicate a distant Ly
α
background of 43 ± 3 Rayleigh brightness (equivalent to 56 ± 4 nW m
−2
sr
−1
), which is present in all directions (i.e., not only in the upstream direction, as previously reported). This result is found independently by: (1) the falloff with distance from the Sun of the IPM Ly
α
brightness observed by NH–Alice in several directions on the sky, and (2) the residual between the observed brightness and a model brightness accounting for the resonantly scattered solar Ly
α
component alone. The repeated observations show that this distant Ly
α
background is constant and uniform over the sky, and represents the local Galactic Ly
α
background. The observations show no strong correlation with the cloud structure of the local IPM. The observed brightness constrains the absorption coefficient of interstellar dust at Ly
α
to 0.2 ± 0.01 kpc
−1
.
The auroral footprint of Enceladus on Saturn PRYOR, Wayne R; RYMER, Abigail M; COATES, Andrew J ...
Nature (London),
04/2011, Letnik:
472, Številka:
7343
Journal Article, Web Resource
Recenzirano
Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling ...between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter's ionosphere. Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh (ref. 12), about an order of magnitude dimmer than Io's footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon's footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters--and as such is probably indicative of variable plume activity.
•Modeling of different initial spatial profile and velocity distribution shows how physical processes affect exospheric distribution.•We have detailed that the inventory from infalling ...micrometeoroids is not the prime contributor to H2 in the lunar exosphere.•The solar wind is a sufficient source of hydrogen that can supply the observed H2 exosphere by chemical sputtering of implant solar wind H.•To produce H2 density of 1200cm−3, 10% of the solar wind would need to be converted to H2.
We investigate the density and spatial distribution of the H2 exosphere of the Moon assuming various source mechanisms. Owing to its low mass, escape is non-negligible for H2. For high-energy source mechanisms, a high percentage of the released molecules escape lunar gravity. Thus, the H2 spatial distribution for high-energy release processes reflects the spatial distribution of the source. For low energy release mechanisms, the escape rate decreases and the H2 redistributes itself predominantly to reflect a thermally accommodated exosphere. However, a small dependence on the spatial distribution of the source is superimposed on the thermally accommodated distribution in model simulations, where density is locally enhanced near regions of higher source rate. For an exosphere accommodated to the local surface temperature, a source rate of 2.2gs−1 is required to produce a steady state density at high latitude of 1200cm−3. Greater source rates are required to produce the same density for more energetic release mechanisms. Physical sputtering by solar wind and direct delivery of H2 through micrometeoroid bombardment can be ruled out as mechanisms for producing and liberating H2 into the lunar exosphere. Chemical sputtering by the solar wind is the most plausible as a source mechanism and would require 10–50% of the solar wind H+ inventory to be converted to H2 to account for the observations.
The
Lyman Alpha Mapping Project
(LAMP) is a far-ultraviolet (FUV) imaging spectrograph on NASA’s
Lunar Reconnaissance Orbiter
(LRO) mission. Its main objectives are to (i) identify and localize ...exposed water frost in permanently shadowed regions (PSRs), (ii) characterize landforms and albedos in PSRs, (iii) demonstrate the feasibility of using natural starlight and sky-glow illumination for future lunar surface mission applications, and (iv) characterize the lunar atmosphere and its variability. As a byproduct, LAMP will map a large fraction of the Moon at FUV wavelengths, allowing new studies of the microphysical and reflectance properties of the regolith. The LAMP FUV spectrograph will accomplish these objectives by measuring the signal reflected from the night-side lunar surface and in PSRs using both the interplanetary HI Lyman-
α
sky-glow and FUV starlight as light sources. Both these light sources provide fairly uniform, but faint, illumination. With the expected LAMP sensitivity, by the end of the primary 1-year LRO mission, the SNR for a Lyman-
α
albedo map should be >100 in polar regions >1 km
2
, providing useful FUV constraints to help characterize subtle compositional and structural features. The LAMP instrument is based on the flight-proven
Alice
series of spectrographs flying on the
Rosetta
comet mission and the
New Horizons
Pluto mission. A general description of the LAMP instrument and its initial ground calibration results are presented here.
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