Observations

Far-ultraviolet (FUV) spectra of the permanently shadowed regions (PSRs) of the Moon acquired by the Lyman Alpha Mapping Project (LAMP) instrument on board the Lunar Reconnaissance Orbiter show relative reddening (> 155 nm) that is best explained by the presence of water ice mixed in with the lunar regolith. Similarly, LAMP dayside observations reveal variations in the FUV spectral slopes also attributed to the diurnal/latitudinal variation of hydration at the lunar surface.

Key Questions:
How much water content do the LAMP FUV hydration signatures correspond to?

The abundance and the spatial distribution of the lunar water is loosely constrained, mainly due to the scarcity of laboratory far-ultraviolet (FUV) reflectance data of Apollo soils and soil-ice aggregates. For instance, the dayside abundance of hydrated species, H₂O or hydroxyl, is estimated at 0-1%, while PSR water abundance are slightly higher,  < 2% if present as an intimate ice-soil mixture or as high as 10% if present as frost.

A detailed understanding of the physico-chemical properties of the lunar soil and volatiles and an accurate assessment of its water abundance and distribution are critical to the in-situ utilization of these resources in future explorations.

Our Effort

Figure 1

We characterize the FUV photometric response of dry and water vapor-exposed lunar simulants and Apollo soils in the Southwest Ultraviolet Reflectance Chamber (SwURC). The SwURC is a high vacuum chamber coupled to a scanning grating monochromator. The rotating grating in concert with slits selects monochromatic light over a 115-200 nm bandpass. This light is incident upon the sample tray which hosts the lunar simulants or Apollo soils samples. The SwURC also houses a CsI-coated channeltron detector, which is installed on a rotating mount and traces a circular track in the principal plane measuring the power of diffuse light reflected by the simulant or Apollo soils at emission angles e spanning -70° to +75° with respect to the zenith (0° polar angle). The optomechanical layout of the SwURC is shown in Figure 1 to the left.

A key feature of the SwURC is the ability to measure absolute bidirectional reflectance made possible by (i) retractable design of the sample tray and (ii) extended dynamic range of the channeltron detector.

Figure 2

We have recently measured phase curves of Apollo soil 10084 and derived key parameters such as the single scattering albedo and scattering phase functions from fits of the simplified Hapke model (Figure 2).

For additional details, click here.

Future Work

We plan to extend such measurements to other Apollo soils samples, but also to further reduce the measured single scattering albedo to optical constants to enable radiative transfer modeling of LAMP albedos to accurately estimate the lunar hydration abundance. We also plan to expose cooled lunar simulants and Apollo soils to precisely known amounts of water vapor to directly establish the relation between the degree of spectral reddening to the amount of water ice present in the soil.

Key Questions

“Radiolytically sputtered” O2 and CO2 exospheres from reactions of surface H2O ice with carbon-bearing ‘organic contaminants’ are common at the Saturnian (Figure 1) and Jovian icy moons. However some icy Ocean Worlds such as Europa have even more diverse surface constituents, possibly including complex oceanic organics and salt-laden material erupted through the icy crust, as well as gaseous and particulate cryovolcanic plume fallout (Figure 2).

What exospheric molecules do these complex ices ‘give off’ when sputtered by Jupiter’s magnetosphere, and in what amounts? What is the dependence on surface composition and irradiation parameters, such as particle energy, dose, and (ion) mass? Even though these questions are essential to understanding exospheric origins at icy moons, and are of rapidly escalating importance to NASA’s research and space programs, they have not been quantitatively and systematically addressed in the laboratory.

Figure 1: Charon’s unique red polar albedo as revealed by the Multispectral Visible Imaging Camera (MVIC) instrument onboard New Horizons.

New Horizons observation and Key Question

Charon’s dark-red colored north polar zone, discovered (Figure 1) by the New Horizon’s spacecraft’s Multispectral Visible Imaging Camera (MVIC), may be one of the most visible examples of exospheric cold trapping in the entire Solar System, owing to the combined effect of Charon’s surface temperature, extreme seasonality, radiation environment, and gravitational interaction with Pluto (see below). By weaving together novel laboratory experiments, state-of-the-art exospheric model, and radiative-transfer photometric model, we seek to understand the physics within the Pluto-Charon system responsible for the unique red albedo on this enigmatic moon.

The Red Material: What is it?

The possibility of reddish ‘tholin-like’ material, synthesized by Ly-a photolysis of cryo-condensed methane gravitationally captured from Pluto by Charon, was proposed soon after the 2015 New Horizons encounter. Methane frozen out of Charon’s exosphere is converted into complex, heavier hydrocarbons by solar Lyman-α light backscattered by the interplanetary hydrogen onto the polar night zone (see by Grundy et al., 2016). Subsequent processing by solar wind and other energetic sources could further transform the stickier hydrocarbons into the red-colored refractory tholins, the details of which we will reveal using modeling and laboratory experiments.

CLASSE Contribution

Building on the seminal work of the New Horizons team, we have combined a Charon exosphere model with novel ‘dynamic’ photolysis experiments to obtain for the first time a distribution of complex photolytic hydrocarbons on Charon’s surface from ~ century-long exposure to Lyman-α. Our models reveal Charon’s exosphere to be subject to extreme seasonal dynamics, as spring sunrise rapidly drives the methane frost off the poles and into the exosphere near every equinox. However, exospheric methane liberated from the spring polar zone is quickly recaptured onto the cooling autumnal polar surface as it recedes into winter night. This seasonal methane swapping between the polar zones has been explored in depth for the first time with our exosphere research. For more details, read our manuscript published in Geophysical Research Letters.

Our novel ‘dynamic photolysis’ experiments provide new limits on the contribution of interplanetary Lyman α to the synthesis of the Charon’s red material. These experiments, during which methane condensation is carried out in an ultra-high vacuum chamber while under exposure to Lyman-α photons, are designed to replicate more accurately than any previous experiments the conditions of Charon’s polar night zones. An important finding from these experiments is that the flux ratio of CH4 molecules to Lyman-α photons is the key determinant of photoproduct yield and refractory tholin composition. The photo-conversion cross section extrapolated from these ultra-realistic experiments is fed back into the exosphere model to estimate the photolytic refractory distribution across Charon’s surface. For additional details, read our manuscript published in Science Advances.

Figure 2: Mystical purple glow in our microwave discharge lamp used in ‘dynamic photolysis’ experiments where methane films were photolyzed during accretion. These experiments simulate the IPM Ly-α photolysis of methane cryo-trapped on Charon’s winter pole with high fidelity.

Next Steps

Our research combining spacecraft data, laboratory experiments, and exospheric modeling, has produced insightful discoveries vital to weaving a complete and consistent understanding of the origins of Charon’s remarkable red albedo.

Yet to be explored is the question of how the solar wind affects the composition and color of Charon’s hydrocarbons. With continued support from NASA’s New Horizons Data Analysis and other scientific research programs, the next phase of our experiments, in tandem with newly upgraded exospheric models, will seek to answer this and other essential Charon questions.

Observation and Motivation

Volatiles at airless bodies, especially water (H2O), are key to several of NASA’s top-level goals, including: Origins, Workings, and Human Exploration. There are spacecraft and ground-based observations of airless bodies that are interpreted as volatiles, often water, at the surfaces of these bodies. However, at the Moon different data sets appear to disagree on exactly where volatiles are found. Laboratory investigations can reveal the fundamental behavior of volatiles interacting with regolith surfaces, aka “gas surface interactions”. Incorporation of the measured gas-surface interactions into exosphere models can help to better reproduce the measurements of exospheres, which enables better prediction of accumulation of volatiles into cold traps.

A second motive for exploring gas-surface interactions at airless bodies is to understand how robotic and/or human activities on the lunar surface alter the surface materials. Key questions include: what is the level and nature of contamination by exhaust from landing and from surface operations? What is the rate and character of volatiles released from the regolith by landing and by surface operations?

Our Effort

We are implementing novel approaches to quantifying gas-surface interactions in progressively more Lunar-like environments and operational scenarios.