Evidence for a Localised Source of the Argon in the Lunar Exosphere

Evidence for a Localised Source of the Argon in the Lunar Exosphere

Abstract

We perform the first tests of various proposed explanations for observed features of the Moon’s argon exosphere, including models of: spatially varying surface interactions; a source that reflects the lunar near-surface potassium distribution; and temporally varying cold trap areas. Measurements from the Lunar Atmosphere and Dust Environment Explorer (LADEE) and the Lunar Atmosphere Composition Experiment (LACE) are used to test whether these models can reproduce the data. The spatially varying surface interactions hypothesized in previous work cannot reproduce the persistent argon enhancement observed over the western maria. They also fail to match the observed local time of the near-sunrise peak in argon density, which is the same for the highland and mare regions, and is well reproduced by simple surface interactions with a ubiquitous desorption energy of 28 kJ mol. A localised source can explain the observations, with a trade-off between an unexpectedly localised source or an unexpectedly brief lifetime of argon atoms in the exosphere. To match the observations, a point-like source requires source and loss rates of  atoms s. A more diffuse source, weighted by the near-surface potassium, requires much higher rates of  atoms s, corresponding to a mean lifetime of just 1.4 lunar days. We do not address the mechanism for producing a localised source, but demonstrate that this appears to be the only model that can reproduce the observations. Large, seasonally varying cold traps could explain the long-term fluctuation in the global argon density observed by LADEE, but not that by LACE.

Jacob A. Kegerreis, Vincent R. Eke, Richard J. Massey, Simon K. Beaumont, Rick C. Elphic, and Luís F. Teodoro

Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK Centre for Sustainable Chemical Processes, Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, UK NASA Ames Research Center, Moffett Field, CA, USA BAER/NASA Ames Research Center, Moffett Field, CA, USA

\@footnotetext

Corresponding author: Jacob A. Kegerreis, jacob.kegerreis@durham.ac.uk


Key Points:

  • We test various proposed explanations for observed features of the lunar argon exosphere.

  • Explaining the “bulge” in argon density over the maria requires either a highly localised source or rapid turnover of argon.

  • Seasonally varying cold traps could explain the long-term variation in the global argon density observed by LADEE.

1 Introduction

The Moon possesses our nearest example of a surface-bounded exosphere, the most common type of atmosphere in the solar system. As the atoms constituting an exosphere do not interact with one another during their ballistic trajectories over the surface, different species form independent systems. Their exospheric densities and variation with local time depend upon the sources, sinks, and surface interactions for that particular species. Hence, studying the lunar exosphere has the potential to teach us about the solar wind, the lunar interior and outgassing, the efficiency of volatile sequestration in polar cold traps, and the kinetics of adsorption and desorption in low pressure environments (Stern1999; Watson+1961; Wieler+Heber2003).

Argon is a particularly well-studied species in the lunar exosphere, having been first detected by the Lunar Atmosphere Composition Experiment (LACE), which measured the Ar/Ar ratio at the surface to be approximately (Hoffman+1973). This implied that the more important source of argon was radioactive decay of K to Ar, rather than solar-wind derived Ar. The LACE results showed that the argon exospheric density decreased through the night and had a rapid increase that began just before sunrise, typical of a condensible gas that adsorbs to the cold nighttime surface and desorbs at dawn (Hodges+Johnson1968). In addition to this daily variation, there was a longer term decrease by a factor of 2 seen during the nine lunar days of observations (Hodges1975).

The Lunar Atmosphere and Dust Environment Explorer (LADEE) orbital mission produced a wealth of data concerning the lunar exosphere at altitudes from 3–140 km (Elphic+2014). As well as measuring the daily and long-term variations in argon density during its 5 month mission, the Neutral Mass Spectrometer (NMS, Mahaffy+2014) also determined the vertical structure of the exosphere and the variation with selenographic longitude. This led to the discovery that there was an enhancement in the argon exospheric density over the western maria, dubbed the argon “bulge” by Benna+2015. The long-term variation in the argon abundance was 28% during the LADEE mission, much smaller than had been seen 40 years earlier by LACE over similar time periods. However, Hodges+Mahaffy2016 noted that “the absence of sensitivity-related tests of the Apollo 17 instrument allows the possibility that the 1973 results were in part artefacts.”

Different models of aspects of the lunar Ar system have been created to help interpret the available data, both in terms of the outgassing rate from the surface and the corresponding sinks that are necessary to yield the measured exospheric density. Hodges1975 used the LACE data and Monte Carlo methods to simulate an argon exosphere to constrain both the source rate and the surface interactions. Grava+2015 also employed a Monte Carlo technique to follow an initial injection of argon atoms through their lifetimes in the exosphere, concluding that approximately 10% of the area of permanently shadowed regions (PSRs, Mazarico+2011) is needed to cold trap atoms in order to provide a sufficiently high loss rate to match the LACE long-term decline in argon exospheric density. If a continuous background source had been included in their model, then larger cold traps would have been required to deplete the exospheric argon density rapidly enough.

Using their model, Grava+2015 suggested that long-term variations in the exospheric density can be ascribed to sporadic moonquakes. Benna+2015 noted the possibility of tidal stress as the source of the LADEE variation. In contrast, Hodges+Mahaffy2016 proposed that seasonal fluctuations in the total cold trap area are responsible for the smooth, mission-long variations in argon density measured by LADEE (Benna+2015).

More than one proposed explanation also exists for the bulge – the persistent enhancement of exospheric argon localised over the western maria. Benna+2015 noted the similarity between the longitudinal variation in argon and the map of near-surface potassium returned by the Lunar Prospector Gamma Ray Spectrometer (LPGRS, Lawrence+1998), suggesting a localised source. However, Hodges+Mahaffy2016 asserted that the lifetimes of argon atoms in the lunar exosphere are too long for them to reflect their source locations. Instead, they suggested that the bulge results from lower desorption energies at these longitudes, which would cause argon to spend less time residing on the surface where it cannot be measured.

In this paper, we develop a new model of the lunar argon exosphere using Monte Carlo methods. This approach is similar to those described by Smith+1978, Hodges1980a, and Butler1997, in their studies of the helium and water exospheres of the Moon and Mercury. We apply our algorithm to address the questions of which – if any – of the proposed models could be responsible for the longitudinal and long-term variations in the argon densities measured by LADEE. Specifically, we produce the first simulations with: spatially varying surface interactions; a source that reflects the lunar near-surface potassium distribution; and seasonally varying cold trap areas.

Section 2 contains a description of the data set used and an overview of the different aspects of the lunar argon exosphere that the LADEE data can constrain. Our model is outlined in section LABEL:sec:model, and the results and their implications for the source, sinks and regolith interactions of argon atoms are described in section LABEL:sec:results.

2 Data

The NMS on LADEE measured the density of argon (and other species) in the lunar exosphere from 22nd November 2013 to 17th April 2014 at a wide range of altitudes, longitudes, and local times of day at latitudes within of the equator. Derived data, including background-subtracted argon number densities at altitude, were obtained from The Planetary Atmospheres Node of NASA’s Planetary Data System.

We apply two cuts to the entire LADEE argon data set, to produce the subset of data used here. In the full data set, any densities that are negative after the background subtraction (due to noise) have had their values set to zero, which causes the mean to be artificially high if these are either included as zeros or discarded. We only use data in bins of local time of day and selenographical longitude for which more than half of the observations are positive. In this way, the medians of the resulting sets of measurements should not be biased by this prior treatment of negative values. Also, an unaccounted-for temperature dependence of the instrument background can affect the densities just after midnight (M. Benna, personal communication, 2016). This problem is only important at this local time of day and for very low densities, where the instrument background was large compared with the signal. We therefore discard data at local times of day 180 (where 0 is noon, 90 is sunset, 180 is midnight, and 270 is sunrise). Fortunately, the LACE data largely fill the overnight gap, and we supplement the LADEE measurements using the results presented by Hodges1975.

Four complementary aspects of the argon exosphere can readily be studied: (1) the change in density with altitude; (2) the long-term variation in the global density during the months of LADEE’s operation; (3) the density distribution with local time of day; and (4) the dependence on selenographical longitude, showing the bulge over the western maria (Benna+2015).

2.1 Densities at Altitude

In order to study the final three of these distributions, we need to account for the measurement altitude varying from 3 to 140 km. Following convention, and for comparisons with the LACE data, we convert all measurements to the corresponding densities that would have been measured at the surface. To do this, we consider the expression derived by Chamberlain1963 linking the number density in an exosphere as a function of height, , to the number density and temperature at the surface, and respectively. For a spherical body with mass and radius , {linenomath*}

(\theequation)

where is the mass of the particle, is the gravitational constant, and is the Boltzmann constant. This is the generalised form of the (isothermal) barometric law. We refer to this as a “Chamberlain distribution”. The altitude dependence used by Benna+2015, which results from assuming a constant acceleration due to gravity, is the first-order expansion of Eqn. (2.1) for small .

The surface temperature varies as a function of latitude and local time, with a particularly rapid variation around the terminators. Lateral transport of molecules implies that the density at altitude will not reflect only the single sub-detector surface temperature, because particles reaching the detector will have originated at a variety of locations and temperatures. Therefore, we expect the real distribution to be a sum of many Chamberlain distributions for different temperatures, weighted by the number of particles that come from each one. As a practical model, we approximate this distribution with a sum of just two Chamberlain distributions at different temperatures and find the best-fit parameters at all times of day using our simulations, as detailed in section. LABEL:app:model:altitude.

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