Investigating Potential Sources of Mercury's Exospheric Calcium

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Investigating Potential Sources of Mercury’s Exospheric Calcium: Photon-Stimulated Desorption of Calcium Sulfide

Alice DeSimone – Georgia Institute of Technology et al.

Deposited 10/23/2018

Citation of published version: Bennett, C., et al. (2016): Investigating Potential Sources of Mercury’s Exospheric Calcium: Photon-Stimulated Desorption of Calcium Sulfide. Journal of Geophysical Research, 121(2). DOI: https://doi.org/10.1002/2015JE004966

RESEARCH ARTICLE Investigating potential sources of Mercury’s exospheric 10.1002/2015JE004966 Calcium: Photon-stimulated desorption Key Points: of Calcium Sulﬁde • Photon-stimulated desorption (PSD) of CaS is investigated at λ = 355 nm Chris J. Bennett1, Jason L. McLain2,3, Menelaos Sarantos3,4, Reuben D. Gann1, Alice DeSimone1, • Desorption cross sections and velocity 1 distributions are determined for Ca0 and Thomas M. Orlando • ’ PSD contributions to Mercury s 1 2 exospheric calcium are modeled Department of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, USA, Astronomy Department, University of Maryland, College Park, Maryland, USA, 3NASA Goddard Space Flight Center, Greenbelt, Maryland, USA, 4Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Baltimore, Maryland, USA Supporting Information: • Text S1 and Tables S1–S4 Abstract Ground-based and MErcury Surface, Space ENvironment, GEochemistry, and Ranging observations Correspondence to: 0 + ’ T. M. Orlando, detected Ca and Ca in the exosphere of Mercury as well as unexpectedly high levels of sulfur on Mercury s [email protected] surface. The mineral oldhamite ((Mg,Ca)S) could be a predominant component of the Mercury surface, particularly within the hollows identiﬁed within craters, and could therefore serve as a source of the

Citation: observed exospheric calcium. Laboratory measurements on the photon-stimulated desorption (PSD) of Bennett, C. J., J. L. McLain, M. Sarantos, CaS powder (an analog for oldhamite) at a wavelength of λ = 355 nm have been conducted, utilizing R. D. Gann, A. DeSimone, and resonance-enhanced multiphoton ionization time-of-flight mass spectrometry to determine the yields T. M. Orlando (2016), Investigating 0 0 fi potential sources of Mercury’s and velocity distributions of Ca . The desorbing Ca could be t using two Maxwell-Boltzmann components: exospheric Calcium: Photon-stimulated a 600 (±30) K thermal component and a 1389 (±121) K nonthermal component, the latter accounting desorption of Calcium Sulfide, – for ~25% of the observed signal. Cross sections for PSD using 3.4 eV photons were found to be J. Geophys. Res. Planets, 121, 137 146, 20 2 0 24 2 + doi:10.1002/2015JE004966. 1.1 (±0.7) × 10 cm for Ca and 3.2 (±0.9) × 10 cm for Ca . Adopting these cross sections, a Monte Carlo model of the release of Ca0 by PSD from the Tyagaraja crater finds the neutral microexosphere Received 29 OCT 2015 created from this process to be substantial even if only 1% CaS is assumed in the hollows. Diffuse Accepted 13 JAN 2016 reflectance UV-visible measurements were made on the CaS powder to determine a bandgap, E ,of Accepted article online 16 JAN 2016 g Published online 19 FEB 2016 2.81 (±0.14) eV via the Tauc method.

1. Introduction Mercury possesses a tenuous surface-bound exosphere which is thought to be replenished primarily through meteoritic impact-induced vaporization, photon-stimulated desorption (PSD), ion-stimulated desorption (ISD), electron-stimulated desorption (ESD), and volatile evaporation. The precise contribution of each of these processes is currently unknown [e.g., Killen et al., 2007; Domingue et al., 2007, 2014; McLain et al., 2011; Schriver et al., 2011; Schmidt et al., 2012; Burger et al., 2012]. Neutral calcium (Ca0) has been identified as a known component of the exosphere of Mercury since observations made with the Keck telescope during the summer of 1998 successfully detected the Ca-I emission line at 422.7 nm [Bida et al., 2000]. These and subsequent observations by Killen et al. [2005] found that the distribution of Ca0 was consistent with two sources: a global thermal source (T < 2000 K) and a localized nonthermal source (T > 20,000 K). Repeated observations of these nonthermal contributions over 4 years indicated that the source is stable over long periods, increasing by a factor of ~2 as the solar cycle approached maximum. Killen et al. [2005] interpreted the high temperatures as either a result of sputtering or the release of molecules such as calcium oxide CaO or CaS into the exosphere. Photodissociation of these molecules could potentially generate nonthermal Ca0 with kinetic energies of up to ~ 1.5 eV (T ~ 17,500 K) [Killen and Hahn, 2015]. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft carried out three initial flybys of Mercury in 2008–2009 and then entered orbital insertion in 2011. From this vantage point, it studied the planet until finally impacting the surface on 30 April 2015. During the mission, the UltraViolet and Visible Spectrometer (UVVS) channel of the Mercury Atmospheric and Surface Composition Spectrometer has been utilized to detect and characterize the distribution of several exospheric species, including Ca0, as well as its singly ionized counterpart, Ca+ [McClintock et al., 2008, 2009; Vervack et al., fl ©2016. American Geophysical Union. 2010; Burger et al., 2012, 2014]. The initial yby observations suggested the presence of an anisotropic source All Rights Reserved. of calcium that is enhanced in the dawn, equatorial region (displaced northward by 5°) of Mercury and that

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shows a seasonal dependence, consistent with a source rate of 1.2 to 2.6 × 1023 Ca atoms s 1. This assumes a hot Maxwell-Boltzmann temperature distribution (T ≥ 50,000 K) as the source. Most recently, Burger et al. [2014] fit several Mercury years of dayside Ca0 data equally well with a dawn-centered, nonthermal source (T ~ 70,000 K). Killen and Hahn [2015] showed that the seasonal dependence of the Ca0 source rate derived by Burger et al. [2014] may be consistent with micrometeoroid impact vaporization. These observations of highly nonthermal Ca0 exospheric emission are in contrast to those for neutral sodium (Na0). The source for Na0 is consistent with a temperature of T ~ 1200 K, with considerable enhancement observed at noon, compatible with a PSD mechanism [Burger et al., 2010; Cassidy et al., 2015]. Both ground observations of Ca0 and those made during the MESSENGER flybys have suggested that the data are insufficient to derive contributions from cooler sources (e.g., PSD), but that these sources could potentially be contributing to the observed Ca0 signal [Killen et al., 2005; Burger et al., 2012]. For instance, high spatial resolution radial profiles of the dayside Ca0 exosphere obtained by MESSENGER from Mercury orbit and analyzed by Burger et al. [2014] generally did not include altitudes < 100 km near dawn and altitudes <200–400 km at the subsolar point. Prior to the arrival of MESSENGER, ground-based observations predicted the presence of several calcium- containing species, including anorthosite, diopside, and oldhamite, among others [Sprague et al., 1995, 2002]. New data from MESSENGER’s instrument suite indicate the likely abundance and form in which calcium may reside on the surface of Mercury. Specifically, an absolute Ca abundance of 5.9 (±1.3)% wt has been identified using the Gamma Ray and Neutron Spectrometer (GRNS) on MESSENGER [Evans et al., 2012]. The X-ray spectrometer (XRS) also on board MESSENGER has detected Ca from 3.7 to 4.9% wt during solar flares [Nittler et al., 2011; Weider et al., 2012]. Emissions were also detected during electron-induced X-ray fluorescence events, and Ca abundances of 6.7 to 9.5% wt were derived [Starr et al., 2012]. Sulfur was detected by XRS at levels of 1.5 to 2.3% wt and by the GRNS at levels of 2.3 (±0.4)% wt, as well as by the XRS during electron events at abundances of 3.9 to 4.5% wt. Sulfur is volatile at the elevated temperatures of Mercury’s surface, so this high abundance for extended periods was not anticipated [Evans et al., 2012; Starr et al., 2012]. Data from the XRS, in particular, showed a strong correlation between the abundance of Ca and S, suggesting the presence of a material containing high levels of both, such as oldhamite ((Ca, Mg, Fe)S), on the surface [Nittler et al., 2011; Weider et al., 2012]. Sulfur-bearing species may originate from either differentiation of impact melt [Vaughan et al., 2012] or the uprising of high-Mg or high-Ca komatiitic magmas [Helbert et al., 2013], which could potentially form what is referred to as low-reflectance material (LRM). Interestingly, Nittler et al. [2011] inferred that the highest levels of both S and Ca were associated with the LRM regions. Additionally, there is substantial evidence that the vola- tilization and loss of sulfur-bearing species could be responsible for the transition between LRM regions to the high-reflectance hollow materials that are typically observed within impact craters, such as Tyagaraja [Blewett et al., 2013, and references therein]. The hollows are believed to be outgassing at present, perhaps due to sublimation. The presence of sulfides typically causes spectral reddening and darkening that is associated with the LRM material but not of the hollow material, which is easily identified as some of the freshest material on the surface of Mercury [Blewett et al., 2011]. Oldhamite is of particular interest since the calcium-rich end-member, CaS, is a known component of enstatite chondritic and achondritic meteorites [Larimer,1968; El Goresy et al., 1988; Burbine et al., 2002; Weider et al., 2012; Helbert et al., 2013], which are believed to contribute to the calcium-bearing species of the surface and exosphere of Mercury [Cintala, 1992; Wurz et al., 2010]. In this paper, we examine whether photon-stimulated desorption (PSD) of Ca0 from calcium sulfide (CaS, a surrogate for the mineral oldhamite) may be contributing to global and local sources of Ca0 in the exosphere of Mercury. We report the PSD (λ = 355 nm) cross section and resulting velocity distributions of the Ca0. The experimentally determined cross sections and velocity distributions were used as input to a Monte Carlo neutral transport model to evaluate the potential contribution to the calcium exosphere from CaS localized in the largest contiguous hollow formation found in the Tyagaraja crater.

2. Experimental Details The experimental setup utilized for the measurement of PSD products, along with their corresponding kinetic energies, has been discussed in detail previously [DeSimone and Orlando, 2014], therefore only a

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3.5x105 2 brief description of methodology speci- 00 2.5 mJ cm· o0 ﬁc to this experiment will be presented 3.0x105 here. For these experiments, CaS pow-

2.5x105 0 der (99% Alfa Aesar; CAS# 20548-54-3) ·c:£l :, was pressed onto a temperature-

~ 2.ox10' 0 controlled nickel mesh sample holder ~ '9 O oo and loaded into an ultrahigh vacuum :ero 1.5x105 Ooo d'i o (UHV) chamber (~1 × 10 10 torr). The tii 00:P 0 C: 9:, 0 Cl 0 <9 cPo sample was resistively heated and "iii 1.ox10' ·o ooo (f'o 0 'o 0 • • o oions ro 2.0x107 tii are injected into a time-of-flight (ToF) C: Cl "iii mass spectrometer by pulsing the 1.0x107 electrostatic extraction lens, typically ' ' held at 100 V. Neutral desorption products require a photoionization 0.0 scheme whereby resonance-enhanced 0.0 2.5 5.0 7.5 10.0 multiphoton ionization (REMPI) was delay time , µs utilized to ionize the Ca0 species. Here Figure 1. Signal intensity of Ca0 versus time for experiments performed a Continuum Nd:YAG (10 Hz) pumped using fluences of (top) 2.5 mJ cm 2 and (bottom) 10.2 mJ cm 2,respec- dye (LDS 867) laser was used in con- tively (open circles). Also shown are the Maxwell-Boltzmann population junction with a KB5O8.4H2O doubling distributions for the thermal (600 K; blue dotted line) and nonthermal crystal to generate a 4 ns pulse at – (1389 1494 K; red dashed line) components and the sum of these λ = 422.67 nm. components (gray line) which were used to fit the observed signal. According to Saloman [1991], the required laser energy to achieve photoionization efficiency from the excited state at the two-photon level near unity at this wavelength is 19 mJ cm 2. To attempt to saturate the two-photon population, the REMPI laser was focused 1.7 (±0.1) mm above the sample down to beam waist of ~32 μm, yielding a power density of ~1227 W cm 2. At this power density, we assume the overall ionization efficiency to be approximately 50%. The kinetic energies of Ca0 were measured by stepping the time delay between the UV PSD laser and the REMPI laser at 50 ns intervals. Due to the time delay for the REMPI generated ions, it is possible to distinguish the signals arising from Ca0 and Ca+ produced directly by the desorption laser pulse; this enabled simultaneous determination of their yields. However, it should be noted that the extraction field prevents the direct determination of the kinetic energy distribution for the Ca+ ions. Since our microchannel plates have an open area ratio (collection efficiency) of ~55%, and the estimated gain provided by our chevron configuration is ~106, our stated yields (and derived cross sections) are expected to be correct to within an order of magnitude [Wiza, 1979; Takahashi et al., 2013]. Additional UV-visible (UV-Vis) measurements were carried out on a pressed CaS pellet in order to determine the bandgap, which can help infer the potential mechanisms leading to desorption (discussed in section 3.2). Here a Cary 5000 UV-Vis near-infrared spectrophotometer was used with the diffuse reflection accessory DRA-2500. Diffuse reflectance measurements were made over the wavelength range 200–500 nm at a scan rate of 2 nm s 1 with a spectral bandwidth of 0.5 nm.

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3.5x105 3. Results and Discussion 00 2.5 mJ cm·2 Oo 3.0x105 3.1. PSD of CaS The Ca0 observed via the REMPI ioniza- 2.5x105 0 ·$c tion scheme for the lowest desorption ::, ﬂuence (2.5 mJ cm 2) and highest ﬂuence 1::- 2.0x105 Jg 2 :0 (10.2 mJ cm ) as a function of time and ro 1.5x105 velocity is shown in Figures 1 and 2,

0 temperature distribution of the sample 0 5.0x104 / oo' / 0 temperature - which was held at 600 0 • .Q 0 (±30) K - were successful, yielding tem- 0.0 4.5x107 peratures of 749 (±65) K and 908 (±23) K for the lower and higher fluences, res- 4.0x107 10.2 mJ cm·2 pectively (with 99% confidence limits). However, analysis of the residuals from $ fi ·c 3.0x107 the t indicated that this single compo- ::, 1::- nent underestimated the contributions Jg at both early and late times indicating :0 ro 2.0x107 the presence of both slower (thermal)

2500 3000 to the remainder of the distribution. For 2 velocity, m s·1 the lower intensity pulse (2.5 mJ cm ), a nonthermal 1389 (±121) K temperature 0 Figure 2. Signal intensity of Ca versus velocity for experiments per- component was required, accounting for fl 2 2 formed using uences of (top) 2.5 mJ cm and (bottom) 10.2 mJ cm , ~25% of the signal (Figures 1 (top) and 2 respectively (open circles). Also shown are the Maxwell-Boltzmann population distributions for the thermal (600 K; blue dotted line) and (top); red dashed line). For the higher- 2 nonthermal (1389–1494 K; red dashed line) components and the sum of intensity pulse (10.2 mJ cm ), a nonther- these components (gray line) which were used to fit the observed signal. mal component of 1494 (±99) K was found to account for approximately 42% of the observed signal (Figures 1 (bottom) and 2 (bottom); red dashed line). These values are in good agreement with previous PSD studies, which show when atomic Na0 is released during PSD it is typically released with energies of around 0.1 eV (900–1300 K) [Madey et al., 1984; Yakshinskiy and Madey, 2000, 2004]. The derived cross sections for neutral Ca0 and ionic Ca+ as a function of the laser pulse energy are shown in Figure 3 and are also tabulated in Table 1, along with the yields and ratio of ions produced. Errors of 50% are shown for Ca0 estimated from the repeated measurements at laser fluences held at 4.6 and 6.4 mJ cm 2. Above a threshold fluence of approximately 3.75 mJ cm 2, the contribution of ionic calcium becomes signif- icant, and the observed cross section increases in a nonlinear fashion with laser fluence, indicating the ions are produced by multiple photon processes. Since the ionization potential of calcium is 6.1 eV, at least two 3.5 eV PSD photons are required. Alternatively, the absorption of a second photon within CaS may open up additional channels capable of generating ions (discussed further in section 3.2). Note that the ablation threshold observed in similar experiments is typically around 1.0 J cm 2 [Brunetto et al., 2006; Moroz et al., 2014], whereas the intensity of the PSD laser used here is typically several orders of magnitude below this (Table 1). Only results below the 3.75 mJ cm 2 threshold are used to derive PSD cross sections. The average yields are derived as 9.6 (±6.3) × 10 6 Ca0 photon 1 and 2.7 (±0.7) × 10 9 Ca+ photon 1. These translate into cross sections of 1.1 (±0.7) × 10 20 cm2 and 3.2 (±0.9) × 10 24 cm2 for Ca0 and Ca+, respectively. These values are in good general agreement with those found in the literature where Yakshinskiy and Madey [1999] derived a cross section of 3 (±1) × 10 20 cm2 for Na0 release by PSD, and Madey et al. [1998] estimated a cross section

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10-17 ----MPD of 1.9 × 10 21 cm2 for K0 atoms released by PSD- 10-18 PSD at λ = 365 nm. The cross sections for ions released by PSD processes are typically 24 2 ~E 10-10 in the range of a few 10 cm [Madey r.., I I C: 0 et al., 1983, 1998; Ageev, 1994]. As pre- 0 I 10-20 I I ti 0 viously noted, our extraction scheme does Q) I I I I 0 0 not allow us to measure the kinetic energy % 0 1/l 10-21 + 0 of the Ca ions released directly from the er.., 0

Cl 0 surface. However, previous PSD experi- (/) 10-22 Q. 0 ments indicate that ions released during 10-23 • ca0 PSD processes are typically born with 0 0 ca· – – 0 0 2 4 eV (23,000 46,500 K) [Madey et al., 10-24 1984; Yakshinskiy and Madey, 2000]. 2 3 4 5 6 7 8 9 10 11 desorption laser intensity, mJ cm·2 3.2. Discussion of Mechanism and

Determination of the CaS Bandgap Figure 3. Estimated PSD (at λ = 355 nm) cross section (in cm 2) for Ca0 (closed circles) and Ca+ (open circles) from calcium sulfide (CaS) The most general description of the PSD versus the desorption laser intensity (fluence) used (mJ cm 2). The process is the Menzel-Gomer-Redhead 0 errorbarsof50%forCa are estimated from the points where multiple (MGR) model. This primarily involves fl 2 experiments were performed at uences of 4.6 and 6.4 mJ cm .The valence excitations of surface-bound ada- dotted line represents the approximate location of the cutoff between tom or terminal surface sites through a photon-stimulated desorption (PSD), initiated by single-photon events, and multiphoton desorption processes (see text for details). Franck-Condon type transition into a repul- sive (antibonding) excited state, leading to desorption [Madey et al., 1983, 1984; Bunton et al., 1992; Alexandrov et al., 2001]. In the case of wide- bandgap semiconductors, such as CaS, the desorption is typically thought to be mediated by defects or disorder-related states with energy levels that extend well below the conduction band [Avouris and Walkup, 1989]. Indeed, there are many instances in the literature where neutral and ionic desorption has been observed during PSD using UV photons on wide-bandgap materials at photon energies below the bandgap and at fluences lower than the threshold for optical breakdown [George et al., 2010; Khan et al., 2011]. Defects are intrinsic to the material. For example, the upper monolayers of surfaces are often undercoordi- nated and are prone to hosting defect sites, particularly along corners and kinks; such defects are commonly found in powders, for example (e.g., three-coordinated sites) [Beck et al., 2006]. Note that sub-bandgap photons have also been demonstrated to generate defects not intrinsically present within materials [Joly et al., 2008; Sushko et al., 2011]. Excitons can be created directly at the surface or diffuse to the surface after creation in the subsurface region. The localization of these excitons with surface defects can give rise to the emission of nonthermal species [Sushko et al., 2011]. Several models, including the MGR model, have been

Table 1. Yields and Cross Sections for the Production of Neutral Ca0 and Ionic Ca+ Released by PSD Processes as a Function of Laser Fluencea Neutral Ca0 Ionic Ca+ Laser Fluence (mJ cm 2) # Photons Pulse 1 Yield Cross Section Yield Cross Section % Ions 2.5 8.9 × 1014 4.1 × 10 6 4.8 × 10 21 2.5 × 10 9 3.0 × 10 24 0.06% 3.1 1.1 × 1015 1.6 × 10 5 2.0 × 10 20 2.1 × 10 9 2.5 × 10 24 0.01% 3.6 1.3 × 1015 8.3 × 10 6 9.8 × 10 21 3.5 × 10 9 4.2 × 10 24 0.04% 4.1 1.4 × 1015 8.1 × 10 6 9.6 × 10 21 3.6 × 10 8 4.2 × 10 23 0.44% 4.6 1.6 × 1015 1.0 × 10 5 1.2 × 10 20 1.1 × 10 7 1.3 × 10 22 1.11% 4.6 1.6 × 1015 1.3 × 10 5 1.6 × 10 20 3.4 × 10 7 4.1 × 10 22 2.50% 5.1 1.8 × 1015 7.5 × 10 5 8.8 × 10 20 1.8 × 10 6 2.1 × 10 21 2.29% 5.6 2.0 × 1015 3.3 × 10 5 3.9 × 10 20 1.2 × 10 6 1.4 × 10 21 3.45% 6.4 2.2 × 1015 2.2 × 10 5 2.5 × 10 20 4.4 × 10 7 5.2 × 10 22 2.00% 6.4 2.2 × 1015 7.6 × 10 5 9.0 × 10 20 1.6 × 10 6 1.9 × 10 21 2.06% 7.6 3.6 × 1016 3.8 × 10 4 4.5 × 10 19 5.6 × 10 6 6.7 × 10 21 1.47% 10.2 5.4 × 1015 2.0 × 10 3 2.4 × 10 18 2.0 × 10 5 2.3 × 10 20 0.98% aThe number of photons per pulse and the percentage that ions contribute to the total signal are also provided.

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2.5 proposed that could explain the release of energetic Ca0 as well as Ca+. These 2.0 require the formation of defects and self-trapping of excitons at the defect sites [Alexandrov et al., 2001; Beck et al., ~ ci 1.5 2006, 2008; Joly et al., 2006, 2008; George et al., 2010; Khan et al., 2011]. Ii:i:' ;> Additionally, these defects have been :S 1.0 shown to be stable on long timescales (minutes to hours) at room temperature [Beck et al., 2006; Sushko et al., 2011]. The formation of surface defects induced by the impact of energetic ions, elec- trons, photons, and meteorites is an 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 ongoing process highly relevant to the photon energy, eV surface of Mercury, as well as its subse- Figure 4. Tauc plot (see text for details) for CaS pellet; the gray line is the quent involvement in surface degrada- actual measurements, and the black line is the same data smoothed tion by space weathering processes ﬁ over ve data points, for clarity. The bandgap (Eg; red dashed line) is [Sushko et al., 2011]. determined as 2.81 (±0.14) eV. The (blue dotted) line at 3.49 eV corresponds to our laser energy at λ = 355 nm. Clearly, to help understand the mechanism of Ca0 desorption, it is necessary to determine where the CaS bandgap lies in relation to the photon energy of our desorption laser (λ = 355 nm; 3.49 eV). CaS is commonly reported as a wide-bandgap semiconductor. However, there is considerable variation on the reported bandgap in the literature, with experimental values ranging from 3.4 to 5.8 eV and theoretical predictions from 2.2 to 7.1 eV [Shaukat et al., 2008; Waroquiers et al., 2013; Raubach et al., 2014]. The bandgap for the CaS powder was determined using the Tauc relation as described in equation (1) [Tauc et al., 1966; Tauc, 1968; Tauc and Menth, 1972]: ÀÁ 1=n ðÞhva ¼ Ahv Eg (1)

where h is Planck’s constant, v is the frequency of vibration, α is the absorption coefficient, Eg is the bandgap, and A is a proportionality constant. The value of the exponent n denotes the nature of the transition, where for a direct allowed transition n = 0.5, whereas for a direct forbidden transition n = 2.0. As discussed by Raubach et al. [2014], there is currently some debate over whether the bandgap in the B1 phase (relevant to our experiments and the surface of Mercury) is governed by direct or indirect transitions between the valence and conductance bands; therefore, we will present both of these results. The diffuse reflectance spectrum initially obtained in terms of R (% Reflectance). The vertical axis is then con- verted via the Kubelka-Munk function ((1 R)2/2R) to give F(R∞), which is proportional to α, and therefore replaces it in equation (1), where R∞ is the reflectance of an infinitely thick layer. Thus, the average bandgap is determined from the intercept of the linear portion of a Tauc plot where hv is plotted against (hvF(R∞))1/n.

Figure 4 shows the results for the direct forbidden transition (n =2.0),whereEg is determined to be 2.81 (±0.14) eV. The Tauc plot for the indirect case (n = 0.5) gives a similar value of 2.94 (±0.54) eV for Eg (not shown, but similar to Figure 4). From our results, the absorption edge seems more consistent with the direct forbidden transition [Tauc et al., 1966], but both determined values lie below the 3.5 eV of the PSD photons, which supports the viability of single-photon desorption processes relevant to the surface of Mercury, including a defect-mediated MGR mechanism. 3.3. Contributions From PSD to the Exosphere of Mercury and the Region Above the Tyagaraja Crater In this section, we estimate the potential contribution of Ca0 photodesorbed from CaS to the Ca0 exosphere of Mercury. The flux of photodesorbed Ca0, Φ(Ca0), can be calculated as ÀÁ ÀÁ Φ 0 ¼ Φ ðÞ½ σ 0 Ca λ>3:5eV cos SZA CaS surface Ca (2) 2 1 where Φλ > 3.5 eV is the flux of photons of energy 3.5 eV to the surface of Mercury, in photons cm s (the flux of photons with E > 3.9 eV was taken) [Bennett et al., 2013], SZA is the solar zenith angle, [CaS]surface is

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106 --- - T = 650 K the abundance of CaS on the surface of 3.9 eV is taken as ~3 -~ 104 . , 16 2 1 (lJ to ~7 × 10 photons cm s at aphe- 0) ·'· ' 3 ·\ lion (0.47 AU) and perihelion (0.31 AU), ~ 10 \ s ,· . respectively. If we assume that the lowest 102 ..c~ ' . abundance of sulfur derived from orbit by (lJ ' . ' the XRS instrument of 1.5% is distributed ~ 101 ' -·-·-·-·-·-·-·-' )-._·_· C ' globally and is accounted for entirely by ~ 10° ' ' CaS (representing oldhamite), then O(!J ' ' ' 12 2 0 ~ 10-' ' [CaS]surface ~7.5 × 10 cm .TheCa PSD cross section is our experimentally ~ '6 10-' ' derived value ~10 20 cm2.Fromequa- ~ ' a.. 0 fl 10" +-,~~~~~~~~~~~~~~~~~~~~~ tion (2), the Ca photodesorption ux 0 100 200 300 400 500 600 700 800 900 1000 under normal incidence is estimated to Altitude, km be 5.1 × 109 Ca0 cm 2 s 1 at Mercury’s aphelion and 1.2 × 1010 Ca cm 2 s 1 at Figure 5. Results from Monte Carlo transport model where the source of Mercury perihelion. Integrated over the Ca0 was assumed to be a 25 × 25 km2 hollows region of 1% assumed CaS abundance within the Tyagaraja crater. The results indicate the vertical entire dayside, including the cosine term, contributions to the Ca0 density as a function of altitude for the two the total (cold) source rate by PSD is temperature components at T = 600 K (blue dashed line) and T = 1389 K ~1027 Ca0 atoms s 1. Note that Burger (red dotted line) as well as their sum (black line) assuming the 3:1 ratio et al. [2012] placed an upper limit on a observed experimentally (see text for further details). Also shown is the global isotropic cold (1000 K) component level at which the Ca0 observations were detected by the UVVS spectrometer on MESSENGER at an altitude of 200 km (gray dash-dotted line). Inset having a maximum source rate of 23 0 1 image of the crater and hollows adapted from Blewett et al. [2011]. ~10 Ca atoms s . Even though this limit was obtained from data behind the terminator, and a cold component from PSD would not be isotropic, it is clear that our derived rate would still be too high by at least 2 orders of magnitude if emission from globally distributed CaS were assumed. The CaS used in these experiments may be more representative of the hollows materials rather than the entire surface. To investigate this possibility, we evaluate the microatmosphere above Tyagaraja, adopting collisionless neutral particle transport to model a local source region ~0.9° × 0.9° large. This is roughly the size of this contiguous hollow region, estimated to be 25 × 25 km2 [Blewett et al., 2011] (Figure 5). Again, the abundance of CaS was assumed to be 1.5% in accordance with the XRS data. The hollow was placed in the subsolar region around Mercury at a True Anomaly Angle (TAA) of ~ 150°. During the simulation, 106 test particles were launched from this region, with initial velocities sampled from a Monte Carlo method with Maxwellian flux distributions of T = 600 K and T = 1389 K at a 3:1 ratio to match our experimental results. The test particle trajectories were then numerically tracked as modified by gravity and radiation pressure until escape, ionization, or return to the surface. These test particles represent the flux of Ca0 particles leaving the surface and their contribution to the Ca0 density as a function of altitude. The weight each particle represents is w = SΦ/N, where Φ is the flux leaving the surface; S is the surface area of a model patch, and N the number of test particles from that surface element. The contribution of each test particle to the exospheric density at a simulation cell is proportional to its total residence time in that cell (i.e., the relevant accumulator for density is n = ∑wΔt/V, where V is the cell volume). This approach has been previously demonstrated to successfully simulate the lunar exosphere [Collier et al., 2014]. The predicted concentration of Ca0 atoms above Tyagaraja is summarized in Figure 5 showing the contributions from the 600 K (blue dashed line), 1389 K (red dotted line), and summation of these components (black line) as a function of altitude. The results predict high densities of ~2 × 105 Ca0 cm 3 near the surface, and, despite the short-scale height of Ca0, the predicted number densities from our local source would exceed the global atmosphere as measured by MESSENGER (1–4Ca0 atoms cm 3 from Burger et al. [2014]; gray dash-dotted line in Figure 5) out to ~400 km. Note that the Ca0 number densities measured peak around the dawn hemisphere at around ~4 Ca0 atoms cm 3 and are a minimum around dusk at around ~1 Ca0 atoms cm 3 [Burger et al., 2014, Figure 8]. The density of this potential Ca0 microatmosphere above Tyagaraja would vary as a function of heliocentric distance and TAA.

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The neutral Ca0 detected in the present set of experiments is not energetic enough to contribute to the hot calcium measured by MESSENGER and telescopic observations. As noted by Burger et al. [2014], the warm component is enhanced toward the dawn equator, whereas PSD processes should lead to increased levels at noon. The seasonal variation of Ca abundance observed as well as its variation with TAA has been demonstrated to correlate well with the column abundances expected from impact vaporization [Killen and Hahn, 2015; Christou et al., 2015]. However, impacts alone cannot account for the hot calcium observed; the ejected material may be subjected to additional processes, leading to the generation of excited species (e.g., electron impact dissociation or photodissociation). Note also that ion and electron bombardment of the surface could lead to additional contributions to the exospheric calcium observed, reaching levels of around 0.1–1Cacm 3 [e.g., McLain et al., 2011; Pfleger et al., 2015]. However, these considerations do not rule out the potential contributions that PSD could contribute to the cold component of exospheric calcium. This cold component of the Ca0 exosphere should be easily detected with MESSENGER UVVS dayside altitude probes down to ~100 km. Note that these simulations indicate that in order to reproduce the measurements made by MESSENGER at an altitude of 200 km, our source rate would need to be reduced by a factor of ~100. There are several factors that can cause our modeled and laboratory source rates to be different than observations. First, surface topography was not considered in our model. This would lower the probability of low-scale height vapors, such as Ca0 from PSD, leaving the hollows crater. Second, pure CaS powder does not accurately represent the calcium- or sulfur-bearing surface materials. The lattice structure of the real mineral may lead to a sufficiently lower photodesorption cross section. Taking these cross sections, it is possible to estimate the erosion loss rate of CaS hollows material due to PSD processes. If we assume a density of 2.6 g cm 3 [Robie et al., 1966] and a molecular mass of 72 g mol 1 for oldhamite, this transfers to loss rates of 120 h nm 1 at aphelion and 50 h nm 1 at perihelion. Therefore, one could expect that ~1 cm of CaS would be eroded in approximately 100,000 years (assuming PSD processes are acting alone). Note that similar erosions rates were reported by Blewett et al. [2011]. This derived erosion rate could be overestimated if our cross section is ~100 times too high; however, this discrepancy is likely accounted for by the fact that additional processes such as ISD, ESD, thermal sublimation, and micrometeorite ablation also contribute to the erosion loss rate. The release energies of Ca0 from CaS by PSD processes suggest that the Ca0 vapors are below the escape velocity, and most of this material returns to the surface regardless of the short photoionization lifetime for Ca0. Therefore, deposition of Ca (and potentially, S) in the regions surrounding hollows are predicted to accumulate over time. It is worth briefly noting that S was not detected during the present PSD experiments. Based on the known mechanisms of PSD, neutral S is expected to desorb [Alexandrov et al., 2001; Avouris and Walkup, 1989; Beck et al., 2008]. However, the detection of neutral S requires a separate REMPI scheme not available in our laboratory when these experiments were carried out.

4. Conclusions and Summary We have performed measurements on the PSD cross sections for Ca0 and Ca+ from CaS powder, acting as an analog for oldhamite, a mineral predicted to be present on the surface of Mercury. Diffuse reﬂectance UV-Vis

measurements were made on the CaS powder to determine a bandgap, Eg, of 2.81 (±0.14) eV via the Tauc method. The impinging 3.5 eV photons induce PSD of Cao primarily by single-photon events under condi- tions that are applicable to Mercury. The desorbing Ca0 could be fit using two Maxwell-Boltzmann components: the thermal component held at 600 (±30) K and a nonthermal component of 1389 (±121) K, the latter accounting for ~25% of the observed signal. The derived PSD cross sections for Ca0 and Ca+ were found to be 1.1 (±0.7) × 10 20 cm2 and 3.2 (±0.9) × 10 24 cm2, respectively. Simulations using our measured Ca0 PSD cross section from CaS show a measurable exospheric density both globally and over large hollows at levels ~100 times higher than what was measured by MESSENGER at 200 km. The possible sources of Mercury’s extensive exospheric calcium are a subject of interest to the planetary science community. Further analysis of the low-altitude campaigns from MESSENGER or the upcoming BepiColombo mission may be able to validate some of the arguments presented here; specifically, if PSD plays a role, significantly enhanced levels of cold Ca0 are anticipated at lower altitudes. The experiments provide evidence that UV PSD effectively desorbs Ca0 neutrals from

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CaS powders. It is suggested that future experiments should be performed not only on oldhamite but also on other calcium-bearing species thought to be present on Mercury (e.g., diopside, anorthosite, and other minerals with natural concentrations of calcium, such as enstatites) using an extended range of photon energies to fully unravel the role of PSD processes acting on Mercury.

Acknowledgments References The authors at Georgia Institute of – Technology gratefully acknowledge Ageev, V. N. (1994), Desorption induced by electronic transitions, Prog. Surf. Sci., 47,55 204. funding from NASA Planetary Alexandrov, A., M. Piacentini, N. Zema, A. C. Felici, and T. M. Orlando (2001), Role of excitons in electron- and photon-stimulated desorption of – Atmospheres Program grant neutrals from alkali halides, Phys. Rev. Lett., 85, 536 539. NNX14AH41G. We would additionally Avouris, P., and R. E. Walkup (1989), Fundamental mechanisms of desorption and fragmentation induced by electronic transitions at – like to thank Janos Simon (Georgia surfaces, Annu. Rev. Phys. Chem., 40, 173 206. fi fi Institute for Technology) for his Beck, K. M., M. Henyk, C. Wang, P. E. Trevisanutto, P. V. Sushko, W. P. Hess, and A. L. Shluger (2006), Site-speci c modi cation of MgO assistance taking the UV-Vis nanoclusters: Towards atomic-scale surface structuring, Phys. Rev. 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