The blue slopes in the darkened basalt and nontronite spectra are likely a function of the large carbon particle size used to darken the samples (90 nm), which falls above the size (< ~50 nm) at which nanophase absorbers contribute reddening to the spectra (e.g., Lucey & Riner, 2011 ). Thus, while the blue slopes of these samples make for imperfect VNIR spectra analogs to Phobos, the presence of finely particulate basalt and/or nontronite on the surface of Phobos should not be discounted based on the slopes of these spectra.

Tagish Lake has been suggested as a compositional analog to D‐class asteroids and Phobos based on its low albedo, red slope, and relatively featureless spectrum at VNIR wavelengths (Hiroi et al., 2001 ; Lynch et al., 2007 ; Murchie & Erard, 1996 ; Pajola et al., 2013 ; Rivkin et al., 2002 ). Of the VNIR spectra measured in this work, that of Tagish Lake (Figure 12 c) is, indeed, the best match to published spectra of Phobos (e.g., Fraeman et al., 2012 , 2014 ; Murchie & Erard, 1996 ; Rivkin et al., 2002 ). It has a low albedo, a red slope, and only three very weak spectral features near ~1.9 μm. The other darkened samples all have low albedos but display blue, rather than red, slopes. The darkened basalt and nontronite samples are mostly featureless, with the exception of a ~650‐nm feature in the nontronite spectrum that has been observed in CRISM spectra of the Phobos red unit (Fraeman et al., 2014 ). The darkened thermally altered nontronite spectrum has a strong red slope shortward of ~840 nm that is not seen in Phobos spectra. It should be noted, however, that the nontronite sample was heated in air, under oxidizing conditions. Thermal alteration under reducing vacuum conditions may result in substantial differences to the VNIR and/or MIR spectra.

A possible source of some of the mismatch between the spectra could be due to differences in the thermal gradient in our particulate spectra acquired in the PARSEC chamber compared to that found in the Phobos regolith. In general, our spectra have brightness temperatures that are higher than the bolometric temperatures of Phobos measured by TES (~250–300 K), which are difficult to achieve in PARSEC. To test the effects of brightness temperature on derived thermal emission spectra, we measured the Tagish Lake fine particulate sample over a range of brightness temperatures (322–369 K) achieved by varying the brightness of the solar lamp shining on the sample. Brightness temperature and derived thermal emission spectra are shown in Figure 14 . Brightness temperature spectra (Figure 14 a) all show the same basic shape, although they are offset by ~10 K each. Emission spectra derived from the brightness temperature spectra (Figure 14 b) all have the same shape, with warmer spectra displaying slightly increased spectral contrast compared to the colder spectra. The spectra are nearly identical near the CF (within the level of noise) and at the longest wavelengths. The maximum emissivity difference between the hottest and coldest spectra is only ~0.003 in the transparency feature region and increases steadily at the wavelengths shortward of the CF to a maximum of ~0.012 near the edge of the reliable range of our measurements. We conclude from these data that the regolith temperature differences between Phobos and the Tagish Lake particulates measured in PARSEC are not likely to be responsible for the spectral differences observed in Figure 13 . Therefore, the primary cause of the mid‐IR spectral differences between Tagish Lake and Phobos must be due to differences in composition.

It is unlikely that differences between the Phobos environment and the simulated airless body environment in the PARSEC chamber are the cause of the spectral differences seen in Figure 13 . Exposure to the space environment has led to space weathering of Phobos' surface. Space weathering has been shown to alter the mid‐IR spectral features of the Moon (Glotch et al., 2015 ; Lucey et al., 2017 ), likely the result of an altered thermal gradient caused by reduced visible albedo associated with optically mature surfaces. Space weathering results in a shift of the CF to longer wavelengths and a reduction in Reststrahlen band and transparency feature strength throughout the mid‐IR (Glotch et al., 2015 ; Lucey et al., 2017 ; Shirley et al., 2018 ). As seen in Figure 13 , the positions of the coarse and finely ground Tagish Lake CF maxima are at much longer wavelengths than that of the Phobos spectrum. The average Phobos spectrum is composed of spectra covering both the Phobos blue and red defined by Murchie and Erard ( 1996 ). However, the generally low albedos, flat‐to‐red slopes, and lack of strong spectral features throughout the VNIR suggest that both the red and blue units could have experienced significant space weathering. Therefore, it is unlikely that the differences between the Phobos spectrum and the Tagish Lake particulate spectra are the result of space weathering.

The Phobos spectral character and derived thermal inertia (Lunine et al., 1982 ; Smith et al., 2018 ) therefore point to a fine particulate material as the best potential analog to the Phobos spectrum. However, both the coarsely ground and finely particulate Tagish Lake sample preparations differ substantially from the Phobos spectrum. As the particle size decreases from chip to coarse particulates to fine particulates, the CF changes shape and shifts to longer wavelengths. The sharp emissivity minimum at ~867 cm −1 disappears and is replaced by a broad transparency feature.

Despite the similarities between Tagish Lake and Phobos at VNIR wavelengths, there are substantial differences between the Tagish Lake and Phobos spectra at TIR wavelengths (Figure 13 ).The closest match between the Tagish Lake and Phobos infrared spectra is for the intact chip. The Phobos and Tagish Lake chip spectra both have broad CF maxima, with roughly similar CF positions. In addition, both spectra have minima in the ~1,200–600 cm −1 region, although they do not occur at similar positions. In addition, the Tagish Lake spectrum has much deeper emissivity at the longest wavelengths, consistent with the presence of substantial magnetite and sulfide in the sample. While there are some similarities, there are numerous substantial differences, indicating that Tagish Lake (or at least the lithology of Tagish Lake used in this work) is not an appropriate compositional analog for Phobos. Despite the similarities in CF position, the other major spectral features are not good matches. In addition, thermal modeling of Phobos (Lunine et al., 1982 ; Smith et al., 2018 ) suggests an average thermal inertia of ~25–80 J · m 2 · K · s 1/2 , consistent with the presence of a fine regolith (likely a few tens of microns; Weschler et al., 1972 ). However, relating thermal inertia to particle sizes on airless bodies is notoriously difficult (Presley & Christensen, 1997 ; Weschler et al., 1972 ) due to grain angularity, roughness, difficulty in deriving accurate thermal inertia values, etc., and as such, we use the thermal inertia constraints listed above as an indicator for fine‐grained particles, rather than attempting to derive robust particle sizes. The interpretation of a finely particulate regolith is also supported by the steep dropoff in emissivity of the Phobos spectrum at wavelengths shortward of the CF.

4.3 Implications for Composition and Origin of Phobos

The fact that Tagish Lake is a poor mid‐IR spectral analog for Phobos calls into question whether D‐class asteroids are suitable compositional analogs for Phobos. The conclusion that Tagish Lake is a poor analog for Phobos is supported by telescopic mid‐IR emissivity spectra of D‐type Trojan asteroids (Emery et al., 2006), which are dissimilar to both Tagish Lake and Phobos. Spectra of other classes of carbonaceous chondrite meteorites, including Allende (CV3) and Murchison (CM2), acquired under similar airless body conditions also appear to be poor matches to the Phobos spectrum (Donaldson Hanna et al., 2017). These spectra have one or more broad, deep features between 400 and 1,000 cm−1 that are wholly absent from the average Phobos TES spectrum and CF maxima that are narrower than that seen for Phobos and occur at lower frequencies (~1,000 cm−1) than the average Phobos TES spectrum (1,133 cm−1). In the absence of mid‐IR spectroscopic evidence for an association between Phobos and a carbonaceous chondrite composition, we have compared the Phobos TES spectrum to other silicates that may provide evidence for Phobos' origin.

The mid‐IR spectral properties of Tagish Lake and other carbonaceous chondrites (e.g., Donaldson Hanna et al., 2017) pose problems for the widely proposed asteroid capture hypothesis (Murchie et al., 1999, 2015; Pajola et al., 2012, 2013; Pang et al., 1978; Pollack et al., 1978; Rivkin et al., 2002) in addition to the dynamical issues related to the required high tidal dissipation rates required to explain Phobos' current orbit (Burns, 1992; Murchie et al., 2015; Rosenblatt, 2011). More recently, a Mars impact origin for Phobos has been suggested by several authors (Canup & Salmon, 2018; Craddock, 2011; Rosenblatt & Charnoz, 2012). This type of origin model is difficult to reconcile with a chondritic composition for Phobos, as would be suggested by an asteroid capture model described above.

Fraeman et al. (2014) analyzed CRISM data of Phobos and found weak 0.65 and 2.8‐μm features associated with the red spectral unit. They hypothesized that these features could be caused by either (1) the presence of a highly desiccated Fe‐bearing phyllosilicate or (2) Rayleigh scattering and absorption by nanophase metallic Fe and surficial O‐H bonds caused by space weathering. To test the first hypothesis, we compare the spectrum of Phobos to thermally altered nontronite acquired under SPE conditions. In addition, we test a potential Mars crustal composition by comparing the Phobos spectrum to <10‐μm Columbia River flood basalt (Thomson et al., 2014). Both samples were darkened by mixing 5 wt.% carbon black to approximate the visible albedo of Phobos.

The average TES spectrum of Phobos is shown with the basalt and thermally altered nontronite spectra in Figure 15. The thermally altered nontronite spectrum is a poor match to the main features seen in the Phobos spectrum. In addition, the CF position of the thermally altered nontronite spectrum occurs at much higher frequencies than is present in the Phobos spectrum, and it exhibits a sharp dropoff in emissivity toward higher frequencies. By contrast, the Phobos spectrum displays a broad CF emissivity maximum centered about 100 cm−1 higher than the altered nontronite CF. It is possible, however, that the sharp band seen at ~1016 cm−1 in the Phobos spectrum could be consistent with another phyllosilicate, as suggested by Giuranna et al. (2011).

Figure 15 Open in figure viewer PowerPoint TES spectrum of Phobos with laboratory SPE spectra of basalt and thermally altered nontronite, both of which were darkened with carbon black to approximate the visible albedo of Phobos.

The darkened thermally altered nontronite spectrum still displays a weak hydration feature near 1,600 cm−1. While the average TES Phobos spectrum has a very weak feature in this region, it is more pronounced in the average spectrum of the red unit (Figure 8). This is consistent with the observation of a weak 2.8‐μm feature seen in VNIR spectra of the red unit (Fraeman et al., 2014). The origin of this feature may be due to the presence of weakly hydrated minerals in the red unit regolith or through the interaction of the solar wind with Phobos' surface. The absence of a 2.8‐μm or ~1,600‐cm−1 feature in the blue unit could be due to dehydration/dehydroxylation of hydrated minerals as a result of the Stickney impact event or because of reduced space weathering of the blue unit. The latter scenario would seem to indicate a relatively young age for the Stickney impact event.

The major feature at ~825 cm−1 in the Phobos spectrum is relatively well matched by the transparency feature in the finely particulate basalt spectrum. The exact positions and shape of the features vary, but minor changes in chemistry (Ca/Na ratio in plagioclase) or mineralogy (plagioclase/pyroxene ratio) can cause shifts in the position of this feature. In addition, the finely particulate basalt displays a broad rounded CF, centered only ~30 cm−1 shortward of the Phobos CF position. The basalt spectrum, however, does not display the strong minima at either 1,016 or 466 cm−1 that are seen in the Phobos spectrum.

If basalt is, indeed, a major component of the Phobos regolith, the general lack of mafic features in VNIR spectra (Fraeman et al., 2012, 2014; Murchie & Erard, 1996; Pieters et al., 2014; Rivkin et al., 2002) must be explained. Mature lunar regolith generally has a visible albedo ranging between ~0.05 and 0.1 (Ohtake et al., 2013), while the Phobos red and blue units have average visible albedos of ~0.05–0.06 and 0.08, respectively (Murchie & Erard, 1996). These albedos are similar, but fresh craters on lunar mare have typical albedos that are a few percent higher. However, Fraeman et al. (2012) showed that spectra of mature lunar soils and an average Mercury surface spectrum are still brighter than Phobos when corrected to the same viewing geometry, and exhibit weak mafic features, unlike the Phobos spectra. Still, the lower albedo of Phobos regolith compared to fresh mare craters could make mafic features more difficult to discern in VNIR spectra of Phobos, as demonstrated by the VNIR spectra of finely particulate basalt darkened by the addition of carbon lamp black (Figure 12).

The source of Phobos' low albedo, then, remains an important question. If Phobos is inherently dark because it is primarily composed of a carbonaceous material, then we would expect MIR spectra of those materials to match those of Phobos. Based on the (admittedly limited) work presented here and by Donaldson Hanna et al. (2017), this does not appear to be the case, although a more thorough analysis of relevant materials is certainly warranted in future work. It is well known that space weathering causes both darkening and reddening (Pieters & Noble, 2016) or, in some cases, just darkening (Lucey & Riner, 2011) of airless body surfaces. The degree of weathering is primarily dependent on the degree solar wind and micrometeoroid bombardment. The space weathering environment of Phobos and Deimos may be particularly intense, with heavy ion sputtering (sourced from Mars) perhaps exceeding that due to solar wind (Poppe & Curry, 2014). Additionally, Reddy et al. (2014) demonstrated that impact melt in the LL chondrite Chelyabinsk has a much lower overall albedo and greatly reduced mafic band depths than the mafic components. This process could not explain the dark, featureless VNIR spectrum of Phobos on its own as shock melt still retains some mafic features, but the presence of shock melt, in addition to the occurrence of intense space weathering, may further act to remove mafic spectral features in VNIR spectra.

An additional factor to consider is average particle size of the Phobos surface. The thermal inertia of Phobos has been estimated to be ~25–80 J · m2 · K · s1/2 (Lunine et al., 1982; Smith et al., 2018). These values correspond to effective particle sizes of at most a few tens of microns (Weschler et al., 1972). On the other hand, the average particle size of lunar regolith is ~60–80 μm, although the mean grain sizes of regolith samples returned by the Apollo astronauts range from 40 to 800 μm (McKay et al., 1991). Very fine particulates, while brighter than coarser particulates of the same material, also exhibit weaker absorption features (e.g., Clark et al., 2003). The combination of lower albedo and finer particle size associated with the Phobos regolith could act to reduce or eliminate mafic absorption features that are common in mafic lunar soils.

If Phobos is composed partially or mostly of basalt, it must have a high porosity to account for its measured bulk density of 1,860 ± 13 kg/m3 (Willner et al., 2014). Rosenblatt (2011) calculated required densities for Phobos and Deimos assuming a variety of compositions. Using lower and upper bounds of 2,500 and 3,500 kg/m3 for a silicate composition, Rosenblatt (2011) estimated a macroporosity of 25–45% for Phobos. A high internal porosity is consistent with the presence of Stickney crater (Andert et al., 2010), as a large impact on a small body like Phobos would likely destroy it in the absence of that porosity (Richardson et al., 2002). In contrast, the bulk density of Tagish Lake (1,670 kg/m3; Hildebrand et al., 2006) is lower than that of Phobos, and Rosenblatt (2011) argues that it would be difficult to compress such a material to the appropriate density for an object of Phobos' size.

A basaltic regolith on Phobos must also include another component to account for the 1,016 and 466‐cm−1 features that are seen in the Phobos spectrum. Phyllosilicates and their thermal alteration and impact products display a variety of features near these positions (Che et al., 2011; Che & Glotch, 2012; Friedlander et al., 2015, 2016; Glotch et al., 2007), so a phyllosilicate or phyllosilicate‐derived phase that we did not measure here could potentially account for those features. At lower frequencies (longer wavelengths), Fe oxides and oxyhydroxides commonly have spectral absorptions (Glotch et al., 2004; Glotch & Kraft, 2008; Glotch & Rossman, 2009), although none display a single, sharp peak at or near 466 cm−1, as seen in the Phobos spectrum.

Sulfides are another intriguing possibility that could account for these spectral features. These minerals, which are common in chondritic meteorites, tend to be featureless throughout most of the mid‐IR, with one or more strong features at <500 cm−1 (Brusentsova et al., 2012). Again, however, none of the phases that have been previously measured display a single sharp peak near ~466 cm−1, as is seen for Phobos. Given the large variety of phyllosilicate chemistry and spectral properties, we suggest that it is most likely that a phyllosilicate phase, perhaps thermally desiccated, or impact‐shocked, accounts for the spectral features of Phobos not well matched by finely particulate basalt. A minor portion of phyllosilicate‐rich material derived from a chondritic impactor, mixed with a major component of Martian crustal material, would be consistent with this scenario.