Volatility of Sodium in Carbonaceous Chondrites at Temperatures Consistent with Low-perihelion Asteroids

The small bodies of the inner solar system are generally grouped into two phenomenological classes: comets and asteroids. The main difference distinguishing these two populations is whether or not the body is observed to show activity in the form of emission of gas and dust. This divide is usually traced to whether the object contains frozen subsurface volatile materials such as water, carbon dioxide, or carbon monoxide and therefore is used as a proxy for where in the solar system the object formed with respect to the protoplanetary ice line of each molecule.

Recently, this clear distinction has been called into question by a newly recognized population of objects. These objects, on asteroid-like orbits, have been seen to show comet-like activity (Jewitt et al. 2015). Referred to by various publications as Active Asteroids, Main Belt Comets, or Active Main Belt Objects, this population can be divided into objects showing activity from impact events, objects with activity consistent with disruption after rotational spin up, and objects with activity consistent with the presence of subsurface water ice.

However, there are a few objects that fail to fit into any of these broad categories. The most notable example is the asteroid (3200) Phaethon, a near-Earth object (NEO) that has been shown to be active at perihelion (Jewitt & Li 2010; Li & Jewitt 2013) and is orbitally associated with the Geminid meteor stream (Gustafson 1989). The perihelion of Phaethon is q = 0.14 au and its aphelion is Q = 2.4 au, making the surface temperatures on the day-lit side well above the sublimation temperature of water ice for the majority of its orbit. Any ice thermally coupled to the surface would be expected to have been removed quickly after Phaethon's dynamical transfer into its current orbit, and not survive to the present day. MacLennan et al. (2021) have shown that over the course of its orbital evolution Phaethon has undergone numerous cycles through high peak temperatures like today, which would have baked off any water near the surface. This means water ice cannot be driving the observed activity, which is seen only in a short temporal period around Phaethon's perihelion. This short burst of activity would not be the behavior expected if water ice was driving the activity. While water could be bound up in hydrated silicates, experiments by Garenne et al. (2014) have shown that these minerals in meteorites begin losing mass below 873 K, well below Phaethon's perihelion temperature. Recent spectral observations by Takir et al. (2020) have indicated an absence of hydrated minerals on the surface, further supporting the conclusion that they are not the source of activity.

Previous investigations have looked into the effects of the extreme temperatures that Phaethon experiences at perihelion (Delbo et al. 2014; Ryabova 2018; Yu et al. 2019; MacLennan et al. 2021). These studies have found that fracturing, driven by thermal cycling, can efficiently fragment boulders with Phaethon's expected surface composition. This would act as a source of dust that could eventually be ejected from the surface to create the observed brightening and the associated meteor stream. However, thermal fracturing alone is unlikely to produce sufficient force to let this dust reach escape velocity and be emitted from Phaethon.

We propose that the driver of the activity on Phaethon at perihelion is the volatilization of sodium bound in minerals such as sodalite and nepheline distributed in the matrix. Other than water ice, which was lost early during Phaethon's transition to a near-Earth orbit, sodium is one of the most volatile elements present at a significant quantity in asteroidal materials (Notsu et al. 1978; Asplund et al. 2009; Sossi & Fegley 2018). Mercury, with a perihelion of q = 0.31 au, is known to have an exosphere populated by vaporized sodium (Cassidy et al. 2015), which means that sodium volatilization does occur in the near-Sun region. It is important to note that Cassidy et al. (2015) show that the temperature profile of Mercury's exosphere implies that the primary mechanism of volatilization is photon-stimulated desorption instead of thermalization. They attribute the lack of thermalized sodium to space weathering, which we would expect to be less significant for Phaethon given that it is a dynamically much younger object.

Additionally, the Geminid meteors have been observed to be depleted in sodium compared to other meteor streams (Kasuga et al. 2005; Abe et al. 2020). Kasuga et al. (2006) investigated the sodium content of meteor showers, finding that while the current orbit of Phaethon does not reach the condensation temperatures for common sodium minerals such as feldspar, small particles will experience higher temperatures and may be devolatilized that way. They suggest that the submillimeter-scale Geminid meteors were depleted in sodium after being ejected from the parent body. However, we propose here the converse: that it is the devolatilization of sodium-bearing minerals on Phaethon that prompted the ejection of the particles that became the Geminids, and thus they were sodium-depleted during their formation.

To investigate this hypothesis, we conducted thermophysical modeling of Phaethon as it orbits the Sun to determine the feasibility of this mechanism. We then performed heating experiments on meteorite samples as an analog for Phaethon to search for sodium loss. Sodium content was determined for a range of peak heating temperatures on individual minerals within each sample, which were analyzed post-heating for changes to their chemistry. This work is intended to be an initial look into the possibility of sodium volatilization causing activity on low-perihelion bodies, and future work will constrain this effect in further detail.

To test the feasibility of our hypothesis that sodium sublimation is driving the activity of Phaethon, we performed thermophysical modeling of that process. We consider Phaethon as a spherical, rotating, and orbiting body consisting of a porous mixture of silicate and sodium grains. Constant model parameters are provided in Table 1. We apply the thermophysical model NIMBUS (Numerical Icy Minor Body evolUtion Simulator) developed by Davidsson (2021; see also Davidsson et al. 2021). NIMBUS considers a number of latitudinal slabs (here 18, evenly distributed in cosine), each split into a number of radial cells, stretching from the body center to the surface (here 123 cells, with a thickness of 200 m at the core, diminishing with geometric progression to 0.003 m at the surface). For this grid, NIMBUS solves a coupled system of differential equations that govern the radial and latitudinal transport of heat and mass throughout the body. The upper boundary condition of the energy conservation equation balances absorbed solar radiation (calculated as a function of orbital position, latitude, and rotational phase) with thermal reradiation into space and heat conducted to or from the surface. At depth, the energy conservation equation accounts for solid-state conduction, radiative transfer of energy in pores, energy consumption of sublimating solid sodium, energy transport (advection) of diffusing vaporized sodium, and energy release when vaporized sodium recondenses into a solid. We note that here we use diffusion to refer to gas flowing through an empty channel (e.g., pore space within the body), as opposed to atomic transport within solids, which we will refer to as "Fickian diffusion." The mass conservation equation ensures that vapor diffuses according to local temperature and pressure gradients, tracks the withdrawal of the sodium sublimation front below the surface, and regulates the release of vaporized sodium into space.

Table 1. Orbital, Rotational, and Physical Parameters Applied in the Thermophysical Model of Phaethon

Symbol	Description	Value	Unit	Reference
a	Semimajor axis	1.271 36	au	MPC
e	Eccentricity	0.889 849 5	⋯	MPC
i	Inclination	22.259 64	°	MPC
ω	Argument of perihelion	322.186 86	°	MPC
Ω	Longitude of the ascending node	265.217 48	°	MPC
${ \mathcal T }$	Perihelion date	2 459 191.093 586 5	JD	MPC
P	Rotation period	3.603 958	h	Hanuš et al. (2016)
λ	Spin axis ecliptic longitude	319	°	Hanuš et al. (2016)
β	Spin axis ecliptic latitude	−39	°	Hanuš et al. (2016)
D	Diameter	4.6	km	Masiero et al. (2019)
A	Bond albedo	0.04	⋯	Golubeva et al. (2020)
ε	Emissivity	0.9	⋯
ρsil	Silicate grain density	3000	kg m−3	Macke et al. (2011)
ρNa	Sodium grain density	968	kg m−3
ψ	Porosity	0.5	⋯
mNa	Sodium abundance	0.5	wt%	Lodders (2003)
rg	Grain radius	100	μm
Lt	Tube length	1	mm
rt	Tube radius	100	μm
ξ	Tortuosity	1	⋯
T0	Initial temperature	260	K

Note. MPC is the Minor Planet Center (https://minorplanetcenter.net).

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A number of temperature-dependent and material-specific functions are used by NIMBUS to regulate the behavior of the physical processes under consideration. For the heat capacity c = c(T) of both rock and sodium, it uses forsterite (Mg₂SiO₄) as an analog, as measured by Robie et al. (1982). The heat conductivity of compacted material κ = κ(T) is taken as that of the H5 ordinary chondrite Wellman measured by Yomogida & Matsui (1983). Because it has been suggested that CK4 carbonaceous chondrites are Phaethon analogs (Clark et al. 2010), we apply the whole-rock density ρ_sil = 3000 kg m⁻³ as measured for such meteorites (Macke et al. 2011). We correct the heat conductivity for porosity ψ by applying the generation n = 2 Hertz factor h = h(ψ) function of Shoshany et al. (2002). This allows us to calculate an instantaneous thermal inertia,

$$\begin{eqnarray}&&{\rm{\Gamma }}=\sqrt{c(T)\rho (1-\psi )\kappa (T)h(\psi )}.\end{eqnarray} \tag{ 1 }$$

Trial runs indicated that ψ = 0.5 would yield 670 ≲ Γ ≲ 1000 J m⁻² s^−0.5 K⁻¹ for the surface temperature range experienced by Phaethon. This is similar to the Γ = 600 ± 200 J m⁻² s^−0.5 K⁻¹ range measured for Phaethon by Hanuš et al. (2016) and the ${\rm{\Gamma }}={880}_{-330}^{+580}\,{\rm{J}}$ m⁻² s^−0.5 K⁻¹ range measured for Phaethon by Masiero et al. (2019), which motivates our choice of porosity.

To describe sodium sublimation we applied the latent heat and Hertz–Knudsen formula parameters tabulated by Huebner (1970), which allowed us to define the corresponding saturation partial pressure ([Pa]) of sodium vapor above metallic sodium:

$$\begin{eqnarray}&&{p}_{\mathrm{sat}}(T)={10}^{1.7372\cdot ({ \mathcal L }-10)}{10}^{\alpha -\beta L/T}\sqrt{\displaystyle \frac{2\cdot {10}^{-3}\pi {k}_{{\rm{B}}}}{{N}_{{\rm{A}}}}},\end{eqnarray} \tag{ 2 }$$

where α = 29.770 and β = 0.21860 are constants, L = 2.46 · 10⁴ cal mole⁻¹ is the latent heat, ${ \mathcal L }=L/{R}_{0}{T}_{{\rm{s}}}\,=10.8$ (where R₀ is the universal gas constant and T_s is the sodium boiling temperature), k_B is the Boltzmann constant, and N_A is the Avogadro constant (Huebner 1970). Whenever the local partial pressure of sodium vapor is below p_sat(T) it will trigger sublimation if solid sodium is present. Conversely, sodium gas will recondense where the local partial pressure exceeds p_sat(T). We consider pure sodium here as the limiting case for what is occurring on Phaethon; in reality any sodium would initially be contained in a silicate mineral phase. The volume mass sublimation and condensation rates are calculated using standard expressions (e.g., Mekler et al. 1990; Prialnik 1992; Tancredi et al. 1994). Gas diffusion fluxes are calculated using the Clausing formula, evaluated as in Davidsson & Skorov (2002). These expressions require a number of geometrical parameters (r_g, L_t, r_t, ξ), for which we apply educated guesses (Table 1). At the modeled sublimation rate, the solution is not strongly sensitive to the diffusivity value that results from these geometrical parameters, as discussed by Davidsson (2021). If the diffusivity changes, the temperature and pressure profiles adjust in such a way that the same net mass-loss rate takes place and the same latent energy is being consumed.

The simulation was initiated at aphelion (Q = 2.40 au), assuming an initial temperature T₀ = 260 K, and terminated after the following perihelion (q = 0.14 au). This provided sufficient time for the near-surface region to lose memory of the arbitrary initial conditions. We also assumed that the (chondritic) sodium abundance m_Na initially was constant throughout the body.

Figure 1 shows that the surface temperature remains below ∼300 K at all latitudes, except during a four-month period around perihelion. Within four weeks of perihelion, the peak temperature exceeds ∼700 K and briefly reaches a maximum of 1050 K near the equator in the upper few centimeters of regolith as shown on the right in Figure 1. These results are comparable to a recently published thermophysical analysis of Phaethon by MacLennan et al. (2021) and Abe et al. (2020). Interestingly, the spin axis is oriented such that the northern hemisphere is illuminated up to perihelion. The equinox occurs close to this point, which keeps the south pole dark and cold until it suddenly receives substantial illumination. The drastic temperature jump of the south pole from nearly 100 K to above 700 K over the course of a week could potentially explain the surge of activity seen near perihelion (Jewitt & Li 2010; Li & Jewitt 2013).

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Figure 2 shows that the temperatures experienced by Phaethon are sufficiently high to cause significant sodium volatilization if the element is present in pure form. Within ∼1 au of the Sun, the sodium production rate increases six orders of magnitude, briefly reaching a molar perihelion production rate that parallels the water production rate of Comet 67P/Churyumov–Gerasimenko at 2.1 au (Fougere et al. 2016). If pure sodium were present and such strong outgassing takes place on Phaethon, it would readily cause comet-like ejection of dust into space. Figure 2 shows that the sodium sublimation front withdraws rapidly below the surface, reaching a depth of 0.4 m at the north pole at perihelion. The simulation did not explicitly model the erosion of the surface because of comet-like activity, therefore these depths should be considered upper limits on the front withdrawal per orbit.

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The pressure peak at the subsurface sublimation front forces sodium vapor to flow both outward and inward. The outward flow is responsible for the outgassing into space, whereas vapor flowing inwards eventually reaches colder material and recondenses. This causes a gradual build-up of sodium at depth. The NIMBUS simulations show that local concentrations that are at least twice as high as the original abundance can form. Such redistribution of solid sodium, especially when occurring slowly over time and when combined with impacts that expose deeper layers to space, could cause strong episodic bursts of activity. Figure 2 also shows the force ${\boldsymbol{F}}=-{dp}/{dr}\,\hat{r}$ caused by the pressure variation p = p(r) with the radial coordinate r. The outward force here reaches a peak of ∼20 N. Given the low surface gravity of Phaethon (g ≈ 0.001 m s⁻²), such a force would be capable of lifting meter-sized boulders. Note that these results assume a pure sodium phase just below the surface and thus are meant to represent a bounding case.

Pure sodium is not common in carbonaceous chondrites and most of the element is hosted by minerals like feldspar, nepheline [(Na,K)AlSiO₄], sodalite, and saponite (Rubin 1997). However, there are at least two mechanisms that could segregate sodium from the host and create a pure phase in bodies like Phaethon that come unusually close to the Sun. First, Čapek & Borovička (2009) assembled atomic diffusivity data for sodium for a range of relevant minerals and demonstrated that substantial Fickian diffusion could occur at temperatures reached by Phaethon on timescales similar to the orbital period, if the grains are sufficiently small. Heating of material at depths from which vapor transport to the surface is limited could build a pure sodium deposit over time as the sodium gas cools and condenses to a solid phase, which later could become exposed at the surface by, e.g., cratering or landslides. Second, laboratory experiments by Russell & Sanders (1994) showed that layers of pure sodium formed on the surface of NaCl particles when irradiated with 0.5 keV electrons. Solar wind electrons typically reach 10–50 eV, but superhalo electrons reach as much as 20–200 keV (Wang et al. 2010). The feasibility of sodium separation from different types of host minerals due to space weathering near the Sun should be studied further. Our simulations of sodium sublimation and outgassing from Phaethon should be considered a "best-case scenario" of how Phaethon's activity could behave in optimum conditions. Under such circumstances, Na-driven comet-like activity seems feasible. However, continued research is needed to better understand potential production mechanisms of pure sodium, the strength of activity at lower abundances of sodium or when the sodium content is present in different mineral phases, and the effect of surface-layer processing on Phaethon during multiple orbits.

3.1. Meteorite Sample

3.2. Experimental Setup

3.3. Experimental Results

The EDS full-chip maps of aluminum and sodium for the unheated and 1073 K chips are presented in Figures 3 and 4. These maps show the distribution of elements present in detectable abundances (>0.1 wt%). The EDS map of the unheated Chip G illustrates a close spatial association of Na with Al and Si but not with Cl, consistent with an interpretation that Na is present in aluminosilicates, likely nepheline and feldspar. Comparison of the EDS maps of the heated chips with the control sample shows that only the sample heated to 1073 K (Chip F) displays lower Na abundances than the control sample, whereas Al and Si are comparable to the control sample. The areas around the phases that should be sodium-rich in the heated sample do not show any increase in sodium content in the EDS maps, suggesting that the reaction that occurred was decomposition rather than Fickian diffusive loss. This suggests that Na-rich aluminosilicates in Chip F lost Na when heated to 1073 K (800 ° C).

EPMA data for Chip F and Chip G are listed in Table 2 for the nepheline and groundmass phases and Table 3 for the sodalite phases. Pyroxene and plagioclase about 20 μm away from the Na-rich phases did not show high Na content, further supporting that the Na-rich phases did not lose Na through Fickian diffusion. Note that after heating, sodalite and nepheline have been altered to different mineral phases. For simplicity, we refer to them as heated sodalite and nepheline. Low oxide totals of heated sodalite in Chip F (<97.6 wt%) could be caused by Na loss, fine grains, or bad polish. Sodalite in unheated Chip G is also fine-grained and similarly polished, but oxide totals are acceptable. Therefore, we regard the bad totals of heated sodalite to be mainly due to Na loss. Although these low totals are below expected values for complete analyses, we included them here to show the changes in Na contents. Compared to unheated sodalite in Chip G, heated sodalite Chip F clearly shows lower Na₂O and higher Al₂O₃, CaO, and MgO, although Cl and FeO appear to be similar between those two chips (Figure 5). It is unclear why Cl did not show comparable changes, though it may be due to the low overall Cl content (see the Appendix, Figures A1 and A2). Further studies are needed to investigate the behavior of Cl in sodalite during heating. The heated nepheline in Chip F contains much higher MgO and CaO than unheated ones in Chip G, although Al₂O₃ contents are comparable between the two chips (Figure 6). An increase in refractory components indicates that a small amount of mafic phases was annealed with the heated sodalite. However, the dilution effects by this mixing are inadequate to explain the decrease in Na. The heated nepheline in Chip F may be groundmass, given that its MgO and CaO contents are similar to altered groundmass in Kimura & Ikeda (1995). Regardless, Na₂O contents in heated nepheline in Chip F are much lower than unheated nepheline in Chip G or unheated groundmass in Kimura & Ikeda (1995). Using average values, heated sodalite and nepheline in Chip F show maximum Na loss of 55% and 35%, respectively. These results demonstrate that at peak temperature (1073 K) for a relatively short time (∼1 hr), Na can be lost from nepheline and sodalite. Although the changes in Chip F may be due to thermal decomposition under our experimental conditions, these results show that heating under vacuum may facilitate the generation of Na vapor, which will be explicitly tested in future work.

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Table 2. Electron Probe Microanalyzer Results—Nepheline/Groundmass

Oxide	Chip F						Chip G				K95 a			W11 a	K95 a
	Nepheline/Groundmass						Nepheline				Nepheline				Groundmass
SiO2	42.6	43.9	43.2	45.7	43.1	44.3	42.7	42.7	42.3	41.4	42.5	42.2	42.8	43.1	44.9	41.7	38.2
TiO2	<0.03	<0.03	0.05	<0.03	<0.03	<0.03	0.07	<0.03	<0.03	<0.03	<0.03	<0.03	<0.03	<0.03	0.27	0.32	0.36
Al2O3	37.0	36.4	33.3	37.1	35.9	33.8	35.9	35.2	35.3	35.3	34.6	32.8	33.7	35.7	26.3	28.2	27.3
Cr2O3	<0.03	0.04	<0.03	<0.03	<0.03	<0.03	0.06	0.09	<0.03	<0.03	<0.03	0.16	<0.03	<0.03	0.65	0.09	0.25
FeO	0.81	0.93	1.40	1.45	1.42	2.30	0.43	0.48	1.49	1.18	0.24	0.13	0.27	0.21	1.45	4.77	4.19
MnO	0.11	0.06	0.03	0.39	<0.02	<0.02	0.02	<0.02	<0.02	<0.02	0.07	0.06	<0.02	0.04	0.03	0.00	0.14
MgO	2.64	2.71	3.20	3.34	4.71	6.78	0.10	0.30	0.27	0.21	0.20	0.19	0.01	0.09	3.49	5.89	4.83
CaO	4.32	2.93	5.85	2.88	2.79	3.16	2.21	2.51	2.29	2.20	2.10	2.18	1.08	0.96	8.33	2.04	4.88
Na2O	11.4	12.8	11.7	9.84	10.2	9.09	16.9	17.5	15.0	15.8	17.9	18.2	19.2	17.4	13.2	14.6	14.2
K2O	0.44	0.49	0.37	0.29	0.24	0.17	2.18	2.12	1.92	1.91	2.09	1.82	1.93	1.50	1.27	1.56	0.01
Cl	<0.02	<0.02	<0.02	<0.02	<0.02	0.08	<0.02	<0.02	0.02	0.04	0.02	<0.02	<0.02	<0.02	<0.02	<0.02	4.66
Total	99.4	100.2	99.1	101.0	98.3	99.6	100.5	101.0	98.6	98.1	99.7	97.7	98.9	99.0	99.8	99.2	99.0

Note.

^a References from the literature: K95 = Kimura & Ikeda (1995), W11 = Wasserburg et al. (2011).

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Table 3. Electron Probe Microanalyzer Results—Sodalite

Oxide	Chip F						Chip G			K95 a		W11 a
SiO2	39.1	40.6	39.2	42.0	39.9	36.9	38.2	36.3	36.7	36.3	36.0	39.1
TiO2	<0.03	0.04	<0.03	<0.03	<0.03	0.08	<0.03	<0.03	<0.03	<0.03	<0.03	<0.03
Al2O3	32.4	33.9	34.4	35.7	34.4	32.7	31.7	31.6	32.3	30.9	31.1	35.2
Cr2O3	<0.03	0.05	0.03	<0.03	0.05	0.04	<0.03	<0.03	0.06	<0.03	<0.03	<0.03
FeO	1.41	0.86	1.50	1.56	1.57	2.10	0.67	1.08	1.49	0.74	0.70	0.18
MnO	<0.02	<0.02	0.09	0.14	0.04	0.22	0.03	<0.02	<0.02	<0.02	<0.02	<0.02
MgO	0.84	1.77	1.18	1.43	2.38	3.23	0.15	0.09	0.39	0.10	0.17	0.08
CaO	2.96	3.42	0.53	0.79	1.13	1.47	0.44	0.54	1.37	0.08	0.17	0.07
Na2O	11.3	11.2	8.4	6.6	7.4	11.5	18.4	23.5	22.3	25.4	24.4	17.9
K2O	0.10	0.07	0.03	0.05	0.06	0.14	0.12	0.03	0.04	0.04	0.02	<0.02
Cl	5.52	5.61	7.89	7.72	6.72	6.76	6.89	7.32	6.78	6.68	7.03	7.10
Total	93.6	97.6	93.4	96.0	93.6	95.1	96.6	100.5	101.4	100.2	99.6	99.6

Note.

^a References from the literature: K95 = Kimura & Ikeda (1995), W11 = Wasserburg et al. (2011).

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Nepheline in carbonaceous chondrites is a secondary mineral formed through thermal metamorphism or metasomatism of primary materials (e.g., Ikeda & Kimura 1995; Kimura & Ikeda 1995; Krot et al. 1998). Another mineral, sodalite [Na₄(Si₃Al₃)O₁₂Cl], could also be a source for Na volatilization. Nepheline and sodalite in carbonaceous chondrites typically occur as accessory phases in trace quantities, but samples with high abundances are present (e.g., Wasserburg et al. 2011). Surface materials on Phaethon could possibly contain significant abundances of nepheline or sodalite that would contribute to the observed vaporization event.

Previous heating experiments of Allende conducted by Wulf et al. (1995) reached higher temperatures than those in our study, with samples heated for longer times. As such, those experiments probed a different regime of thermal exposure than our work, more appropriate to the study of silicate vaporization. In those experiments, powder and chips of Allende and Murchison were heated to 1323–1623 K (1050 °C–1350 °C) under different oxygen fugacities. Results by Wulf et al. (1995) showed that more Na is lost under reduced conditions. Na vaporization can be written in a generalized reaction form: $\tfrac{1}{2}$ Na₂O (s or l) = Na (g) + $\tfrac{1}{4}$ O₂ (g). Thus, the lower oxygen fugacity drives this reaction toward the right-hand side. At 1323 K, only 26% bulk Na was lost from samples heated in air. In contrast, our experiments showed up to 55% loss of Na from sodalite and nepheline. We attribute this difference to the fact that Na is hosted in this sample in both refractory grains (albite) and secondary phases (nepheline and sodalite). While our analysis focused only on the secondary phase minerals, the bulk analysis contains both refractory and secondary phases and therefore would have a more muted sodium loss. More importantly, our experimental results demonstrate that Na can be lost from secondary minerals such as nepheline in a short heating period that is predicted from the thermophysical modeling above. Thus, the observed losses combined with the predictions from models support our theory that sodium can be a driver of activity on small, near-Sun asteroids.

The OSIRIS-REx spacecraft has observed particles recently launched in short-term stable orbits around the carbonaceous NEO (101955) Bennu (Lauretta et al. 2019). These centimeter-scale objects were observed to be ejected from the surface, typically in the late afternoon on low-energy orbits. Numerous possible ejection mechanisms including thermal fracturing, electrostatics, and phyllosilicate dehydration were investigated, but no direct cause was confirmed. The surface of Bennu does not reach the same peak temperatures as that of Phaethon, making sodium volatilization unlikely to be responsible for these events. Still, Bennu stands as another interesting example of unexpected forms of activity on NEOs in addition to Phaethon.

Recent work by MacLennan et al. (2021) describes the long-term evolution of the thermal history of Phaethon, showing that it experiences cyclical changes to its maximum heating from the Sun. Based on those results, Phaethon currently is just past the peak of its ∼18 kyr heating cycle, which would imply that sodium volatilization should be ramping down. This heating history is consistent with Phaethon having had higher levels of sodium-driven activity in the past few thousand years. This could then explain how Phaethon created the Geminid meteor stream in the recent past but has a present-day activity insufficient to produce it. Additional models by MacLennan et al. (2021) predict that asteroid (155140) 2005 UD, which may have been formed from an earlier splitting of Phaethon, is at the beginning of a new heating cycle, and we would expect sodium volatilization to increase over the next ∼5 kyr from a currently quiescent state.

When considering the broader NEO population, Granvik et al. (2016) demonstrated that there is a significant lack of low-albedo asteroids with very low-perihelion distances. They attribute this lack of small, dark objects to extreme thermal events, though for many of the asteroids with low perihelia the time to reach these orbits is nontrivial. We would therefore not expect a breakup scenario like what is observed for low-perihelia comets, as these NEOs spend a significant time heated to well above the temperature for ice sublimation. Any existing ice must be deeply buried and would not respond rapidly (or catastrophically) to heating. Sodium volatilization, then, could act as the needed trigger to remove some of the dehydrated surface material and expose enough ice-rich subsurface to trigger the catastrophic breakups needed to explain the observations of Granvik et al. (2016).

Both our theoretical thermophysical modeling and laboratory analyses indicate that sodium has the potential to mobilize dust on the surface of small bodies when other more volatile agents (e.g., water) are not present. This mechanism could then explain the activity seen on objects with very low perihelia such as Phaethon. However, this activity would depend strongly on the mineral phases present and the specific surface geology of these objects.

Further work is needed to make more specific predictions for Phaethon, including conducting heating experiments in a vacuum and using meteorite materials that are closer analogs to Phaethon than Allende. Likewise, future investigations of larger quantities of heated analog material would enable testing of the efficiency of deposition of a pure sodium layer that could later be rapidly triggered to cause outgassing. Results of these studies then would lead to improved models of dust mobilization on the Phaethon's surface.

The DESTINY+ mission that is currently being pursued by JAXA (Arai et al. 2018, 2019) will provide in situ observations of Phaethon for the first time. These data may allow us to determine if, and to what extent, sodium contributes to the observed activity on Phaethon. If Phaethon's dust emission is indeed driven by sodium desorption, this mechanism provides another avenue for mass loss on small solar system bodies, especially those close to the Sun.

More broadly, simulations of the orbital evolution of the whole NEO population by Marchi et al. (2009) indicate that approximately 2% of the current NEO population had or will have a perihelion distance within 0.1 au at some point in their dynamical lifetimes. This means that sodium volatilization would not be expected to be restricted to just Phaethon, but instead may drive activity on a small but important subset of the whole NEO population. Investigations of other asteroids with small perihelia will allow us to determine the extent to which this novel form of activity is present among the asteroids.

The authors thank Vishnu Reddy for assistance in acquiring the meteorite samples used for this work. The authors also thank the anonymous referees, who provided suggestions that improved this work. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D004). Funding for this work was provided by a JPL Lew Allen award.

Maps of Cl location within the chips for the control sample (Figure A1) and heated sample (Figure A2), for comparison with the sodium maps.

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