Near-infrared observations of active asteroid (3200) Phaethon reveal no evidence for hydration

Asteroid (3200) Phaethon is a near-Earth asteroid (NEA) classified as an Apollo-type object. It's renowned as the parent body of the Geminid meteor shower, one of the most prolific meteor showers observable from Earth. While initially considered inactive, observations have revealed that Phaethon develops a small dust tail for a short period after perihelion passage. The origin of this dust tail remains a key question in planetary science. Theories have included thermal fracturing of the surface due to extreme heating from its close solar approach, or the possible sublimation of volatiles, similar to cometary activity. However, the high temperatures reached by Phaethon at perihelion (0.14 AU) make the survival of water ice on its surface highly improbable. The asteroid's activity thus presents a unique puzzle. Its dynamical similarity to asteroid 2005 UD suggests a shared origin, possibly as fragments of a larger, primitive precursor object. Understanding Phaethon's activity is crucial for both dating the asteroid and gaining insights into the nature of this significant meteor shower. Phaethon's size, estimated at 5.1 ± 0.2 km in effective diameter and 6.1 km in equatorial diameter, categorizes it as one of the largest potentially hazardous asteroids (PHAs). The asteroid's physical characteristics, including albedo (ranging from 0.08 to 0.16), rotational period (approximately 3.604 hours), and spectral classification (B-type according to the SMASS taxonomy), have been studied previously. Notably, many B-type asteroids, including (2) Pallas and (101955) Bennu, exhibit hydration features indicated by a strong absorption band near 3 µm. However, the extreme temperatures Phaethon experiences make the presence of hydrated minerals on its surface questionable. The planned DESTINY+ mission will provide crucial close-up observations of Phaethon, offering a valuable opportunity for further investigation. This study uses near-infrared observations to investigate the presence of hydration on Phaethon’s surface, aiming to elucidate the mechanism behind its unique activity.

Previous studies on (3200) Phaethon have focused on characterizing its orbit, spin state, and thermophysical properties. Hanuš et al. (2016) provided a detailed characterization of these parameters, while Jewitt and Li (2010) and Jewitt et al. (2013) reported on the asteroid's activity and the origin of its dust tail. Kareta et al. (2018) conducted rotationally resolved spectroscopic observations in the near-infrared (NIR) range (0.7–2.5 µm), observing minimal surface variation, apart from slight curvature in the NIR reflectance spectrum. Studies like those of de León et al. (2010) have proposed a compositional and dynamical connection between Phaethon and the B-type asteroid (2) Pallas, noting similarities in their spectral features in the 0.5–2.5 µm spectral region. Masiero et al. (2019) used NEOWISE observations to estimate Phaethon's geometric albedo, finding it consistent with that of Pallas. However, unlike Phaethon, Pallas exhibits a prominent 3 µm absorption band indicative of phyllosilicates. The study of Bennu by Hamilton et al (2019) and Lauretta et al (2019) provided insights into a similar B-type asteroid showing evidence of hydrated minerals and weak activity, raising questions about a potential common origin or evolutionary pathway with Phaethon.

This study employed rotationally resolved spectroscopic observations of asteroid (3200) Phaethon using the long-wavelength cross-dispersed (LXD) mode of the SpeX spectrograph/imager at the NASA Infrared Telescope Facility (IRTF). Ten LXD datasets were acquired on December 12, 2017, aiming to cover a complete rotational phase, primarily focusing on the northern hemisphere and equatorial region. Data reduction involved flat-field correction, subtraction of OH emission and sky thermal emission, and background removal. Telluric absorption corrections were performed using a solar analog star (SAO 39985). Wavelength calibration was achieved using argon lines (λ < 2.5 µm) and telluric absorption lines (λ > 2.5 µm). Spextool (v4.0) software was used to process the data. The long-wavelength spectra (1.9-4.2 µm) contained both thermal and reflected components requiring thermal excess removal. The Near-Earth Asteroid Thermal Model (NEATM), based on the Standard Thermal Model (STM), was utilized. Phaethon's visible geometric albedo (pv = 0.122 ± 0.008) and slope parameter (G = 0.06) were used as inputs. The beaming parameter (f) was adjusted to best fit the model thermal excess to the measured thermal flux for each dataset. Bolometric and spectral emissivities were assumed to be 0.9. A K to V scale (K/V – 0.7) was applied to reconcile reflectance values at different wavelengths. Band depths at 2.90 µm were calculated using a specific formula considering the reflectance at 2.90 µm (R2.90) and the continuum reflectance at 2.45 µm (Rc), along with associated uncertainties.

The key finding of this research is the absence of a detectable 3-µm absorption band in the observed spectra of (3200) Phaethon. The analysis of ten rotationally resolved spectra, covering most of the asteroid's northern hemisphere and equatorial regions, revealed that the 2.90 µm band depths, indicative of hydration, were consistently within the 2σ uncertainty range, indicating no significant evidence of hydrated minerals on the surface. This means that the observed surface of Phaethon shows no significant signs of water-bearing minerals, at least not in the depth probed by the near-infrared spectroscopy. Table 1 in the paper shows the derived band depths at 2.90 µm and associated uncertainties for each of the ten observation sets (a–j). The values range from -5.43% to 8.52%, with uncertainties exceeding the magnitude of the band depth in most cases. This lack of a significant negative band depth contrasts with previous research that had reported a weak absorption band around 0.43 microns, which was attributed to hydrated minerals. This discrepancy might be explained by the different spectral range and potentially different spatial coverage of the observations. The absence of a 3 µm band, even taking into account the uncertainties, suggests that the observed portion of Phaethon’s surface is not hydrated. This conclusion is further supported by the comparison with the spectrum of (2) Pallas, another B-type asteroid which exhibits a significant 3 µm absorption band due to the presence of phyllosilicates on its surface. Figure 3 provides a visual comparison of the spectra of (2) Pallas and (3200) Phaethon.

The absence of hydration features in the near-infrared spectra suggests that volatile sublimation and phyllosilicate devolatilization are unlikely to be the primary drivers of Phaethon's observed activity. This finding supports the hypothesized connection between Phaethon and the Pallas family of asteroids. If Phaethon originated from the Pallas family, it might have initially contained hydrated minerals but subsequently lost its volatiles due to intense heating during its close approach to the Sun. The high surface temperatures (estimated to exceed 1000 K) are capable of causing dehydration of even the most thermally stable hydrated minerals. Alternatively, Phaethon may have formed from anhydrous material, negating the presence of water ice or hydrated minerals in its composition. The uniform dry appearance of Phaethon's surface could be the result of extensive heating over thousands of years, homogenizing any original compositional variations. The comparison with the active asteroid (101955) Bennu, which shows evidence of both hydration and weak activity, raises questions about the possible shared evolutionary pathways between these two B-type NEAs. The current data does not allow for a definitive determination between whether Phaethon was originally hydrated and later dehydrated, or was simply formed from anhydrous materials. Further investigations, particularly the DESTINY+ mission, will be essential in resolving this question.

This study provides strong evidence against the presence of significant hydrated minerals on the observed surface of asteroid (3200) Phaethon. The lack of a 3 µm absorption feature indicates that volatile sublimation of phyllosilicates is unlikely to be the main driver of its observed activity. The results support the potential connection of Phaethon to the Pallas family, suggesting either an originally anhydrous composition or substantial dehydration during its evolutionary history. Future missions like DESTINY+ will be crucial for resolving remaining uncertainties about Phaethon's origin and the mechanisms underlying its unique activity.

The study's primary limitation is the incomplete spatial coverage of the observations. While the observations covered a significant portion of the asteroid's surface, primarily the northern hemisphere and equatorial region, the southern polar region was not observed. The possibility of localized hydration in the unobserved areas therefore cannot be entirely ruled out. Furthermore, the near-infrared spectroscopy primarily probes the surface layers, potentially missing deeper hydration within the asteroid's interior.