Sodium Brightening of (3200) Phaethon near Perihelion

Abstract

Sunskirting asteroid (3200) Phaethon has been repeatedly observed in Solar Terrestrial Relations Observatory (STEREO) Heliospheric Imager 1 (HI1) imagery to anomalously brighten and produce an antisunward tail for a few days near each perihelion passage, phenomena previously attributed to the ejection of micron-sized dust grains. Color imaging by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectrometric Coronagraph (LASCO) during the 2022 May apparition indicates that the observed brightening and tail development instead capture the release of sodium atoms, which resonantly fluoresce at the 589.0/589.6 nm D lines. While HI1's design bandpass nominally excludes the D lines, filter degradation has substantially increased its D line sensitivity, as quantified by the brightness of Mercury’s sodium tail in HI1 imagery. Furthermore, the expected fluorescence efficiency and acceleration of sodium atoms under solar radiation readily reproduce both the photometric and morphological behaviors observed by LASCO and HI1 during the 2022 apparition and the 17 earlier apparitions since 1997. This finding connects Phaethon to the broader population of sunskirting and sungrazing comets observed by SOHO, which often also exhibit bright sodium emission with minimal visible dust, but distinguishes it from other sunskirting asteroids without detectable sodium production under comparable solar heating. These differences may reflect variations in the degree of sodium depletion of near-surface material and thus the extent and/or timing of any past or present resurfacing activity.

The Wagging Plasma Tail of Comet C/2020 S3 (Erasmus)

“…Sodium atoms have a very large solar radiation pressure acceleration (e.g., >100 × solar gravity), and so closely track the antisolar direction. For example, a sodium tail was recently reported on asteroid 3200 Phaethon near perihelion using the same instruments as we use in this work (Hui 2023;Zhang et al 2023). However, our data provide no clear evidence for a neutral sodium tail perhaps for the reason that S3 is not as bright as comet C/1995 O1 Hale-Bopp (Cremonese et al 1997).…”

The DESTINY + Dust Analyser — a dust telescope for analysing cosmic dust dynamics and composition

“…Recurrent dust ejection and a dust tail at perihelion were reported in Refs. [11,13,14], while an observed tail at perihelion was more recently attributed to sodium emission [15]. No coma or tail was observed around 1.0-1.5 AU [11,[16][17][18].…”

“…sublimation and thermal fracturing) for Phaethon because of the nature of activity observed during perihelion passage. Thermal decomposition of rock-forming minerals and subsequent release of volatiles including sodium is another potential mechanism as sodium release from other celestial bodies in the inner solar system has also been observed [15,21,22]. DESTINY + will fly by Phaethon at a distance of approximately 500 km, and the DDA instrument will be able to directly analyse dust released from Phaethon's surface [18,23].…”

The Sensitivity of NEO Surveyor to Low-perihelion Asteroids

“…Although numerous studies of this object were undertaken, detection of any activity was not seen until a small tail was identified by Jewitt et al (2013) in STEREO data during the object's 2009 and 2012 perihelion passes. Multiple theories have been put forth for the driver of observed activity, including thermal fracturing (Ryabova 2018) and volatilization of sodium (Masiero et al 2021;Zhang et al 2023), but all are a result of the extreme temperatures Phaethon experiences at perihelion.…”