Activity in Geminid Parent (3200) Phaethon

Abstract

The asteroid (3200) Phaethon is widely recognized as the parent of the Geminid meteoroid stream. However, it has never shown evidence for ongoing mass loss or for any form of comet-like activity that would indicate the continued replenishment of the stream. Following an alert by Battams & Watson, we used NASA's STEREO-A spacecraft to image Phaethon near perihelion, in the period UT 2009 June 17-22, when the heliocentric distance was near 0.14 AU. The resulting photometry shows an unexpected brightening, by a factor of two, starting UT 2009 June 20.2 ± 0.2, which we interpret as an impulsive release of dust particles from Phaethon. If the density is near 2500 kg m −3 , then the emitted dust particles must have a combined mass of ∼2.5 × 10 8 a 1 kg, where a 1 is the particle radius in millimeters. Assuming a 1 = 1, this is approximately 10 −4 of the Geminid stream mass and to replenish the stream in steady state within its estimated ∼10 3 yr lifetime would require ∼10 events like the one observed, per orbit. Alternatively, ongoing mass loss may be unrelated to the event which produced the Phaethon-Geminid complex. An impact origin of the dust is highly unlikely. Phaethon is too hot for water ice to survive, rendering the possibility that dust is ejected through gas drag from sublimated ice unlikely. Instead, we suggest that Phaethon is essentially a rock comet, in which the small perihelion distance leads both to the production of dust (through thermal fracture and decomposition cracking of hydrated minerals) and to its ejection into interplanetary space (through radiation pressure sweeping and other effects).

The Active Asteroids

“…All these objects may be related to a precursor body that disintegrated ∼10 3 yr ago (Ohtsuka et al 2006). Evidence for modern-day mass loss comes from the observed brightening of Phaethon by a factor of two within a few hours of perihelion (R = 0.14 AU) in 2009 (Jewitt & Li 2010). This rapid brightening cannot be due to scattering from the ∼5 km diameter nucleus alone and, instead, indicates transient mass loss with the ejection of ∼10 8 a 1 kg of particles each a 1 mm in radius.…”

“…Young's moduli Y = (10-100) × 10 9 N m −2 are typical for rock (Pariseau 2006, p. 474). If converted into kinetic energy with efficiency η, thermal strain energy can generate fractured material with speeds (Jewitt & Li 2010) …”

“…These are sufficient to trigger thermal fracture and to dehydrate waterbearing minerals, if they are present (Jewitt & Li 2010). It has been suggested that Phaethon was recently scattered inward from a Pallas-like orbit (a = 2.771 AU, e = 0.281, i = 33 • ; de León et al 2010), where a large proportion of the asteroids are volatile-rich B and C types.…”

The Main Belt Comets and ice in the Solar System

“…There are other hypothesised effects that have yet to be conclusively demonstrated to explain observed 'activity', such as thermal cracking, electrostatic levitation of dust, or radiation pressure accelerating dust away from the surface (Jewitt et al 2015c). We also exclude dynamically asteroidal objects (i.e., with T J > 3.05) outside of the asteroid belt for which activity has been detected, e.g., (3200) Phaethon (Jewitt and Li 2010;Li and Jewitt 2013;Hui and Li 2017) and 107P/(4015) Wilson-Harrington (Fernandez et al 1997). See Jewitt et al (2015c) and references therein for more extensive discussion of active asteroids beyond our definition of 'MBC'.…”

Comet 322p/Soho 1: An Asteroid With the Smallest Perihelion Distance?*

“…Although these objects are almost certainly active when in the SOHO fields of view (0.15 au), this activity is not necessarily due to the traditional "cometary" mechanism of sublimation of volatile ices. Jewitt & Li (2010) and Li & Jewitt (2013) argue that (3200) Phaethon produces dust near perihelion (0.14 au) via non-traditional means such as thermal fracture, while Kimura et al (2002) and others have shown that silicates and other refractory materials begin to sublimate at even smaller distances.…”