Wednesday, April 10, 2013

Activity on Phaethon

Phaethon is an asteroid with an unusual orbit that brings it very close to the Sun. For any object orbiting the Sun, the point in its orbit where it is closest to the Sun is known as perihelion. At perihelion, Phaethon comes as close as 0.14 AU from the Sun and its surface gets intensely heated to a temperature of ~1000 K. In Greek mythology, Phaethon was the name of the son of the sun god Helios. The asteroid Phaethon measures about 5 km in diameter and is believed to be the parent body of the Geminid meteoroid stream. On Earth, meteors from the Geminid meteoroid stream usually peak around the 13th to 14th December.

NASA’s STEREO spacecraft detected anomalous optical brightening of Phaethon each time when the asteroid was at perihelion in 2009 and 2012. At perihelion, Phaethon brightened by a factor of ~6 and the brightening lasted for a duration of about 2 days. Such a brightening is far too large and too long-lived to be attributed simply to the variation in brightness caused by rotation of the irregularly-shaped asteroid every ~3.6 hours. The longevity of the brightening also means that it cannot be attributed to reflection from a mirror-like patch on the asteroid’s surface. Furthermore, the sublimation of water ice cannot be the cause of brightening because surface temperatures on Phaethon are too high for water ice to exist in the first place. Even the estimated interior temperature of Phaethon is too high for deeply buried water ice to survive for long.

Image: Artist’s representation of P/2012 F5 - an asteroid discovered in March 2012 from the Mount Lemmon Observatory in Arizona (USA). It has a trail of dust particles caused by internal rupture or collision with another asteroid. Credit: Servicio de Información y Noticias Científicas (SINC)

A plausible explanation for the anomalous optical brightening of Phaethon at perihelion is the presence of ejected dust particles. For this to occur, the combined cross-sectional area of all ejected dust particles has to be larger than the cross-sectional area of Phaethon itself so as to scatter enough sunlight to account for the brightening of the asteroid. If Phaethon has a density of 3000 kg/m3 and the average grain size is 1 mm, the required mass of dust is ~4×108 kg. This amount of dust is negligible in comparison to the estimated mass of Phaethon at ~2×1014 kg. Thermal fracture and thermal decomposition of surface minerals serve as possible mechanisms for the ejection of dust grains from Phaethon at perihelion.

As a result, Phaethon is like a “rock comet” where a coma of ejected dust grains forms around the asteroid during each perihelion passage. The subsequent decline in brightness can be explained as the illuminated side of Phaethon and its coma of dust grains gradually face away from STEREO. In addition, sublimation or disintegration of dust grains can also aid in the decline in brightness. The ejection of dust from Phaethon during each perihelion passage is likely to contribute somewhat to the Geminid meteoroid stream.

Jing Li and David Jewitt (2013), “Recurrent Perihelion Activity in (3200) Phaethon”, arXiv:1304.1430 [astro-ph.EP]