Recurrent Perihelion Activity in (3200) Phaethon (1304.1430v1)

Abstract: We present a study of planet-crossing asteroid (3200) Phaethon at three successive perihelia in 2009, 2010 and 2012, using the NASA STEREO spacecraft. Phaethon is clearly detected in 2009 and 2012, but not in 2010. In both former years, Phaethon brightened unexpectedly by ~1 magnitude at large phase angles, inconsistent with the ~1 magnitude of steady fading expected from a discrete, macroscopic body over the same phase angle range. With a perihelion distance of 0.14 AU and surface temperatures up to ~1000 K, a thermal origin of this anomalous brightening is strongly suspected. However, simple thermal emission from Phaethon is too weak, by a factor >1000, to explain the brightening. Neither can ice survive on this body, ruling out comet-like sublimation. Our preferred explanation is that brightening occurs as a result of dust produced and ejected from Phaethon, perhaps by thermal fracture and/or thermal decomposition of surface minerals when near perihelion. A contribution from prompt emission by oxygen released by desiccation of surface minerals cannot be excluded. We infer an ejected mass of order 4x10^8 a_mm kg per outburst, where a_mm is the mean dust radius in millimeters. For plausible dust radii, this mass is small compared to the estimated mass of Phaethon (~2x10^14 kg) and to the mass of the Geminid stream (10^12 kg to 10^13 kg) with which Phaethon is dynamically associated. Perihelion mass-loss events like those observed in 2009 and 2012 contribute to, but do not necessarily account for the Geminids stream mass.

• The paper analyzes unexpected photometric brightening observed during asteroid (3200) Phaethon's 2009 and 2012 perihelion passages, which are dynamically linked to the Geminid meteor stream.
• The study proposes dust emission via thermal fracture or breakdown of hydrated minerals near perihelion as the cause for brightening, ruling out ice sublimation, thermal emission, and solar wind.
• Findings suggest that while current perihelion activity contributes dust, Phaethon and related objects likely originated from a significant past breakup event, opening avenues for future spectroscopic and observational research.

An Examination of Recurrent Perihelion Activity in Asteroid (3200) Phaethon

This paper presents a comprehensive analysis of the asteroid (3200) Phaethon, an active object dynamically associated with the Geminid meteor stream. Notably, Phaethon exhibits surprising photometric behavior near its perihelion, characterized by unexpected brightening observed during its 2009 and 2012 perihelion passages, while no such activity was detected during its 2010 passage. The paper leverages data from the NASA STEREO spacecraft to analyze these observations and proposes a plausible explanation for the anomalous brightening.

Observational Findings

Phaethon was investigated using the Heliospheric Imagers onboard the NASA STEREO spacecraft, which provide a unique vantage point for capturing photometric data of celestial objects at small solar elongations. The paper meticulously detailed the observational conditions across the years 2009, 2010, and 2012, noting specific challenges such as variations in observing geometry that impacted the ability to continuously monitor Phaethon. Clear detections of Phaethon were made in both 2009 and 2012, with each occurrence of brightness enhancement preceding perihelion by approximately half a day. However, during the 2010 perihelion window, Phaethon was not visible due to unfavorable geometrical conditions, which severely limited the observational data available during that time.

Brightening Cause Analysis

The paper concludes that the unexpected brightening phenomena observed near Phaethon's perihelion cannot be attributed to common cometary activities such as sublimation of water ice or other volatile components, given the extreme surface temperatures that preclude the survival of ice. Thermal emission calculations show that the temperature at perihelion, though exceedingly high (up to 1000 K), falls significantly short of explaining the observed optical brightening in the STEREO bandpass. Similarly, the solar wind, another potential source of energy for brightening, is noted as an insufficient cause due to its negligible kinetic energy under these conditions.

Dust emission presents a more viable explanation. Thermal fracture and the breakdown of hydrated minerals due to perihelion temperatures are hypothesized mechanisms for dust production. The authors estimate a dust mass on the order of 4×10^8 a_mm kg per event, suggesting that despite the frequency and consistency required for this process to account for the entire Geminid mass, it remains a plausible contributor to the observed optical phenomenon.

Implications and Future Prospects

The findings offer intriguing insights into the relationship between Phaethon and the Geminid meteor stream. They suggest that although ongoing, perihelion-driven activity contributes to the stream, it is unlikely to be the sole origin. Instead, Phaethon and related bodies might have resulted from a significant breakup event in the distant past.

The paper points to several areas for future investigation, including the determination of the mean grain size in the dust ejected, understanding the delayed onset of brightening post-perihelion, and more detailed spectroscopic observations for gaseous emissions. These future works could provide further elucidation on the mechanisms at play and refine the understanding of the thermally-induced processes on Phaethon. They could also explore potential meteorite detections tied to recent perihelion events, providing a broader narrative about the asteroid's ongoing evolution and impact within the solar system.

In conclusion, this paper underscores the complexities inherent in the coupled dynamics and composition of Phaethon. It lays a foundation for future observational and theoretical work aimed at unraveling the intricate physical processes occurring in small solar system bodies as they transit extreme thermal environments.