Dynamical Evolution and Thermal History of Asteroids (3200) Phaethon and (155140) 2005 UD

Abstract

The near-Earth objects (NEOs) (3200) Phaethon and (155140) 2005 UD are thought to share a common origin, with the former exhibiting dust activity at perihelion that is thought to directly supply the Geminid meteor stream. Both of these objects currently have very small perihelion distances (0.140 and 0.163 au for Phaethon and 2005 UD, respectively), which results in them having perihelion temperatures of or exceeding 1000 K. NEO population models compared to discovery statistics suggest that low-perihelion objects are destroyed over time by a temperature-dependent mechanism that becomes relevant at heliocentric distances < 0.3 au. Thus, the current activity from Phaethon is relevant to the destruction of NEOs close to the Sun, which most likely has produced meteor streams linked to asteroids in the past.We model the past thermal characteristics of Phaethon and 2005 UD using a combination of a thermophysical model (TPM) and orbital integrations of each object. Temperature characteristics such as maximum daily temperature, maximum thermal gradient, and temperature at varying depths are extracted from the model, which is run for a predefined set of a and e. Next, dynamical integrations of orbital clones of Phaethon and 2005 UD are used to estimate the past orbital elements of each object. These dynamical results are then combined with the temperature characteristics to model the past evolution of thermal characteristics such as maximum (and minimum) surface temperature and thermal gradient.We find that dwarf planet (2) Pallas is unlikely to be the parent body for Phaethon and 2005 UD, and it is more likely that the source is in the inner part of the asteroid belt in the families of, e.g., (329) Svea or (142) Polana. The orbital histories of Phaethon and 2005 UD are characterized by cyclic changes in e, resulting in perihelia values periodically shifting between present-day values and 0.3 au. Currently, Phaethon is experiencing relatively large degrees of heating when compared to the recent 20, 000 yr. We find that the subsurface temperatures are too large over this timescale for water ice to be stable, unless actively supplied somehow. The near-surface thermal gradients strongly suggest that thermal fracturing may be very effective at breaking down surface regolith. Observations by the DESTINY+ flyby mission will provide important constraints on the mechanics of dust-loss from Phaethon and, potentially, reveal signs of activity on 2005 UD.

Volatility of Sodium in Carbonaceous Chondrites at Temperatures Consistent with Low-Perihelia Asteroids

“…Still, Bennu stands as another interesting example of unexpected forms of activity on NEOs in addition to Phaethon. MacLennan et al (2020) describe the long-term evolution of the thermal history of Phaethon, showing that it experiences cyclical changes to its maximum heating from the Sun. Based on those results, Phaethon currently is just past the peak of its ∼ 18 kyr heating cycle, which would imply that sodium volatilization should be ramping down.…”

“…Any ice thermally coupled to the surface would be expected to have been removed quickly after Phaethon's dynamical transfer into its current orbit, and not survive to the present day. MacLennan et al (2020) have shown that over the course of its orbital evolution Phaethon has undergone numerous cycles through high peak temperatures like today, which would have baked off any water near the surface. This means water ice cannot be driving the observed activity, which is seen only in a short temporal period around Phaethon's perihelion.…”

“…Previous investigations have looked into the effects of the extreme temperatures that Phaethon experiences at perihelion (Delbo et al 2014;Ryabova 2018;Yu et al 2019;MacLennan et al 2020). These studies have found that fracturing, driven by thermal cycling, can efficiently fragment boulders with Phaethon's expected surface composition.…”

Investigating the Relationship between (3200) Phaethon and (155140) 2005 UD through Telescopic and Laboratory Studies

“…However, given Phaethon's non-zero inclination and high eccentricity, the number of plausible parent bodies or families is not extremely numerous. The recent work by MacLennan et al 2020 shows that the high-inclination inner Main Belt Svea family might be a better match than previous candidates like the family of (2) Pallas (de León et al 2010), so further characterization of objects in that family is of interest.…”

“…If the two objects really are only coincidentally similar, then there should be multiple pathways into Phaethon / Geminid -like orbits. While there are serious issues to be resolved in understanding whether or not Phaethon is a heated and devolatilized member of the Pallas Collisional Family (de León et al 2010;Kareta et al 2018;MacLennan et al 2020), 2005 UD's spectrum is a very poor match for Pallas or any member of the Pallas Collisional Family (they are all strongly blue in the near-infared, many with Phaethon-like concave up spectra), suggesting that those problems would be even more challenging to resolve for it. Furthermore, the surface of the less studied candidate member of the Phaethon-Geminid-Compolex 1999 YC (Ohtsuka et al 2008) is an even worse fit (Kasuga & Jewitt 2008), so the problems continue in that arena.…”