Instantaneous thermally-driven erosion can explain dearth of dark near-Sun asteroids

TL;DR

Problem: the dearth of dark near-Sun asteroids and the processes driving their disruption. Approach: SHINeS vacuum irradiance experiments on CI-type carbonaceous simulants to model thermally driven erosion at $r$ between $0.07$ and $0.25\,\mathrm{au}$. Findings: CI-like material erodes rapidly, with destruction times decreasing at smaller $r$ and detectable volatile release; destruction can be nearly instantaneous at the closest distances. Significance: supports a purely thermally driven disruption pathway that can shape NSA distributions and informs interpretations of activity in objects like (3200) Phaethon and 322P/SOHO 1, with implications for asteroid population models and hazard assessment.

Abstract

Recent models of the near-Earth asteroid population show that asteroids must be super-catastrophically destroyed when they evolve to orbits with perihelion passages well inside of Mercury's orbit. The heliocentric distances at which the disruptions typically occur are tens of solar radii, which is too far from the Sun for asteroids to be destroyed by sublimation and tidal disruption. The typical disruption distance also appears to be larger for darker asteroids. Here, by carrying out irradiance experiments in vacuum that replicate the conditions in the near-Sun environment, we show that CI meteorite simulants are destroyed within minutes when exposed to the level of solar irradiance encountered at heliocentric distances of up to about 0.2 au. Our results provide an explanation for the scarcity of dark, carbonaceous asteroids with perihelion distances less than 0.2 au, and for the observed mass-loss rate of the asteroid-like object 322P/SOHO~1 assuming its composition is similar to CI carbonaceous chondrites.

Figures (5)

• Figure 1: Snapshots of an experiment with CI simulant and an irradiance equivalent to that of the Sun at a heliocentric distance $r=0.14\,\mathrm{au}$.
• Figure 2: An example of the measured activity as a function of time at an irradiance level corresponding to $r=0.14\,\mathrm{au}$. The normalized cumulative distribution of the detected events (cyan) and the fitted sigmoid function (blue). The colored dots represent the beginning of each of the three phases of the disruption process as described.
• Figure 3: Disruption time versus simulated heliocentric distance. The red crosses do not provide information about the length of the experiments.
• Figure 4: Residual gas analysis scanned peak intensities immediately before (blue) and during irradiation (red). The compounds corresponding to some of the highest intensities are labeled.
• Figure 5: The perihelion distances, $q$, and absolute magnitudes, $H$, for all near-Earth asteroids (NEAs) with $q<0.4\,\mathrm{au}$ are shown with gray dots. NEAs with spectral types C, B, D, P, K, and L and/or with albedos $p_v<0.1$ are assigned as dark and are highlighted with blue points and NEAs with spectral types S, Q, V, R, and M and/or with albedo $p_v>0.1$ are assigned as bright and are highlighted with red points. The black line corresponds to the average disruption distance as a function of $H$Granvik2016. The histograms are showing the distributions of the respective groups of asteroids in absolute magnitude and perihelion distance, and are scaled by area.