# Surface thermophysical properties investigation of the potentially hazardous asteroid (99942) Apophis

## TL;DR

This study determines the thermophysical surface properties of asteroid Apophis using infrared observations and advanced modeling, revealing its surface likely has fine regolith and suggesting a Main Belt origin influenced by the Yarkovsky effect.

## Contribution

First detailed thermophysical analysis of Apophis combining multiple infrared datasets and advanced modeling to estimate its surface properties and implications for its origin.

## Key findings

- Apophis has wavelength-dependent thermal emissivity with a maximum around 20 μm.
- Thermal inertia is low, indicating possible surface regolith.
- High surface roughness suggests limited regolith migration.

## Abstract

In this work, we investigate the surface thermophysical properties (thermal emissivity, thermal inertia, roughness fraction and geometric albedo) of asteroid (99942) Apophis, using the currently available thermal infrared observations of CanariCam on Gran Telescopio CANARIAS and far-infrared data by PACS of Herschel, on the basis of the Advanced thermophysical model. We show that the thermal emissivity of Apophis should be wavelength dependent from $8.70~\mu m$ to $160~\mu m$, and the maximum emissivity may arise around $20~\mu m$ similar to that of Vesta. Moreover, we further derive the thermal inertia, roughness fraction, geometric albedo and effective diameter of Apophis within a possible 1$\sigma$ scale of $\Gamma=100^{+100}_{-52}\rm~Jm^{-2}s^{-0.5}K^{-1}$, $f_{\rm r}=0.78\sim1.0$, $p_{\rm v}=0.286^{+0.030}_{-0.026}$, $D_{\rm eff}=378^{+19}_{-25}\rm~m$, and 3$\sigma$ scale of $\Gamma=100^{+240}_{-100}\rm~Jm^{-2}s^{-0.5}K^{-1}$, $f_{\rm r}=0.2\sim1.0$, $p_{\rm v}=0.286^{+0.039}_{-0.029}$, $D_{\rm eff}=378^{+27}_{-29}\rm~m$. The derived low thermal inertia but high roughness fraction may imply that Apophis could have regolith on its surface, and less regolith migration process has happened in comparison with asteroid Itokawa. Our results show that small-size asteroids could also have fine regolith on the surface, and further infer that Apophis may be delivered from the Main Belt by Yarkovsky effect.