Title: Surface thermophysical properties investigation of the potentially hazardous asteroid (99942) Apophis,
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.