Solar-radiation heating effects on 3200 phaethon

Apollo-type near-Earth asteroid 3200 Phaethon, having a small perihelion distance of q ∼ 0.14 AU, is classified as F- or B-type, one of subclasses among the C-complex (C-, G-, B-, and F-types) asteroids. The F/B-type asteroids and dehydrated CI and CM carbonaceous chondrites, which are regarded as being linked to each other, underwent a thermal history of high-temperature heatings at more than hundreds of degrees and dehydration for a certain period of time after aqueous alteration in their parent bodies. However, their primary heating mechanism and its timing are less certain and still controversial. We have investigated solar-radiation heating effects on Phaethon at the present planetary-epoch. As a consequence, we have found that the effects on Phaethon, if it is still hydrated, might indeed be a likely candidate for the primary metamorphic heat source. We also found that solar-radiation heating on Phaethon is a function of the latitude, since Phaethon has a highly tilted polar axis. Thus, the northern hemisphere would be selectively more heated than the southern hemisphere. Therefore, we hypothesized that the northern hemisphere, especially the north pole-northern midlatitude region, would be more thermally metamorphosed and dehydrated, if solar-radiation heating is the primary metamorphic heat source of Phaethon. This may provide the latitude-dependent color variations on Phaethon's surface, although this has not been proven by the existing Phaethon's spectral data.