a staff, U+2695 ⚕), confusing Hygieia with her masculine counterpart. These symbols are now both largely obsolete. In this century, 10 Hygiea has seen some minor astrological use, and its symbol was confused once again, with Asclepsius's rod replaced by Mercury's caduceus: , though in a more elaborate form (U+2BDA ⯚) than the symbol of the planet Mercury. The caduceus has long been mistaken for the rod of Asclepius. Physical characteristics Based on spectral evidence, Hygiea's surface is thought to consist of primitive carbonaceous materials similar to those found in carbonaceous chondrite meteorites. Aqueous alteration products have been detected on its surface, which could indicate the presence of water ice in the past which was heated sufficiently to melt. The primitive present surface composition would indicate that Hygiea had not been melted during the early period of Solar System formation, in contrast to other large planetesimals like 4 Vesta. However, observations in 2019 suggest Hygiea had suffered a head-on collision which had disrupted it, with its re-accretion resulting in its present spherical shape. No deep basins are visible in the VLT images, indicating that any large craters must have flat floors, consistent with an icy C-type composition. In images taken with the Very Large Telescope imager in 2017, a bright surface feature is visible, as well as at least two dark craters, which have been informally named Serpens and Calix after the Latin words for snake and cup, respectively. Serpens has a size of 180 km, Calix is about 90 km in diameter. Hygiea is the largest of the class of dark C-type asteroids that are dominant in the outer asteroid belt, beyond the Kirkwood gap at 2.82 AU. Hygiea has an average diameter of 434 ± 14 km. While early studies (Ragazzoni et al., 2000) have found a noticeably oblate shape with a semimajor axis ratio of 1.11 (much more than for the other objects in the "big four"—2 Pallas, 4 Vesta and the dwarf planet Ceres), recent results indicate that Hygiea is more spherical, with an axis ratio of 1.06, consistent with a MacLaurin ellipsoid. Aside from being the smallest of the "big four", Hygiea, like Ceres, has a relatively low density, which is more comparable to the icy satellites of Jupiter or Saturn than to the terrestrial planets or the stony asteroids. Although it is the largest body in its region, due to its dark surface and