and spectroscopy Photometric observations of Bennu in 2005 yielded a synodic rotation period of . It has a B-type classification, which is a sub-category of carbonaceous asteroids. Polarimetric observations show that Bennu belongs to the rare F subclass of carbonaceous asteroids, which is usually associated with cometary features. Measurements over a range of phase angles showed a phase function slope of 0.040 magnitudes per degree, which is similar to other near-Earth asteroids with low albedo. Before OSIRIS-REx, spectroscopy indicated a correspondence with the CI and/or CM carbonaceous chondrite meteorites, including carbonaceous-chondrite mineral magnetite. Magnetite, a spectrally prominent water product but destroyed by heat, is an important proxy of astronomers including OSIRIS-REx staff. Water Predicted beforehand, Dante Lauretta (University of Arizona) then stated that Bennu is water-rich- already detectable while OSIRIS-REx was still technically in approach. Preliminary spectroscopic surveys of the asteroid's surface by OSIRIS-REx confirmed magnetite and the meteorite-asteroid linkage, dominated by phyllosilicates. Phyllosilicates, among others, hold water. Bennu's water spectra were detectable on approach, reviewed by outside scientists, then confirmed from orbit. OSIRIS-REx observations have resulted in a (self-styled) conservative estimate of about 7 x 108 kg water in one form alone, neglecting additional forms. This is a water content of ~1 wt.%, and potentially much more. In turn this suggests transient pockets of water beneath Bennu's regolith. The surficial water may be lost from the collected samples. However, if the sample return capsule maintains low temperatures, the largest (centimeter-scale) fragments may contain measurable quantities of adsorbed water, and some fraction of Bennu's ammonium compounds. Activity Bennu is an active asteroid, sporadically emitting plumes of particles and rocks as large as , (not dust, defined as tens of micrometers). Scientists hypothesize the releases may be caused by thermal fracturing, volatile release through dehydration of phyllosilicates, pockets of subsurface water, and/or meteoroid impacts. Before the arrival of OSIRIS-REx, Bennu had displayed polarization consistent with Comet Hale-Bopp and 3200 Phaethon, a rock comet. Bennu, Phaethon, and inactive Manx comets are examples of active asteroids. B-type asteroids displaying a blue color in particular, may be dormant comets. If the IAU declares Bennu to be a dual-status object, its comet designation would be P/ (LINEAR). Surface features All geological features on Bennu are named after various species of birds and bird-like figures in mythology. The first features to be named were the final four candidate OSIRIS-REx sample sites, which were given unofficial names