Presentation #202.01 in the session “Main-Belt Asteroids 3: Very Large Asteroids”.
M-type asteroids appear to be metal-rich and may be the core fragments of large planetesimals that had differentiated by the time they underwent cataclysmic collisions. This class of objects exhibits thermal inertias and radar albedos indicative of dense surfaces and metallic, Fe-Ni compositions. However, recent detections of spectral features indicative of silicates and even hydrated minerals are inconsistent with purely metallic surfaces. Observations at radio frequencies, sensitive to thermal emission from the near subsurface, provide a complementary view to both radar and reflectance spectroscopy. We observed the M-type asteroid 16 Psyche with the Atacama Large Millimeter Array (ALMA) in Chile on June 19, 2019 (UT). Observations were made at a frequency of 230 GHz (wavelength of 1.3 mm), and the intensity and polarization of 16 Psyche’s thermal emission were measured. At this frequency, we likely detect emission from the upper ~1-2 cm of the surface, intermediate between the regions sensed by optical/infrared observations and active radar observations. Individual observations were obtained every few minutes during a 2.6-hour interval, covering more than half of the 4.2-hour rotation period at this temporal sampling and at a spatial resolution of 0.02” (30 km on 16 Psyche at the time of observation). The resultant thermal images of 16 Psyche will be presented, and the implications of the surface temperature and polarization results will be discussed in the context of potential scenarios for the surface of this unusual object.