Artist concept of asteroid Psyche
Image Credit: NASA/JPL-Caltech/ASU/Peter Rubin
The metal-rich asteroid Psyche
Recent Earth-based observations – ranging from radar imaging (Arecibo) and millimetre-wavelength observations (ALMA) to adaptive optics imaging (Keck and VLT) and stellar occultations – have refined our knowledge of Psyche’s size and morphology. Psyche can be approximated by a 278 x 238 x 171 km tri-axial ellipsoid with a spherical-equivalent diameter of about 222 km. Its highly irregular and non-spherical shape already indicate a complex topography with several large depressions up to 50 – 75 km wide and about 4 km deep. With a bulk density of 4172 +/- 145 kg/m^3 – which is only about half the density of pure iron (7900 kg/m^3) – and a porosity likely less than 20 %, Psyche is believed to be a mixture of metal and silicate.
Psyche was long thought to be the exposed iron core of a differentiated planetesimal, a relic of the violent hit-and-run collisions that characterized the early solar system. While its low bulk density initially raised questions about this interpretation, the absence of an associated asteroid family does not rule out such an origin, as any family formed in the early solar system would have dispersed over time. Current evidence, such as its high radar albedo and thermal inertia, supports the hypothesis that Psyche is the remnant of a differentiated body. Core formation remains the most plausible mechanism for concentrating metal, making it the currently leading hypothesis. However, the exact metal fraction and the body’s history are still uncertain, and alternative scenarios, such as collisions with differentiated or metal-rich undifferentiated bodies, are being considered. Only in-orbit observations will provide definitive answers.
Although the detection of ferrovolcanism would be of significant scientific interest, spectral data does not currently provide evidence for its occurrence on Psyche. Modeling studies have demonstrated that ferrovolcanism is theoretically possible, but it remains unconfirmed. The most likely volcanic activity, if present, would involve sulfur-rich or troilitic regions, which could be identified from orbit on any preserved original surfaces. Studying Psyche offers a unique opportunity to gain insights into the formation and evolution of planetary cores, including that of Earth, should it indeed prove to be the exposed core of a differentiated planetesimal.