Unlike its two largest homologs, Ceres and Vesta, Pallas has so far remained unknown and unvisited by a spacecraft mission. This situation is in part due to Pallas' unusual and highly tilted orbit out of the ecliptic plane, making it hard to reach with a spacecraft. Images of Pallas acquired by our team (PI: P. Vernazza; Laboratoire d'Astrophysique de Marseille) taking advantage of the extreme adaptive-optic system SPHERE on the European Southern Observatory (ESO)'s Very Large Telescope (VLT) unveiled a surface so thoroughly dimpled by craters so as to resemble a golf ball. The tilt of Pallas' orbit may be the key as to why Pallas appears to be so much more cratered than any main belt asteroid ever seen before. Because of this tilt, Pallas finds itself weaving its way up and down through the main asteroid belt throughout its entire 4.6 year orbit around the Sun. This makes Pallas subject to collisions at higher impact speeds than a typical asteroid having a circular orbit in the mid-plane of the asteroid belt. 

The two hemispheres of (2) Pallas as seen by VLT/SPHERE. The average size of Pallas is 512 km (about one-seventh the size of the moon). Numerous large craters are visible on both hemispheres, and a bright spot reminiscent of salt deposits on Ceres is found on the southern one. Credit: ESO/M. Marsset et al./MISTRAL algorithm (ONERA/CNRS)

What do craters on Pallas tell us about its composition and origin? The response of a planetesimal's crust to an impact directly relates to the composition of its subsurface. For water-rich bodies, relaxation of the crust is driven by viscous flowing of warm ice and clays in the subsurface, resulting in a flatter topography than in the case of a purely rocky body. Unlike Vesta, both Ceres and Pallas are believed to be water-rich worlds that accreted from a mixture of ice and silicate dust. This is shown by their low bulk density, as well as by the detection of diagnostic spectral signatures of hydrated silicates and, in the case of Ceres, ices, in their visible and near-infrared (0.3-4.0 μm) reflectance spectrum. The high and seemingly more stable topography of Pallas' craters compared to Ceres' now provides evidence that Pallas must contain less water in its subsurface and, in all likelihood, in its interior than Ceres. This agrees with the higher bulk density of Pallas (2,890 kg/m³) with respect to Ceres (2,160 kg/m³) derived from a 3D shape reconstruction of the asteroid based on the SPHERE images. 

Fragments from Pallas originating from collisions with smaller asteroids are believed to be ubiquitous throughout the inner Solar System, from Pallas' vicinity at around 2.8 astronomical units from the Sun, down to Earth-crossing orbits. In particular, it has been suggested that the near-Earth asteroid (3200) Phaethon, the parent body of the Geminids meteor shower observable each year on Earth in December, could have originated from Pallas based on dynamical simulations and spectral similarities between the two objects. Remarkably, the Geminids are known for their unusually wide diversity of sodium content compared to other meteors. The detection of a bright spot that could indicate a salty patch on Pallas similar to those found on Ceres (e.g., the Cerealea Facula in the Occator crater) opens the possibility for such diversity to originate from an early enrichment in salts through aqueous alteration in the interior of Pallas. While this currently remains highly speculative, the upcoming  DESTINY+ spacecraft mission of the Japan Aerospace Exploration Agency to Phaethon will soon shed new light on the composition and origins of Phaethon and the Geminids. 

The ESO VLT observatory is composed of four telescopes, each with an 8.2-meter-wide primary mirror. For three years, our team has been using one of these large telescopes to image asteroids and learn more about their origin and past collisional evolution. Credit: ESO/B. Tafreshi

Long-exposure image of the 2012 Geminid meteor shower over Yunnan Province, China. The Geminids are bits and pieces of the near-Earth asteroid (3200) Phaethon, which itself is believed to be a fragment of (2) Pallas. Credit: Getty Images/Stocktrek Images/Jeff Dai