The structure of the asteroid Leto is primarily composed of rock and metal. In this image, it is labelled B.
Just like other asteroids in the asteroid belt, Leto is a remnant from the early solar system, a leftover building block that never coalesced into a full-fledged planet. Its composition consists of silicate minerals and metallic elements such as iron and nickel.
These materials are typical of asteroids in the inner asteroid belt which lies between the orbits of Mars and Jupiter. Leto's surface is likely pockmarked with impact craters, evidence of collisions with other objects over billions of years.
In terms of its physical characteristics, Leto is irregularly shaped, as most asteroids are, and it lacks the gravitational pull to form a spherical shape.