Thetis: an asteroid that may have melted

What pyroxene and olivine in Thetis’s spectrum say about its hot early history.

Published 2026-10-23

Many asteroids are thought to be leftover building blocks that never melted. Thetis may be different.^[2]

Reading minerals from light

Minerals absorb sunlight at characteristic wavelengths. By measuring an asteroid’s reflected spectrum, astronomers can identify what its surface is made of.

What Thetis shows

• Both low- and high-calcium pyroxene.^[2]
• Less than 20% olivine.^[2]
• High-calcium pyroxene making up 40% or more of the pyroxene.^[2]

The interpretation

That much high-calcium pyroxene indicates a history of igneous rock deposits. It suggests Thetis underwent differentiation by melting, producing a surface of basalt.^[2]

Differentiation explained

When a young body heats up enough to melt, heavier metal sinks toward the centre and lighter rock rises, producing layers. Vesta is the best-known example in the asteroid belt and the source of the basaltic HED meteorites.^[4]

Supporting evidence

Thetis’s density of about 3.21 g/cm³ fits a solid rocky body.^[2] It is classed as S-type (stony), with a geometric albedo of about 0.19.^[1]

Numbered citations link to the sources list on the Thetis overview page.