28th Largest Asteroid, 24 Themis

24 Themis is a large main-belt asteroids and the largest member of the Themistian asteroid family. Themis is the 28th largest asteroid currently known.

Discovery

24 Themis was discovered on 5 April 1853 by Annibale de Gasparis at Naples.

Naming

Themis the asteroid was named after Themis, the Greek goddess of law.

Themis is an ancient Greek Titaness. She is described as "of good counsel", and is the embodiment of divine order, law and custom.

Themis was named by Angelo Secchi, who was Director of the Observatory at the Pontifical Gregorian University (then called the Roman College). He was a pioneer in astronomical spectroscopy, and was one of the first scientists to state authoritatively that the Sun is a star.

Stats

Diameter (mean): 198 km
Aphelion: 3.539 AU
Perihelion: 2.732 AU
Semi-major axis: 3.316 AU
Orbital Period: 5.55 years
Rotation period: 8.37 hrs
Date discovered: 1853.4.5
Class: C
Group: Themis Group
Type: Main-belt Asteroid
(data from JPL Small-Body Database)

Orbit and rotation

Themis is in an elliptical orbit around the Sun with an eccentricity of 0.1306 and an inclination of 0.76°.

Themis is part of the Themis family of asteroids, which is located in the outer part of the main belt. The family consists of a core of large objects surrounded by a cloud of smaller objects; 24 Themis is a member of the core.

Surface ice

On October 7, 2009, the presence of water ice was confirmed on the surface of Themis using NASA’s Infrared Telescope Facility. The surface of Themis appears completely covered in ice. As this ice layer is sublimated, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.

Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. The presence of ice on 24 Themis supports this theory. Because of its proximity to the sun (~3.3 AU), the widespread ice on the surface of 24 Themis is somewhat unexpected. The surface ice may be replenished by a sub-surface reservoir of water.

An alternative mechanism to explain the presence of water ice on Themis is similar to the hypothesized formation of water on the surface of the Moon by solar wind. Trace amounts of water would be continuously produced by high-energy solar protons impinging oxide minerals present at the surface of Themis.