24 Themis

Ice

On 7 October 2009, the presence of water ice was confirmed on the surface of this asteroid using NASA’s Infrared Telescope Facility.^[13][14] The surface of the asteroid appears completely covered in ice. As this ice layer is sublimated, it may be getting replenished by a reservoir of ice under the surface.^[15][16]

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.^[15] Because of its proximity to the Sun, the widespread presence of ice on the surface of 24 Themis is somewhat unexpected. The surface ice may be replenished by a subsurface reservoir of ice or impact gardening—a phenomenon in which impact events overturn surface material at a rate of 1 m/ Gyr (billion years).^[15]

An alternative mechanism to explain the presence of water ice on 24 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 the asteroid. The hydroxyl surface groups (S–OH) formed by the collision of protons (H⁺
) with oxygen atoms present at oxide surface (S=O) can further be converted in water molecules (H
₂O) adsorbed onto the oxide minerals surface. The chemical rearrangement supposed at the oxide surface could be schematically written as follows:

2 S-OH → S=O + S + H
₂O

or,

2 S-OH → S–O–S + H
₂O

where S represents the oxide surface.^[14]

Organics

Organic compounds were also detected on the surface of Themis^[15][16] in the form of tholins, high-molecular weight organics found in the outer solar system, distinguished by a brown or reddish color in optical spectra. Compounds found in the spectra of Themis include ice tholin (the residual of an irradiated mixture of water ice and ethane), asphaltite, carbonaceous meteorite material, and polycyclic aromatic hydrocarbons.^[16][17]