 Deconvolved image of Themis byVLT-SPHERE | |
| Discovery | |
|---|---|
| Discovered by | Annibale de Gasparis |
| Discovery date | 5 April 1853 |
| Designations | |
| (24) Themis | |
| Pronunciation | /ˈθiːmɪs/[1] |
Named after | Themis |
| 1947 BA; 1955 OH | |
| Main belt (Themis) | |
| Adjectives | Themistian[2]/θɪˈmɪstiən/[3] |
| Orbital characteristics[4] | |
| Epoch 17.0 October 2024 (JD 2460600.5) | |
| Aphelion | 524.4Gm (3.505AU) |
| Perihelion | 415.9 Gm (2.780 AU) |
| 470.2 Gm (3.143 AU) | |
| Eccentricity | 0.1153 |
| 2035 d (5.57yr) | |
| 346.1° | |
| Inclination | 0.7368° |
| 36.39° | |
| 109.0° | |
| Earth MOID | 1.79422 AU |
| Jupiter MOID | 1.50402 AU |
| TJupiter | 3.199 |
| Physical characteristics | |
| Dimensions | 232 × 220 × 176 ± 14 km[5] |
| 208±3 km[5] 198±20 km[6] | |
| Flattening | 0.24[a] |
| Mass | (6.2±2.9)×1018 kg[5] (1.13±0.43)×1019 kg[6] 2.39×1019 kg[7][b] |
Meandensity | 1.31±0.62 g/cm3[5] 2.78±1.35 g/cm3[6] |
Equatorialsurface gravity | 0.15+0.08 −0.07 m/s2[6] |
Equatorialescape velocity | 87+15 −20 m/s[6] |
| 0.34892 d(8 h 22 min)[4] 8.374187±0.000001 h[5] | |
| 18°[5] | |
| 146°±3°[5] | |
| 73°±3°[5] | |
| 0.060 (calculated)[5] 0.067[4] | |
| C/B[4] (B-V=0.68) | |
| 7.24[4] 7.08[5] | |
24 Themis is one of the largestasteroids in theasteroid belt. At roughly 200 kilometres (120 mi) in diameter, it is also the largest member of theThemistian family. It was discovered byAnnibale de Gasparis on 5 April 1853. It is named afterThemis, the personification of natural law and divine order inGreek mythology.
24 Themis was discovered on 5 April 1853 byAnnibale de Gasparis ofNaples, though it was given its name by fellow Italian astronomerAngelo Secchi. The asteroid was named afterThemis, the Greek goddess of law.[8] Gravitational perturbations in the orbit of Themis were used to calculate the mass ofJupiter as early as 1875.[9]
On 24 December 1975, 24 Themis had a close encounter with2296 Kugultinov with a minimum distance of 0.016 AU (2.4×10^6 km). By analyzing the perturbation of Kugultinov's orbit due to the gravitational pull of Themis, the mass of Themis was determined to be approximately2.89×10−11solar masses (9.62×10−6Earth masses).[10]
Themis is in anelliptical orbit around theSun with aneccentricity of 0.1306 and aninclination of 0.76°.[11] It has anorbital period of 5.54 years. The distance between Themis and the Sun ranges from 2.71 AU atperihelion and 3.55 AU ataphelion,[12] with a mean distance of 3.1302 AU.[11] Themis is part of theThemis family of asteroids, which is located in the outer part of themain 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.[12]
On 7 October 2009, the presence ofwater ice was confirmed on the surface of thisasteroid usingNASA’sInfrared Telescope Facility.[13][14] The surface of the asteroid appears completely covered in ice. As this ice layer issublimated, it may be getting replenished by a reservoir of ice under the surface.[15][16]
Scientists hypothesize that some of the first water brought toEarth was delivered by asteroid impacts after the collision that produced theMoon. 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 orimpact gardening—a phenomenon in whichimpact 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 bysolar wind. Trace amounts of water would be continuously produced by high-energy solarprotons impingingoxide minerals present at the surface of theasteroid. Thehydroxyl surface groups (S–OH) formed by the collision of protons (H+
) withoxygen 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:
or,
where S represents the oxide surface.[14]
Organic compounds were also detected on the surface of Themis[15][16] in the form oftholins, 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, andpolycyclic aromatic hydrocarbons.[16][17]
