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24 Themis

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24 Themis
Discovered byAnnibale de Gasparis
Discovery date5 April 1853
(24) Themis
Named after
1947 BA; 1955 OH
Main belt (Themis)
AdjectivesThemistian[2] /θɪˈmɪstiən/[3]
Orbital characteristics[4]
Epoch 23 July 2010 (JD 2455400.5)
Aphelion529.4 Gm
(3.539 AU)
Perihelion406.8 Gm
(2.719 AU)
468.1 Gm
(3.129 AU)
2021 d (5.54 yr)
Physical characteristics
Mean diameter
208±3 km[5]
198±20 km[6]
Mass(6.2±2.9)×1018 kg[5]
(11.3±4.3)×1018 kg[6]
2.39×1019 kg[7][b]
Mean density
1.31±0.62 g/cm3[5]
2.78±1.35 g/cm3[6]
Equatorial surface gravity
Equatorial escape velocity
0.34892 d (8 h 23 min)[4]
0.060 (calculated)[5]
C/B[4] (B-V=0.68)

Themis (minor planet designation: 24 Themis) is one of the largest asteroids in the asteroid belt. It is also the largest member of the Themistian family. It was discovered by Annibale de Gasparis on 5 April 1853. It is named after Themis, the personification of natural law and divine order in Greek mythology.

Not to be confused with 269 Justitia, named for Justitia, Themis' Roman name.

Discovery and observations[edit]

24 Themis was discovered on 5 April 1853 by Annibale de Gasparis of Naples, though it was given its name by fellow Italian astronomer Angelo Secchi. The asteroid was named after Themis, the Greek goddess of law.[8] Gravitational perturbations in the orbit of Themis were used to calculate the mass of Jupiter as early as 1875.[9]

On 24 December 1975, 24 Themis had a close encounter with 2296 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 approximately 2.89×10−11 solar masses (9.62×10−6 Earth masses).[10]

Orbit and rotation[edit]

Themis is in an elliptical orbit around the Sun with an eccentricity of 0.1306 and an inclination of 0.76°.[11] It has an orbital period of 5.54 years. The distance between Themis and the Sun ranges from 2.71 AU at perihelion and 3.55 AU at aphelion,[12] with a mean distance of 3.1302 AU.[11] 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.[12]

Surface materials[edit]


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 (~3.2 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 or impact gardening—a lunar phenomenon in which the moon overturns surface material at a rate of 1  m/ Gyr.[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
) 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


2 S-OH → S–O–S + H

where S represents the oxide surface.[14]


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]

See also[edit]


  1. ^ Flattening derived from the maximum aspect ratio (c/a): , where (c/a) = 0.76±0.08.[5]
  2. ^ 0.12×10−11 M = 2.39×1019 kg


  1. ^ Noah Webster (1884) A Practical Dictionary of the English Language
  2. ^ Astronomy now, Volume 22 (2008)
  3. ^ "Themistian". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  4. ^ a b c d e "JPL Small-Body Database Browser: 24 Themis". 17 March 2010. Retrieved 27 May 2010.
  5. ^ a b c d e P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. Astronomy & Astrophysics 54, A56
  6. ^ a b c d e Baer, James; Steven R. Chesley (25 June 1999). "Astrometric masses of 21 asteroids, and an integrated asteroid ephemeris". Celest. Mech. Dyn. Astron. 100 (2008). Springer Science+Business Media B.V. 2007: 27–42. Bibcode:2008CeMDA.100...27B. doi:10.1007/s10569-007-9103-8.
  7. ^ Michalak, G. (2001). "Determination of asteroid masses". Astronomy & Astrophysics. 374 (2): 703–711. Bibcode:2001A&A...374..703M. doi:10.1051/0004-6361:20010731.
  8. ^ Schmadel, Lutz D. (2003). Dictionary of minor planet names (5th ed.). Springer. p. 17. ISBN 978-3-540-00238-3.
  9. ^ "Our Astronomical Column" (PDF). Nature. 13 (316): 48. 18 November 1875. Bibcode:1875Natur..13...47.. doi:10.1038/013047d0. S2CID 8491861.
  10. ^ García, A. López; Medvedev, Yu. D.; Fernández, J. A. Moraño (1997). "Using Close Encounters of Minor Planets for the Improvement of their Masses". Dynamics and Astrometry of Natural and Artificial Celestial Bodies. Poznań, Poland: Kluwer Academic Publishers. pp. 199–204. ISBN 978-0-7923-4574-9.
  11. ^ a b The Astronomical Almanac. United States Naval Observatory and United Kingdom Hydrographic Office. 2011. p. G2. ISBN 978-0-7077-4103-1.
  12. ^ a b "Dictionary of Astronomy". Oxford Dictionary of Astronomy. Oxford University Press. 27 May 2010. p. 528.
  13. ^ Cowen, Ron (8 October 2009). "Ice confirmed on an asteroid". Science News. Archived from the original on 12 October 2009. Retrieved 9 October 2009.
  14. ^ a b Atkinson, Nancy (8 October 2009). "More water out there, ice found on an asteroid". International Space Fellowship. Archived from the original on 11 October 2009. Retrieved 11 October 2009.
  15. ^ a b c d Campins, Humberto; Hargrove, K; Pinilla-Alonso, N; Howell, ES; Kelley, MS; Licandro, J; Mothé-Diniz, T; Fernández, Y; Ziffer, J (2010). "Water ice and organics on the surface of the asteroid 24 Themis". Nature. 464 (7293): 1320–1. Bibcode:2010Natur.464.1320C. doi:10.1038/nature09029. PMID 20428164. S2CID 4334032.
  16. ^ a b c Rivkin, Andrew S.; Emery, Joshua P. (2010). "Detection of ice and organics on an asteroidal surface". Nature. 464 (7293): 1322–1323. Bibcode:2010Natur.464.1322R. doi:10.1038/nature09028. PMID 20428165. S2CID 4368093. (pdf version Archived 7 February 2023 at the Wayback Machine accessed 28 Feb. 2018).
  17. ^ Emery, Joshua P.; et al. "Advances in Solar System Science: Research Enabled by 2.4 – 25 μm Spectroscopy with SOFIA" (PDF). SOFIA Science Center. Retrieved 28 February 2018.

External links[edit]