View source for 24 Themis

{{Short description|Main-belt Themistian asteroid}}
{{For|the spurious tenth satellite of Saturn|Themis (hypothetical moon)}}
{{Use dmy dates|date=June 2017}}
{{Infobox planet
| minorplanet = yes
| background = #D6D6D6
| name = 24 Themis
| image = 24 Themis VLT (2021), deconvolved.pdf
| caption = 
| discoverer = [[Annibale de Gasparis]]
| discovered = 5 April 1853
| pronounced = {{IPAc-en|ˈ|θ|iː|m|ᵻ|s}}<ref>Noah Webster (1884) ''A Practical Dictionary of the English Language''</ref>
| mpc_name = (24) Themis
| alt_names = 1947 BA; 1955 OH
| named_after = [[Themis]]
| adjectives = Themistian<ref>''Astronomy now'', Volume 22 (2008)</ref> {{IPAc-en|θ|ᵻ|ˈ|m|ɪ|s|t|i|ə|n}}<ref>{{OED|Themistian}}</ref>
| mp_category = [[Main belt]] ([[Themis family|Themis]])
| orbit_ref = <ref name="jpldata">{{cite web
|date=12 December 2024
|title=JPL Small-Body Database Browser: 24 Themis
|url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=Themis
|access-date=22 December 2024}}</ref>
| epoch = 17.0 October 2024 ([[Julian date|JD]] 2460600.5)
| semimajor = 470.2 Gm<br />(3.143 AU)
| perihelion = 415.9 Gm<br />(2.780 AU)
| aphelion = 524.4 [[Giga|Gm]]<br />(3.505 [[Astronomical unit|AU]])
| eccentricity = 0.1153
| period = 2035 d (5.57 [[Julian year (astronomy)|yr]])
| inclination = 0.7368°
| asc_node = 36.39°
| arg_peri = 109.0°
| moid = 1.79422 AU
| jupiter_moid = 1.50402 AU
| mean_anomaly = 346.1°
| tisserand = 3.199
| dimensions = {{nwr|{{val|232|x|220|x|176}} ± 14 km<ref name=VLT/>}}
| mean_diameter = {{val|208|3|u=km}}<ref name=VLT>P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. ''Astronomy & Astrophysics'' 54, A56</ref><br />{{val|198|20|u=km}}<ref name="astrometric">{{cite journal
| last = Baer
| first = James
| author-link = James Baer
|author2= Steven R. Chesley
|author2-link= Steven R. Chesley
| title = Astrometric masses of 21 asteroids, and an integrated asteroid ephemeris
| journal = Celest. Mech. Dyn. Astron.
| volume = 100
| issue = 2008
| pages = 27–42
| publisher = Springer Science+Business Media B.V. 2007
| date = 25 June 1999
| doi = 10.1007/s10569-007-9103-8 |bibcode = 2008CeMDA.100...27B | doi-access = free
}}</ref>
| flattening = 0.24{{efn|1=Flattening derived from the maximum aspect ratio (c/a): <math>f=1-\frac{c}{a}</math>, where (c/a) = {{val|0.76|0.08}}.<ref name=VLT/>}}
| mass = {{val|6.2|2.9|e=18|u=kg}}<ref name=VLT/><br />{{val|1.13|0.43|e=19|u=kg}}<ref name="astrometric" /><br />{{val|2.39|e=19|u=kg}}<ref name=Michalak2001>{{cite journal
 |last=Michalak
 |first=G.
 |title=Determination of asteroid masses
 |journal=Astronomy & Astrophysics
 |volume=374
 |issue=2
 |pages=703–711
 |date=2001
 |doi=10.1051/0004-6361:20010731
 |bibcode=2001A&A...374..703M
|doi-access=free
 }}</ref>{{efn|name=Themismass|1={{val|0.12|e=-11|u={{Solar mass}}}} = {{val|2.39|e=19|u=kg}}}}
| density = {{val|1.31|0.62|u=g/cm3}}<ref name=VLT/><br />{{val|2.78|1.35|u=g/cm3}}<ref name="astrometric" />
| surface_grav = {{val|0.15|+0.08|-0.07|u=m/s2}}<ref name="astrometric" />
| escape_velocity = {{val|87|+15|-20|u=m/s}}<ref name="astrometric" />
| rotation = {{val|0.34892|u=d}} {{nowrap|(8 h 22 min)}}<ref name="jpldata"/><br>{{val|8.374187|0.000001|u=h}}<ref name=VLT/>
| axial_tilt = 18°<ref name=VLT/>
| spectral_type = [[C-type asteroid|C]]/[[B-type asteroid|B]]<ref name=jpldata/> (B-V=0.68)
| abs_magnitude = 7.24<ref name="jpldata"/><br>7.08<ref name=VLT/>
| pole_ecliptic_lat = {{val|73|3|u=deg}}<ref name=VLT/>
| pole_ecliptic_lon = {{val|146|3|u=deg}}<ref name=VLT/>
| albedo = 0.060 (calculated)<ref name=VLT/><br />0.067<ref name="jpldata"/>
}}

'''24 Themis''' is one of the largest [[asteroid]]s in the [[asteroid belt]]. It is also the largest member of the [[Themistian asteroid|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 ==
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.<ref>{{cite book | title = Dictionary of minor planet names | page = 17 | author = Schmadel, Lutz D. | url = https://books.google.com/books?id=KWrB1jPCa8AC&q=dictionary+of+astronomy+themis&pg=PA17 | edition = 5th | publisher = Springer | date = 2003 | isbn = 978-3-540-00238-3}}</ref> Gravitational perturbations in the orbit of Themis were used to calculate the mass of [[Jupiter]] as early as 1875.<ref>{{cite journal | journal = Nature | title = Our Astronomical Column | date = 18 November 1875 | url = https://www.nature.com/nature/journal/v13/n316/pdf/013047d0.pdf?foxtrotcallback=true| page = 48 | volume=13 | issue=316 | doi=10.1038/013047d0|bibcode = 1875Natur..13...47. | s2cid = 8491861 | doi-access = free }}</ref>

On 24 December 1975, 24 Themis had a close encounter with [[2296 Kugultinov]] with a minimum distance of {{convert|0.016|AU|e6km|abbr=on}}. By analyzing the perturbation of Kugultinov's orbit due to the gravitational pull of Themis, the mass of Themis was determined to be approximately {{val|2.89|e=-11}} [[solar masses]] ({{val|9.62|e=-6}} [[Earth mass]]es).<ref name="CloseEncounters">{{cite conference | title = Using Close Encounters of Minor Planets for the Improvement of their Masses | book-title = Dynamics and Astrometry of Natural and Artificial Celestial Bodies | author = García, A. López | author2 = Medvedev, Yu. D. | author3 = Fernández, J. A. Moraño | pages = 199–204 | publisher = [[Kluwer Academic Publishers]] | location = Poznań, Poland | url = https://books.google.com/books?id=sJe_2VBDYNEC&q=Using+close+encounters+of+minor+planets+for+the+improvement+of+their+masses.&pg=PA199 | isbn = 978-0-7923-4574-9 |date = 1997}}</ref>

== Orbit and rotation ==
Themis is in an [[elliptical orbit]] around the [[Sun]] with an [[Orbital eccentricity|eccentricity]] of 0.1306 and an [[inclination]] of 0.76°.<ref name="AstroAlmanac">{{cite book | title = The Astronomical Almanac | date = 2011 | isbn = 978-0-7077-4103-1 | publisher = United States Naval Observatory and United Kingdom Hydrographic Office | page = G2 }}</ref> It has an [[orbital period]] of 5.54 years. The distance between Themis and the Sun ranges from 2.71&nbsp;[[astronomical unit|AU]] at [[perihelion]] and 3.55&nbsp;AU at [[aphelion]],<ref name="Oxford dictionary"/> with a mean distance of 3.1302&nbsp;AU.<ref name="AstroAlmanac"/> 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.<ref name="Oxford dictionary">{{Cite encyclopedia
| title = Dictionary of Astronomy
| encyclopedia = Oxford Dictionary of Astronomy
| pages = 528
| publisher = [[Oxford University Press]]
| date = 27 May 2010
}}</ref>

== Surface materials ==
===Ice===
On 7 October 2009, the presence of [[ice|water ice]] was confirmed on the surface of this [[asteroid]] using [[NASA]]’s [[Infrared Telescope Facility]].<ref>{{cite magazine
| first=Ron
| last=Cowen
| date=8 October 2009
| title=Ice confirmed on an asteroid
| magazine=Science News | url=http://www.sciencenews.org/view/generic/id/48174/title/Ice_confirmed_on_an_asteroid
| access-date=9 October 2009 | archive-url= https://web.archive.org/web/20091012075224/http://www.sciencenews.org/view/generic/id/48174/title/Ice_confirmed_on_an_asteroid| archive-date= 12 October 2009 | url-status= live}}</ref><ref name="Atkinson2009">{{cite web
| last = Atkinson
| first = Nancy
| date = 8 October 2009
| title = More water out there, ice found on an asteroid
| work = International Space Fellowship
| access-date = 11 October 2009
| url = http://spacefellowship.com/2009/10/08/more-water-out-there-ice-found-on-an-asteroid/
| archive-url= https://web.archive.org/web/20091011051040/http://spacefellowship.com/2009/10/08/more-water-out-there-ice-found-on-an-asteroid/| archive-date= 11 October 2009 | url-status= live}}</ref> The surface of the asteroid appears completely covered in ice. As this ice layer is [[Sublimation (chemistry)|sublimated]], it may be getting replenished by a reservoir of ice under the surface.<ref name="ReferenceA">{{ Cite journal |last1=Campins |first1=Humberto |last2=Hargrove |first2=K |last3=Pinilla-Alonso |first3=N |last4=Howell |first4=ES|author4-link=Ellen Howell |last5=Kelley |first5=MS |last6=Licandro |first6=J |last7=Mothé-Diniz |first7=T |last8=Fernández |first8=Y |last9=Ziffer |first9=J |title=Water ice and organics on the surface of the asteroid 24 Themis |journal=[[Nature (journal)|Nature]] |volume=464 |issue=7293 |pages=1320–1 |date=2010 |doi=10.1038/nature09029 |pmid=20428164|bibcode = 2010Natur.464.1320C |s2cid=4334032 }}</ref><ref name= "Rivkin2010" />

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.<ref name="ReferenceA"/> 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 &nbsp;m/ Gyr.<ref name="ReferenceA"/>

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 [[proton]]s impinging [[oxide]] minerals present at the surface of the [[asteroid]]. The [[hydroxyl]] surface groups (S–OH) formed by the collision of protons ({{chem|H|+}}) with [[oxygen]] atoms present at oxide surface (S=O) can further be converted in water molecules ({{chem|H|2|O}}) 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 + {{chem|H|2|O}}
or, <br />
:2 S-OH → S–O–S + {{chem|H|2|O}}
<br />
where S represents the oxide surface.<ref name="Atkinson2009" />

===Organics===
Organic compounds were also detected on the surface of Themis<ref name="ReferenceA"/><ref name= "Rivkin2010">{{Cite journal |last1=Rivkin |first1=Andrew S. |last2=Emery |first2=Joshua P. |title=Detection of ice and organics on an asteroidal surface |journal=[[Nature (journal)|Nature]] |volume=464 |issue=7293 |pages=1322–1323 |date=2010 |doi=10.1038/nature09028 |pmid=20428165 |bibcode = 2010Natur.464.1322R |s2cid=4368093 }} ([http://somosbacteriasyvirus.com/surface.pdf pdf version] {{Webarchive|url=https://web.archive.org/web/20230207123007/https://somosbacteriasyvirus.com/surface.pdf |date=7 February 2023 }} accessed 28 Feb. 2018).</ref> in the form of [[tholin]]s, 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), [[Gilsonite|asphaltite]], [[carbonaceous chondrite|carbonaceous meteorite material]], and [[polycyclic aromatic hydrocarbon]]s.<ref name= "Rivkin2010"/><ref>{{cite web |last=Emery |first=Joshua P. |display-authors=et al |url=https://www.sofia.usra.edu/sites/default/files/Poster_5.10_Emery.pdf |title=Advances in Solar System Science: Research Enabled by 2.4 – 25 μm Spectroscopy with SOFIA |access-date=28 February 2018 |website=SOFIA Science Center}}</ref>

== See also ==
* [[Extraterrestrial liquid water]]

== Notes ==
{{notelist}}

== References ==
{{Reflist|30em}}

== External links ==
* [http://news.bbc.co.uk/2/hi/science_and_environment/10090128.stm Asteroid Themis has 'frosted surface']
* [http://www.nasa.gov/topics/earth/features/water_ice_asteroid.html NASA detection of Water Ice and organics on the surface of 24 Themis]
* {{AstDys|24}}
* {{JPL small body}}

{{Minor planets navigator |23 Thalia |number=24 |25 Phocaea}}
{{Authority control}}

{{DEFAULTSORT:Themis}}
[[Category:Themis asteroids|000024]]
[[Category:Discoveries by Annibale de Gasparis]]
[[Category:Named minor planets]]
[[Category:C-type asteroids (Tholen)|000024]]
[[Category:B-type asteroids (SMASS)|000024]]
[[Category:Astronomical objects discovered in 1853|18530405]]
[[Category:Extraterrestrial water]]