Editing Sedna (dwarf planet) (section)

== Physical characteristics ==
Sedna has a [[V band]] [[Absolute magnitude#Solar System bodies (H)|absolute magnitude]] of about 1.8, and is estimated to have an [[albedo]] (reflectivity) of around 0.41, giving it a diameter of approximately 900&nbsp;km.<ref name=herschel/> At the time of discovery it was the brightest object found in the Solar System since Pluto in 1930. In 2004, the discoverers placed an upper limit of 1,800&nbsp;km on its diameter;<ref name="Grundy2005"/> after observations by the [[Spitzer Space Telescope]], this was revised downward by 2007 to less than 1,600&nbsp;km.<ref name="spitzer"/> In 2012, measurements from the [[Herschel Space Observatory]] suggested that Sedna's diameter was {{nowrap|995 ± 80 km}}, which would make it smaller than Pluto's moon Charon.<ref name=herschel>{{cite journal |last1=Pál |first1=A. |last2=Kiss |first2=C. |last3=Müller |first3=T. G. |last4=Santos-Sanz |first4=P. |last5=Vilenius |first5=E. |last6=Szalai |first6=N. |last7=Mommert |first7=M. |last8=Lellouch |first8=E. |last9=Rengel |first9=M. |last10=Hartogh |first10=P. |last11=Protopapa |first11=S. |last12=Stansberry |first12=J. |last13=Ortiz |first13=J.-L. |last14=Duffard |first14=R. |last15=Thirouin |first15=A. |last16=Henry |first16=F. |last17=Delsanti |first17=A. |title="TNOs are Cool": A survey of the trans-Neptunian region. VII. Size and surface characteristics of (90377) Sedna and {{mp|2010 EK|139}} |doi=10.1051/0004-6361/201218874 |journal=Astronomy & Astrophysics |volume=541 |pages=L6 |year=2012 |bibcode= 2012A&A...541L...6P |arxiv=1204.0899|s2cid=119117186 }}</ref> In 2013, the same team re-analyzed Sedna's thermal data with an improved thermophysical model and found a consistent value of {{val|906|314|258|u=km}}, suggesting that the original model fit was too precise.<ref name="Lellouch A60">{{Cite journal |last1=Lellouch |first1=E. |last2=Santos-Sanz |first2=P. |last3=Lacerda |first3=P. |last4=Mommert |first4=M. |last5=Duffard |first5=R. |last6=Ortiz |first6=J. L. |last7=Müller |first7=T. G. |last8=Fornasier |first8=S. |last9=Stansberry |first9=J. |last10=Kiss |first10=Cs. |last11=Vilenius |first11=E. |last12=Mueller |first12=M. |last13=Peixinho |first13=N. |last14=Moreno |first14=R. |last15=Groussin |first15=O. |date=29 September 2013 |title="TNOs are Cool": A survey of the trans-Neptunian region: IX. Thermal properties of Kuiper belt objects and Centaurs from combined Herschel and Spitzer observations⋆⋆⋆ |journal=Astronomy & Astrophysics |volume=557 |pages=A60 |doi=10.1051/0004-6361/201322047 |issn=0004-6361|doi-access=free |bibcode=2013A&A...557A..60L |hdl=10316/80307 |hdl-access=free }}</ref> Australian observations of a [[stellar occultation]] by Sedna in 2013 produced similar results on its diameter, giving [[Chord (astronomy)|chord]] lengths {{val|1025|135|u=km}} and {{val|1305|565|u=km}}.<ref name="Rommel2020"/> The size of this object suggests it could have undergone [[Planetary differentiation|differentiation]] and may have a [[Planetary oceanography|sub-surface liquid ocean]] and possibly [[Planetary geology|geologic]] activity.<ref name="Emery2007"/>

As Sedna has no known moons, the direct determination of its mass is as yet impossible without either sending a [[space probe]] or perhaps locating a nearby object which is gravitationally [[Perturbation (astronomy)|perturbed]] by the planetoid. It is the largest trans-Neptunian Sun-orbiting object not known to have a natural satellite.<ref name = "Lakdawalla2016a" /> As of 2024, observations from the [[Hubble Space Telescope]] in 2004 have been the only published attempt to find a satellite,<ref name="HubbleProposal" /><ref name="Mystery" /> and it is possible that a satellite could have been lost in the glare from Sedna itself.<ref name="BannisterTwitter" />

Observations from the SMARTS telescope show that Sedna, in [[visible light]], is one of the reddest objects known in the Solar System, nearly as red as [[Mars]].<ref name="mikebrown"/> Its deep red [[spectral slope]] is indicative of high concentrations of [[organic chemistry|organic material]] on its surface.<ref name="Emery2007" /> Chad Trujillo and his colleagues suggest that Sedna's dark red color is caused by an extensive surface coating of [[hydrocarbon]] sludge, termed [[tholins]]. Tholins are a reddish-colored, amorphous, and heterogeneous organic mixture hypothesized to have been transmuted from simpler organic compounds, following billions of years of continuous exposure to [[ultraviolet]] radiation, interstellar particles, and other harsh environs as the dwarf planet either comes close to the Sun or transits interstellar space.<ref name="Trujillo2005"/> Its surface is homogeneous in color and [[spectrograph|spectrum]]; this may be because Sedna, unlike objects nearer the Sun, is rarely impacted by other bodies, which would expose bright patches of fresh icy material like that on [[8405 Asbolus]].<ref name="Trujillo2005"/> Sedna and two other very distant objects – {{mpl|2006 SQ|372}} and {{mpl|(87269) 2000 OO|67}} – share their color with outer [[classical Kuiper belt object]]s and the [[Centaur (planetoid)|centaur]] [[5145 Pholus]], suggesting a similar region of origin.<ref name="Sheppard2010"/>

Trujillo and colleagues have placed upper limits on Sedna's surface composition of 60% for methane ice and 70% for water ice.<ref name="Trujillo2005"/> The presence of methane further supports the existence of tholins on Sedna's surface, as methane is among the organic compounds capable of giving rise to tholins.<ref name="Emery2007"/> Barucci and colleagues compared Sedna's spectrum with that of [[Triton (moon)|Triton]] and detected weak [[absorption bands]] belonging to methane and nitrogen ices. From these observations, they suggested the following model of the surface: 24% Triton-type tholins, 7% [[amorphous carbon]], 10% nitrogen ices, 26% [[methanol]], and 33% methane.<ref name="Triton"/> The detection of methane and water ice was confirmed in 2006 by the Spitzer Space Telescope [[mid-infrared]] photometry.<ref name="Emery2007"/> The [[European Southern Observatory]]'s [[Very Large Telescope]] observed Sedna with the SINFONI [[near-infrared]] spectrometer, finding indications of tholins and water ice on the surface.<ref name="Barucci_et_al_2010">{{cite journal |last1=Barucci |first1=M. A. |last2=Morea Dalle Ore |first2=C.|author2-link=Cristina Dalle Ore |last3=Alvarez-Candal |first3=A. |last4=de Bergh |first4=C. |last5=Merlin |first5=F. |last6=Dumas |first6=C. |last7=Cruikshank |first7=D. |title=(90377) Sedna: Investigation of Surface Compositional Variation |journal=[[The Astronomical Journal]] |volume=140 |issue=6 |pages=2095–2100 |date=December 2010 |doi=10.1088/0004-6256/140/6/2095 |bibcode=2010AJ....140.2095B |s2cid=120483473 |doi-access=free }}</ref>

In 2022, low-resolution near-infrared (0.7–5 μm) [[spectroscopy|spectroscopic]] observations by the [[James Webb Space Telescope]] (JWST) revealed the presence of significant amounts of [[ethane]] ice (C<sub>2</sub>H<sub>6</sub>) and of complex organics on the surface of Sedna. The JWST spectra also contain evidence of the existence of small amounts of [[ethylene]] (C<sub>2</sub>H<sub>4</sub>), [[acetylene]] (C<sub>2</sub>H<sub>2</sub>) and possibly [[carbon dioxide]] (CO<sub>2</sub>). On the other hand little evidence of the existence of [[methane]] (CH<sub>4</sub>) and nitrogen ices was found at variance with the earlier observations.<ref name="Emery2024"/>

The possible presence of nitrogen on the surface suggests that, at least for a short time, Sedna may have a tenuous atmosphere. During the 200-year portion of its orbit near perihelion, the maximum temperature on Sedna should exceed {{convert|35.6|K|C}}, the transition temperature between alpha-phase solid N<sub>2</sub> and the beta-phase seen on Triton. At 38&nbsp;K, the N<sub>2</sub> [[vapor pressure]] would be 14&nbsp;microbar (1.4&nbsp;Pa). The weak methane absorption bands indicate that methane on Sedna's surface is ancient, as opposed to being freshly deposited. This finding indicates that Sedna's surface never reaches a temperature high enough for methane on the surface to evaporate and subsequently fall back as snow, which happens on Triton and probably on Pluto.<ref name="Emery2007"/>