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Amalthea (moon)
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== Physical characteristics == The surface of Amalthea is the reddest in the [[Solar System]].{{sfn|Thomas Burns et al.|1998}}<ref>[https://science.nasa.gov/jupiter/moons/amalthea/ Amalthea - NASA Science]</ref> This color may be due to [[sulfur]] originating from [[Io (moon)|Io]] or some other non-ice material.{{sfn|Thomas Burns et al.|1998}} Bright patches of less-red tint appear on the major slopes of Amalthea, but the nature of this color is currently unknown.{{sfn|Thomas Burns et al.|1998}} The surface of Amalthea is slightly brighter than surfaces of other [[inner satellites of Jupiter]].{{sfn|Simonelli Rossier et al.|2000}} There is also a substantial asymmetry between the leading and trailing [[Sphere|hemispheres]]: the leading hemisphere is 1.3 times brighter than the trailing one. The asymmetry is probably caused by the higher velocity and frequency of [[impact crater|impacts]] on the leading hemisphere, which excavates a bright material—presumably ice—from the interior of the moon.{{sfn|Simonelli Rossier et al.|2000}} [[File:Jupiter's moon Amalthea photographed by Galileo.jpg|thumb|left|''[[Galileo probe|Galileo]]'' images showing Amalthea's irregular shape (1997)]] [[File:Amalthea.gif|thumb|170px|The most detailed existing image of Amalthea (2.4 km/pix).{{sfn|Burns Simonelli et al.|2004}} Anti-Jupiter side. [[Ida Facula]] and [[Lyctos Facula]] are on the left side (on the [[terminator (solar)|terminator]]). Bright spot underside is associated with crater [[Gaea (crater)|Gaea]]. Photo by ''Galileo'' (2000)]] [[File:Amalthea (moon).png|thumb|170px|Leading side of Amalthea. North is up, and Jupiter is beyond the right side. Crater Pan is seen on the upper right edge, and Gaea on the lower. Ida Facula and Lyctos Facula are on the left end (upper and lower brightenings respectively) (1979)]] Amalthea is irregularly shaped, with the best [[ellipsoid]]al approximation being {{nowrap|250 × 146 × 128 km}}.{{sfn|Thomas Burns et al.|1998}} From this, Amalthea's surface area is likely between 88,000 and 170,000 square kilometers, or somewhere near 130,000. Like all other inner [[moons of Jupiter]], it is [[tidally locked]] with the planet, with the long axis pointing towards Jupiter at all times.{{sfn|Burns Simonelli et al.|2004}} Its surface is heavily scarred by [[Impact crater|crater]]s, some of which are extremely large relative to the size of the moon: [[Pan crater|Pan]], the largest crater, measures 100 km across and is at least 8 km deep.{{sfn|Thomas Burns et al.|1998}} Another crater, [[Gaea (crater)|Gaea]], measures 80 km across and is likely twice as deep as Pan.{{sfn|Thomas Burns et al.|1998}} Amalthea has several prominent bright spots, two of which are named. They are [[Lyctos Facula]] and [[Ida Facula]], with a width reaching up to 25 km. They are located on the edge of ridges.{{sfn|Thomas Burns et al.|1998}} Amalthea's irregular shape and large size led in the past to a conclusion that it is a fairly strong, rigid body,{{sfn|Burns Simonelli et al.|2004}} where it was argued that a body composed of ices or other weak materials would have been pulled into a more [[sphere|spherical]] shape by its own gravity. However, on 5 November 2002, the [[Galileo spacecraft|''Galileo'' orbiter]] made a targeted flyby that came within 160 km of Amalthea, and the deflection of its orbit was used to compute the moon's mass (its volume had been calculated previously—to within 10% or so—from a careful analysis of all extant images).{{sfn|Thomas Burns et al.|1998}} In the end, Amalthea's density was found to be as low as {{nowrap|0.86 g/cm<sup>3</sup>}},{{sfn|Anderson Johnson et al.|2005}}{{sfn|Swiss Cheese Moon}} so it must be either a relatively icy body or very porous "[[rubble pile]]" or, more likely, something in between. Recent measurements of infrared spectra from the [[Subaru (telescope)|Subaru telescope]] suggest that the moon indeed contains [[hydrous]] minerals, indicating that it cannot have formed in its current position, since the hot primordial Jupiter would have melted it.{{sfn|Takato Bus et al.|2004}} It is therefore likely to have formed farther from the planet or to be a captured [[Solar System]] body.{{sfn|Anderson Johnson et al.|2005}} No images were taken during this flyby (''Galileo''{{'}}s cameras had been deactivated due to radiation damage in January 2002), and the resolution of other available images is generally low. Amalthea radiates slightly more heat than it receives from the [[Sun]], which is probably due to the influence of Jovian heat flux (<9 [[kelvin]]s), sunlight reflected from the planet (<5 K), and charged particle bombardment (<2 K).{{sfn|Simonelli|1983}} This is a trait shared with [[Io (moon)|Io]], although for different reasons. === {{anchor|Craters}} Named geological features === <!-- anchor used to redirect here --> There are four named geological features on Amalthea: two craters and two {{Linktext|faculae}} (bright spots).{{sfn|USGS: Jupiter: Amalthea}} The faculae are located on the edge of a ridge on the anti-Jupiter side of Amalthea.{{sfn|Thomas Burns et al.|1998}} Craters are named after characters in [[Greek mythology]] associated with [[Zeus]] and Amalthea, and faculae are named after locations associated with [[Zeus]].<ref>{{cite web |url = http://planetarynames.wr.usgs.gov/jupiter/amalTOC.html |title = USGS: Amalthea nomenclature |archive-url=https://web.archive.org/web/20040808102340/http://planetarynames.wr.usgs.gov/jupiter/amalTOC.html |archive-date=8 August 2004 |url-status=dead}}</ref> {| class="wikitable" ! style="width:10em" | [[Planetary nomenclature|Feature]] !! Pronunciation !! [[Diameter]] !! Approval<br />year !! [[Eponym]] !! {{Ref.}} |- | [[Gaea (crater)|Gaea]] || {{IPAc-en|ˈ|dʒ|iː|ə}} || align=right | 80 km || align=center | 1979 || [[Gaia (mythology)|Gaia]], Greek mother earth goddess who brought Zeus to Crete || {{WGPSN|2053}} |- | [[Pan (crater)|Pan]] || {{IPAc-en|ˈ|p|æ|n}} || align=right | 100 km || align=center | 1979 || [[Pan (god)|Pan]], Greek goat-god son of Amalthea and Hermes || {{WGPSN|4569}} |- | [[Ida Facula]] || {{IPAc-en|ˈ|aɪ|d|ə}} || align=right | 50 km || align=center | 1979 || [[Mount Ida]], [[Crete]] || {{WGPSN|2646}} |- | [[Lyctos Facula]] || {{IPAc-en|ˈ|l|ɪ|k|t|ɒ|s}} || align=right | 25 km || align=center | 1979 || [[Lyctus]], [[Crete]] || {{WGPSN|3528}} |}
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