Editing Phobos (moon) (section)

== Orbital characteristics ==
[[File:Phobos fly-by animation ESA223006.gif|thumb|Orbits of Phobos and [[Deimos (moon)|Deimos]]]]
The [[orbital motion]] of Phobos has been intensively studied, making it "the best studied [[natural satellite]] in the Solar System" in terms of orbits completed.<ref name="Bills">{{cite journal |last1=Bills |first1=Bruce G. |first2=Gregory A. |last2=Neumann |first3=David E. |last3=Smith |first4=Maria T. |last4=Zuber |date=2005 |title=Improved estimate of tidal dissipation within Mars from MOLA observations of the shadow of Phobos |journal=Journal of Geophysical Research |volume=110 |issue=E07004 |pages=E07004 |doi=10.1029/2004je002376 |bibcode=2005JGRE..110.7004B |doi-access=free }}</ref> Its close orbit around Mars produces some distinct effects. With an altitude of {{convert|5989|km|abbr=on}}, Phobos orbits Mars below the [[synchronous orbit]] radius, meaning that it moves around Mars faster than Mars itself rotates.<ref name="Close Inspection for Phobos" /> Therefore, from the point of view of an observer on the surface of Mars, it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice each Martian day (every 11 h 6 min). Because it is close to the surface and in an [[equator]]ial orbit, it cannot be seen above the horizon from [[latitude]]s greater than 70.4°. Its orbit is so low that its [[angular diameter]], as seen by an observer on Mars, varies visibly with its position in the sky. Seen at the horizon, Phobos is about 0.14° wide; at [[zenith]], it is 0.20°, one-third as wide as the full [[Moon]] as seen from Earth. By comparison, the Sun has an apparent size of about 0.35° in the Martian sky. Phobos's phases, inasmuch as they can be observed from Mars, take 0.3191 days (Phobos's [[Synodic period|synodic]] period) to run their course, a mere 13 seconds longer than Phobos's [[sidereal period]].
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=== Solar transits ===
{{Main|Transit of Phobos from Mars}}
[[File:Phobos_transit_in_real_color.webm|thumb|Phobos [[Astronomical transit|transits]] the [[Sun]], as viewed by the [[Perseverance (rover)|''Perseverance'' rover]] on 2 April 2022]]
An observer situated on the Martian surface, in a position to observe Phobos, would see regular [[astronomical transit|transits]] of Phobos across the Sun. Several of these transits have been photographed by the Mars Rover ''[[MER-B|Opportunity]]''. During the transits, Phobos casts a shadow on the surface of Mars; this event has been photographed by several spacecraft. Phobos is not large enough to cover the Sun's disk, and so cannot cause a [[total eclipse]].<ref>{{cite web |url= https://www.theverge.com/2022/4/21/23035514/nasa-perseverance-rover-mars-eclipse-phobos|title= Check out NASA's latest footage of a solar eclipse on Mars|author= Mary Beth Griggs|date= 21 April 2022|publisher= The Verge|access-date=19 April 2022}}</ref>

=== Predicted destruction ===
[[Tidal deceleration]] is gradually decreasing the orbital radius of Phobos by approximately {{convert|2|m|abbr=on}} every 100 years,<ref name="Falling apart">{{cite web |last=Zubritsky |first=Elizabeth |url=https://www.nasa.gov/feature/goddard/phobos-is-falling-apart |title=Mars' Moon Phobos is Slowly Falling Apart |work=[[NASA]] |date=10 November 2015 |access-date=19 July 2021}}</ref> and with decreasing orbital radius the likelihood of breakup due to [[tidal force]]s increases, estimated in approximately 30–50 million years,<ref name="Falling apart" /><ref name="Bills" /> or about 43 million years in one study's estimate.<ref name="Efroimsky">{{Cite journal |last1=Efroimsky |first1=Michael |last2=Lainey |first2=Valéry |title=Physics of bodily tides in terrestrial planets and the appropriate scales of dynamical evolution. |date=2007 |journal=Journal of Geophysical Research |volume=112 |number=E12 |page=E12003 |doi=10.1029/2007JE002908 |arxiv=0709.1995 |bibcode=2007JGRE..11212003E |s2cid=9480498 }}</ref>

Phobos's grooves were long thought to be fractures caused by the impact that formed the Stickney crater. Other modelling suggested since the 1970s support the idea that the grooves are more like "stretch marks" that occur when Phobos gets deformed by tidal forces, but in 2015 when the tidal forces were calculated and used in a new model, the stresses were too weak to fracture a solid moon of that size, unless Phobos is a rubble pile surrounded by a layer of powdery regolith about {{convert|100|m|abbr=on}} thick. Stress fractures calculated for this model line up with the grooves on Phobos. The model is supported with the discovery that some of the grooves are younger than others, implying that the process that produces the grooves is ongoing.<ref name="Falling apart" /><ref>Hurford, Terry A.; Asphaug, Erik; Spitale, Joseph; Hemingway, Douglas; et al.<!-- Rhoden, Alyssa; Henning, Wade; Bills, Bruce; Kattenhorn, Simon; Walker, Matthew -->; "Surface Evolution from Orbital Decay on Phobos", Division of Planetary Sciences of the American Astronomical Society meeting #47, National Harbor, MD, November 2015</ref>{{Inconsistent|date=June 2019|reason=Chapter 'Physical characteristics' says the grooves were caused by rolling boulders}}

Given Phobos's irregular shape and assuming that it is a pile of rubble (specifically a [[Mohr–Coulomb theory|Mohr–Coulomb body]]), it will eventually break up due to tidal forces when it reaches approximately 2.1 Mars radii.<ref name="death">
{{Cite journal |first=Keith A. |last=Holsapple |date=December 2001 |title=Equilibrium Configurations of Solid Cohesionless Bodies |journal=Icarus |volume=154 |issue=2 |pages=432–448 |doi=10.1006/icar.2001.6683 |bibcode=2001Icar..154..432H |s2cid=10781522 |url=http://pdfs.semanticscholar.org/131a/1b59e2f721e98b95653327f691ef171a4e63.pdf |archive-url=https://web.archive.org/web/20200412142548/http://pdfs.semanticscholar.org/131a/1b59e2f721e98b95653327f691ef171a4e63.pdf |url-status=dead |archive-date=12 April 2020 }}</ref> When Phobos is broken up, it will form a [[Ring system|planetary ring]] around Mars.<ref>{{Cite web |last=Sample |first=Ian |url=https://www.theguardian.com/science/2015/nov/23/gravity-will-rip-mars-moon-apart-dust-rubble-ring |title=Gravity will rip Martian moon apart to form dust and rubble ring |work=[[The Guardian]] |date=23 November 2015 |access-date=17 July 2016}}</ref> This predicted ring may last from 1 million to 100 million years. The fraction of the mass of Phobos that will form the ring depends on the unknown internal structure of Phobos. Loose, weakly bound material will form the ring. Components of Phobos with strong cohesion will escape tidal breakup and will enter the Martian atmosphere.<ref>Black, Benjamin A.; and Mittal, Tushar; (2015), "The demise of Phobos and development of a Martian ring system", ''Nature Geosci'', advance online publication, doi:10.1038/ngeo2583</ref>