Phobose mõõtmed on {{nowrap|27 × 22 × 18 km}} ning ta on liiga väike selleks, et muutuda ümaraks iseenda gravitatsiooni toimel. Phobose pindala on 1548,3 km<sup>2</sup>, mis on umbkaudu midagi [[Saaremaa]] ja [[Hiiumaa]] pindala vahepealset. Phobosel puudub oma väikese massi ja gravitatsiooni tõttu atmosfäär.<ref>{{cite web|url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Phobos |title=Solar System Exploration: Planets: Mars: Moons: Phobos: Overview |publisher=Solarsystem.nasa.gov |accessdate=19 August 2013}}</ref> KaSamuti on ta üks kõige vähem valgust peegeldavpeegeldavaid objektobjekte [[Päikesesüsteem]]is. Spektroskoopiliselt paistab ta olevat sarnane [[D-tüüpi asteroidid]]ega<ref name="c-type">{{cite web|url=http://www.physorg.com/news115483748.html|title=New Views of Martian Moons}}</ref> ja on koostiselt ilmselt sarnane [[süsinikkondriit]]idega.<ref>{{Cite book|title=Physics and Chemistry of the Solar System|last=Lewis|first=J. S.|authorlink=John S. Lewis|page=425|publisher=Elsevier Academic Press|year=2004|isbn=0-12-446744-X}}</ref> Phobose tihedus on liiga väike selleks, et tegu võiks olla lihtsalt ühe monoliitse kivikamakaga. Ilmselt on Phobose pinnas üsna poorne ja taevakeha koostises on palju jääd.<ref>{{cite web|title=Porosity of Small Bodies and a Reassesment of Ida's Density|url=http://www.aas.org/publications/baas/v31n4/dps99/65.htm|quote=When the error bars are taken into account, only one of these, Phobos, has a porosity below 0.2...}}</ref><ref name="Close Inspection for Phobos">{{cite web|title=Close Inspection for Phobos|url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31031|quote=It is light, with a density less than twice that of water, and orbits just {{Convert|5989|km|sp=us}} above the Martian surface.}}</ref><ref>{{Cite journal|last=Busch|first=M. W.|coauthors=''et al.''|year=2007|title=Arecibo Radar Observations of Phobos and Deimos|journal=Icarus|volume=186|issue=2|pages=581–584|doi=10.1016/j.icarus.2006.11.003|bibcode=2007Icar..186..581B}}</ref> Spektraalne analüüs siiski ei näita, et Phobose pinnal leiduv [[regoliit]] oleks hüdratiseerunud<ref>{{Cite journal|last=Murchie|first=S. L.|coauthors= Erard, S., Langevin, Y., Britt, D. T., Bibring, J. P., and Mustard, J. F.|title=Disk-resolved Spectral Reflectance Properties of Phobos from 0.3-3.2 microns: Preliminary Integrated Results from PhobosH 2 |journal=Abstracts of the Lunar and Planetary Science Conference| volume=22|page=943|year=1991 |bibcode=
1991pggp.rept..249M}}</ref><ref>{{Cite journal|last=Rivkin|first=A. S.|coauthors=''et al.''|date=March 2002|title=Near-Infrared Spectrophotometry of Phobos and Deimos|journal=Icarus|volume=156|issue=1|page=64|bibcode=2002Icar..156...64R|doi=10.1006/icar.2001.6767}}</ref>, kuid jää olemasolu regoliidikihi all pole siiski välistatud.<ref>{{ cite journal | last1 = Fanale | first1 = F. P. | first2 = J. R. | last2 = Salvail | year = 1989 | title = Loss of water from Phobos | journal = Geophys. Res. Lett. | volume = 16 | number = 4 | pages = 287–290 | doi = 10.1029/GL016i004p00287 |bibcode = 1989GeoRL..16..287F }}</ref><ref>{{ cite journal | title = Evolution of the water regime of Phobos | first1 = Fraser P. |last1 = Fanale | first2 = James R. | last2 = Salvail | doi = 10.1016/0019-1035(90)90089-R | date = Dec 1990 | journal = Icarus | volume = 88 | pages = 380–395 |bibcode = 1990Icar...88..380F }}</ref>
