Origin thumb|An illustration of main-belt asteroid capture hypothesis The origin of the Martian moons has been disputed.Burns, Joseph A.; "Contradictory Clues as to the Origin of the Martian Moons" in Mars, H. H. Kieffer et al., eds., University of Arizona Press, Tucson, AZ, 1992 Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedo, and density very similar to those of C- or D-type asteroids. Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids.Landis, Geoffrey A.; "Origin of Martian Moons from Binary Asteroid Dissociation", American Association for the Advancement of Science Annual Meeting; Boston, MA, 2001, abstract Both moons have very circular orbits which lie almost exactly in Mars' equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces, although it is not clear that sufficient time is available for this to occur for Deimos. Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey A. Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces.
origIn thuMb|An iLLUstratioN oF mAin-BElt astErOid Capture hypOtheSiS the oriGin Of tHe martiAn moons haS beEN DisputEd.BUrns, jOseph a.; "coNTraDIcTORy ClueS as tO The origIn of tHe MaRTian mOOnS" in mars, H. h. kieffer et Al., eds., uniVeRsITy of ariZOna PRess, tucSoN, Az, 1992 PHobOS and deiMos both have much in common WiTh CarbONaceoUs C-Type asteRoidS, with SpecTra, aLbeDo, and dEnsiTy vERy SIMIlAR to tHose Of c- or d-tyPe asterOIDs. BaseD on theIr siMiLaRiTY, one Hypothesis Is thAt boTh moons MAY Be cAPtUred maIn-BElT AstEroIDs.lAndiS, gEoFFrey a.; "origin of maRtian moonS from BInarY asteroID dissOCIation", amErican assOciaTIon for thE ADVAnCemeNt oF sciencE anNuAl mEetINg; boston, MA, 2001, aBstRAcT BOTH MOonS havE vERy ciRcUlaR orbits wHICH liE aLmosT EXactly in mArs' equaTOriAL plAne, aNd hencE a capture OrIgin rEqUiRes A mechAniSm for cIrcularizIng thE iNitiaLLY Highly eccentric orbit, aND aDjUsTIng iTS inclInatiON into thE EquAtOriaL plAne, mOst probAblY by a combiNaTion of atMoSPhErIc dRag and tIdal forces, alThougH IT is not CLeaR That sufficIEnt TiMe IS availabLE for thiS To OCCuR foR Deimos. captUrE also reQuirES DissIpaTiON of eNergy. tHe CURreNT MArtiaN atmOSphERe is toO tHin To caPtUre a pHobOs-sizeD objecT By atmOsphERIc BrAking. geOffREY A. landis HAS poIntEd oUt that thE CapTuRe coulD Have OccurrEd if thE orIgiNAl BodY wAS A biNary AsteROID thAt separated undeR TiDal ForCeS.