Phobos Grooves Analysis: do They Favor the In Situ or the Asteroidal Capture Origin?

Abstract:

Despite 43 years of spacecraft observations (Duxbury et al., 2014), Phobos origin is still a matter of great debate within the scientific community. Two main scenarios have been presented in the last decades: the in situ formation and the asteroidal capture origin. Both approaches present pros and cons that do not uniquely demonstrate the debated moon’s origin; above that, Schmedemann et al. (2014) suggest that Phobos has an ancient cratering surface age of about 4.3-3.7 Ga, dating back to a period where the two proposed origins can be supported.

Within such context frame, the work we present suggests a new interpretation of Phobos multiple grooves, which may point at one of the two scenarios.

Phobos parallel grooves have been debated since their first observation on Viking images in 1976 (Veverka and Duxbury, 1977). Multiple origins have been proposed for their formation i) as the result of Mars tidal stress (Dobrovolskis, 1982); ii) as fractures directly related to the formation of the 9 km Stickney crater (Fujiwara and Asada, 1983) or iii) a formation caused by rolling boulders coming from the Stickney impact crater (Wilson and Head, 1989). Another formation scenario is that the grooves are chains of secondary impacts resulting from primary impact events on Mars (Murray and Iliffe, 2011), but recently Ramsley and Head (2014) indicated that the volume of ejecta coming from Mars to Phobos is insufficient to produce the grooves as secondary craters.

Grooves are common on asteroids, as presented in 951 Gaspra (Veverka et al., 1994), 243 Ida (Belton et al., 1994), 433 Eros (Thomas et al., (2002), Buczkowski et al., 2008), 21 Lutetia (Massironi et al., 2011), 4 Vesta (Buczkowski et al., 2012). In this work we will show the preliminary results of a novel approach on the analysis of the grooves distribution, suggesting they are the remnants of an ancient parent body from which the moon could have been originated providing important implications on the origin of Phobos.