Phobos grooves, a lunar analogy

The global network of grooves on Phobos was partially seen in the Mariner 9 images, at the limit of camera spatial resolution. The Viking Orbiters had many close flybys of Phobos, most occurring naturally as the orbit evolved. However, the closest flybys of less than a few hundred km were targeted. The groove network was seen early in the Viking mission, but their extent took over a year to accumulate the global, high-resolution coverage. By the end of the Viking mission, the grooves were well mapped (Thomas, et al., 1978) and a majority appeared to be related to the formation of the large crater Stickney. Possible origins of the grooves might be related to surface fractures from the interior generated by Mars tidal forces while undergoing large impacts, tidal stresses during capture into Mars orbit (if Phobos was an asteroid), secondary craters chains from Phobos crater ejecta, crater ejecta rolling on the surface, or from secondary crater chains from Mars crater ejecta (Head, et al., 1978, Thomas, et al., 1979, Murchie, et al., 1989, Wilson, et al., 1989, and Murray, et al., 2006). The study of the grooves continued during the Soviet Phobos 88 mission and continues today with the ESA Mars Express (MEX) and US Mars Reconnaissance Orbiter (MRO) missions. Mars Express has many close encounters naturally, as its orbit extends beyond the orbit of Phobos that is over 6,000 km from the Martian surface, as well as targeted closer flybys. Even though the MRO orbit is only a few hundred km from the surface of Mars, its high-resolution HiRISE and CRISM instruments easily resolve the Phobos surface at better than 10 m/pixel for HiRISE. The highest spatial resolution images of the grooves have come from the HRSC camera during the targeted MEX close flybys, near the 1-meter level. With all of these direct observations of the Phobos grooves by spacecraft at Mars, it may be that our own moon will provide insight into the possible origin of some of these grooves. Recent Lunar Reconnaissance Orbiter Camera (LROC) images of our moon show linear features similar to the Phobos grooves. On lunar crater walls and central peaks, many boulder tracks, formed by rolling blocks of ejecta and mass-wasting of steep crater walls and peaks, are observed. These boulder tracks follow the local surface topography, leaving long trails, sometimes crossing and sometimes with a braided appearance due to the irregular shapes of the boulders, similar to the morphology of Phobos grooves. They tend not to hop but stay in contact with the surface, unless encountering major topographic boundaries. The main difference between the trails on our moon and some of the Phobos grooves is that the boulders remain at the end of the lunar trails. Since very few boulders are seen on Phobos, they must have left the surface when the frictional forces with the Phobos regolith became less than the radial velocity component of the rolling boulders. The LROC images give direct information on groove morphology to boulder size and shape within the lunar gravitational and topographic environment for comparative studies. Also relating the tremendous amount of grooves on Phobos to boulders, mostly from one crater, may reflect on the origin of Phobos being an accretion of Mars ejecta, providing a large inventory of boulders to be ejected during the largest of crater impacts. Head, J. and Cintala, M. , NASA TM-80339, 19, 1978 Murchie, S., et al., LPSC Abstract, 20.744, 1989 Murray, J., et al., LPSC Abstract, 37.2195, 2006. Thomas, P., et al., Nature, 273, 282-283, 1978. Thomas, P., et al., J. Geophys Res., 84, 8457-8477, 1979 Wilson, L., and Head, J., LPSC Abstract 20.1211, 1989