The meteoroid environment and impacts on Phobos Apostolos A. Christou a,n , Jürgen Oberst b,c , Valery Lupovka d , Vasily Dmitriev d , Maria Gritsevich g,f,e a Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK b German Aerospace Centre, Institute of Planetary Research, Rutherfordstrasse 2, D-12489 Berlin, Germany c Technische Universität Berlin, Institute for Geodesy and Geoinformation Science, Planetary Geodesy, Strasse des 17. Juni 135,10623 Berlin, Germany d Moscow State University for Geodesy and Cartography, Extraterrestrial Laboratory, 4, Gorokchovsky pereulok,105064 Moscow, Russia e Russian Academy of Sciences, Dorodnicyn Computer Centre, Department of Computational Physics, Vavilova ul. 40,119333 Moscow, Russia f Ural Federal University, Institute of Physics and Technology, Department of Physical Methods and Devices for Quality Control, Mira ul.19, 620002 Ekaterinburg, Russia g Finnish Geodetic Institute, Department of Remote Sensing and Photogrammetry, Geodeetinrinne 2, P.O. Box 15, FI-02431 Masala, Finland article info Article history: Received 19 February 2013 Received in revised form 11 July 2013 Accepted 26 July 2013 Available online 22 August 2013 Keywords: Meteoroids Phobos Impact cratering Comets Asteroids abstract We review current knowledge of the flux of meteoroids on Phobos, a key to interpreting its cratering record and understanding the origin of the Martian satellite system. Past observational attempts to estimate the flux of small (mm to cm) meteoroids as meteors in the Martian atmosphere highlight the need for customised instrumentation onboard future missions bound for Mars. The temporal distribution of cometary meteoroid streams as predicted by recent work is non-uniform; we advocate emplacing seismic stations on Phobos or cameras optimised to monitor the Martian atmosphere for meteors as a means to elucidate this and other features of the meteoroid population. We construct a model of the sporadic flux of metre-sized or larger meteoroids and use it to predict a leading/trailing ratio in crater density of  4 if crater production by asteroidal meteoroids dominates over that by cometary ones. It is found that the observed distribution of craters Z100 m as determined from spacecraft images is consistent with a 50/50 contribution from the two meteoroid populations in our model. A need for more complete models of the meteoroid flux is identified. Finally, the prospects for new observational constraints on the meteoroid environment are reviewed. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction As with every other body in the solar system, the Martian system is subject to a continuous influx of meteoroids ranging in size from under a μm to several km. Their effect on the Martian atmosphere is to produce ionisation layers (Pätzold et al., 2005) and meteors (Adolfsson et al., 1996) with the larger ones reaching the Martian surface and creating impact craters, crater clusters or meteorites (Flynn and McKay, 1989; Popova et al., 2003; Chappelow and Sharpton, 2006). Impacts of “large” (metre-sized or larger) meteoroids on the airless surface of Phobos is the primary process for producing the craters seen in spacecraft images. In addition, seismic shaking and the production of ejecta contribute to the displacement and transport of surface material. On the other hand, impacts by “small” (decimetre or smaller) meteoroids can launch ejecta in circum-martian space, contribut- ing to the dust environment in the orbital vicinity of this moon (e.g. Krivov and Hamilton, 1997). Consequently, as can be read in the relevant chapters of this volume, an independent determina- tion of the meteoroid flux is needed to compare with models of the production rate of craters and boulders on Phobos, the impact flux on Mars itself now and in the past as well as the dust torus that is postulated by several studies. A study of the Phobos meteoroid environment also has value in itself. It represents an opportunity to learn more about the population of meteoroids which do not intersect the Earth's orbit. The physical properties of meteoroids causing meteors in the Earth's atmosphere have been studied theoretically in a number of works (Lebedinets, 1987; Babadzhanov, 1994; Kikwaya et al., 2006). According to the classical physical theory of meteors, these are considered to be solid bodies similar to meteorites of stony/ iron composition with bulk densities ranging from 3:5 g cm 3 to 7:7 g cm 3 (Levin, 1956). Lebedinets (1987) and Babadzhanov (1994) obtained an average bulk density of 3:3 g cm 3 with values for individual cases in the range from 0:1 g cm 3 to 8:0 g cm 3 . Bellot Rubio et al. (2002) gave even lower estimates ranging from 0:1 g cm 3 to 4:5 g cm 3 . In contrast, a wide range of the labora- tory measurements for meteorites, micrometeorites and interpla- netary dust particles did not confirm such a low limit for the bulk Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/pss Planetary and Space Science 0032-0633/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2013.07.012 n Corresponding author. Tel./fax: þ44 2837 522928/+ 44 2837 527174. E-mail addresses: aac@arm.ac.uk, aac@star.arm.ac.uk (A.A. Christou), juergen.oberst@dlr.de (J. Oberst), v.lupovka@miigaik.ru (V. Lupovka). v.dmitriev@miigaik.ru (V. Dmitriev), gritsevich@list.ru (M. Gritsevich). Planetary and Space Science 102 (2014) 164–170