A Survey for Outer Satellites of Mars: Limits to Completeness
  We surveyed the Hill sphere of Mars for irregular satellites. Our search covered nearly the entire Hill Sphere, but scattered light from Mars excluded the inner few arcminutes where the satellites Phobos and Deimos reside. No new satellites were found to an apparent limiting red magnitude of 23.5, which corresponds to radii of about 0.09 km using an albedo of 0.07. 
Scott S. Sheppard, David Jewitt, Jan Kleyna