  We evaluate gravitational lensing as a technique for the detection of
extrasolar moons. Since 2004 gravitational microlensing has been successfully
applied as a detection method for extrasolar planets. In principle, the method
is sensitive to masses as low as an Earth mass or even a fraction of it. Hence
it seems natural to investigate the microlensing effects of moons around
extrasolar planets. We explore the simplest conceivable triple lens system,
containing one star, one planet and one moon. From a microlensing point of
view, this system can be modelled as a particular triple with hierarchical mass
ratios very different from unity. Since the moon orbits the planet, the
planet-moon separation will be small compared to the distance between planet
and star. Such a configuration can lead to a complex interference of caustics.
We present detectability and detection limits by comparing triple-lens light
curves to best-fit binary light curves as caused by a double-lens system
consisting of host star and planet -- without moon. We simulate magnification
patterns covering a range of mass and separation values using the inverse ray
shooting technique. These patterns are processed by analysing a large number of
light curves and fitting a binary case to each of them. A chi-squared criterion
is used to quantify the detectability of the moon in a number of selected
triple-lens scenarios. The results of our simulations indicate that it is
feasible to discover extrasolar moons via gravitational microlensing through
frequent and highly precise monitoring of anomalous Galactic microlensing
events with dwarf source stars.
