  Electrons on the surface of a strong topological insulator, such as Bi2Te3 or
Bi1-xSnx, form a topologically protected helical liquid whose excitation
spectrum contains an odd number of massless Dirac fermions. A theoretical
survey and classification is given of the universal features, observable by the
ordinary and spin-polarized scanning tunneling spectroscopy, in the
interference patterns resulting from the quasiparticle scattering by magnetic
and non-magnetic impurities in such a helical liquid. Our results confirm the
absence of backscattering from non-magnetic impurities observed in recent
experiments and predict new interference features, uniquely characteristic of
the helical liquid, when the scatterers are magnetic.
