  Infrared radiation emitted from an asteroid surface causes a torque that can
significantly affect rotational state of the asteroid. The influence of small
topographic features on this phenomenon, called the YORP effect, seems to be of
utmost importance. In this work, we show that a lateral heat diffusion in
boulders of suitable sizes leads to an emergence of a local YORP effect which
magnitude is comparable to the YORP effect due to the global shape. We solve a
three-dimensional heat diffusion equation in a boulder and its surroundings by
the finite element method, using the FreeFem++ code. The contribution to the
total torque is inferred from the computed temperature distribution. Our
general approach allows us to compute the torque induced by a realistic
irregular boulder. For an idealized boulder, our result is consistent with an
existing one-dimensional model. We also estimated (and extrapolated) a size
distribution of boulders on (25143) Itokawa from close-up images of its
surface. We realized that topographic features on Itokawa can potentially
induce a torque corresponding to a rotational acceleration of the order 10^-7
rad day^-2 and can therefore explain the observed phase shift in light curves.
