  We investigate the distribution of Faraday rotation measure (RM) in the M87
jet at arc-second scales by using archival polarimetric VLA data at 8, 15, 22
and 43 GHz. We resolve the structure of the RM in several knots along the jet
for the first time. We derive the power spectrum in the arcsecond scale jet and
find indications that the RM cannot be associated with a turbulent magnetic
field with 3D Kolmogorov spectrum. Our analysis indicates that the RM probed on
jet scales has a significant contribution of a Faraday screen associated with
the vicinity of the jet, in contrast with that on kiloparsec scales, typically
assumed to be disconnected from the jet. Comparison with previous RM analyses
suggests that the magnetic fields giving rise to the RMs observed in jet scales
have different properties and are well less turbulent than these observed in
the lobes.
