  We report the detection of infrared emission from the jet of the nearby FR I
radio galaxy 3C 31. The jet was detected with the IRAC instrument on Spitzer at
4.5 micron, 5.8 micron, and 8.0 micron out to 30" (13 kpc) from the nucleus. We
measure radio, infrared, optical, and X-ray fluxes in three regions along the
jet determined by the infrared and X-ray morphology. Radio through X-ray
spectra in these regions demonstrate that the emission can be interpreted as
synchrotron emission from a broken power-law distribution of electron energies.
We find significant differences in the high energy spectra with increasing
distance from the nucleus. Specifically, the high energy slope increases from
0.86 to 1.72 from 1 kpc to 12 kpc along the jet, and the spectral break
likewise increases in frequency along the jet from 10-100's of GHz to ~20 THz.
Thus the ratio of IR to X-ray flux in the jet increases by at least an order of
magnitude with increasing distance from the nucleus. We argue that these
changes cannot simply be the result of spectral aging and that there is ongoing
particle acceleration through this region of the jet. The effects of mass
loading, turbulence, and jet deceleration, however these processes modify the
jet flow in detail, must be causing a change in the electron energy
distribution and the efficiency of particle acceleration.
