  We report wind measurements within Martian dust devils observed in plan view
from the High Resolution Imaging Science Experiment (HiRISE) orbiting Mars. The
central color swath of the HiRISE instrument has three separate charge-coupled
devices (CCDs) and color filters that observe the surface in rapid cadence.
Active features, such as dust devils, appear in motion when observed by this
region of the instrument. Our image animations reveal clear circulatory motion
within dust devils that is separate from their translational motion across the
Martian surface. Both manual and automated tracking of dust devil clouds reveal
tangential winds that approach 20-30 m/s in some cases. These winds are
sufficient to induce a ~1% decrease in atmospheric pressure within the dust
devil core relative to ambient, facilitating dust lifting by reducing the
threshold wind speed for particle elevation. Finally, radial velocity profiles
constructed from our automated measurements test the Rankine vortex model for
dust devil structure. Our profiles successfully reveal the solid body rotation
component in the interior, but fail to conclusively illuminate the profile in
the outer regions of the vortex. One profile provides evidence for a velocity
decrease as a function of r^-1/2, instead of r^-1, suggestive of surface
friction effects. However, other profiles do not support this observation, or
do not contain enough measurements to produce meaningful insights.
