  A search for temporal changes on Pluto and Charon was motivated by (1) the
discovery of young surfaces in the Pluto system that imply ongoing or recent
geologic activity, (2) the detection of active plumes on Triton during the
Voyager 2 flyby, and (3) the abundant and detailed information that observing
geologic processes in action provides about the processes. A thorough search
for temporal changes using New Horizons images was completed. Images that
covered the same region were blinked and manually inspected for any differences
in appearance. The search included full-disk images such that all illuminated
regions of both bodies were investigated and higher resolution images such that
parts of the encounter hemispheres were investigated at finer spatial scales.
Changes of appearance between different images were observed but in all cases
were attributed to variability of the imaging parameters (especially geometry)
or artifacts. No differences of appearance that are strongly indicative of a
temporal change were found on the surface or in the atmosphere of either Pluto
or Charon. Limits on temporal changes as a function of spatial scale and
temporal interval during the New Horizons encounter are determined. The longest
time interval constraint is one Pluto/Charon rotation period (~6.4 Earth days).
Contrast reversal and high-phase bright features that change in appearance with
solar phase angle are identified. The change of appearance of these features is
most likely due to the change in phase angle rather than a temporal change. Had
active plumes analogous to the plumes discovered on Triton been present on the
encounter hemispheres of either Pluto or Charon, they would have been detected.
The absence of active plumes may be due to temporal variability (i.e., plumes
do occur but none were active on the encounter hemispheres during the epoch of
the New Horizons encounter ...
