What are the white round holes at Antarctica and Arctica visible at pictures from Cryosat-2 satelite program?

Mapperz and lluke are correct in that this gap is due to CryoSat-2's orbit, which flies up to 88 degrees north and south from the equator and so doesn't acquire any measurements beyond it (hence the hole). This orbit was chosen to both maximise coverage of Arctic sea ice and to provide the high density of repeat measurements needed to measure changes in Antarctica and Greenland further away from the poles.

Cheers, Tom