The total data returned by the mission was 5.2 million bits (about 634 kB). All instruments operated successfully with the exception of a part of the ionization chamber, namely the Geiger–Müller tube, which failed in February 1965. In addition, the plasma probe had its performance degraded by a resistor failure on December 8, 1964, but experimenters were able to recalibrate the instrument and still interpret the data. The images returned showed a Moon-like cratered terrain, which scientists did not expect, although amateur astronomer Donald Cyr had predicted craters. Later missions showed that the craters were not typical for Mars, but only for the more ancient region imaged by Mariner 4. A surface atmospheric pressure of and daytime temperatures of were estimated. No magnetic field or Martian radiation belts or, again surprisingly, surface water was detected.