Upper limits to the hard X-ray fluxes from Mars, Venus, and Jupiter

A balloon experiment was conducted on August 10, 1966, that searched for photons with energies between 30 and 465 Kev from Mars, Venus, and Jupiter. No increases above background were observed from any of the planets although the emissions from the Crab Nebula were detected. The quiet-sun upper limit on the planetary fluxes between 40 and 100 key was 1.8 × 10−2 photon/cm² sec. Mars and Venus are not expected to be X-ray sources because they are neither unusual radio sources nor are they surrounded by trapped radiation. The upper limit on the strength of the Jovian X-ray source of 2 × 1020 ergs/sec was equivalent to 10−2 of the solar-wind energy at the Jovian magnetosphere. The quiet-time Jovian source strength to be expected is discussed.