Mariner 10 - Mercury atmosphere

Reduction of data from the three Mariner 10 encounters to this date have allowed identification of helium and hydrogen as atmospheric constituents. Subsolar point densities are estimated at 4500 cm−3 for He and 8 cm−3 for the thermal component of H. A non-thermal component in H with a scale height of ∼ 70 km has been observed near the limb off the subsolar point, providing a total apparent number density of 90 cm−3. Upper limits on other atmospheric constituents have been reduced an order of magnitude. A very tentative identification of O has been obtained but the uncertainty is large and we require further data reduction for confirmation.