Nitric oxide density in the lower thermosphere (97–150 km) has been measured from the polar‐orbiting Student Nitric Oxide Explorer (SNOE) satellite as a function of latitude, longitude, and altitude for the 2 1/2 year period from 11 March 1998 until 30 September 2000. The observations show that the maximum density occurs near 106–110 km and that the density is highly variable. The nitric oxide density at low latitudes correlates well with the solar soft X‐ray irradiance (2–7 nm), indicating that it is the solar X‐rays that produce thermospheric nitric oxide at low and midlatitudes. Nitric oxide is produced at auroral latitudes (60°–70° geomagnetic) by the precipitation of electrons (1–10 keV) into the thermosphere. During high geomagnetic activity, increased nitric oxide may be present at midlatitudes as the result of meridional winds that carry the nitric oxide equatorward.

Spatially resolved infrared and ultraviolet wavelength spectra of Europa's leading, anti-jovian quadrant observed from the Galileo spacecraft show absorption features resulting from hydrogen peroxide. Comparisons with laboratory measurements indicate surface hydrogen peroxide concentrations of about 0.13 percent, by number, relative to water ice. The inferred abundance is consistent with radiolytic production of hydrogen peroxide by intense energetic particle bombardment and demonstrates that Europa's surface chemistry is dominated by radiolysis.

[1] A model of NO abundance in the lower thermosphere is described. The model includes time dependence, an energetic electron flux calculation that includes transport, neutral and ion photochemistry, and vertical diffusion. We show that a steady state calculation is inadequate for calculating NO abundance. We examine the relationship between observed NO abundance and the integrated energy input to the lower thermosphere over the previous day. It is shown that the relationship between the energy input and the NO abundance varies with the local time of the NO measurement and with the length of daylight. These dependencies arise due to the role of photodissociation as a loss mechanism for NO. This model is designed for analysis of NO observations and will be used in the analysis of observations by the SNOE spacecraft .