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Abstract:

Mars Express was conceived and built by ESA as a successor of the unsuccessful Russian Mars-96 mission. It was planned from the onset as an orbiter and lander mission to be able to carry out long-term, remote sensing and in-situ scientific investigations of the planet Mars and its environment. As an exceptionally successful workhorse and a backbone of the Agency’s Science Programme in operation at Mars since end December 2003, Mars Express has proven to be a highly productive mission returning excellent scientific value for the investments made by ESA and its Member States. This paper is intended as the introduction to the series of papers that make this special collection. It briefly reviews the history of the mission, its science goals, its uniqueness while establishing its complementarity with other Mars missions in a collaborative context. It also lists the teams and operational aspects and innovations that made this mission a success. Then the paper highlights Mars Express’s scientific achievements throughout its 20-year lifetime. Mars Express results and discoveries continue playing an essential role in understanding the geological, atmospheric and climate evolution of the Red Planet and determining its potential past habitability. To conclude, a preview of the science and other topics covered by this collection is given. Mars Express, a pioneering mission for Europe at Mars, is currently continuing on its long scientific journey around the Red Planet.

Abstract:

Context . Venus exhibits strong and changing contrasts at ultraviolet wavelengths. They appear to be related to the clouds and the dynamics in the cloud layer, but to date their origin continues to be unknown. Aims . We investigate the nature of the UV contrasts exhibited by Venus’ clouds by examining possible correlations between the thermal structure inferred from radio occultation data and UV brightness from imagery data, both observed with Venus Express. Methods . We analysed Venus Express images obtained from 11 hours before to a few hours after the time of radio occultation measurements of the same area. We accounted for the advection of clouds by zonal and meridional winds and applied a phase angle correction to compensate for the changing viewing geometry. Results . We find a possible anti-correlation between UV brightness and atmospheric temperature around an altitude of 67 km for low latitudes, with a one percent probability of this finding being due to chance (p value = 0.01). Heating in this altitude and latitude region due to an increase in the UV absorber has been predicted by radiative forcing studies. The predictions roughly match our observed temperature amplitude between UV-dark and UV-bright regions. Conclusions . This could be the first observational evidence of a direct link between UV brightness and atmospheric temperature in the 65–70 km altitude region in the clouds of Venus.

Abstract:

Whether or not there is a particulate layer in the lowest 10 km of the Venusian atmosphere is still an open question. Some of the past in situ experiments showed the presence of a detached particulate layer, and a few suggested the existence of finely dispersed aerosols, while other instruments supported the idea of no particulate matter in the deep atmosphere. In this work, we investigate the presence of a near‐surface particulate layer (NSPL) using in situ data from the Venera 13 mission. While the original spectrophotometric data from Venera 13 were lost, we have reconstructed a part of this data by digitizing the old graphic material and selected the eight most reliable Venera 13 downward radiance profiles from 0.48 to 0.8 μ ${\upmu }$ m for our retrievals. The retrievals suggest the existence of the particulate layer with a peak in the altitude range of 3.5–5 km. They further indicate a log‐normal particle size distribution with a mean radius between 0.6 and 0.85 μ ${\upmu }$ m. The retrievals constrain the real refractive index of the particles to lie around the range of 1.4–1.6, with the imaginary refractive index of a magnitude of 10 − 3 ${10}^{-3}$ . Based on refractive index retrievals, uplifted basalt particles or volcanic ash could be responsible for near‐surface particulates. In comparison, volatile condensates appear less likely to be behind the formation of NSPL.