Mercury is a very savage place with temperatures ranging from -170 C at night to over 400 C at day. When observable it is bright enough to cast faint shadows in favourable conditions. Telescopic observations are much easier than Mercury with Venus visible for several hours after sunset or before sunrise. Nearly all our information on Venus has come from satellite missions. Both Soviet and American orbiters, particularly Magellan, have now mapped most of the surface via radar.
Radar mapping is essentially due to the thick toxic clouds which fill the atmosphere.
Probes have also been deployed to the surface sampling soil types and atmospheric conditions. The Venetian surface is vast and rolling with signs of recent volcanic activity. Perhaps of more interest is the atmosphere. It is predominantly carbon dioxide with sulphuric acid clouds. Clearly one of the most inhospitable places in the solar system.