Mercury is the nearest planet to the Sun at an average distance of 58 million km and can be rather elusive at best. Due to its slow rotation, its daytime side roasts and reaches peak temperatures of around 465 degrees but due to the very thin atmosphere, the night time side plummets to -184 degrees. Because of the lack of atmosphere Mercury’s surface hasn’t been protected from constant pounding from meteoroids so its surface resembles the Moon, peppered by craters.
Venus on the other hand has a very dense carbon dioxide atmosphere which seems to have protected it from all but the largest meteoric impacts. The dense atmosphere has another effect too, it serves as an insulation blanket! The heat and energy from the Sun passes through the thick Venusian atmosphere, heats up the surface and then gets re-radiated at a slightly different wavelength. This re-radiated energy can’t escape through the thick atmosphere and so the planet gets warmer and warmer. The average temperature on Venus is a staggering 449 degrees and the presence of the atmosphere means the night time side is not much cooler than the daytime.
Space craft that have visited these two planets have recorded pretty hostile conditions, the Venera landers which visited Venus recorded the high surface temperatures but also detected sulphuric acid rain and an atmospheric pressure of around 90 times the atmospheric pressure we experience on Earth.
The surface of Mercury is peppered with craters but the surface of Venus is scarred by evidence of a very geologically active past with volcanoes, vast canyon’s and solidified lava flows.
Trying to look at these two objects through telescopes is challenging and in fact the only useful thing that can be picked out from telescopic ground based observation is the phase. In the case of Venus, perhaps some atmospheric detail in the atmosphere can also be seen. Both planets show phases just like the Moon, which, is merely the effect of the changing relative positions of the Sun, Earth and planet (or Moon). Looking at Venus through a normal visual light telescope will only allow us to look at the tops of the thick clouds. This is one of the reasons that Venus looks so bright in the night sky, due to the highly reflective atmosphere.
Mark’s Observing Tip
The best time to observe the two innermost planets is when they appear to be at the most extreme points in their orbit as viewed from Earth. These points are known as greatest eastern or western elongation depending on which side of the Sun they are. From Earth and at this point, they will be at the greatest distance from the Sun in the sky and so easier than usual to spot. Mercury is the most elusive of the two but both planets will only ever be visible in the evening or morning skies, never in the depth of the night.