The earliest atmosphere on Mercury

Mercury is a most unusual planet. The smallest planet in the solar system, and the closest planet to the sun, it is in a 3:2 spin resonance, slowly turning and experiencing scorching heat up to 430 degrees Celsius, and the night side frigid, down to -170 degrees Celsius. Due to its much larger iron-rich core compared to Earth, it has the second-highest average density in the solar system, just 1.5 percent below Earth's. Despite its proximity to the sun, the surface of Mercury was, surprisingly, ...