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, found to be rich in volatile elements such as sodium and sulfur.
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Source: Phys.org