Picture Mercury in your mind, and what's the first word you'd choose to describe it? Small? Puny even? Thorough observation proves that Mercury is indeed the smallest major planet in our solar system. But ask yourself how it got so small, and the picture becomes unclear.
The most popular scenario scientists use to explain Mercury's relatively small stature is an impact that occurred when our solar system was in its infancy. It's suggested that at this time, Mercury was more than twice its current size, consisting of a dense metallic core surrounded by a mantle of molten rock and a terrestrial crust -- that is until a rocky body called a planetesimal collided with it, stripping it of much of its outer layers. The remaining matter would have condensed around the core until it became the Mercury we recognize.
Another theory suggests that Mercury's size is the result of vaporization. In our sun's early development, it would have created temperatures on Mercury as high as tens of thousands of degrees Fahrenheit. This extreme heat would likely burn Mercury's silicate-rich crust into a vapor that would then be swept away by solar winds, and only a fraction of the original mass would remain.
A third hypothesis involves a tug of war between planet and star. As the young Mercury slowly gathered rocky material around its core, our sun's gravity may have diverted the lighter material away, leaving only heavy material behind as substance for the forming planet.
Somehow, Mercury's size was dramatically reduced. And as if it weren't small enough, new evidence shows that it is getting smaller. Images taken by the Messenger Spacecraft show fractures called scarps on Mercury's surface where the crust is buckling. This is due to the shrinking of Mercury's core -- a process that scientists say has lasted billions of years, thus raising the question "How long till Mini Mercury is missing from our sky?"