Lopsided Mercury

Generally, planets will be wider at their equatorial region than at their poles. Being flatter at the poles of a sphere creates what is called an oblate spheroid.  However, Mercury is strange.

There is a large crater on the surface of Mercury called the Caloris Basin or Caloris Planitia .  It is a large impact crater on the surface that was formed about 4 billion years ago.  The impact was so great, that it is believed that seismic waves from the original impact pushed up the area directly opposite the Caloris Basin.  This area is on the antipode of the impact crater and is referred to the Chaotic Terrain, or my favorite, Weird Terrain.  There are relatively few impact craters in this area which tells us that the area is relatively young.

Because of the uplift on the antipode of the Caloris Basin and the Basin itself, the radii from the center to the Basin and to the antipode are not the same.  So the planet is a little lopsided.

NASA.gov

 

NASA.gov

There is also a series of concentric rings around the Caloris Basin, much like any crater in the solar system.  These rings were created from ejecta from the impact.

And here are craters on Mercury that have a strange shape:

Space.com