This cake was my first round cake and was challenging, but it certainly was fun to make. I am quite happy with the way it turned out, even with all the issues I had!
Callisto is an ice moon of Jupiter whose craters appear luminescent among the dark background of the ice. To achieve this look on the surface of the moon, we tried using a light colored icing (we just used plain white buttercream) and used cocoa powder to darken it up a bit. We carved out some craters to achieve the moon surface effect.
For the interior of the moon, which includes multiple layers and here we used a blue colored white cake for the ocean and a white cake with chocolate chips to represent the core. Each layer of the cake was baked according to directions on the box (we used easy to make box cake) in a half-sphere cake pan. I used two different sizes. The trick here is propping up your sphere cake pan so that it doesn’t tip over. That happened to me but it wasn’t too bad. You can tell the spherical layers are not quite even due to this.
For the round cake, you need to cook two half-spheres of each layers and assemble them after they cool. This was the biggest challenge as you will have to cook the smaller spheres first, then place them in your cake batter-filled larger sphere to cook the second layer. You will need to use a knife to even everything out and use extra white icing to stick them together. After you have your sphere assembled, it’s time to ice the cake!
Post baking 
Post trimming
To achieve the best results for your planet or moon cake, I recommend following directions laid out here https://cakecrumbs.me/2013/07/24/jupiter-structural-layer-cake/.
the intra- and extra-terrestrial microbe 