... Mars . ... Supernova showered Earth with radioactive debris . ... Scientists predict that rocky planets formed from т??pebblesт?? ... Conventional solar system formation models generate good analogs to Earth and Venus but predict that Mars should be of similar-size or even larger than Earth. ... Using a new process in planetary formation modeling, where planets grow from tiny bodies called "pebbles," Southwest Research Institute (SwRI) scientists can explain why Mars is so much smaller than Earth. ...