But like any siblings, the two planets changed as they grew older. Earth’s liquid iron core creates a magnetic field, which protects the planet from blistering solar rays. Somehow, Mars’ geomagnetic dynamo died. Earth, the bigger planet, held onto its atmosphere; little Mars, with only 38 percent of Earth’s gravity, saw its air stripped away. Earth kept its oceans and its water cycle, and Mars became the dusty, dry and dead world we know today.