Because it doesn't have a liquid magnetic core that rotates in a way that creates a magnetic field. It's clear that at one point Mars did have a system similar

Because it doesn't have a liquid magnetic core that rotates in a way that creates a magnetic field. It's clear that at one point Mars did have a system similar to Earth's, but it stopped and nobody knows why. The theory is that one of the large collusions early in Mars' planetary formation caused surface heat that disrupted the convection of the magnetic dynamo. http://www.universetoday.com/30538/was-mars-magnetic-field-blasted-away/ http://www.universetoday.com/30538/was-mars-magnetic-field-b...