In addition to learning whether Mars' core is liquid, solid or a mix of both, scientists are curious why the fourth planet from the sun evolved so differently than Earth.
"Mars is our real first attempt to be able to understand what these terrestrial bodies go through in their early evolution," said Jim Green, NASA's planetary sciences director.
"We know that that the interior of the Earth has been modified over time through its plate tectonics and its evolution. Mars, we're really clueless about," Green said.
The solar-powered lander, which is based on the successful 2008 Phoenix lander, would be designed to last two years.
InSight competed for NASA funding and a ride to space against two other projects selected last year, along with InSight, for $3 million study grants.
The contenders were a robotic boat to sail around a lake on Titan, the largest moon of Saturn and the only other body in the solar system found to have liquids on its surface; and a robotic hopper that would study a comet.