In InSight's case, the question is what is inside of Mars. It includes a seismometer, to measure "Marsquakes" if any exist, and a subsurface thermometer to determine how much heat is being released from the planet's core.
"This has been something that has interested the scientific community for many years," NASA's associate administrator for science John Grunsfeld told reporters during a conference call late Monday.
"Seismology, for instance, is the standard method by which we've learned to understand the interior of the Earth and we have no such knowledge for Mars," he said.
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.
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Grunsfeld said each had excellent science, but reviewers felt InSight had a better chance of being built within its budget cap and on time.
Image: Artist's rendering of InSight working on the surface of Mars. Credit: NASA/JPL-Caltech