An illustration of the InSight lander on Mars. (NASA/JPL-Cal Tech)

What do you call an earthquake when it happens on Mars?

This is not the opener to a nerdy joke. It's a central question facing NASA's newest spacecraft, InSight.

The rocket carrying the probe launched Saturday from California's Vandenberg Air Force Base into a foggy predawn sky, carrying instruments to take the temperature and pulse of the Red Planet's deep interior. Even the subtlest shake — known as, you got it, a marsquake — could carry clues about how the planet formed and what goes on today beneath its surface.

Most intriguing of all, InSight aims to help scientists understand why Earth and Mars, which formed from the same primordial ingredients more than 4.5 billion years ago, now look so different.

“Earth has plate tectonics, so its initial crust is essentially gone, it's all been recycled,” explained Suzanne Smrekar, the mission's deputy principal investigator. “Mars gives us an opportunity to see the materials, the structure, the chemical reactions that are close to what we see in the interior of Earth, but it's preserved from the first 10 million years [of the solar system]. It gives us a chance to go back in time.”

Liftoff Saturday was NASA's first interplanetary launch from the West Coast. And if all goes according to plan, InSight will be the first mission to study seismic waves on another planet. (NASA tried, and failed, to do this with its twin Viking landers in the 1970s.)

The probe had company atop its Atlas V launch vehicle: two briefcase-size satellites called Mars Cube One, or MarCO. The twin spacecraft will fly behind InSight on its 300-million-mile, six-month journey, allowing NASA to test new, miniaturized deep-space communication equipment. If they make it to the planet, they can relay back data from InSight as it plunges through the Martian atmosphere and touches down on the surface.

InSight's ability to send back scientific data does not depend on the success of the MarCO satellites, though. The Mars Reconnaissance Orbiter, which has circled Mars since 2006, also will be recording broadcasts from the lander.

But success “means that when we land on Mars in the future, we can choose to have something like MarCO go with it, and we'd be able to land in places that the orbiter might have a hard time hearing the landing happening,” said Joel Krajewski, an engineer at the Jet Propulsion Laboratory and the project manager for MarCO. “And we can contemplate doing this kind of relay for missions to other bodies that right now don’t have orbiters around them to serve this useful function.”

InSight has been postponed once; in 2015, NASA pushed back launch by 26 months after finding leaks in the vacuum enclosure for the seismometer. The delay added $154 million to the cost of the mission.


Engineers and technicians evaluate InSight's solar arrays during a final test before launch. (NASA/JPL-Cal Tech/Lockheed Martin)

After touching down, InSight, which stands for “Interior Exploration using Seismic Investigations, Geodesy and Heat Transport,” will spend the next two years sitting patiently in the middle of Elysium Planitia, a vast, flat plain near the Martian equator.

The lander's dome-shaped seismometer resembles the tools used to detect quakes on Earth. But the biggest seismic waves on this planet are generated by tectonic plates drifting and colliding. Tremors on Mars — if they happen — are probably caused by its cooling.

The ultrasensitive instrument also can detect seismic rumblings of other origins: the thump of a meteorite impact, shivers produced by dust storms.

Whatever the source, the seismic waves that ripple through the planet will be distorted by changes in the materials they encounter. InSight's seismometer is capable of detecting those distortions, giving scientists insight (get it?) into lingering questions about the planet's interior: Where is the boundary between crust and mantle? Are there plumes of active volcanoes or reservoirs of liquid water hiding beneath the surface?

Another instrument, the Heat Flow and Physical Properties Probe, will drill almost 16 feet into the planet’s surface — farther than any craft has dug. Its goal is to measure the geothermal heat coming out of Mars as a result of radioactive decay. That in turn should reveal the kinds of materials from which Mars first formed, Smrekar explained.

Meanwhile, two antennas will track how the north pole “wobbles” during the planet's orbits around the sun — an indicator of the size and composition of the Martian core.

“It's kind of like the difference between how a raw egg and a hard-boiled egg spins,” Smrekar said. “We'll be trying to determine whether the core is liquid or solid.”

The results might be the key to a four-billion-year-old mystery. Magnetization in ancient rocks suggests that the Red Planet had a global magnetic field like Earth's in its very early history, one that was powered by a spinning liquid mantle and metallic core. That field probably protected the planet from solar wind, helping it hold on to its atmosphere. In that era, Mars was a warm, wet place, brimming with oceans whose shorelines can still be seen today. That planet and ours may have been nearly twins.

But Mars's internal dynamo died, and its magnetic field dissipated. Unprotected, its water and atmosphere were stripped away by a bombardment of charged particles streaming from the sun. InSight's revelations about Mars's core could explain how that disaster played out, resulting in the apparently lifeless world we know today.

Unfortunately, the planet's hostility extends to the poor, puny spacecraft sent by humans. Mars missions fail about 50 percent of the time, in part thanks to an atmosphere that's too thin to slow down an incoming object, but thick enough to generate friction as the craft hurtles toward the ground. Mars's fearsome reputation is so well established that scientists refer to the entry, descent and landing process as “the seven minutes of terror.

But those terrifying minutes are still six months away. On Saturday, NASA and those Californians who were willing to wake up early simply enjoyed  the launch: a flare of light streaking southward, en route to another world.

Read more:

Mars: A virtual reality tour of the Red Planet

'A fantastic find': Mars hides thick sheets of ice just below the surface

Curiosity’s five-year journey across Mars — in one stunning photo