A simulation of a Martian crater based on images from NASA’s Mars Reconnaissance OrbiterUniversity of Arizona / JPL-Caltech / NASA

After Earth, Mars may be the most well-studied planet in the solar system.

Since 1965, humankind has sent spacecraft after spacecraft to examine our celestial neighbor. The flybys captured the first close-up photographs of the Red Planet, revealing a rocky, cratered surface. The orbiters found towering volcanoes, dried-up riverbeds, and enormous storms that choked the thin atmosphere with dust. The landers searched for signs of life in the cinnamon-colored soil but found none. The rovers carved tire tracks into the ground and detected organic molecules embedded in 3-billion-year-old rock, a sign that perhaps some beings may have existed long ago.

But even after more than 50 years of research and exploration, our understanding of Mars remains surface-deep. Spacecraft have drilled into the rock and scooped up soil samples, but they haven’t made it very far.

“We know a lot about the surface of Mars, we know a lot about its atmosphere, and even about its ionosphere, but we don’t know very much about what goes on a mile below the surface, much less 2,000 miles below the surface down to the center,” explains Bruce Banerdt, a scientist at NASA’s Jet Propulsion Laboratory who studies the evolution of Mars.

Scientists such as Banerdt will soon learn more. A NASA spacecraft unlike any other touched down on Mars Monday afternoon. In the weeks ahead, a robotic arm will unpack some scientific instruments and place them gently on the ground. The tools will spend their entire lifetimes in these spots, pressing themselves into the surface to probe what’s going on deep below the surface.

For the first time, human beings will be able to study another rocky planet in the same depth as they have their own.

The mission is called the Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport, which the acronym connoisseurs at NASA have shortened to InSight. InSight has spent the last seven months coasting through space at a cool 6,200 miles an hour, and the last several days preparing for a turbulent descent through the Martian atmosphere. InSight touched down in Elysium Planitia, a flat, smooth plain mostly devoid of rocks and hills, just 373 miles north of its NASA neighbor, the Curiosity rover.

Observations have shown that Mars, like Earth, has three distinct layers: a thin, rocky crust; a mantle; and a metal core. Beyond that, scientists are still in the dark about the planet’s inner workings. “The limits and constraints imposed by the measurements carried out to date do not enable us to design even a very simple model of Mars’ internal structure,” a scientific overview for InSight laments.

The InSight mission will attempt to improve this model with its suite of diverse instruments. A round, dome-shaped seismometer will sit on the surface and patiently wait for vibrations or, as they’re known on Mars, marsquakes. (A previous NASA mission, Viking, brought two seismometers to Mars, but only one deployed successfully, and scientists suspect that the sole seismic event the instrument managed to record may have been a product of powerful winds.)

Another instrument will burrow nearly 16 feet into the soil to measure the heat coming from the interior of the planet, which, like Earth, is still cooling from its fiery creation 4.5 billion years ago. A third instrument will focus on the lander itself, tracking its position as the planet wobbles during its orbit around the sun, which can reveal information about the size of the core.

Scientists hope that the InSight mission will help explain why Earth and Mars, forged from the same nebulous cloud billions of years ago, grew into such different worlds. “Our measurements will help us turn back the clock and understand what produced a verdant Earth but a desolate Mars,” Banerdt, who leads the mission, said recently.

Since the invention of the telescope, humanity has been using technology to extend its reach into the cosmos without ever stepping off the ground. NASA’s suite of Mars missions, particularly the rovers and landers, have delivered analogues of the human body to roam the planet’s surface, inspect the fine grains, and dig into the soil of an alien world. Though it will be weeks before InSight returns its first measurements of the Martian depths, Monday’s landing marks a scientific transformation. Geology—the most earthly of all sciences—is about to become interplanetary.

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