NASA has sent orbiters to investigate the atmosphere of Mars and rovers to examine their surface. Now the agency is planning to look into the planet. The $ 814 million InSight Lander, scheduled for launch next month, carries three instruments that look through the rusted marsupial. This includes a seismometer that detects "Marsquakes". "We have a black hole that starts 5 meters below the surface right down to the center," says Bruce Banerdt, Principal Investigator of InSight and Geophysicist at the Jet Propulsion Laboratory (JPL) in Pasadena, California. He and his colleagues hope that by measuring the thickness and composition of the Earth's crust, Earth's mantle, and core, InSight provides clues to how Mars has lost its magnetic field and whether it once housed plate tectonics.
The mission came close to cancellation after finding a leak in the vacuum of the seismometer. Finally, the launch was postponed for 2 years to find a solution. "We are a much better mission than the ones we had two years ago," says Philippe Lognonné, a planetary seismologist at the University of Paris Diderot, who leads the seismological instrument.
InSight marks NASA's return to planetary seismology after four decades. Apollo astronauts deployed five seismometers that detected moonquakes that identified the core of the moon. The two Viking Landers on Mars carried both seismometers, one of which failed and the other did not send reliable signals. Ambitious efforts since then to place several seismic stations on Mars have spat. But the geophysicists kept pushing, says Banerdt. "I get up at meetings and scold people for not having been behind for decades," he says. "Sometimes I think they chose my mission just to silence me."
InSight was developed by JPL with Lockheed Martin and European partners and is based on the same platform as the 2008 Phoenix Lander. Like its predecessor, it will use parachutes and retrorockets to reach the surface. The destination landing place is a smooth lava plane near the equator – perhaps "the most geologically boring place on the planet," says Banerdt. For one reason: InSight could probably do its job from anywhere on Mars. Therefore, the team chose a location with few landing hazards and, thanks to its tropical location, plenty of sunlight for the solar panels of the probe. After the ship touches down in late November, its robotic arm uses the volleyball seismometer and a heat probe that drives a bar five meters into the surface with thousands of blows from a tungsten hammer.
The heat probe measures how much heat escapes the planet and how fast – an indication of its history. From the chemical analysis of the Marsbrocken, which come as meteorites on the earth, researchers have a feeling for the composition of the Marsmantels. In combination with InSight's thermal gradients and internal dimensions, they can estimate how much of Mars' internal heat comes from radioactive elements inside the planet. The rest is the original energy left over from the Martian Formation. Based on the rates at which these two heat sources crash, researchers can estimate when volcanoes were strongest on Mars. "The evolution of a planet is driven entirely by heat in space," says Steven Hauck, a planetary scientist at Case Western Reserve University in Cleveland, Ohio.
Tiny Doppler shifts in radio broadcasts sent from Earth to the receivers of InSight will reveal further details of Martian interiors. The signals will track how the planet wobbles in its rotation, reflecting the intrinsic traction of its core and mantle. Just as raw eggs with their liquid interiors turn differently than cooked ones, squirrels should point out the size, density and partially melted grains, says Véronique Dehant, geophysicist at the Belgian Royal Observatory in Brussels. This information could in turn clarify its composition and the question of whether it crystallizes from the inside out, like the core of the earth, or from outside to inside. Ultimately, the results will improve models of how the planet has lost the magnetic field. This has once created its core, says George Helffrich, geophysicist at the Earth-Life Science Institute at the Tokyo Institute of Technology. "The core of Mars could represent what the Earth will look like in the future."
A third set of clues will come from Marsquakes. Since Mars does not contain tectonic plates that grind together on Earth, its vibrations are probably 100 times less common than earthquakes. "We could see five to ten over InSight's two-year mission," says Banerdt. "Or maybe we'll see 1000." In his dreams, the Lander sees dozens of Marsquakes in excess of five – a premium that could help InSight identify its sources, even though it will be a single seismic station.
Typical Three stations are needed to triangulate the source of an earthquake from so-called body waves that dive through the planet. But the InSight team has developed a workaround for Mars, relying on waves that sway along the surface. On Earth, features such as the oceans quickly dampen such waves. But on Mars surface waves of large quakes should race around the planet several times. By detecting surface waves from three different global paths, researchers hope to pinpoint the source of each tremor so they can get an idea of how the body creates waves, changes speed, or reflects internal structures.
If all goes so hopefully, the resulting Mars seismic X-rays will reveal the dimensions of its crust, its mantle and its core, and any stratification in them. A thick crust would mean that Mars has melted thoroughly at launch, allowing larger amounts of less dense minerals to rise and collect on the surface. Banerdt says that a thick crust that does not break into slabs also suggests that Mars had no plate tectonics.
If InSight survives until 2021, a second seismic station could join in and attach a gauge to the landing pad for Europe's ExoMars rover. But even InSight is a blessing, says Yosio Nakamura, a planetary seismologist at the University of Texas at Austin, who began his career at Apollo. "A single station is much, much better than no station."