InSight’s arm will deploy two probes to the martian surface after landing this November.
LOCKHEED MARTIN/JPL-CALTECH/NASA
By Paul Voosen
NASA has sent orbiters to study the atmosphere of Mars and rovers to study its surface. Now, the agency plans to look inside the planet. The $814 million InSight lander, due to launch next month, carries three instruments designed to peer through Mars’s rusty shell, including a seismometer that will detect “marsquakes.” “We’ve got a black hole that starts 5 meters below the surface and goes all the way down to the center,” says Bruce Banerdt, InSight’s principal investigator and a geophysicist at the Jet Propulsion Laboratory (JPL) in Pasadena, California. He and his colleagues hope that by measuring the thickness and composition of the planet’s crust, mantle, and core, InSight will provide clues to how Mars lost its magnetic field and whether it once hosted plate tectonics.
The mission came close to cancellation after a leak was found in the seismometer’s vacuum. Ultimately, the launch was delayed for 2 years to find a fix. “We are a much better mission compared to the one we had 2 years ago,” says Philippe Lognonné, a planetary seismologist at Paris Diderot University who leads the seismology instrument.
InSight marks NASA’s return to planetary seismology after 4 decades. Apollo astronauts deployed five seismometers that detected moonquakes—tremors that helped identify the moon’s core. The two Viking landers on Mars both carried seismometers, though one failed and the other sent no reliable signals. Ambitious efforts since then to put multiple seismic stations on Mars have sputtered. But the geophysicists kept pushing, Banerdt says. “I’ve been getting up at meetings and berating people for not getting behind this for decades,” he says. “Sometimes I think they selected my mission just to shut me up.”
Developed by JPL with Lockheed Martin and European partners, InSight is built on the same platform as 2008’s Phoenix lander. Like its predecessor, it will use parachutes and retrorockets to reach the surface. The target landing site is a smooth plain of lava near the equator—perhaps “the most geologically boring site on the planet,” Banerdt says. That’s for a reason: InSight could probably do its job from anywhere on Mars, so the team picked a site with few landing hazards and, thanks to its tropical location, plenty of sunlight for the probe’s solar panels. After the craft touches down in late November, its robotic arm will deploy the volleyball-size seismometer and a heat probe, driving a rod 5 meters into the surface with thousands of strokes of a tungsten hammer.
The heat probe will measure how much heat is escaping from the planet, and how quickly—a clue to its history. From chemical analysis of the chunks of Mars that arrive on Earth as meteorites, researchers have a sense of the martian mantle’s composition. Combining this with the heat gradient and interior dimensions divined by InSight, they can estimate how much of Mars’s internal heat comes from radioactive elements in the planet’s interior. The remainder is primordial energy leftover from Mars’s formation. Based on the rates at which those two heat sources ebb, researchers can estimate when volcanoes were most vigorous on Mars. “The evolution of a planet is driven completely by how heat moves out to space,” says Steven Hauck, a planetary scientist at Case Western Reserve University in Cleveland, Ohio.
Radio antennas Hammered rod Instrument tethers Solar panel InSight lander A look inside Mars To be launched next month, NASA’s InSight mission will land in late November to study the planet’s interior. Parachutes and retrorockets will set the car-size lander down on plains near the equator to maximize sunlight. Wobble tracker Doppler shifts in radio signals from Earth will point to wobbles in Mars’s spin, a clue to the composition of its core. Heat probe A rod the diameter of a quarter and the length of a forearm will be ham- mered down 5 meters to sense the flow of the planet’s heat. The probe can slip around obstructing rocks. Quake watch A wind and thermal shield will cover a seismometer, containing six sensors to measure high and low frequency shaking in each of three directions. Armed and ready Over several weeks, an arm will place instruments away from the lander to avoid noise from the spacecraft.
C. BICKEL and A. Cuadra/SCIENCE
Tiny Doppler shifts in radio broadcasts sent from Earth to receivers on InSight will reveal other details of the martian interior. The signals will track how the planet wobbles in its rotation, which reflects the internal tug of its core and mantle. Just as raw eggs, with their liquid interiors, spin differently than cooked ones, Mars’s wobbles should hint at the core’s size, density, and whether it is partially molten, says Véronique Dehant, a geophysicist at the Royal Observatory of Belgium in Brussels. That information could, in turn, shed light on its composition and whether it is crystallizing from the inside out as it cools, like Earth’s core, or from the outside in. Ultimately, the results will improve models of how the planet lost the magnetic field that its core once generated, says George Helffrich, a geophysicist at the Tokyo Institute of Technology’s Earth-Life Science Institute. “Mars’s core could represent what Earth’s will look like in the future.”
A third set of clues will come from marsquakes. Because Mars lacks the tectonic plates that grind together on Earth, its tremors are likely to be 100 times less frequent than earthquakes. “We might see five to 10” over InSight’s 2-year mission, Banerdt says. “Or we might see 1000.” In his dreams, the lander will see dozens of marsquakes with a magnitude more than five—a bounty that could help InSight identify their sources, even though it will be a single seismic station.
Typically, three stations are needed to triangulate an earthquake’s source from so-called body waves, which dive through the planet. But the InSight team has devised a workaround for Mars by relying on waves that vibrate along the surface. On Earth, features such as the oceans quickly dampen such waves. But on Mars, surface waves from big quakes should race around the planet multiple times. By detecting surface waves from three different global paths, the researchers hope to pinpoint each tremor’s source, which will enable them to make sense of how the body waves it produces change speed or reflect off structures in the interior.
If all goes as hoped, the resulting seismic x-rays of Mars will reveal the dimensions of its crust, mantle, and core and any layering within them. A thick crust would mean that Mars melted thoroughly at its start, allowing larger amounts of less dense minerals to rise and collect at the surface. Banerdt says a thick crust, resistant to fracturing into plates, would also suggest that Mars never had plate tectonics.
If InSight survives until 2021, a second seismic station could join it and corroborate its readings: an instrument mounted on the landing platform for Europe’s ExoMars rover. But even by itself, InSight is a boon, says Yosio Nakamura, a planetary seismologist at the University of Texas in Austin, who started his career working on Apollo. “A single station is much, much better than no station at all.”
Source: Science Mag