Insightful to the End
Just before our break for the winter holidays, we said a fond farewell to one of our beloved Planetary Science Division (PSD) missions. With dust build-up on its solar panels, InSight’s power levels had become precipitously low, and the spacecraft communicated with Earth for the final time on December 15, 2022. Although we had been anticipating losing contact with InSight for many months, and it was well into its extended mission, saying goodbye to one of our spacecraft is always sad — somewhat akin to losing a member of a family. I was glad, therefore, to spend time over the break relaxing and recharging with my own family and friends and reflecting on InSight’s many successes.
InSight (short for Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) was the twelfth mission in the Discovery Program. It launched in May 2018, shortly after I started my tenure as Director of NASA’s Planetary Science Division, so it has been especially meaningful for me to see this mission complete its whole “life.” Unlike our rovers on Mars, which are sent to explore geologically and astrobiologically interesting locations, InSight’s landing site in Elysium Planitia was specifically chosen to be pretty “boring” — but for good reason!
InSight’s primary goal was to investigate the interior structure and composition of Mars. To do that, the lander needed to be at a site where it could remain as still and quiet as possible on the Mars surface for the entire mission. Elysium Planitia was therefore selected not for its geology and surface features, but rather for safety considerations. InSight touched down on November 16, 2018, in the western portion of the plane, less than 5° north of the equator, in an area that is characterized by flat topography and few large nearby rocks.
InSight carried three science instruments: the Seismic Experiment for Interior Structure (SEIS), the Heat Flow and Physical Properties Package (HP3), and the Rotation and Interior Structure Experiment (RISE). With this payload, the spacecraft was designed to address fundamentally important questions about the formation and evolution of Mars, such as the size and structure of the planet’s core, mantle, and crust, the planet’s heat flow, and the planet’s current seismic activity.
During InSight’s four-and-a-half-year sojourn on Mars, SEIS detected more than 1300 “marsquakes.” Most of these quakes arose from activity on faults and rock fractures — caused by the buildup of stress as the planet continues to cool and slightly contract. In a surprise to the team, the majority of the largest quakes (up to about magnitude 5) can be traced to a region known as Cerberus Fossae, which is thought to be volcanically active enough to have had lava flowing within the last few million years. With these seismic data, the team has so far been able to confirm that the crust of Mars is between about 20 to 37 kilometers (12 to 23 miles) thick (thinner than expected) and may have two or three sublayers; that the mantle reaches to a depth of almost 1500 kilometers (about 1000 miles) below the surface; and that the core of Mars is molten, with a radius of about 1830 kilometers (1140 miles).
Despite the primary success in revealing the interior structure of Mars, InSight also faced its share of setbacks and challenges. Most significant were the valiant attempts to bury the HP3 “mole.” The heat probe had been intended to dig about 5 meters (16 feet) into the surface, via the self-hammering mole, and had been designed to encounter soil that was loose and sandy, like we have found at other mission landing sites. The ground at InSight’s location, however, turned out to be unexpectedly clumpy, which scuppered the mole’s burrowing progress. The valiant engineers at the Jet Propulsion Laboratory and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, or DLR) devised several ingenious and innovative methods to further the mole’s journey through the soil, but the probe was eventually buried only slightly below the surface. Nevertheless, the lessons learned and the data collected by HP3 are still valuable for understanding the physical and thermal properties of Mars’ soil and will be useful for future human or robotic missions.
In a case of appropriate celestial timing, InSight received one of its most exciting gifts on Christmas Eve (December 24), 2021. SEIS recorded a magnitude 4 quake, which the team later realized had been caused by a meteoroid striking Mars (the largest of such impact-driven quakes detected by InSight). This discovery was made by examining images from the Mars Reconnaissance Orbiter (MRO) HiRISE camera, which revealed a fresh crater almost 150 meters (500 feet) in diameter. It is thought that the meteoroid would have been about 5 to 12 meters (15 to 40 feet) in size — small enough that it would have burned up in Earth’s thicker atmosphere. The contemporaneous operation of InSight and MRO provided the opportunity to make this discovery and this was an invaluable opportunity to examine the seismic effects of a relatively large impact. What’s more, the HiRISE images also revealed boulder-sized chunks of ice that were excavated by the impact. This detection of ice is particularly exciting because it is the first time ice has been seen on Mars at such low latitudes, with important implications for future missions to the planet.
InSight has been a treasured part of PSD’s fleet for many years and is a fantastic example of how the Discovery Program continues to change our understanding of the solar system. In fact, we will be celebrating Discovery’s thirtieth anniversary this year, including a symposium in Washington, D.C., in the fall — stay tuned for more information. Among other PSD highlights for 2023 — including the launch of the European Space Agency’s JUICE mission in April, the delivery of OSIRIS-REx’s Bennu sample in September, and the launch of Psyche in October — we will also be seeing the first Commerical Lunar Payload Services (CLPS) deliveries to the Moon. Future CLPS deliveries will place seismic suites on the lunar surface, and will be the next step in planetary seismology — a legacy started with Apollo and continued superbly with InSight.
— Lori S. Glaze, Director, NASA’s Planetary Science Division, January 2023