Insight Instruments

[Joseph]

Aplanet's deep interior holds evidence related to details of the planet's formation that set the stage for what happens on the surface. The interior heat engine drives the processes that lift some portions of the surface higher than others, resulting in a landscape's elevation differences. The interior is the source of most of a planet's atmosphere, its surface rocks, water and ice, and its magnetic field. It provides many of the conditions that determine whether a planet will have environments favorable for the existence of life.

Seismic investigations study vibrations of the ground set off by marsquakes (the Mars equivalent to earthquakes) and meteorite impacts, including the analysis of how these vibrations pass through interior materials and bounce off boundaries between layers. For this research technique, InSight will deploy a seismometer provided by France. Seismic investigations can be compared to how physicians use sonograms and X-rays to see inside a body.

Geodesy is the study of a planet's exact shape and its orientation in space, including variations in the speed of rotation and wobbles of its axis of rotation. The axis of rotation is very sensitive to conditions deep inside Mars. For this research technique, the lander's radio link to Earth will provide precise tracking of a fixed location on the surface as the planet rotates, throughout the course of a full Mars year. This investigation of the planet's motion can be compared to examining a patient's reflexes during a medical check-up.

Study of heat transport is a way to assess a planet's interior energy and its dissipation. For this research technique, InSight will sink a German-made probe more than 3 meters (10 feet) into the ground to measure how well the ground conducts heat and how much heat is rising toward the surface. This investigation can be compared to how a physician reads a patient's temperature as an indicator of internal health.

Other components of the InSight lander's science payload are auxiliary instruments for monitoring the environment to aid the primary investigations, and a deployment system with a robotic arm and two cameras for the task of placing the main instruments onto the ground.

Some of these additional sensors will monitor wind, variations in magnetic field and changes in atmospheric pressure because these factors could affect seismometer readings. Others will monitor air temperature and ground-surface temperature, which will help in subtracting effects of those temperatures from heat probe and seismometer data. These supplemental instruments will also enable additional investigations, such as magnetic soundings of the Martian interior by the magnetometer and weather monitoring by the atmospheric sensors.

The auxiliary sensors and the two color cameras will provide information about the environment surrounding the InSight lander on the surface of a broad Martian plain near the equator, but for this mission, the science emphasis is to learn about depths that cannot be seen.

Several reports setting scientific priorities for planetary science have stressed the importance of investigating the interior of Mars. While the Mars Viking missions of the 1970s were still active, a report by the National Research Council's Committee of Planetary and Lunar Exploration, Strategy for Exploration of the Inner Planets: 1977-1987, said, "Determination of the internal structure of Mars, including thickness of a crust and the existence and size of a core, and measurement of the location, size and temporal dependence of Martian seismic events, is an objective of the highest importance."

A stationary lander capable of placing sensitive instruments directly onto the surface and monitoring them for many months is a mission design exactly suited to studying the interior of Mars. InSight will be the first Mars mission to use a robotic arm to grasp objects (in this case, scientific instruments) and permanently deploy instruments onto the ground. The mission has no need for a rover's mobility. The heat probe and seismometer stay at a fixed location after deployment. The precision of the geodesy investigation gains from keeping the radio in one place.

InSight uses many aspects of a stationary-lander mission design already proven by NASA's Phoenix Mars lander mission, which investigated ice, soil and atmosphere at a site in the Martian arctic in 2008. The robotic arm for InSight, rather than scooping up samples for laboratory analysis as Phoenix did, will hoist the heat probe, seismometer and a protective shelter for the seismometer one at a time from the lander deck and place them onto the ground.

The first time a seismometer was placed on a world other than Earth was during the Apollo 11 Moon landing in 1969. The only seismometers previously used on Mars stayed on the decks of two Viking landers in 1976. Those were much less sensitive and more exposed to wind effects than InSight's seismometer will be. Nearly 50 years after Apollo, InSight will be the first seismometer placed directly on the surface of the Mars.

InSight's science payload and science team draw heavily on international collaboration and shared expertise. The national space agencies of France and Germany are providing the two main instruments. Austria, Belgium, Canada, Italy, Poland, Spain, Switzerland and the United Kingdom are also participating.

InSight is part of NASA's Discovery Program of competitively selected missions for exploring our solar system. The Discovery Program enables scientists to use innovative approaches to answering fundamental questions about our solar system. Bruce Banerdt of NASA's Jet Propulsion Laboratory, Pasadena, California, now the principal investigator for InSight, led the team that prepared the mission proposal -- originally called Geophysical Monitoring Station, or GEMS -- submitted in September 2010. That proposal and 27 other proposals for missions to various destinations throughout the solar system were evaluated in a competition for the 2016 launch opportunity of the Discovery Program. InSight was selected in August 2012.

The four inner planets of the solar system, plus Earth's Moon, are called terrestrial worlds because they share a closer kinship with each other, including Earth, than with the worlds farther from the Sun. Diverse as they are, they each have rocky surfaces; they are also called the rocky planets. They each have high density -- the ratio of volume to mass -- indicating their interiors have even denser ingredients than their surface rocks.

Some of the ever-increasing number of exoplanets identified around stars other than our Sun may be similarly rocky and layered, though Earthlike worlds are smaller than the giant exoplanets whose size makes them easiest to find.

A key challenge in planetary science half a century into the Space Age is to understand factors that affect how newly forming planets with the same starting materials evolve into worlds as diverse as the terrestrial planets. As a particularly interesting corollary: What does it take to make a planet as special as Earth?

Planets start as growing coagulations of primordial particles in a disc-shaped swarm around a formative star -- the proto-Sun in the case of our solar system. Meteorites provide information about composition of the planet-forming raw material. Earth formed from the same material as its neighboring planets, but none of the planets now matches the mineral composition of those starting ingredients. They evolved.

As the forming planets grew larger, they heated inside, with energy from pieces coming together and natural radioactivity. Melting due to the heat enabled enough mobility for heavier ingredients to sink toward the center. Temperature and pressure affected the chemistry of the ingredients. Cooling caused some minerals to crystallize out of the melt at different temperatures than others. Multiple models have been proposed for the steps in how different minerals were produced and stratified as Earth's evolution proceeded. Each of these models of terrestrial planet evolution fits the evidence known from studying Earth. Gaining knowledge of a different case -- Mars -- should rule out some of the models. Achieving that will yield both a better understanding of why Earth turned out the way it did and a conceptual framework for studying rocky planets of other stars.

The most accessible world for studying terrestrial planets is Earth. In the past century, research using InSight's main methods -- seismology, geodesy and heat transport -- has substantially rewritten humans' understanding of Earth's interior and planetary history. But Mars offers advantages making it the right choice for a mission seeking to learn more about the formation and early evolution of terrestrial planets.

The major process in Earth that geological science has elucidated in the past century is plate tectonics, a recycling of crust driven by convection in the mantle as heat moves out from the core. The mantle has been vigorously stirred by convective motion driven by warmed material rising and cooled material sinking. The rising generates fresh crust at mid-ocean ridges; the sinking drags downward at some plate edges. The churning has erased from both crust and mantle most structural evidence of the first several tens of millions of years of Earth's history after the planet formed about 4.5 billion years ago.

Mars lacks plate tectonics. Likely because of its smaller size, compared to Earth, that process has not churned the mantle and crust of Mars. Therefore, its interior could provide clues unavailable on Earth about the accretion and early evolution of Earth, Mars and other rocky planets. For example, the mantle of Mars may retain differences in composition at different depths, which convection has blended together on Earth.

Investigations of the Earth's Moon, including analysis of lunar rocks returned to Earth, indicate that, although the Moon followed many of the same evolutionary steps as Earth, the path of its evolution was distinctly different because of its much smaller size. For example, it never underwent certain geochemical changes related to the greater interior pressure of the Earth.

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