Science

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.

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.

Unlike the Moon, Mars is big enough to have undergone most of the same processes as early Earth. Unlike Earth, it is small enough not to have erased as much evidence of its early activity. Compared to Venus and Mercury, Mars provides a more accessible destination and less harsh surface environment for sensitive robotic hardware to operate for many months of data collection.

As added benefits, knowledge about the surface and atmosphere of Mars that has been gained from a series of successful missions there will help researchers interpret information InSight adds about the deep interior, and InSight's findings will improve context for understanding those missions' results.

Heat Flow and Physical Properties Probe

One of InSight's three main investigations -- the geodesy study -- does not require its own dedicated science instrument: The Rotation and Interior Structure Experiment (RISE) will use InSight's direct radio connection with Earth to assess perturbations of Mars' rotation axis. These measurements can provide information about the planet's core.

The perturbations resemble the wobble of a spinning top, and occur on two time scales. The longer wobble takes about 165,000 years and is the same as the process that makes a top wobble, called precession. The speed of this precession is directly related to the proportion of the body's mass that is close to the center, in the iron-rich core. The shorter-period wobbles, called nutations, occur on time scales of less than a year and are extremely small. Their cause is unrelated to a toy top's wobble. A closer analogy is the traditional method for determining whether an egg is hard-boiled by spinning it. An egg with a solid center spins easily. The liquid center of a raw egg perturbs the spin.

With InSight as the marker for a specific point on the Martian surface, radio tracking will monitor the location of that point in space to within a less than 4 inches (10 centimeters). This will provide information about how much the rotation axis of Mars sways with motion that is an indicator about the size of the core.

Radio tracking of the location of NASA's Mars Pathfinder lander for three months in 1997, combined with tracking data from the Viking Mars landers in the 1970s, provided information about long-term changes (precession) in Mars' spin axis. Researchers were able to confirm that Mars has a very dense core. A different radio-science investigation, analyzing gravitational effects of Mars on NASA's Mars Global Surveyor orbiter, indicated some portion of the planet's outer core is molten, based on how much Mars bulges from tidal pull of the Sun.

A longer tracking period with a stationary lander is the next step for measuring nutations to determine the core's exact size and density, and how much of the core is molten. This is not an experiment suited to Mars rovers, because they change their locations on the planet.

The tools for the RISE investigation are the X-band radio on the InSight lander and the large dish antennas of NASA's Deep Space Network at stations in California, Australia and Spain. This is the same direct radio link by which the spacecraft will receive commands and can return data, though it will use relayed radio links through Mars orbiters for most of its data return.

The lead investigator for RISE is William Folkner of JPL, who led the 1997 investigation of Mars' core using the radio link between Earth and NASA's Mars Pathfinder.