In recent weeks, NASA’s InSight lander has made changes to the seismometer laid down on the Martian surface on Dec. 19. It has now reached another milestone by placing a domed shield on the seismometer to help the instrument collect accurate data. The seismometer will give scientists their first look inside the Red Planet, helping them understand how Mars and other rocky planets formed.
The Wind and Thermal Shield helps protect the super-sensitive instrument from being shaken by blowing wind, which can add “noise” to its data. The dome’s aerodynamic shape causes the wind to push it toward the planet’s surface, ensuring that it doesn’t tip over. A sort of skirt made of chain mail (metal rings crisscrossed with fabric) and thermal insulation wraps around the bottom, allowing it to easily settle on any rock, though there are very few in InSight’s location.
An even bigger concern for InSight’s seismometer, called the Seismic Experiment for Interior Structure or SEIS, is the change in temperature, which can cause metal springs and other parts inside the seismometer to expand and contract. At the site where InSight landed, temperatures fluctuate about 94 degrees Celsius over the course of a Martian day, or sol.
“Temperature is one of our biggest bogeymen,” said InSight lead researcher Bruce Banerdt of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. JPL leads the InSight mission and built the domed shield.
“Think about the shield, it’s like you’re putting protection over the food on your table. In the same way you’ll keep SEIS from getting too hot during the day or too cold at night. Basically we want to keep the temperature as stable as possible.”
On Earth, seismometers are often buried in holes about 1.2 meters underground, which helps keep the temperature stable. InSight can’t dig a hole on Mars, so the mission uses other strategies to protect its seismometer. The shield is the first line of defense.
A second line of defense is the SEIS itself, which is specially designed to compensate for the wide temperature swings on the Martian surface. The seismometer has been constructed so that while some parts expand or contract, others do so in the opposite direction to partially negate thermal effects. In addition, the instrument is vacuum-sealed in a titanium sphere that insulates its sensitive interior and reduces the influence of temperature.
But even this is not enough. The sphere is enclosed within another insulating container: a copper-colored hexagonal box visible during SEIS placement. The walls of this box are honeycombed with cells that trap air and prevent its movement. Mars provides an excellent gas for this insulation: its thin atmosphere is composed primarily of carbon dioxide, which has particularly low thermal conductivity at low pressure.
With these three insulating barriers, SEIS is well protected from the thermal “noise” that filters into the data and masks the seismic waves that the InSight team wants to study. Finally, most of the additional interference from the Martian environment can be detected by InSight’s weather sensors and then filtered out by mission scientists.
With the seismometer on the ground and covered, the InSight team is preparing for the next step: placing the heat flow probe, called the Heat Flow and Physical Properties Package (HP3), on the Martian surface. This is expected to happen next week.
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