NASA's InSight lander continues to ramp up for the science phase of its mission, having now dropped its the second instrument pack on the Martian surface. Images sent back by the unmanned probe confirm that the Heat Flow and Physical Properties Package (HP3) had been placed within a meter of the previously deployed seismometer. When operational, the HP3 will drill deeper into the Red Planet than any previous attempt.

InSight's HP3 is designed to measure the interior temperature of the planet and how heat flows through the subsurface, so scientists can gain a better understanding of how rocky planets form. This means the lander has to place the sensors deep in the soil.

In a few days InSight will attempt to dig down to a depth of 16 ft (5 m) – many times deeper than the 8.6 in (22 cm) reached by the Viking 1 lander's scoop or the 7 in (18 cm) reached by the Phoenix lander. At the pointy end of the operation will be a device called a mole – a 16-in (40 cm) long, spiked-shape probe that hammers itself into the Martian surface, tailing an instrument tether behind it.

The tether measures the subsurface temperature while the mole measures the thermal conductivity. That is, how heat flows in the soil. Being underground, the instruments are isolated by the temperature fluctuations caused by the passage of day and night or the successions of the seasons on the surface.

"Our probe is designed to measure heat coming from the inside of Mars," said InSight Deputy Principal Investigator Sue Smrekar of NASA's Jet Propulsion Laboratory in Pasadena, California. "That's why we want to get it belowground. Temperature changes on the surface, both from the seasons and the day-night cycle, could add 'noise' to our data."

The mole will drill down in 19-in (50-cm) stages, stopping for two days in between to let heating caused by friction to dissipate, before being heated to 50º F (10º C) over a 24-hr period so that scientists can calculate the heat conductivity of the planet. The hope is that there won't be any large rocks in front of the drill – if it doesn't reach a minimum depth of 10 feet scientists say it will take a Martian year (two Earth years) to make an accurate reading, because "noise" from surface temperature changes will need to be filtered out of the data.

"We picked the ideal landing site, with almost no rocks at the surface," says JPL's Troy Hudson, a scientist and engineer who helped design HP3. "That gives us reason to believe there aren't many large rocks in the subsurface. But we have to wait and see what we'll encounter underground.

"[The HP3] weighs less than a pair of shoes, uses less power than a Wi-Fi router and has to dig at least 10 ft on another planet. It took so much work to get a version that could make tens of thousands of hammer strokes without tearing itself apart; some early versions failed before making it to 16 ft, but the version we sent to Mars has proven its robustness time and again."

Source: NASA

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