Image copyright NASA/JPL-Caltech Image caption On the surface: HP3 is on the left; the domed SEIS is on the right

The US space agency's (Nasa) InSight mission has positioned the second of its surface instruments on Mars.

Known as HP3, the heat-flow probe was picked up off the deck of the lander with a robot arm and placed next to the SEIS seismometer package, which was deployed in December.

Together with an onboard radio experiment, these sensor systems will be used to investigate the interior of the planet, to understand its present-day activity and how the sub-surface rocks are layered.

The Heat Flow and Physical Properties Package (HP3) is a German-led instrument.

It incorporates a "mole" to drill down up to 5m below the surface.

The French-led SEIS system will be listening for "Marsquakes" and meteorite impacts. This is information that can be used to build a picture of the planet's overall structure - from its core to its crust.

But SEIS will also be monitoring HP3 when its burrowing activity gets going, because the local vibrations will say something as well about the underground materials in the immediate area.

The Nasa mission landed on Mars on 26 November.

Touchdown occurred on flat terrain close to the equator in a region referred to as Elysium Planitia.

The mission's experiments will run initially for one Martian year (roughly two Earth years).

Image caption On schedule: SEIS and HP3 were due to be positioned within weeks of landing

Image copyright NASA/JPL-Caltech Image caption Seismometer deployment began at the end of December

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos