From the Wikipedia article on Pathfinder:
Using Doppler tracking and two-way ranging, scientists added earlier measurements from the Viking landers to determine that the non-hydrostatic component of the polar moment of inertia is due to the Tharsis bulge and that the interior is not melted. The central metallic core is between 1,300 and 2,000 km (810 and 1,240 mi) in radius.[28]
Pathfinder was stationary and only 35W of power. How could it determine info about the core of the planet?
ETA: title is supposed to say “on Mars”, not “on Amara”. Gah!
Scientists are looking at the precession rate and slight perturbations in the rotation of the planet to infer how much of the subsurface is “non-hydrostatic” (meaning not liquid) versus the hysteresis in the molten core. Unlike Earth, which has a molten mantle, Mars has a mostly frozen mantle with a large tidal bulge so it is ‘easier’ to distinguish between that and the core, although there were uncertainties about how much of the mantle may be partially flowing and how big the core is.
However, since those measurements, both the Mars Global Surveyor orbiter was able to get more detailed measurements of planetary rotation, and the and the [Mars InSight lander] (InSight Science Instruments) was able to get direct seismic information including the refraction of seismic waves from meteorite impacts to make even more accurate surveys of the planetary interior. This is why it is critical to have repeated followups to planetary missions; because everything gleaned from one mission—including all of the unanswered questions—feeds into the design and objectives of subsequent mission to maximize the scientific yield.
Stranger