Doug Ellison asked:

"Was that a symptom of trajectory? I thought MER was a heavier payload in an only slightly larger heatshield."

Many parameters determine heat shield thickness. Ballistic coefficient is defined as: Mass / (Cd * Area). A bigger ballistic coefficient often means a bigger peak heat flux (if everything else is the same). The ballistic coefficents for MER and Pathfinder (MPF) at peak heat flux were:

MER: 86.9 kg/m^2
MPF: 62.3 kg/m^2

However the stagnation point thermal protection system (TPS) thickness was the following:

MER: 0.62 inches
MPF: 0.75 inches

Both MER and MPF used the same TPS material, SLA-561V. Why is MPF thicker when its ballistic coefficient is less? The answer can be seen in the entry velocity:

Atmosphere relative entry velocity:

MER: 5.78 km/sec
MPF: 7.48 km/sec

What really drives heating is kinetic energy. Specific kinetic energy or "C3" is defined as: 0.5 * velocity^2.

C3:

MER: 33.4 [km/sec]^2
MPF: 56.0 [km/sec]^2

Because MPF had such a high entry C3, it required more TPS thickness than MER even though MER had a higher ballistic coefficient.

Gary