> He said the heat shield ablates for Mars trips but not Earth trips (not sure why)

Falcon 9 presently lands completely without any heat shield, because it just uses it's engines to reduce velocity until they are not needed. In principle, you can always replace heat shields with fuel like this.

Mars trips need to bleed velocity aerodynamically because the craft cannot bring enough fuel. Presumably a sub-orbital hop on Earth can save enough fuel to act as the brakes.

Falcon 9 has a heat shield around the engines. Since they are going into the atmosphere engines-first it's going to be hot there. The engine bells sticking out from there don't care (much, since the entry burn reduces speed enough). Most of the speed reduction for the landing happens via aerobraking, not by using the engines.

The first stage of a Falcon 9 also flies much slower than a ship arriving to Mars or coming back to earth from orbit. It just needs to slow down a bit with engines when entering the thickest part of the atmosphere.

The shield ablates on Mars trips because the ship is using the atmosphere to slow itself down, it will pass threw many times and bleed of speed.

On earth you just need to survive, the speed is primarily reduced with the engines.

> the heat shield ablates for Mars trips but not Earth trips (not sure why).

I suppose the reentry speed on mars is much higher, since it happens at the end of an interplanetary trip.

For E2E routes, the rocket doesn't have to be given as much kinetic energy - it doesn't have to escape Earth's gravity well.

Dragon 1 has already gone through a couple of iterations of heat shields. Mars entry is expected to be hotter, that's straightforward math.