There isn't a single perfect synonym for "Earth's crust" as it's a specific term in geology. However, here are some words that convey similar meaning, depending on the context:

General terms:

* Lithosphere: This refers to the rigid outer layer of Earth, which includes the crust and the uppermost mantle.

* Outermost layer: This is a more general term that could refer to the crust, but it doesn't specify the geological context.

* Surface: This refers to the very top layer of the Earth, but it doesn't specifically denote the geological structure.

More specific terms:

* Continental crust: This refers to the crust that makes up the continents.

* Oceanic crust: This refers to the crust that makes up the ocean floor.

The best alternative depends on the specific context of your writing.