There isn't a single perfect antonym for "biosphere," as it's a broad concept. However, depending on the context, some possible alternatives include:

* Abiotic sphere: This refers to the non-living parts of the Earth, like rocks, water, and the atmosphere.

* Inorganic sphere: This emphasizes the lack of life, focusing on the non-biological aspects.

* Non-living environment: This is a more general term that simply describes the absence of life.

* Extraterrestrial space: This contrasts the biosphere with the vastness of space beyond Earth.

Ultimately, the best antonym for "biosphere" depends on the specific context and what you want to emphasize.