Yes, ASIC S9 can mine Ethereum. However, Ethereum is designed to be ASIC-resistant, meaning that it is difficult to mine with an ASIC. There are a few reasons for this:
1) Ethereum has a memory-hard algorithm. This means that in order to be successful at mining, an ASIC needs to have a large amount of memory.
Unfortunately, most ASICs do not have enough memory to be successful at mining Ethereum.
2) Ethereum also has a GHOST protocol. This protocol makes it difficult for an outsider to see which blocks are being mined by whom.
This makes it difficult for an ASIC manufacturer to create an ASIC that is optimized for mining Ethereum.
3) Finally, Ethereum is constantly changing its mining algorithm in order to stay ahead of ASIC manufacturers. This makes it difficult for an ASIC manufacturer to create an ASIC that can keep up with the changes.
Overall, while it is technically possible for an ASIC S9 to mine Ethereum, it is not practical due to the difficulty of the algorithm and the constant changes being made to it.