Here's why:

* 70S refers to the sedimentation coefficient of the ribosome, a measure of how fast it settles in a centrifuge.

* Prokaryotic ribosomes are smaller and less complex than eukaryotic ribosomes (which are 80S).

* 70S ribosomes are composed of two subunits: a 30S subunit and a 50S subunit.

Let me know if you have any more questions!