Why was Medusa the only mortal of the Gorgons?

Question

As far as I understood the myth of Medusa, she was punished by Athena (either for letting herself being raped by Poseidon or for succumbing to Poseidon's woes). Her sisters, Euryale and Stheno (all daughters of Phorcys and Ceto) stood with Medusa, and Athena transformed all of them in gorgons.

I'm not surprised about the punishment of the gods, neither to Medusa for her offenses nor to her sisters for staying with her (most Greek mythos depict similar punishments for something that is almost responsibility of the gods), but I'm surprised by the fact that Medusa is depicted as the only mortal of the Gorgons:

And again, Ceto bore to Phorcys the fair-cheeked Graiae, sisters grey from their birth: and both deathless gods and men who walk on earth call them Graiae, Pemphredo well-clad, and saffron-robed Enyo, and the Gorgons who dwell beyond glorious Ocean in the frontier land towards Night where are the clear-voiced Hesperides, Sthenno, and Euryale, and Medusa who suffered a woeful fate: she was mortal, but the two were undying and grew not old.

_{Source: Hes. Th. 275}

Why this difference between the three of them? Is something that is attributable to Athena when transforming the sisters? Or for any reason they were different in spite of being daughters of the same father and mother? Is is just due to the definition of "immortal" (meaning from "not growing old" to "can't be killed or die")?

Answer 1

I believe there is simply no clear explanation offered for this.

Two sources I'm aware of make reference to the mortality of Medusa. They are:

• Hesiod, Theogony, 276

  Gorgons who dwell beyond glorious Ocean in the frontier land towards Night where are the clear-voiced Hesperides, Sthenno, and Euryale, and Medusa who suffered a woeful fate: she was mortal, but the two were undying and grew not old. With her lay the Dark-haired One in a soft meadow amid spring flowers.

• Pseudo-Apollodorus, Library, 2.4.2

  ...he flew to the ocean and caught the Gorgons asleep. They were Stheno, Euryale, and Medusa. Now Medusa alone was mortal; for that reason Perseus was sent to fetch her head.

Neither of these offers any explanation of the phenomenon.

Possible explanations:

• Athena's punishment - While both of the quotes sources refer to Medusa having offspring with Poseidon, the first mention I can find of Athena's (more accurately Minerva's) punishment is Ovid's Metamorphosies, Book 4, in response to the question of "...why Medusa, alone among her sisters, had snakes twining in her hair." Which is somewhat difficult to reconcile with the quoted Apollodorus myth, in which all three are described as similarly monstrous:

  But the Gorgons had heads twined about with the scales of dragons, and great tusks like swine's, and brazen hands, and golden wings, by which they flew; and they turned to stone such as beheld them.

• Perhaps it just works out that way sometimes. Looking at the offspring of Phorcys and Ceto, some few of them appear to have been killed. In one version of the story, Ladon was killed by Heracles, in pursuit of the Apples of the Hesperides (his 11th labor). Echidna was killed by Argus Panoptes, as related in pseudo-Apollodorus Library.

  The idea that, by some chance of fate, she was born mortal seems more in line with my impression from the phrasing used in translations of Hesiod, with phrases like "suffered a woeful fate" or "suffered grievously", in the two translation I've seen. That turn of phrase seems interesting to me. Analysis of the original text might be valuable here, but I am unequipped to provide it.

Answer 2

Unfortunately, the actual answer is... there is no answer. The reason being, it is like all myths that have been adapted, changed, added to, corrupted and retold over a long period of time - the story becomes incoherent and cannot be reconciled in a manner which would make a sensible narrative.

The sisters were a later addition to a myth that had been around for centuries. Another incoherent part of the story that one might use as an example is the fact that it is often the Parthenon that is claimed as the temple in which Medusa served. However, the original Medusa myths predate the construction of the Parthenon by centuries. Remember, it was supposedly Perseus (Ruler of Mycenae, around 1300 BCE) that ended her life.

Sorry, that won't be the answer you are probably hoping to hear... but it IS the correct answer :)