Close to Critical

From back cover Ballantine paperback July 1975:

CRISIS ON TENEBRA Shrouded in eternal gloom by its own thick atmosphere, Tenebra was a hostile planet... a place of crushing gravity, 370-degree temperatures, a constantly shifting crust and giant drifting raindrops. Unpromising -- yet there was life, intelligent life on Tenebra. For more than 20 years, Earth scientists had studied the natives from an orbiting laboratory... and had even found a way to train and educate a few of them. Then the unexpected happened! A young Earth girl and the son of a powerful, hot-tempered alien diplomat were marooned in a bathyscape, drifting toward the planet's deadly surface. Only the Tenebrans could rescue them!