How long it had stayed cool they didn’t guess. But both men felt sure that Mercury must have periodic upheavals as heat accumulated inside it — heat coming not from radio-activity but from tidal energy. Mercury’s orbit is highly eccentric. At perihelion, tidal force tries to pull it apart along the planet-to-sun line, while at aphelion the tidal force is less and the little world’s own gravity tries to bring it back to a spherical shape: The real change in form is not great, but a large force working through even a small amount of distance can mean a good deal of energy.

If the energy can’t leak out — and Mercury’s rocks conduct heat no better than those of Earth — the temperature must rise.

Sooner or later, the men argued, deeply buried rock must fuse to magma. Its liquefaction would let the bulk of the planet give farther under tidal stress, so heat would be penetrated even faster. Eventually a girdle of magma would have to form far below the crust all around the twilight strip, where the tidal strain would be greatest. Sooner or later this would melt its way to the surface, giving the zone a period of intense volcanic activity and, incidentally, giving the planet a temporary atmosphere.

The idea was reasonable. It had, the astronomer admitted, been suggested long before to account for supposed vulcanism on the moon. It justified the careful examination that Schlossberg and Zaino gave the plain before they descended the ladder; for it made reasonable the occasional changes which were observed to occur in the pattern of cracks weaving over its surface.

No one was certain just how permanent the local surface was — though no one could really justify feeling safer on board the Albireo than outside on the lava. If anything really drastic happened, the ship would be no protection.

The sun, hanging just above the horizon slightly to the watcher’s right, cast long shadows which made the cracks stand out clearly; as far as either man could see, nothing had changed recently. They descended the ladder carefully — even the best designed space suits are somewhat vulnerable — and made their way to the spot where the tractors were parked.