Each made final checks on the other's suit; then they descended one more level to the airlock. This occupied part of the same deck as the fusion plants, below the wings and reaction mass tanks but above the main engine. Its outer door was just barely big enough to admit a spacesuited person. Even with the low air pressure carried by spaceships, a large door area meant large total force on jamb, hinges and locks. It opened onto a small balcony from which a ladder led to the ground. The two men paused on the balcony to look over the landscape. This hadn't changed noticeably since the last time either had been out, though there might have been some small difference in the volcanic cones a couple of miles away to the northeast. The furrows down the sides of these, which looked as though they had been cut by water but were actually bone-dry ash slides, were always undergoing alteration as gas from below kept blowing fresh scoria fragments out of the craters. * * * * * The spines--steep, jagged fragments of rock which thrust upward from the plain beyond and to both sides of the cones--seemed dead as ever. The level surface between the _Albireo_ and the cones was more interesting. Mardikian and Schlossberg believed it to be a lava sheet dating from early in Mercury's history, when more volatile substances still existed in the surface rocks to cut down their viscosity when molten. They supposed that much--perhaps most--of the surface around the "twilight" belt had been flooded by this very liquid lava, which had cooled to a smoother surface than most Earthly lava flows. 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 radioactivity 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 generated 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.