Charles Sheffield - Proteus in the Underworld

“That’s not what was worrying me. What’s this about suits? I didn’t think we would need them. Why should we wear suits in the Underworld?”

“No reason at all. But we’re not going there.” Trudy’s blue-green eyes glittered. “We’re going up, Bey, not down—up and up, all the way to the surface.”

“I assume it’s safe?”

Even shielded by a kilometer or more of rock and twelve hundred kilometers of distance, Bey had felt (or imagined) a planetary surface in massive tumult. Soon it would be much closer.

“Safe as can be. Nothing hits this near to the equator. But it’s still spectacular. Wait and see.” Trudy stood a step in front of Bey on the spiral escalator, built in oddly-connected discrete segments, that bore them steadily up through a wide shaft in the compacted rubble of the regolith. They had ridden a little rail car to the foot of the escalator direct from Melford Castle, and put on their suits at a way station halfway up the kilometer-long rise. The lock of that station signaled a sharp change in outside conditions. According to Bey’s suit indicators the temperature was now twenty below zero and the pressure had dropped to forty millibars. Three hundred meters more, and they would be at the surface.

Bey had asked his question about safety more from curiosity than real fear or discomfort. The suits were lightweight, but for all practical purposes foolproof. He had also been in far stranger and more threatening environments than the surface of Mars—even if that surface had now become one of the most active in the solar system, rivaling Io’s sulfur-spitting vulcanism.

The vibration at his feet was certainly not imagined. It brought signals that did not carry well in air so thin. When they emerged onto the surface Bey took one glance at the rising sun to orient himself, then turned north-west.

“Other way.” Trudy placed gloved hands on his shoulder and spun him around, just in time to see one to the south. A ball of fire came flaming across the southern sky from west to east. It vanished from sight in twenty seconds. One minute later a brighter flash of crimson light lit the south-eastern horizon. The sky in that direction already glowed with incandescent streaks and plumes.

“Now the other.” She had Bey’s arm and was turning him again, this time toward the north. “Get ready for the quakes, they come every few minutes.”

A second fireball ripped the northern sky, again traveling from west to east. Before it could pass out of sight the shock of an earlier impact was arriving. A surface wave came rippling in from the south and shifted the ground beneath Bey’s feet in a double up-and- down that had him swaying and sent die rubble-strewn desert into new patterns of cracks and small fissures.

Bey hardly followed the trajectory of the second object. The ground beneath your feet was not supposed to move like that. He felt much less safe.

“That was a big one.” Trudy still had her hand on his arm, steadying him. “Close to maximum size, at a guess.”

Which meant it was about a hundred meters in diameter; a rough-edged chunk of water ice, dirtied throughout with smears of ammonia ice, silicate rock and metallic ore, had smashed into the surface and vaporized on impact.

“What’s the energy release?” Bey felt a second, smaller ripple of movement.

“About a thousand megatons, for one that size.”

Like a really big volcanic explosion back on Earth. Bey was watching events that were equal in energy to several Krakatoa eruptions—except that these were happening every few minutes rather than every few decades. It was the hail-storm of the Gods, with hailstones the size of Melford Castle hitting the ground at forty kilometers a second; and mortal humans, not gods, were responsible for it.

The chunks of ice had been on their way for a long time. Even with a strong initial boost the journey in from the middle of the Oort Cloud, a quarter of a light-year out, took a comet fragment at least thirty years. And even with the most precise direction by the Cloudlanders during the first phase of the trajectory, a fragment’s fusion motor usually needed a small corrective burn as it came closer to Mars. The specification was a tight one: tangential impact along a due west-to-east line of travel, striking between latitudes twenty and twenty- five degrees north or south of the equator. The thin atmosphere of Mars ablated a little from the bolide, but most of it would make it all the way to the surface and strike at over forty kilometers a second.

Space-based lasers in orbit high above Mars watched for correction rocket malfunction. At the first sign of a guidance problem the fragment would be disintegrated in space, long before it could become a danger to dwellers on the planet.

The rain of comets had begun a century ago and continued ever since. It was slow work. Even with a hundred years of added volatiles from orbit and the help of bespoke ground- based organisms to split oxygen from iron oxide, it took a Martian eye to see much difference in the planet’s atmosphere. The water vapor was up to only a thirtieth that of Earth, the oxygen content one fortieth.

The contribution of the comet fragments to changing the Martian day was even harder to appreciate. Arriving tangentially at forty-two kilometers a second, every one made an addition to the planets angular momentum. Mars was gradually being spun up like a gigantic top, whipped by in falling chunks of frozen volatiles; but a century of impacts had shortened the period by less than a second. If anyone hoped to see a time when the Mars day of twenty-four hours and thirty-nine minutes was reduced to exactly equal that of Earth, they would have to be prepared to live a long, long time.

Bey scanned the horizon. The sky to both north and south was streaked and filmed with white haze. Most of the added dust and water vapor in the air came not from the comet fragments themselves, but from surface ejecta, vaporized rocks and permafrost of the upper few hundred meters blown high into the stratosphere.

As a spectacle it was astonishing, just as Trudy had promised. But since Bey was strictly an amateur in all science except his own form-change specialty, it was clearly not the reason for the trip to the surface. He was not surprised when she nodded to him and started to walk across the powdered and rubble-strewn surface. As he caught up with her he noticed that the ground was not quite as desolate as it seemed. The grains of rock were coated with a varnish of dark green. More bespoke organisms, hardened against solar ultraviolet, eking out an existence using dissolved minerals and captured water vapor; their photosynthesis was making its own contribution to the oxygen in the Mars atmosphere.

A surface structure—the first that Bey had seen—lay a hundred meters to the north. An air car was waiting inside.

“Put up with the bumpiness for twenty seconds.” Trudy motioned Bey inside. “After that it will be all right.” Even with outsized tires the run to takeoff was rough.

The landing strip had presumably once been smooth and flat, but continuous ground movement had created pits and potholes ranging in size from fist-sized to big enough to swallow a house. The pressurized car was more than smart enough to note the obstacles and choose a safe course to reach flight speed, but it had less objection than its passengers to sudden swoops and swerves and changes of direction. The promised twenty seconds seemed much longer to Bey before they were at last safely aloft.

Melford Castle had been placed in a location at the extreme northern edge of Martian development. To Bey’s surprise they now set a course that took them farther north, due north, toward the danger zone of comet impact. He was paranoid enough to reflect that if he were to vanish somewhere in the broken wilderness below it would be a long time before anyone found him.