Getting in and out of anything, habitats or pressured rovers, was so laborious, they kept the lock pass-throughs (an ancient NASA term) to a minimum. With every one they brought in fine red dust, even with the two-shower system designed to wash them away. But she needed to go out, so she went through the procedures. Rules said she was supposed to go with a partner, but she wanted to be alone.

Once outside, she marveled. Marsforming had already brought thin cirrus to the skies. Silvery, they caught the late afternoon sun in filmy layers. It had taken a century to settle Mars and create a substantial local industrial capability and population. Within half a century more, robo-factories venting fluorocarbon gases had warmed the planet by 8°C. That brought water and carbon dioxide outgassing from the soil, thickening the atmosphere and raising the planetary temperature a further 20°C.

Still, the peroxides hadn’t met enough water to gush forth their captured oxygen—the big event everyone anticipated. That awaited big rains. Marsforming had benefited from nano-bio machines galore, but some brute chemical facts still ruled. Still, within her lifetime—which she cheerfully expected to exceed the 150 years now standard—Rachel hoped to be able to walk here with just a lightweight compressor to boost the oxygen she needed.

Even so transformed now, this Mars taught hard lessons. For one, how much Mother Earth did for humans without their noticing. Recycling air, water, and food demanded an intricate dance of chemistry and physics. The habitat had to tinker with its systems constantly. Let the CO₂ rise, and they could all be dead before anyone noticed anything wrong; the sensors stood sentinel, carefully tended. Those also watched the moisture content of the hab’s air, lest they all get “suit throat”––drying out until voices rasped.

Though wetter and warmer now, Mars still had plenty of nasty tricks. The pesky peroxides got in everywhere—even her underwear!—and seals eroded, so they had to be replaced with each outing. Air scrubbers needed adjustment and filter changes. They fought a steady battle to keep dust down.

Rachel hiked to a nearby hill and watched the high clouds shadow the rust-brown land to the north. The vent was a dark spot a few kilometers away, thronged by monitors and gear. Their dung dump was a dark spot on the opposite horizon. Marsforming had already made some water melt out of the permafrost. Muddy rivers flowed and light rain fell here and there. Radiation doses on the surface were falling as the atmosphere grew. Near the equator the first photosynthetic microbes were spreading in blue-green mats.

“Rachel!” came a call. She turned, irritated, and saw Stefano approaching. He must have followed her out. She wasn’t pleased; she liked getting the feel of Mars alone.

“Beautiful, isn’t it?” he said, huffing inside his suit from the climb. His eyebrows furrowed. “But look at this.”

He led her to a gully over the crest of the hill. It yawned two meters deep, newly carved. “Some permafrost erupted here last month. Flowed down, into the plain. That will work through into the Marsmat caverns. We’re intruding on a fragile biosphere.”