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By Root 2486 0

known and loved, every dyad and every clump of campion, even the speed of water as it lofted off stone and plashed down in its smooth silver shapes. The way moss felt under the finger pads. His visitors were people for whom Mars was forever an idea, a nascent state, a political situation. They lived in the tents and they might as well have been in a city anywhere, and their devotion, while real, was given to some cause or idea, some Mars of the mind. Which was fine. But for Nirgal now it was the land that mattered, the places where water arrived just so, trickling over the billion-year-old rock onto pads of new moss. Leave politics to the young, he had done his part. He didn’t want to do anymore. Or at least he wanted to wait until Jackie was gone. Power was like Hiroko, after all— it always slipped away. Didn’t it? Meanwhile, the cirque like an open hand.

• • •

But then one morning when he went out for a dawn walk, there was something different. The sky was clear, its purest morning purple, but a juniper’s needles had a yellowish tinge to them, and so did the moss, and the potato leaves on their mounds.

He plucked the yellowest samples of needles and sprigs and leaves, and took them back to the workbench in his greenhouse. Two hours’ work with microscope and AI did not find any problem, and he went back out and pulled up some root samples, and bagged some more needles and leaves and blades and flowers. Much of the grass had a wilted look, though it wasn’t a hot day.

Heart thudding, stomach taut, he worked all day and into the night. He could discover nothing. No insects, no pathogens. But the potato leaves in particular looked yellow. That night he called Sax and explained the situation. By coincidence Sax was visiting the university in Sabishii, and he drove up the next morning in a little rover, the latest from Spencer’s co-op.

“Nice,” Sax said as he got out and looked around. He checked Nirgal’s samples in the greenhouse. “Hmm,” he said. “I wonder.”

He had brought some instruments in his car, and they lugged them into the boulder and he went to work. At the end of a long day he said, “I can’t find anything. We’ll have to take some samples down to Sabishii.”

“You can’t find anything?”

“No pathogen. No bacteria, no virus.” He shrugged. “Let’s take several potatoes.”

They went out and dug potatoes from the field. Some of them were gnarled, elongated, cracked. “What is it?” Nirgal exclaimed.

Sax was frowning a little. “Looks like spindle tuber disease.”

“What causes it?”

“A viroid.”

“What’s that?”

“A bare RNA fragment. Smallest known infectious agent. Strange.”

“Ka.” Nirgal felt his stomach clamping inward. “How did it get here?”

“On a parasite, probably. This kind seems to be infecting grass. We need to find out.”

So they gathered samples, and drove back down to Sabishii.

Nirgal sat on a futon on the floor of Tariki’s living room, feeling sick. Tariki and Sax talked long after dinner, discussing the situation. Other viroids had been appearing in a rapid dispersal from Tharsis; apparently they had made it across the cordon sanitaire of space, arriving on a world that had been previously innocent of them. They were smaller than viruses, much smaller, and quite a bit simpler. Nothing but strands of RNA, Tariki said, about fifty nanometers long. Individuals had a molecular weight of about 130,000, while the smallest known viruses had molecular weights of over a million. They were so small that they had to be centrifuged at over 100,000 g in order to be pulled out of suspension.

The potato-spindle-tuber viroid was well understood, Tariki told them, tapping around on his screen and pointing at the schematics called up. A chain of merely 359 nucleotides, lined out in a closed single strand with short double-strand regions braiding it. Viroids like this one caused several plant diseases, including pale cucumber disease, chrysanthemum stunt, chlorotic mottle, cadang-cadang, citrus exocortis. Viroids had also been confirmed as the agent in some animal brain diseases, like scrapie, and kuru, and Creutzfeldt-Jakob