Martian life needed access to the oxidizing surface. There the sun’s hard ultraviolet light had made the dust superactive, laced with peroxides. Raw dust seared anything alive with a burn like the worst kitchen sterilizers. That superoxidation was like photosynthesis on Earth, a big energy driver. Pumping up molecules with oxygen balanced the anaerobic decay underground, which ran on hydrogen sulfide. Their samples always reeked of rotten eggs.

Mars was an upstairs/downstairs world. Surface oxidized species handed down energy to chemically balance the reduced, oxygen-starved Marsmat. That was why there were active vents, with discharges of otherwise valuable water vapor. The cycle had to close.

Hassan’s voice came over intercom. “Satellite chem analyzer says that plume’s methane fog. The light beam goes up three kilometers.”

The display faded, and after a few moments the crowd started to drift away, murmuring with speculations. Hassan appeared at Rachel’s elbow. His eyes were wide, alarmed. “What’s going on? The Mat is disturbed. There hasn’t been a display like this in decades.”

Zuminski said, “The First Expedition saw one, yes? When the return ship lifted off. They thought the Mat was saying good-bye.”

“Maybe the Mat likes what’s happening,” she said. “The Sphinx might be a way to talk to it.”

Hassan shook his head and stalked away. Probably going to file another report.

And so did she, it turned out. As soon as she was away from them, showered, in a now-deserted meditation zone to center herself—here came Prefect Stiles. She could not hide her surprise.

He held a finger to his lips, shook his head. “We have concealed ourselves, as I promised in the Aldrin. We detected subsurface rumblings hours ago—Fourier analyzed them, compared with the centuries-old seismic rhythms measured by the Julia Team, the discoverers. So we came here, observed.”

“You were watching all along?”

“Of course.” He sealed the room. “We have methods, built into the infrastructure here. Now you will tell me all you have learned.” He sat.

She had forgotten how easily this irksome man could intimidate her.

“Well…”

“Quickly. Then I will melt away.”

She couldn’t sleep. Stiles could do that to her. He had sucked the info out and whooshed off, and she felt washed out.

The data from the alien Sphinx device had stalled her, too, so she had sent it Earthside, to her friends at the SETI Library. Had they looked at it yet, hard, fast?

Her email held 143 messages. A whole team had jumped on the readout from the Sphinx. The Sphinx Signal, as they dubbed it, had pulsed parts like a telegraph, long and short blips. Crypto people called those “tokens”—symbols or letters or words—a term first used by those trying to decipher an ancient Indus script language. The team studied the correlations between strings of tokens. In a true language, the probability that a given token would appear depended on the token just in front of it. In lists or names of objects, there was no correlation. Some SETI messages had proved to be just hierarchical lists of deities, like prayers. They showed up as a scattered set of probabilities. Real languages, though, had probabilities that clustered together. Revealingly, that was true of the Ythri song-speech, too. Coevolution again, this time mental.