With their lamps off she took video shots of the ghostly lesion images with her microcam, though she was pretty sure the level of illumination was too low to turn out. She would memorize all this and write it down in the rover. Careful notes…
“Their lines just keep going,” Marc said, looking down as they descended.
“I’m nearly halfway through my oxygen.”
“So would they have been, when they got this far.”
“This goes nearly straight down. Not like the way we went.”
“There’s been no plate tectonics for a long time, remember. Nothing to shear a volcanic passage like this, twist it around. So lava just came pretty much straight out. This tube, it’s probably a couple billion years old.” Marc seemed a bit spooked by the mat, but more confident with geology.
“Getting narrower though.”
“This mat is getting thicker, too.”
Her lamp was on high, poking down, so she saw it first. “What’s that?”
Far below was an oatmeal-colored floor. They stopped just above where the two Airbus cables forked straight through the middle.
“Where’d they go?”
“They got through this thing,” she said.
It looked like two massive, cupped palms pressed together at the center. The whole structure was perhaps three meters across. Maybe not an accident that it’s here, where the vent narrows down.
“Some kind of valve?” she speculated.
“Looks pretty solid.”
“Reminds me of stomates,” she said. “Plant cells that guard openings in leaves. The plant opens or closes the holes by pumping fluid into the stomate cells, changing their shape.”
“The mat is a plant?”
“No, it’s something we have no category for. A film, a biofilm—but one incredibly more advanced than the simple ones that grew in the early oceans of Earth. These have had billions of years to follow a different path.”
“Well, it’s sure good at blocking our path.”
“But it didn’t stop Chen and Gerda.”
“Maybe it was open when they came through?”
“That’s it. This structure seals the tube, maybe to protect the lower vent—”
“From what?”
“Peroxide dust? Maybe they irritated it, so it closed up.”
“So if we poke at it…”
“Good idea.”
She lowered directly onto the thing, boots sinking in. “It can hold my weight. Wow, that’s strong.”
“For a plant, yeah.”
She walked around on it. “Some give to it, but—wait, I have an idea.” She winched down so she could sit down. “Ugh, not easy in these suits.”
“What’s up?”
“Maybe my air exhaust will tickle it.”
Abruptly it flexed. She automatically reached for her winch control, but the membrane gave way faster. It retracted and she lost her footing. A hole opened at the middle and she skidded through. The surface was slick now and she stopped halfway through the opening.
“Hey!” Marc called.
She stabbed at her winch control and played out the line, slipping fully through. As she looked up the opening widened. She was dangling just below the roof of—
“My God, it’s huge,” she said.
Below and around her was a murky vault that stretched beyond view. As her lamp swept around the fog reflected back its glare. But to the side she could make out a sweep of radiance that dwindled into the distance—the ceiling of a vast cavern.
“You okay?” Marc peered down at her through the opening.
“Fine. Come on through.”
“What if it closes up on us?”
“We’ll kick our way back out.”
