We all have some climbing experience, but only the Lotatis and Ed have very much. It is, however, a very easy climb down in 38 percent gravity. The ropes are well secured to the plain above, the slope is usually less than vertical. Mike and Karen Svenson will remain on the rim with the bulk of the equipment until we are all safely down. They call themselves the “human robots” and have been in excellent humor. When Randi and Dr. Lotati scale the other wall, they will fire a rope across. The Lotatis will then pull over a larger rope on which a tram will carry the equipment. Meanwhile, at the bottom, Dr. Tierzo will supervise sample gathering by the rest of us. That is the plan.
It becomes dark quickly as we descend. The sky contracts to a starry band overhead, one edge of which glows a faint, frozen, shadowy pearl, a reflection lit by a reflection of sunlight on distant peaks. We turn on our helmet lights, and their glare banishes any other source of illumination. They spread sparkly pools of light on the wall—tiny crystals everywhere. I am conscious of a fine mist or fog, just on the edge of the perceivable. Our suits allow the skin’s waste gases to diffuse slowly outward, our footsteps create microscopic dust clouds on which it may condense, our helmet lights evaporate hydrogen gas that has condensed on the wall. Our progress appears tinged with the ethereal.
We are three hundred meters down: a football field on end. The descent is easy, simple, routine. You ease rope out under friction, take a pair of steps down, and ease out some more. This descent is demanding and terrifying. You dare not lose concentration. Pitons can pull out, crampons can slip, and you will be just as dead from a fall of a kilometer in Mercury’s gravity as Earth’s. Yes, it will take a few seconds longer and an academic might note that you hit with less velocity—only about five times instead of fifteen times what is needed to smash your helmet and the skull within it.
You dare not cause problems for everyone else. One foot after another.
A person in fear of his or her life needs no more excitement—but if you want it, you glance at the wall in front of you, at layers of ice laid down when dinosaurs were young. This is not on a screen, not a simulation, but never-seen-before reality that puts ice hard in front of your own eyes.
“Ed,” I ask, “do you wonder if anyone might have been here a billion years ago?”
“Eh? Interesting thought, that. The feature would attract someone with enough curiosity to build spaceships, I should think. But the crack itself wouldn’t be that old, now, would it?”
“No, I guess not.” When the layers were laid down, of course, this had been a part of the plain.
Still, my eyes scan every layer, hoping.
“Has everyone got positive pressure in their suits?” Dr. Lotati asks, and receives chuckles. “There is,” he continues, “a significant build-up of nitrogen gas, and a bare trace of nitrogen triflouride, which I would not recommend breathing.”
I call up a temp display and find that it is cold in the crack—about eighty kelvins, versus a hundred twenty at the surface. As I think about it, I notice traces of frost on the outside of my gloves: our insulation is that good. I have no idea what the biological implications of nitrogen trifluoride are, but I would rather someone else perform the experiments. A drop rolls down my face plate.
“Watch your footing, Juanita,” Dr. Lotati says. “It’s slippery here.”
So far, my crampons dig into the dirty clathrate walls with ease, but I can tell it is wet. The wall is mostly ice, but ice that is heavily mixed with crater ejecta, pocked with more volatile ices, and stiffened far harder than anything on Earth by cryogenic temperatures. Dr. Lotati says it’s something like sandstone, but if it weren’t for the gas in the ice, it would be like concrete. There are few cracks, but the piton gun works well, as does the ax.
“Nitrogen trifluoride data,” I ask. Floating in the wall, by virtue of my helmet display hardware, are glowing numbers telling me that nitrogen tri-fluoride is liquid over a range of about 80 kelvins—from about 77 up to 145—which is over 120 Celsius degrees below the freezing point of water. Somehow, stating such temperatures in kelvins above absolute zero is less scary than using negative Celsius degrees below freezing. The vapor pressure of this big, heavy molecule is almost nil at the low end of this temperature range—a wet vacuum.
A hundred meters to go, and I can see the bridge clearly in the shifting pools of our helmet lights.
“A sliver of wall appears to have detached itself and slid down until it jammed,” Dr. Tierzo tells us. “The top is a jumble with, here and there, flat spaces that may have been part of the original surface, including part of a crater. I wonder if that’s what knocked it down?”
“Hello down there,” Karen Svenson calls. “Yes, that crater would explain what looks like a ray network around where you went down. Now that I know what I’m looking at, this spiderweb network of cracks is a real giveaway.”
I hadn’t remembered any cracks. I ask my suit to play back my recording of our approach to the side. The surface in the depression was smooth. My pulse races. “Playback two hours ago,” I command. In a ghostly video window, my suit shows me almost falling in to the crevasse, but…
“Hey, everyone!” I shout. “Those cracks weren’t there before. Dr. Lotati, I’ve got it on channel six.”
There is a moment of silence.
“Quickly now,” Dr. Lotati speaks briefly and very businesslike. “Those of you still on the wall come down as quickly as possible without panic and without yanking on anything. Mike and Karen, set another belay well back of the cracked area.”
I have my full attention on climbing down, gingerly as possible. Dr. Tierzo is off belay just below me. I remember to lock my crampon spikes out—the bridge is slippery.
Meanwhile, Dr. Lotati has set an ice bolt in at the far end of our bridge. Dr. Tierzo sets another one in the biggest hunk of bridge she can find at our end and pulls the line tight.
Then I am off the wall. I quickly clip my line to the bridge line and release myself from the wall ropes. “Off belay.”
Miranda Lotati and the grad student, Eloni, are still on the wall.
I hear, I think, at very low frequency, a kind of groan.
“We have the protection in,” Karen says. “The cracks are larger!”
“Wojciech can get your line, Ed,” Dr. Lotati says, “release from the wall immediately!”
“No worry. Here.” He tosses me the end of his spare line and I hit it with the loop of a smart ’biner, which opens, takes the line, and shuts faster than I can see—like that old magic trick with hoops. I take up the slack.
Ed releases and scrambles off the wall, holding the taut line for balance until he reaches me. We move further down the bridge.
“Come on Eloni,” Ed says encouragingly. The young Kenyan woman is the least experienced of our group—she descends slowly, but flawlessly, a few meters right of where Ed came down. “Toss me the end of your spare line.”
She stops to find it. Miranda Lotati’s feet are but a few meters above her. I hear another groan.
“Come on…” Ed says again.
Finally, Eloni tosses a coiled line toward Ed. It jerks short and dangles below her, a hopeless tangle.
“Sorry, I try to do this too fast.”
“That’s OK, mate,” Ed says. “Just come on down now. We’ll improvise.”
We feel a slight tremor. She freezes.
“Down, Eloni. Fast,” Randi says.
Eloni starts moving again. I can see her tremble.