The Maretian

by Kris Overstreet

Chapter 14: Sol 20

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In the dawn of the solar system dozens of rocky planetoids swarmed inside the orbit of Jupiter, colliding with one another and sweeping up the fragments back into themselves.

One such early planet took a glancing blow from a smaller planetoid, losing its first primordial atmosphere to the impact. The strike carved out a massive basin which filled first with lava and then with water, as volcanoes, comets, and the remaining bits of the smaller planetoid created a new atmosphere, thinner but still substantial. Other fragments coalesced in orbit as a series of moons of varying size in less than stable orbits.

The planet was large enough and hot enough to have a differentiated interior, complete with an iron inner and outer core whose rotations and convections created a magnetic field that protected the atmosphere from the solar winds. Above that a water-rich mantle thrust bubbles of magma up through an already thick crust, finding weak points to vent the little world’s internal heat.

At one such point, not far from the original edge of the great basin, a long rift formed, part of the planet's billion-year flirtation with plate tectonics. A chain of small volcanoes formed, insignificant compared to the titans which would come later. Each had its short three or four thousand years of glory, spewing sulfur and ash and light fractionated lava, before its caldera cooled and sealed. Fresh magma rose from the depths, found no vent, and remained, waiting, deep below the surface. As more magma rose, the subterranean pools collected and joined, the internal heat and fresh supplies of lava from the depths of the young planet keeping the reservoirs hot and fluid.

Millions of years passed. The basin became a great ocean, swallowing up the volcanoes and washing away their ashy layers, replacing them with sediment from a great river. Beneath, the magma chamber cooled and warmed, freezing and melting as fresh lava sought a path to the surface only to be denied by the pressure of uncounted waters.

The ocean water seeped through the compacting sediment, through the remains of the volcanic shield, down to the hot magma chamber. The water eroded pockets in the older volcanic rock around the magma, pockets which filled with gas and then with hypercritical water. The magma rose and fell, its heavier components sinking back through the cracks into the mantle, the lighter components mixing with water to form a mineral-rich mixture. The relatively low gravity of the planet allowed these pockets of air and mineral water to grow larger than on any on its nearby rocky cousins, despite the weight of rock and sediment layered atop them.

The planet cooled, reheated briefly by the impact of its largest moon and uncounted smaller asteroids as the gas giants Jupiter and Saturn migrated outwards in the chaotic dance of planetary orbits. The crust thickened, but for a very long time enough heat flowed through the closing rift to keep the magma chamber viable. With every surge and ebb of fresh magma new pockets were created, merging with one another, filling with mineral-rich water that began, slowly, to redeposit its mineral wealth on the walls of the pockets.

Deep inside the planet the iron core froze completely, ending the magnetic dynamo which had protected the planet. Solar radiation began bombarding the world, breaking apart water and whisking the atmosphere away a whisper at a time. The planet’s orbit widened, slowly tugged away from its star by the gradual migration of the gas giants, eventually bringing it to the edge of the asteroid belt created by the gravitational chaos of the giants’ passing. Receiving less heat from the sun, losing its atmospheric blanket to the vacuum of space, the planet cooled even faster.

The great basin, which was the planet’s first ocean, was also its last, as the poles froze, as the ice retreated into permafrost or into polar deposits that ebbed and grew with the long seasons. Without water the early tectonic plates ceased to move, first seizing up and then freezing up as the rifts connecting to the mantle choked with congealing lava.

As the waters retreated they continued to erode away the remnants of the ancient volcanoes birthed by that first immense impact. But beneath the surface the great magma chamber retained enough heat, even as it died, to create children, a field of new volcanoes that spewed water and ice instead of molten rock.

And under the surface the water remained, still liquid, still dissolving and redepositing its minerals, inside the great gas pockets.

But nothing lasts forever. Even meteor impacts, even the cracks around the boundary of the ancient basin, could no longer sustain the magma chamber. A billion years after its birth it froze solid, never to melt again.

Inside the air pockets, the water drained or froze or sublimated away, leaving behind the work of uncounted millennia.

Billions of years passed. The axis of the planet tilted back and forth. The polar deposits of ice and carbon dioxide thawed, shifted, and refroze. The crust, despite its thickness, flexed and shifted. The giant volcanoes elsewhere on the planet continued to grow, changing the planet's balance on its axis and occasionally unleashing cataclysmic eruptions that launched lava and stones beyond escape velocity, out into the vast gulf between planets.

As the all-but-dead world changed ever so slowly with the eons, the layers of compacted soil and rock eroded away from above the magma chamber. The ever-diminishing winds of an already rust-covered planet blew across the dry ocean floor, carrying away material to form dunes around the poles. Now and again small asteroids would penetrate the wispy atmosphere and strike the surface, one of which penetrated the soil on top of the magma chamber and created a new hole that accelerated the process of erosion.

Slowly, slowly, the frozen magma chamber emerged from its coat of looser material. The crater at its top gave it a superficial resemblance to the nearby dead ice volcanoes it had sired. It shed its remaining detritus at its feet, blending in with the surroundings, its hardened core bidding defiance to the now feeble and tenuous wind of an almost dead world.

And then, as the next world closer to the sun entered a cycle of ice ages punctuated with brief warm periods, one of the great air pockets, with its deposits left behind by the ancient boiling waters of Mars, broke through to the surface. Dust and the occasional runoff of perchlorate-tainted ice water flowed into the open chamber little by little. The winds and dunes sealed the opening, then revealed it, then resealed it, each cycle depositing a bit more soil and ice into the immense chamber below.

The chamber sat, still mostly buried under a gray gravestone itself mostly buried by sand, and waited for its treasure to be discovered.



For the three thousand four hundredth and umpty-second time Spitfire wished that her space suit had wings.

She’d never really appreciated, even when injured and temporarily grounded, how confining it felt to be required to go everywhere on hoof like an earth pony or unicorn. But that was before she’d spent over two weeks either trapped in her space suit or inside an alien structure some ten yards on a side. That was before she’d spent all that time, except for a couple of minutes, with only the faintest scrap of pegasus magic, unable to fly properly or sense the air properly or, well, do anything properly.

Every day she woke up feeling like somepony had replaced her horseshoes with magic weights that made her feel fifty pounds lighter but held her to the ground like a magnet. She woke up grumpy and went to bed grumpier.

And she couldn’t tell anypony about it, because Wonderbolts don’t whine.

She could have taken it better if she’d had proper duties, a daily schedule, some structure to work in. But Spitfire had been a last-minute addition to the crew, a political decision to keep the balance of pony races and space agencies. She’d been promoted up and out of the Wonderbolts and then been placed at the bottom of the crew seniority rankings. And she’d been given a whole whopping three weeks of supplemental field medic training to go with the standard stuff she’d had when she first joined the Wonderbolts Reserve on her way up.

So this was her life now: standing on a hillside on an alien planet with no magic, almost no air, and with her wings securely bound to her sides by the Celestia-bedamned spacesuit, her only duty something for which she was minimally trained and not in the least talented, shadowing a unicorn ex-con who was such a bucking genius that she kept experimenting with new and innovative ways to commit suicide via magical burnout, and all the time wishing for somepony to either ask her for orders or to give her some.

But the only pony around at the moment was Starlight bucking Glimmer.

“Look,” she said to the unicorn in question, “we brought that battery for a reason. Use that instead of your reserves and maybe we’ll get somewhere before you pass out.”

“I only need to run Rarity’s spell for a couple of seconds at a time,” Starlight Glimmer insisted. “And with just enough power to take a bearing. I can do that easily so long as I don’t keep the spell running.”

“We’ve taken five bearings since we got back to this lump,” Spitfire said. “And it’s kind of hard to triangulate a location when none of the bearings converge.”

A few steps away, the alien Mark, face hidden by the reflective plate of his own spacesuit, stood patiently, one of his little shovels in his hands.

“I’m sure there are gems here somewhere!” Starlight insisted. “The spell’s picking up large deposits! We just have to keep searching!”

Spitfire shook her head. The one duty she’d been given, and half the time her patient and her superior officer wouldn’t let her do it. “Fine, cast it again,” she said. “But we’ve been walking all over this hill for hours! We didn’t find anything in the trench we carved coming down, and we haven’t found anything around the hillside!”

“But the spell says it’s got to be here!” Starlight Glimmer’s horn lit up, and for a second or two the ground around them glowed a faint blue in the dim orange light of Mars’s far-too-distant sun. The unicorn’s head jerked down hard and she cut the spell instantly. “See? This time the spell says it’s right underneath us! Quick, start digging!”

Sighing, Spitfire began pawing at the ground with her forehooves, The top layer of fine dust moved aside easily. The second layer of coarser material compacted by billions of years parted more reluctantly. The third layer, reached far sooner than Spitfire expected… was solid gray rock. “Huh… you may be right, Starlight. Come look at this.” She also waved to Mark, who straightened slightly at the sign of a potential discovery and walked towards them.

Three steps later the tall alien’s leg plunged into the ground, and he flopped back onto his flank, his arms flailing, his shovel flying off to one side.

“Mark!” Starlight Glimmer abandoned the exposed bit of rock and dashed over to where the alien sat on the dirt.

“Don’t move him!” Spitfire urged. “We don’t know what he fell into! His suit might be damaged!”

Starlight pulled up well before reaching him. “Oh. Right.” She carefully worked her away around Mark, avoiding the sand trickling down into the growing cavity around his right leg, until she could approach him from behind. Spitfire was afraid she’d try lifting him out of the hole, which after a day of spellcasting would definitely leave her exhausted or worse- again. But instead the unicorn put her helmet visor gently up against the back of Mark’s helmet. Spitfire heard over their suit-to-suit comms two carefully pronounced words in Mark’s language: “Syuut. Okeh?

Mark nodded. As Starlight put her helmet back into contact with his, Spitfire heard a distorted noise which sounded vaguely like, “syuut okeh.” With a slight shift of his weight he brought his right knee back above the surface of the sand, then very slowly and carefully scooted himself back, raising the half-buried leg higher until the boot surfaced, leaving him free again. Even then he continued to scoot back, Starlight keeping to his side, until he was a good three pony-lengths from where he’d fallen in.

With Mark’s leg freed, sand poured into the void where it had been. An overhang appeared as the Martian dust shifted into the growing hole and then- thank Celestia!- a row of large white crystals like teeth in a monstrous upper jaw.

“Success!” Starlight Glimmer cheered, stepping forwards. Mark grabbed her with his gloves and hauled her back, pointing to the continual cascade of sand and dirt into the growing chasm.

“Listen to him!” Spitfire said. “Don’t do anything until it stabilizes. That overhang might collapse at any time, or the sands might suck you down into the hole.”

“But I can just-“

“And absolutely no magic!” Spitfire snapped. “We can go get the battery from Mark’s carriage if we need it, but you’ve cast enough spells on your own resources for one day!”

“Fine,” Starlight said grumpily, as Mark finally got to his feet and nudged her farther back from the hole.

After about twenty minutes the small landslide stopped. The sinkhole had grown several hooves wide, the part under the overhang about two hooves high, not high enough to crawl under. Mark retrieved his shovel and gave the now uncovered ledge that had prevented him from falling through a hard strike, and it broke off and crumbled, revealing more of the hole, with a sparkling mixture of crystal fragments mixed with the usual Martian soil. Waving the other two away, the alien began digging, flinging little bits of soil away with the too-small shovel, occasionally inverting it to beat a bit of harder material around the edges of the hole into submission.

Twenty minutes later the hole was large enough for him to stand in, and he did, focusing on scooping dust and sand out of the overhang. After another ten minutes of this he set the shovel on the edge of the hole, turned on the flashlight built into his suit’s right arm, and stuck his arm inside the overhang, waggling it around.

Then the alien stiffened, still with his arm stuck in the ground, unmoving.

“He’s got caught on one of those crystals!” Starlight insisted.

“No, I don’t think so,” Spitfire said cautiously. She didn’t know what he was doing; what was the point of shoving his flashlight into the hole when his eyes weren’t at a level to see what was inside?

Then, very carefully, Mark pulled his arm out, turning off the flashlight. He picked up the shovel again, motioned the ponies to stand back, and then attacked the sand under the overhang with a passion that defied all common sense. Despite the awkwardness of his suit and the small size of the shovel blade, dust flew.

“What is he doing?” Spitfire mused aloud.

“I’m going to get the battery,” Starlight said. “He’ll hurt himself if he keeps going like that. Why does he want to go deeper anyway? We’ve got perfectly good gems right there!”

“Hm… yeah, I think you better do that,” Spitfire said.

It took about fifteen minutes for Starlight to make the round trip from the rover, battery pack strapped to her spacesuit with improvised belts made by Dragonfly for the purpose. (It had cost half a food ration, after which the changeling had spent half an hour behind the Curtain of Infernal Stench before emerging with a changeling-rope harness perfectly fitted for the battery. She’d said, “Don’t ask,” and nopony had wanted to.) “I’m going to tell him what I’m about to do,” she said as soon as she got back.

“Can you spare the charge?” Spitfire asked.

“The battery’s showing thirteen percent charge,” Starlight said. “Twenty percent was enough to pick up the entire Amicitas. All I’ll be doing is shoveling loose dirt.”

“You be careful anyway,” Spitfire warned, knowing it wouldn’t help.

Starlight stood atop the overhang and waved her forehooves until she got Mark’s attention. Only then did she light up her horn again, and then only long enough to tell him to move. Mark waved one of his arms in a gesture Spitfire didn’t recognize, and Starlight responded by pointing a hoof imperiously, ordering him out of the hole. Mark shrugged and picked himself out of the hole, which he’d expanded enough that it was chest-deep to him now.

Satisfied, Starlight unstrapped the battery and set it down. She put one forehoof directly on the exposed leads as she flipped the switch on with the other. Almost instantly a large scoop made of turquoise light appeared in the air above the hole, plunging into the sand and flinging it well downslope and out of the way.

After about a minute of this Mark waved his own arms for attention, and Starlight cancelled her digging spell and shut off the battery, still showing most of its charge. Her horn lit up again, and the unicorn’s magic surrounded her helmet and Mark’s as they exchanged a few more words. Finally Mark pointed into the hole, through the overhang where he’d been digging. It was Starlight’s turn to shrug and obey, a little weak-kneed as she cancelled the translation spell and eased her way down into the hole.

“You’re pushing yourself again,” Spitfire said.

“Spitfire,” Starlight said, sounding quite confused, “he wants me to go into the cave and look at something. And apparently a ‘green lamp’ has something to do with it. I don’t understand.”

“I thought you were working on that translation spell,” Spitfire said.

“It still doesn’t do idioms well,” Starlight admitted. “I don’t know how to fix that, and anyway it’s better if we just learn the language.”

“Mind your hooves,” Spitfire said. “If you get stuck don’t try to free yourself. Mark and I will get you out.”

“It’s all right. The surface is just like a sand dune back home…” Starlight paused. “Gotta turn on my suit lights, it’s dark in h- oooooooh my Faust.”

“What?” Spitfire danced on her hooves, wanting to follow Starlight into the little cave, afraid of what might happen.

“It’s… it’s incredible,” Starlight gasped. “It’s like the caverns under Canterlot!”

Spitfire stopped dancing. “You mean the big, crystal-filled, prime security risk caverns civilians like you aren’t supposed to know exist?” she asked.

“I…. may have learned something about them when I was still a crazy Twilight-Sparkle-stalking supervillain,” Starlight Glimmer admitted. “Anyway, can you bring the battery to the mouth of the cave? I need to make it big enough for Mark to come inside. He needs to see all of this.”

“See what? That we’ve got enough gems to feed Fireball for a while?” Spitfire shook her head and silently damned all geniuses whose brains ran ahead of their mouths. “That’s nice, but what else is there?”

“What else is there?” The unicorn’s voice over the magic comm link was triumphant. “The solution to all our problems, that’s what else!!”


Greetings from the Fortress of Solitude!

Well, not really. I’m writing this from Rover 2. We’ve run out of EVA time for the day, and once I finish writing this down we’re going back to the Hab for more tools, more planning, and probably that birthday cake that was in the ponies’s refrigerator. It’s going to go stale if it sits any longer, and today’s find deserves a celebration!

This morning we went back to the crash site- Starlight, Spitfire and myself. Site Epsilon is the only chance we have without modifying the rover to find the kind of gems or crystals that Starlight wants. When we started out I didn’t think there was any real hope of finding anything of the kind. But it’s critical to the ponies that we get some kind of gems, for Fireball’s sake if nothing else, so I thought we’d give it a try.

We spent a good four hours wandering all over the northeast side of Site Epsilon. Starlight seemed to be dowsing or something for the crystals, but every time she did it she pointed a different direction. So we made circles, digging down in the soil a couple of feet, hitting rock we couldn’t penetrate, and giving up. NASA never imagined a need or desire for an Ares crew to engage in heavy mining operations, so all I had with me was a sample shovel, a hammer for breaking small rocks with, and a chisel for breaking whatever I want with. None of that is going to penetrate bedrock.

Eventually it wasn’t the pony magic that discovered it. It was good old human clumsiness. My right foot found a hollow patch under the soil surface and punched straight through. Fortunately the initial hole was only a bit wider than my suit leg, so I wasn’t swallowed up completely. Even more fortunately, the surface was only moderately compacted sand and nothing harder or sharper, so my suit didn’t get torn or damaged. Otherwise this log would only be continued if the ponies took typing lessons from Strong Bad.

I very carefully extracted myself from the sinkhole. Sand continued to pour in once I removed my leg, and the abrasion widened the hole pretty quickly. Apparently the void under the surface was pretty big, because a good portion of the hillside eventually got swallowed up by it, revealing an overhang that looked kind of like the upper jaw of a troll, complete with diamond teeth. (Okay, not diamond, because diamonds don’t work like that. White quartz. But still very toothy-looking. I wouldn’t want to be bit by that mouth, anyway.)

We’d found what we came for, completely by accident and in spite of every bit of common sense. Which, to be honest, is par for the course for Mars. Every probe and crew that have landed here have found something totally contrary to what they expected, so why should I be any different? Of course, having found it, we immediately explained it away so it wasn’t surprising any more, but hindsight is always easy to peer review.

But I wanted to see just how much we had to work with and how hard we’d have to work to get it. Quartz, if that’s what those crystals are, is really hard stuff- it’s one of the defining levels of the Mohs scale, 7 or 8, I forget which. I don’t think I’ve got anything that’ll cut it, so I went digging, first widening the hole so I could work in it, then working my way under the troll teeth, looking for some broken or fallen bits that we could just pick up and take home.

The dirt that had fallen into the hole had piled up and turned out to be solid enough to stand on. That let me climb into the hole, make it deeper, and then work on clearing out the space under that overhang. There was a danger that the sinkhole would sink further or that I would get trapped in sand again, but I didn't care. We needed those gems, and I was going to get them, one way or another.

Once I had a good sized opening I turned on the camera and flashlight on my right arm. All the Ares surface suits have them. Because our ability to see side to side is restricted by the spacesuits, we have to turn our whole bodies to see things not directly in front of us. The cameras project an image into our helmets so we don’t have to stop and turn all the time. Plus the camera feed can be viewed by the crew still in the Hab and retransmitted to Earth for further review. It’s not a perfect system- I’d have put it on my left arm so I can use the light and have a tool in my right hand at the same time- but it works pretty good.

But if the mission had gone to spec, and if I’d done something like this, NASA would have ordered me strapped to a bunk for the rest of the mission, and Lewis would have done it, because it would have meant I’d gone crazy. NASA never considered the dangers of jumping into a Martian sinkhole and sticking one arm of your spacesuit up to the shoulder into a strange hole on an alien planet because they all expected our mothers would have taught us that when we were five. Really, they’re obvious, as obvious as the sharp pointy crystals right in my faceplate at the top of the hole.

The first view made one thing obvious: the hole was deep and went a long, long way back into the volcano. At first I thought it might be a lava tube, but that didn’t explain the crystals. I don’t know if crystals can grow in a lava tube, but I know I never saw the two together in nature during my training as Lewis’s geology backup. And that training wasn’t all it could have been, because we were so busy training for ten thousand other things, and anyway all NASA really wanted from us was to say what stuff looked like, pack up the really weird bits, and haul five hundred kilograms of it back to Earth for the real geologists to poke at for the next hundred years.

But then I caught sight of the sides of the hole. Crystal. Big-ass crystals. Crystals absolutely everywhere. There were even a couple of shafts of crystal that looked as thick as I am that went from ceiling through the floor.

I began digging the hole out bigger so Starlight could go in and see for herself. After a few minutes of this Starlight pulled out her Box o’ Magic Juice and ordered me out of the hole. In about a minute she’d done more than I’d managed in half an hour with the sample shovel. (Maybe she ought to be supervising the Hab soil project instead of Cherry? Just a thought.) Then she went in (after I told her she needed to go look)… and she stayed in. Spitfire took the battery in to her, then came back out and shoved me until I got the idea that we should, as the saying goes, “de-assify the area”.

Once we were clear, Starlight did something, probably magical, that sent tons of loose dirt flying out of that cave mouth like ammo from a marshmallow gun. When she was done the magic battery was empty, but so was the mouth of what turned out to be a really big cavern once you got past the entrance.

There were a couple of tight spots, but we were able to go back quite a long way- hundreds of meters, anyway. And let me tell you, it’s truly amazing. It looks like a gigantic geode that grew and absorbed smaller geodes. Most of the crystals are white, but there were a lot of yellow and red ones and even some purple. They come in all sizes, and I do mean all, from tiny enough to be set in a ring on up to shafts as big around as I am.

And they’re hard. I had a knife in my tool pouch, so I tried to scratch several with it. No good. After the fifth failed attempt, I scraped the flat of the blade across one of the crystal points, and it left a shallow gouge down the steel. Definitely quartz or something harder.

We didn’t find any loose broken bits on the floor, but there’s a reason for that. The floor is hard-packed Mars soil, sloping down from the entrance. I have to bend a bit to get in without risking a scrape from the troll teeth, but ten paces in I can stand straight with no problems, and twenty paces in I can’t reach the ceiling. Apparently this cave or lava tube or geode or whatever it is opens to the surface periodically. Sand blows or falls into the hole until it fills, and then it hardens by compaction, leaving a brittle shell up top and opening a hole underneath. Eventually something happens- a meteor strike or a dust storm or something- the hole reopens, and the cycle repeats. And every time it does more sand slides further back into the cave, filling it in a bit at a time.

I think I might have seen a twinkle of crystals on the floor on the edge of my suit lights when we finally turned around to get out of the cave. Other than that, there’s at least a thin layer of dirt all the way down. Hell, it could be a really deep layer. We have no way of knowing how deep the original cave went. But any crystals that broke off of the walls and ceiling would have been buried by sand and dust ages ago.

Anyway, we now have crystals. Santa came early this year. I haven’t got a damn thing that will cut them, but Starlight doesn’t seem bothered about that. She’s much more excited about the dirt and the open space inside.

And I think I’m on the same page as she is, but to be sure we need to do a proper exploration of the cave. That means we’re coming back tomorrow. Yes, we retreat for now, but we shall return… armed with SCIENCE!

Author's Notes:

Yes, this is the "bullshit!" moment in this story, if ever there's going to be one. Andy Weir gave us wind forces that couldn't possibly work (particularly the storm at the start of the story). I'm giving the ponies- potentially- an underground farm and more gems than they'll be able to use.

Now, here's why it's bullshit.

First, as mentioned, Acidalia Planitia is an ancient ocean floor. It's a southern extension of the great Borealia impact basin, formed a bit more than four billion years ago by our best estimates. Debris from the impact that created it formed Phobos and Deimos (recent studies show that they have a mixture of Mars and non-Mars material, making it unlikely that they're captured asteroids) and probably other moons which eventually crashed back into the planet, creating some of the other large basins.

As an ocean floor, and particularly as adjacent to the Mawrth Valles river valley network running off of Arabia Terra, Acidalia is covered in sediment. There are a number of small hills here and there which might be ice volcanoes or remains of rocky prominences that are part of Arabia Terra's geological formations- we don't know for sure and won't know without a closer look. Under no circumstances would you expect volcanic remnants anywhere near the surface, much less conveniently placed for Watney to stick his foot in one.

But that's the lesser of this cave's sins. The really big sin is the difference between basalt and granite.

As a very vague rule of thumb, on Earth granite is what makes up almost all the continents, and basalt is what makes all the ocean floors. Basalt is what happens when lava and magma cool quickly, without having time to form proper crystal structures. Every ash-spewing or lava-spurting volcano you've ever see is a basalt formation in progress. And the crystals you usually get from such volcanoes are small, soft, and prone to rapid erosion in the presence of water.

For hard stones you need either metamorphic rocks- rocks that are melted and re-frozen by tectonic subduction or by the folding and pressing of layers of rock- or granite formations. The heat and pressure over a long period of time has two main effects: it allows liquid rock to fraction into heavier and lighter components (iron, magnesium, etc. settling to the bottom, oxides floating to the top), and it allows crystals to form and to grow for very long periods at a time.

And there's one other ingredient you need for really large crystals to form: water. Because water is how you get geodes. SImplifying the process down, you get an empty pocket in the rocks near a source of heat and minerals, and water seeps in to fill the pocket. So long as the water sticks around, the minerals dissolved inside it will crystallize on the insides of the pocket, forming a geode.

On Earth geodes are really common. Earth's crust is full of silicon, mostly in the form of silicon dioxide (SiO2), more commonly known as quartz. The parts of beach sand that aren't diatoms or shell fragments are mostly quartz, as are the greater part of desert sands. A huge number of precious and semi-precious stones are varieties or forms of quartz, either as crystals or as layered deposits.

On Mars quartz- and granitic formations in general- are rare as hell. We've only detected traces of quartz and feldspar from orbit in a couple of locations, one on Arabia Terra and the other in the caldera of one of Mars's giant volcanoes. The theory is that, since Mars's plate tectonics died over three billion years ago, its remaining volcanoes just sat atop the magma plumes that fed them forever, giving their magma chambers time to settle out into lighter and denser components, which allowed some granite to form in select areas. But the vast majority of Mars is covered in basaltic minerals, and if you found a crystal, you'd have to assume it was olivine or gypsum or something of that sort.

On Mars, so far as we currently know, quartz is rarer than diamonds.

So why am I going this route?

Well, it's not just about Fireball. My first idea was to send Leonid the Yak up as the fifth crewperson and sole male of Amicitas. And with enough improvising I could probably have thought of a way to get all six people off of Mars without a single spare gem, if I absolutely had to.

But the food issue was critical. The Hab farm in the book was barely going to be able to feed Mark, and it was too small to be viable beyond feeding him through Sol 900... if the airlock hadn't blown out. Under no circumstances would it sustain two people, let alone six. So the question was, how to give the characters a larger space to farm in?

The pony ship, which has less interior space than the Hab, was never a serious option. The MDV had part of its hull stove in by flying debris during the Sol 6 storm, so that was out too. The landing stage of the MAV, left behind when it launched, contained the fuel plant that Mark could (and still can) use to compress Mars air if he needs it, but no habitat space. Mark has only six square meters of spare Hab canvas, and in the course of the book he uses practically all of it, the last portion becoming the ragtop roof for his ride home.

So, what's left? He hasn't enough materials to build an airtight enclosure three or four times the surface area of the Hab. All he can do is find a place, a cave of some kind, where he can seal it off, pump it full of air, add some lights and get to work.

So what kind of caves are on Mars at all? Limestone caves, the most common on Earth, are out. Every limestone rock you see is the cemetery marker for billions of ancient diatoms that settled to an ocean floor ages before the first of our ancestors hopped from one shallow pond to another. The odds of finding limestone on Mars are vanishingly small. (But the good news is, if we ever do, it means absolute confirmation of ancient life there.)

Sandstone caves exist- the Anasazi and Pueblo tribes built their homes in sandstone overhangs- but they're structurally weak. Pressure differentials and interior heating to provide a livable environment would destroy such a cave in short order, if such a cave would hold air at all. I doubt it would.

Ice caves are possible, but once exposed to Mars's environment they'd rapidly sublimate away, a process making them inhabitable would accelerate.

So the best, and practically the only, option is volcanic caves- which is why my first idea was for the crew to find an ancient lava tube.

That plan didn't last long. We know lava tubes exist on Mars; we have orbital photographs that let us trace their path. And the reason we can do this is because those tubes have collapsed. Lava tubes are notoriously fragile and unstable. And, being basaltic deposits, they practically never have any crystals worth exploiting.

So I fudged. I fudged big time, and I gave our heroes the biggest geode conceivable, and I made it quartz for its strength, durability, and its comparative ease of making airtight. (No stone is truly airtight, but a quartz geode is as close as I could come up with without going back to college and getting a geology degree.)

Three examples from Earth for your edification:

The Giant's Causeway, Ireland - It looks like giant stone crystals, but it's not, at least not quite. This is a huge ancient lava flow that, when it cooled, split along the rough molecular structure of basalt.

Heineman's Crystal Cave, Ohio, USA - The largest known geode on Earth, which after close to a century of being mined for its mineral wealth is now large enough to allow a couple dozen people inside at a time to ooh and aah at the (somewhat reduced) wonder of nature. It's not quartz- it's a metamorphic geode formed inside a limestone deposit as much as three billion years old.

By comparison, the largest known quartz geode is an amethyst structure just big enough for three people to lie in head to foot.

Cueva de los Crystales (Crystal Cave), Mexico - The cave with the largest crystals known on Earth. Not a geode, though- the deposits are gigantic gypsum crystals, grown for millions of years in a water-filled chamber. The water has been allowed to return, but when it was open scientists could only spend twenty minutes at a time inside, even with protective suits, due to the lethal heat and humidity three hundred meters below the surface.

So, here it is: my one big you-get-to-live gift to Mark and the ponies. They get a not-100%-impossible-but-wildly-improbable quartz cave on a scale dwarfing anything on Earth (just like Olympus Mons dwarfs any mountain on Earth, come to think of it). They get gems for batteries, gems to rebuild the Sparkle Drive if Starlight Glimmer wants, gems for Fireball to eat... but more important, they get a large cave with a narrow entrance and a floor full of soil.

Now they have to make it a farm... and Santa's bag just went empty.

EDIT: And yeah, apparently I got it wrong anyway. You DO get geodes, even quartz geodes, from basaltic formations. Maybe I'll come back and fix this at one point, but in the meantime I'm just going to go watch Mythbusters and wonder why I wasn't this interested in rocks back when I was actually in college...

EDIT2: And I went back to change it this morning, and ended up only adding about twenty words. I mentally had the process wrong (because every source told me quartz is a granitic rock and doesn't appear in volcanoes), but more or less by accident I'd gotten the process that creates geodes almost right. Maybe I should forget the hard science and just have Starlight train Mark as an apprentice wizard.

Next Chapter: Sol 21 Estimated time remaining: 29 Hours, 12 Minutes
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