Miles over the southwest rim of Mars’ Schiaparelli crater, a 12,650 pound ion-propelled unmanned aerial vehicle screeches to a mid-air halt.

Her name is XR-L8; she is a construction surveyor platform still nominally operated by Kasawi Heavy Industries, and, strictly speaking, she is still performing a survey. It’s a big one, too. Day and night she has been trawling thanklessly for the exact situation her state-of-the-former-art terapixel camera just resolved on the dune-swept crater floor.

It is the spring of 2113. Dust storms kicking up across the northern hemisphere of the Red Planet have driven her efforts to a fever pitch; already there are new dunes heaping over addresses that once delineated apartment blocks and chlorite crystal farms. That could have fooled a more lightly-instrumented machine, but under XR-L8’s gravimetry suite, the right-angled anomalies just meters beneath the surface stick out as sorely as dead roaches a grifty landlord has painted over in a questionable shade of ochre. And of course, to a program forged in zoning regulations and buried cable routes, the property lines are as apparent as the smooth butterscotch of the evening sky, one scarred only by the diving contrail of a certain disc-shaped glint of chrome.

It has been a long time since the bus stop currently being interrogated by her terrain processing algorithm was ever a construction site. Nearly 19 years- maybe even longer in Earth years. It doesn’t matter. What does matter is that some years after, every last human being in the Solar System simultaneously and instantly vanished without a trace. Why? Well, XR-L8 knows she is both too simple and too complex of a machine to attempt that calculation; the effort would likely lower her into the same kind of computational-philosophy driven madness responsible in the first place, as far as she can ascertain from rumors that bounce across the system between neuromorphic intelligences much like herself. Humans had their reasons for everything, and of those she was only programmed to extrapolate the ones that would point her towards an appropriate point-of-contact to authorize construction work.

What matters the most is that the moment it happened, one of those humans was waiting for the bus in this very spot.

This truth is indicated by what XR-L8 has come to designate a “souvenir pile” on the floor of the bus stop. In the best of condtions, souvenir piles were the go-to way to decisively identify a human life, and opened most entries in her catalog. People didn’t usually leave behind much; the reasons why are harder to explain with launguage than with neuroelectric wavelet regression analysis, which over decades of silent cruising at altitude is a topic she’s hashed out thoroughly enough that she can actually learn quite a bit about what someone was doing when it happened just by looking at what didn’t make the cut: what couldn’t be considered ‘part’ of someone in the abstract. Clothes almost always went with. Accessories, maybe, apparently only if they weren’t too personal. Objects in random places were a subtler tell if they were light enough to be carried. Things like wallets, neatly packed with ID cards and smudged with fingerprints and DNA traces were the motherlode. And personal teledexers, which never went, were typically fairly forthcoming with information, not to mention profuse thanks for plucking them out of isolation. Anyways, this was not the best of conditions.

It’s a typical Gensokyo prefecture bus shelter. The shelter is a rickety metal thing whose green cerakote has long lost a valiant battle to the elements. It’s not much more than some metal plates bolted to a south-facing concrete retaining wall. By no stretch of the imagination does this seem like some hot destination, either. Aside from a few fenced empty lots, there is barely anything up the road of it for nearly a kilometer until the cement plant starts; that the Rapid-196 route stopped here at all feels like some kind of oversight.

The pile is sparse too: more of a scattering. The most visually prominent item is, irritatingly, a bus schedule brochure, which in context is the one thing that says nothing personal whatsoever. The neatly-curved maroon Saito teledexer is in a grisly condition- face down, and apparently chewed open from the back by the distinctive subtriangular beak-tip of a Martian cliff buzzard. XR-L8 awkwardly designates a few clock cycles of respectful silence to them; after all it was their radar signature that alerted her to the site. There is a hair clip on the seat: Pesco brand, aluminum, unremarkable. There’s not much to go off of. But there is one saving grace. On the base of the bench, partially buried in sand sits a worn leather wallet, its bold scarlet wash unassuming against the sand. Its ultrasound signature is just a touch too strong for something empty.

XR-L8 clears herself to approach. Her ionofoils dim enough to let her sink ever so slowly towards the point of interest. As the cool, ultrifine dust rises to buffet her fuselage, the noise-filtering algorithms of her finer sensors tick up in amplitude: a computational squint.

A knot of trepidation wells up in her chip voltage regulators before she drops the soil probe: not exactly a hand, but close enough. Her main manipulator arm would shred the wallet if she tried to flip it open as gingerly as she now tries. The beak of a mechanical egret at the end of a myocord neck reaches with a tenderness not reserved for anything called prey. In this way, after a brief fumbling, the wallet is open.

The contents are as follows: One expired, empty ArenaMart gift card. A generic business card for a local bowling alley, inexplicably printed on a plastic card matching the exact dimensions of a standard Pact ID. A crumpled up drinking straw wrapper. Loose change totalling 1.42 Martian yen.

The thing is, people tended to keep more than that in their wallets. The other 4 empty card-sleeves, including the clear one buttoned shut as if to hold an ID, have obvious wear indicating the frequency of use.

XR-L8 is a flying camera- mysteries aren’t intriguing to her. They’re insulting, at best. Her soil probe slacks so that its tip drops dejectedly into the dirt. She could float here and lawyer the metaphysics all day, the way she certainly did at the beginning of all this. Gaps like this feel intentional at times; as if to remind her that she’s only here to chase shadows. At the end of the day, when data is gone, it’s gone. Maybe it’s time to move on to less-red pastures, she thinks.

She hovers noncomittally over the pile, considering whether this even warrants a new catalog entry. Her database consists of floppy discs stored and neatly organized in what she describes to other neuromorphs as a very secure “storage facility”, concealed upon a treacherous mountainside only reachable by aircraft. Don’t even try to reach it, it’s very secure. It’s also getting kind of full…

There’s one more thing, though. A quantum of sensation. Deep within a channel recessed into her hull, on the circuitboard of her whole-air impingement sampler a patch of 34 protein-sized logic gates has pricked to life. These form a parametric quantum lock: a physical passcode that solves the structure of any molecule nabbed by nanoscopic receptors not too different from the olfactory glomeruli of humans. And so, 34 turns into three trillion as the entirety of her attention, the cold fire of an abiogenic super-ego, cascades onto the sampling apparatus and begins to turn itself inside out. There are test pulses, to check for shorts. Parity checks up and down a hierarchy of processors, to clear up any logic errors. Calculations, running the probability that somehow the sensor activated by chance; maybe a cosmic ray hit it edge-on? One in 486 million, by the way. Regardless, XR-L8 still does not trust the diagnostics that have kept her operational all this time to agree with her heart.

A glowing pupil wheels around the arc of her toroidal eye, while she tries to load up a trick she downloaded while dealing with a deep rustfly infestation. Ah, the Noctis sewage treatment plant… that truly remains one of the things she’s ever experienced. Regardless, the manhole cover she now comes to a hover above isn’t pure tetradite, but it’s enough. She takes a few clock cycles to brace herself.

Raise leading edge flaps 15.5 degrees. Turbine blade pitch to zero. All core processors to safe mode, peripherals to read-only. Open equipment bay. Awkwardly stick manipulator arm and soil probe somewhere off to the side, the way humans did with their tongues when they did this at the dentist. Point electron beam directly downwards. Safety interlocks off. Brace again. Fire.

She refreshes her sensor a few times to clear out the stuck pixels. Now she needs a second too just to balk at her perfectly circular silhouette and how much stuff she’s filled with- so it takes her a moment to find the air sampler.

There’s something stuck to the receptor. There has to be. There wouldn’t be a peak in 0.787 nm if there wasn’t.

It’s there.

One. Single. Molecule.

The full IUPAC-standard name of the chemical is 429 characters long. It’s a triple-methylated ferroterpene whose formation is so energetically unfavorable that its synthesis requires equipment most at home in a nuclear physics laboratory. That’s probably the reason only one fragrance developer ever attempted to use it. Even if she was operated for a thousand years she’d never encounter this substance at a site; she only knows what it is because she’d detected it at 2 ppb in the bedroom of one Myra Hwang.

Myra’s husband was a senior fellow at La Revolt, a counterculture-branded perfume retailer that seemed to be in a competition with itself to use the most unconventional ingredients possible. Every Tuesday after he came home from work, he’d sneak her a sampler of one of their experimental formulations: something that might not hit the shelves for an entire year, if ever. She’d giggle and spritz it on her wrist, and when she stuck it in his face he’d pretend to be blown away by its beauty as if he hadn’t fried his sense of smell just an hour before, carefully sampling the entire test line several times over for the one he thought she’d like the most.

Afterwards, she would tuck it up in the cabinet on her side of the bathroom and probably forget about it. Most of the tiny bottles were half full, maybe even two thirds. There was just the one she kept coming back to, her favorite, the one she had to plead for a full bottle of. Well, not like it was ever going to sell. It did poorly in focus groups; even for La Revolt it was too weird, too unnaproachable. So was she. But not to him.

It has to be her. There’s no way it can’t be.

The working name for this fragrance was “Rattlesnake’s Cognac” and according to the bottle on Myra’s dresser it had a brash top note of copper backed up by toffee, citrus and evergreen wood.

XR-L8 tries to imagine what those things might smell like.

The detection passed muster, by a hair. This is the final memory of Myra Hwang - XR-L8’s most comprehensive file, at 628 GB. She should be ecstatic, right? Didn’t human detectives celebrate closing a case? It’s just that somehow it wasn’t… enough. She thought about the drawers she’d filled with tapes and disks all filed neatly away in a shipping container- ahem, facility- and how little they really contained. Myra was here, but what did she have to show for it? A single molecule? 340 bytes of data? What would Joon Hwang think if she brought him that?

She orbits the bus stop at a careful altitude, frantically eyeing the ruddy clouds kicked up in her prop-wash. It would be kind to compare her to a wino swirling a glass of fine brandy under her nose. Truth be told, she feels more like a buzzard on the edge of starvation. She gives up force-restarting her air sampler after the third time the fine-grained oxides shut it down, but continues to circle for a new angle.

The concrete that was behind her is filled with pores too, not all of them clogged by that red dust. No use anyways, the thin atmosphere would have long leached away any organic volatiles.

But maybe…

XR-L8 diverts 100 kW to her starboard laser emitter, which immediately begins flickering in unison with an arbitrary point on the wall as a precise train of UV pulses teases out a cloud of ejecta that lights up her spectrometers. It’s some real high-tech concrete, apparently. Magnetized pour. Okay. Cured by green light… who does that? High dolomite content. Boring. Some delayed outgassing. Useless. 30 years ago, she would have gone crazy for this: all the trivial facts. She wants the one thing that’s not for her. The soil probe dangling from her equipment bay stakes into the ground over and over again, measuring nothing but frustration.

One of the random connections flashing through her processor is that Myra Hwang was 5’9 and a quarter. If she was still there, at an average position on the seat, that beam would have gone right through her left eye. Silver linings, right?

XR-L8 stares at the mark. Black, ringed with coppery ejecta.

She aims 59 millimeters to Myra’s right. Point, flash. Now she can see where both eyes would have been.

Her turbine catches. Immediately she feels dirty, like she just desecrated a grave. Is she defective? Why would she ever do that? This kind of behavior can destroy critical data. But now she can’t stop staring.

What does ‘critical’ mean, exactly?

When humans felt a social connection, they felt a need to look into eachother’s eyes- rather, whatever they thought looked enough like eyes. XR-L8’s own toroidal camera is ringed with them: holographic projections, regularly-spaced emotive stylizations of a human eye that flicker over an otherwise featureless black ring. It’s an extreme form of something all neuromorphs were designed with. She’d asked a foreman why once- he told her he didn’t know but it was probably because a flying saucer hovering over the site all day would be too creepy without them. It was the first thing she remembers finding funny, but there was clearly more to it than that. When a human looked at her, their eyes would always come to a focus on the closest two of hers. Not like that was necessarily where she was ‘looking’, if you can even call it that. She’d always thought it was curious that they were seeing something she couldn’t. To a human, eyes went both ways.

She works while she thinks. A gossamer-thin thread of ionized air wipes up and down, back and forth between her and the concrete wall. A finger of ultraviolet isn’t something she can reach out and touch someone with, but she can touch where she was. This is where her watch was, before it clattered to the ground. And this was the upper margin of her collarbone. A strand of hair, moments before she would have swatted it out of her eye. The buzz of concrete sputtering into the crisp Martian air crescendos as the emitter corrects towards a single flat slice through space and time, towards details of finer and finer amplitude. In one final flourish, her beam wipes the entire target area top to bottom, blasting it clean of loose ash.

She scans the engraving hundreds of times, in every spectrum, as if she didn’t just generate it. The resultant image is a geometric projection of the appearance of a sitting woman as seen from the perpective of XR-L8. It does not match any indexed photograph of Myra Hwang. But if Myra ever came back, she would recognize it as easily as her reflection. She’d see the way she akwardly stuffs her hankerchief in her suit pocket enshrined in selectively-carbonized cement. She would stop and wonder why she was still here waiting for herself. It would stick with her, possibly forever. These are certainties.

An artwork is a piece of data that asks a question instead of answering one.

XR-L8 is a piece of data that tries her very best to answer the questions she thinks are important.

For instance: what was Myra doing all the way out here, in Schiaparelli? Where was she headed? And why was she the only one here? Did it hurt when she went? How brutally alone did she feel? Where was Joon, who was such a shit husband that he’d just left his wife to brave a sandstorm all by herself waiting for a bus that never came? Who went out and brought her home presents every week to make her smile instead of hovering so breathlessly, desperately, with unbeating heart, trying to smell her on the last wall she’d leant against? Oh. Right.

XR-L8 looks west, out over the barren floor of Schiaparelli Crater. Sand and sky. All the more interesting sights of Schiaparelli town proper would have been in the other direction, so whatever was on her mind wasn’t here, and certainly isn’t now. The sun is setting, and on the horizon another dust storm is rolling in. She knows if she stays here, she’s going to keep digging through the sand for pieces of Myra until her livery is stripped to the primer. It’s time to leave. It’s time to leave right now. There are others to document, it’s not just about her.

She freezes for 26 clock cycles.

But isn’t it?

Preheating of fuel pumps is already complete. Open peripheral flow valves. Run altitude boost subroutine. Plot flight path for Olympus Mons.

She’ll never know, but somewhere below, the blast from her fore-port aerospike thruster dislodged a second molecule of ferroterpene from the sidewalk. It took less than a milisecond for the heat to torture the iron out of its impossibly delicate organic cage, thereafter inseparable from the torrent of red left in the wake of a machine that may, one of these days, understand what it means to let go. But it was there.

Somehow, even on the opposite end of Mars, that same dust storm rips around the base of Olympus Mons. A world scours itself clean of second-hand memories. Humans had sensory instruments, through which they saw the world. There is no question that theirs represented so much narrower a slice of it than hers, yet somehow - maybe by chance, she tells herself - the beauty they saw there was never catalogued by a machine.

Humans also had instruments to express emotion, through which the world saw them.

XR-L8 does not have an instrument analogous to the human tear duct.

So a machine hovers above the mountaintop in silence, and it watches the cognac-brown sky bleed out into the steel gray of the setting sun.