In the cube grid outside my office, everybody popped up and looked around. We walked towards the hallway, as if it was a fire drill or something, before halfway through the evacuation process we kind of milled around and ascertained that, yes, everyone else felt that, too. Our next collective move was to the internet (the USGS maintains pretty spiffy live monitors on their web site). I’m sure the entire population of California was laughing at us, but the East Coast doesn’t get earthquakes.

I’d never felt an earthquake before, and this was definitely unmistakable. The psychological effects lingered a bit longer: every now and then for the next ten minutes, I felt like I couldn’t quite trust my inner ears.

I stayed at work late; when I left the parking lot was mostly empty. As I looked out over the expanse of flat asphalt, I thought that there’s nothing to remind us that the physical processes that drove our planet to the shape and form and state it is in now are still active than when the ground moves under our feet. I walked to my car, thinking about how a planet is a dynamic system and how much I take it for granted that the ground is going to stay still so I can drive home. And I wondered what it would be like if that little quake happened again. It was one of those moments when I couldn’t escape feeling how much bigger the world is than I am. It gave me pause for a minute; it was a small moment when I held a larger perspective of the world.

Of course: one of the truly wonderful things about the way this universe works is that, small as we are, human beings can learn to understand it. But even though I know about earthquakes, and know the mechanisms that cause and sustain them, feeling even a little one is a wholly different experience.

I have come into the possession of a most extraordinary object, which I procured rather fortuitously before the auction of goods from an insolvent boutique on the East Boulevard. I do not know how long it lay, disused and uncared-for, in a dusty drawer at that establishment, or when the boutique acquired it. The artifact in question is a curious map of the southern continent. I have scrutinized the place names and cross-referenced the markers corresponding to cities and towns with the atlases and charts in the City Library, and I have determined that this map dates from approximately 530 A.E. It covers the area from the North Barovin Mountains in its upper-left extremity, to historic Vorsvenbal in the south and all of South Brenin, Kalatchal, and part of Olahira to the east.

The dòm Gurand Map

The famous dòm Gurand Map of our southern continent does not only provide interesting historical and societal context, but contains some surprisingly accurate geographic information. One can examine the map for geological purposes, for evidence of historical wind patterns, and for characteristics of the climate of the year 530. Drainage areas of rivers are readily apparent, for instance, and the cartographer has captured some of the different qualities in the mountain ranges.

In his legend, the cartographer Ferin dòm Gurand clearly betrays his prejudices: he specifies only the three post-Imperial Nations by name. In fact, dòm Gurand’s title suggests, by placing Aurinon, Ereval, and South Brenin all on equal footing, that the cartographer hails from one of the many city-states of South Brenin (which, of course, had been defunct as a cohesive Nation for over one hundred sixty years at the time this map was drawn). The cartographer’s surname also traces him to South Brenin, perhaps Coseine or Rom Upale, which are labeled as cities despite their small populations in 530; I think likely this map was commissioned by the feudal lord of one of those city-states and dòm Gurand sought to please.

The Mountains

For example, the representation of the Pitchkalen Mountain range is as a series of fairly regular conical shapes, indicative of the volcanic origin of those mountains which mark the eastern subduction zone. Contrast these with the Barovin Mountain chain in the west, which have much more sculpted and varied shapes. While the symbols of the Barovin range on this map do not have a clean correspondence with the eroded ridges left  over from the continental collision and Barovin orogeny, dòm Gurand clearly apprehended the distinct character of these features.

It is curious, in a way, that dòm Gurand did not embellish South Brenin more than he did; however, one must remember that the Brenin countryside was politically volatile throughout the period from 370-840 A.E. Following the Fall of the Empire, this peninsula almost immediately fractured into independent city-states, while the former Imperial provinces of Aurinon and Ereval maintained their national identities. The great Imperial cities of Stonevale, Hollam Lake, and Dowill of course became the most powerful domains, but either a lack of ambition on the part of their leaders or the consequences of infighting and territorial squabbling prevented any of these from expanding beyond their feudal locales until 640 when the Four Nations period began in South Brenin. Smaller communities, several of which are marked on this map, rose and fell, rose and fell. Only the larger cities and the colonial holdings and fortresses of Aurinon (to wit: Farsoth and Empala) remained stable before 640.

South Brenin

The lush character of the central latitude band is due to the prevailing westward winds, which carry moisture from the Helonas Ocean over the lowlands of South Brenin and then, after picking up more humidity from Serinale, into Aurinon and Ereval. The Brenin countryside consists mainly of low-lying grasslands, interspersed with deciduous forests, with winding rivers draining rainfall to both sides of the peninsula. Even in dòm Gurand’s time, Brenin was fertile farmland (excepting the colder climes of the bogs at its southern extremity).

The winds coming over the generally flat terrain of South Brenin then encounter the hilly terrain of eastern Aurinon, and, rising with the land, they begin to rain out. This precipitation feeds the verdant Westwood. The combination of varied terrain (the product of deformational and metamorphic processes) and fortuitous location with respect to seas blessed Aurinon with great natural resources that were easily accessible even in 530. Frequently, the prevailing wind veers northward over the Serinale Sea to rain out in the Arrith Valley. The remainder of the land, southwest to the barrier of the Barovin Mountains, is the vast, grassy plain of Ereval. Grazing livestock and wild grains have been present in Ereval for most of recorded history.

Aurinon

We come now to dòm Gurand’s treatment of Aurinon, the nation that best weathered the Fall of the Empire. Indeed, in 530 A.E. the kings and queens of Aurinon would have had their subjects believe that the Empire simply continued under different rule. The timber-rich Westwood supplied Aurin navies; the fertile farmland along the Serinale coast, bay of Aneda, and Idenford fed the Aurin people, minerals in the central mountains and the spur of the Barovins provided vital metals, and the strategic location of the country allowed easy trade with other nations while remaining defensible. After the Fall of the Empire, Aurinon enjoyed a relatively warm relationship with Ereval, save the incidents leading to the annexation of Arrith Valley and the Battle at the Cressit Stone, and it largely lacked interest in Oghura except as an occasional trading partner. Generations of kings in Aurin, the former seat of the Imperial House, dreamed of restoring Imperial glory and engaged in campaigns in South Brenin.

Oghuran Desert

I am quite surprised, in fact, at the level of detail this map exhibits in the desert land of Oghura, as it reveals that either dòm Gurand was in fact very well traveled or he had access to substantial resources with a great deal of accumulated knowledge. If he had contact with the Oghuran archivists, it is likely that they could have provided him with much of this information. Still, I suspect that the location of the desert stronghold Rukhas is but speculation, and the locations (or even existence) of all the indicated oases in 530 were undoubtedly uncertain.

Before the era of recorded history on this continent, lush jungles and forests spanned the plains between the North Barovin Mountains and the Pitchkalen Mountains. The early Kalatchali civilization was widespread throughout this area. However, a combination of factors changed the climate dramatically over a period of a mere several thousand years: the southward shift of the jetstream, the final drying of equatorial seas to the north, and the buildup of the active Pitchkalen volcanic range, all contributed to depriving what would become the Oghuran desert of surface water. Naturally, these phenomena had been ongoing for thousands or millions of years; the contributing factors simply passed the tipping point within the lifetime of the Kalatchali civilization. The great forests withered and died.

If you can get them to sit with you long enough to tell tales, the Kalatchalis wax poetic about one subject in particular: the halcyon days when their forest domain stretched from the edge of the seas to the backbone of the continent, the majestic Barovin Mountains. Nowadays this claim seems hard to believe, but nonetheless there exists substantial evidence. Natural historians have even determined that certain species of tree in the Westwood or Arrith Valley forests bear striking resemblances to Kalatchali jungle woods. Why the great forests should have disintegrated and become desert we do not yet know – but this is clearly one of the major sources of historical strife between Kalatchal and Oghura. (Indeed, some would say a preserved Kalatchali sense of loss is the source of that conflict.) This great shift in the land affected even the Empire, evidently: the outpost of Biran Tol marked the border of the desert in the early days of the Empire. By 530 A.E. it was a ruin.

Now, the entire plain in the rain shadow of the Pitchkalen appears at first glance to be a parched land. The highlands to the northwest do possess a substantial water reserve, however, and the bedrock underlying the Oghuran desert is quite porous. Water drains towards the Serinale in underground rivers, or seeps through the bedrock from the mountains to the desert plain. The most dramatic and long-lived example of these features is the Sugra oasis, which is fed by a large spring sources at Asoka and falls into the Ura River. Much smaller springs can appear unpredictably throughout the desert, and disappear as drainage finds new underground courses, resulting in the famous shifting oases of Oghura.

Kalatchal

Kalatchal itself is a humid, vegetation-choked land of plains and foothills between the ocean and the Pitchkalen Mountains. Several dendritic river systems drain precipitation away to a vast bay. Here, the dòm Gurand map was in fact substantially inaccurate: river courses match artistic impression more than anything else, and only two topographic features are noted. The boundaries of the rain forest are reasonable, and the Pitchkalen range is clearly composed of (active) andesitic volcanoes, but beyond this we cannot draw conclusions.

dòm Gurand never visited xenophobic Kalatchal, that much is evident. I certainly never would have, living in 530, and I cannot fault him for this – he would have likely been killed outright once identified within Kalatchali borders. Still, he records the existence, at least, of the three known Kalatchali settlements, and he marks the legendary hilltop temples of sacrifice to the Kalatchali gods. These few markers are deceptive: the Tiliprrek living in these jungles spread throughout the trees, and did not always congregate in cities as men do. The population of Kalatchal in 530 was likely comparable to that of Aurinon. In geographic terms, though, the splinter Tiliprrek realm at the end of the South Brenin peninsula is likely better recorded on this map.

Other nations and locales (Olahira, Tung Gras, and Vorsvenbal among them) are faithfully represented. Certainly Olahiran navigators possessed charts that could be purchased in one of the trading cities for accurate representations of these coastlines, saving our cartographer from a dangerous trek into the rugged lands of the Vorsven raiders or from trespassing on the Fire Islands. On the whole, I am impressed. This is by far the most complete map of the southern continent I have seen dating from earlier than 760 A.E.

Naturally, this notion tripped my sci-fi circuits.

This artist's conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. Image credit: NASA/JPL-Caltech

We live in an age in which new planetary systems are being discovered at an incredible rate. We are getting closer and closer to the ability to detect other Earth-like worlds around other stars. In fact, just a few days ago a study found that certain climate models of Gliese 581d (that would be potential-planet Zarmina‘s until-now-slightly-less-sexy sister) may support a liquid water cycle.

So what would it take for one of these free-flying, starless planets to be habitable?

The immediate answer that may come to you, the average person, is, “Joe, you are crazy.” But wait a moment!

All life requires is an energy input and certain chemicals, right? Well, all sorts of chemicals exist in gas planets. And there are plenty of possible energy inputs from the gas dynamics going on in their atmospheres – not to mention magnetic fields and other esoteric stuff like that that Earth life generally doesn’t incorporate into its metabolism.

But forget gas-giant balloon-life. Suppose we constrain our notion of habitability to the usual anthropocentric meaning: liquid water on a rocky surface.

In order for a rocky planet to have liquid surface water, it needs two things: heat and pressure. (Pressure so that the water doesn’t just sublimate or boil off into space, and heat so that it doesn’t freeze.) The “pressure” part we can take care of by giving our rocky world an atmosphere. However, we need a heat source – not only to keep the water from freezing, but to keep the atmosphere itself from freezing onto the planet, too. How do we get this heat source? Radioactive heating from the planet interior isn’t going to warm the surface to 273 K. Stars are all going to be too far from these planets to do any good. Emission nebulae are way too cold and rarified, even if the planet is right in the middle of them. The planet is going to pretty efficiently radiate away any heat inputs before that energy goes into heating ice to make water. (I suppose we could stick the planet right in the way of a black hole’s polar jet or some other source of hard radiation for our energy source – but then we’re back to getting really alien alien life. Fun to think about! And what happens to those alien civilizations that thrive on a dark planet bathed in X rays when their planet finishes traversing the zone of hard radiation?!)

I’m pretty convinced that liquid surface water is not going to appear on any free-flying rocky planets. Unless…

Suppose, when a Jovian planet got ejected from its birth star system, it carried its moon system away with it. Maybe some heat can come off of that gas giant and hit the moon! It’s not going to be reflected light, though, because there’s no star to provide bright enough light. No, the energy will have to come from the Jovian itself. This condition means that we’ll have to look at something like brown dwarfs: astronomical bodies that are just slightly too small to ignite under their internal pressure and turn into the hydrogen fusion furnaces that are stars. But they do have some fusion going on in their dense cores.

Take Teide 1, the first brown dwarf to have its existence confirmed. It has a surface temperature of around 2500 K, a luminosity of about 0.001 L_sun, and a radius around 0.1 R_sun. Suppose that a rocky (Earth-density) satellite orbits Teide 1 at its Roche limit, the closest orbital radius it can have without tides tearing the moon apart into a pretty but uninhabitable ring. (By a quick calculation, I get about 337,000 km for Teide 1 – coincidentally close to the Earth-Moon distance.) At that distance, the moon would receive around 1 million watts per square meter from the Jovian. If that’s the input power, the Stefan-Boltzmann law gives the output radiation of the planet in equilibrium. With a couple assumptions about albedo (Earthlike) and assuming that the moon receives incoming radiation over its cross-sectional area but radiates out over its entire surface (and that it’s the size of Earth’s Moon), my quick hand scratchings give a surface temperature near 50 K. Hmm…no liquid surface water there.

But there’s another possible heat input to a moon around a gas giant: the tides of the Jovian world.

Consider Jupiter: it has four big moons, and Jupiter raises such huge tides on these moons that the rocky mini-worlds actually flex, generating heat from friction. On Europa, this tidal heating in its central rocky part is sufficient to melt the inner bit of its water-ice coating into an ocean. Heck, scientists combing Galileo probe data just determined that tidal heating is sufficient to keep pretty much all of Io’s interior molten. That world is made of lava, with a thin crusty shell. And it’s all because the moon orbits a gas giant in a resonance with some other moons. the interaction between their orbits keeps the tidal energy coming.

So let’s give our moon some companions and an orbital resonance. Solar radiation is negligible compared to tidal heating even for Jupiter, so we know that that could give our moon liquid water…at least under the surface, like Europa.

But add an atmosphere, and you get an insulating blanket around the moon’s surface. More internal heat stays trapped on the moon’s surface instead of radiating away into space. I haven’t done the calculations, but if tidal heating can liquify rock on Io I bet it could be enough to melt Europa’s ice layer all the way through for slightly different orbital parameters. And with an atmosphere, the moon gets pressure to keep that liquid water from boiling. Like Titan. Put Titan where Io is…and what do you get? I’m not sure, but it would be really interesting. And it wouldn’t require the Sun.

Cool, huh? It certainly hasn’t been confirmed, and I don’t have a detailed model, of course, but I think the theoretical grounds exist for these free-flying dark planets to have liquid-water surfaces. Imagine vacationing on a beach next to a steaming ocean that is basically a global-scale hot spring, where it’s perpetual night and every couple (Earth) days you see the shadowy form of the gas giant loom overhead, visible more because of the stars it blocks out than from any external light source, except for the occasional immense spark of lightning through its clouds…

I spent a pretty good weekend doing some world-building. Since discovering the maps in the first pages of The Lord of the Rings, Redwall, and the like, I have really enjoyed sketching out maps of imaginary worlds and outlining details of the cultures and histories that play out over those maps. My maps started as knockoffs of Tolkien’s (with the bad guys sequestered in a nice, rectangular wall of mountains around some barren lands) or parallel-universe versions of the terrain around my house. Since then, though, I’ve started to inject a lot more realism into the worlds I create. Want to know where the tectonic plates and prevailing winds are on my map of Oghura? I could show you!

Map of Oghura

Beyond the maps, some of my imagined cultures have fully fleshed-out languages, religions, and customs. Slowly, slowly, I’ve been compiling reference documentation on the Oghuran desert and people, the fantastical Cathedral Galaxy, and the future-universe of the Four Colonies. This weekend I was spending my time in the Cathedral Galaxy, putting together a master list of the major galactic regions and polities, along with distinguishing characteristics. Now I know a bit more about why the Imperium of the Triumvirate is split in three, how the far-from-galactic-center Traders’ Rim came to be populated by merchants and entrepreneurs, and the tumultuous history of conflict between Amseile and Shobah. I’ve also got the beginning of a couple more stories – one concerning an Imperium gladiator’s bid for freedom and another describing the Waygehn people, who evolved to sentience near the death of their star and outlived the event, leaving them homeless in the galaxy. That’s one of the most fun things about deciding to build a universe purely for short stories: I get to invent worlds, and then immediately show them off with snippets of detail!

Though the Cathedral Galaxy has some distinctly space-fantasy elements, I decided early on that it would be a universe based on hard science – though not necessarily our hard science. My short story “Conference” illustrates the point, as it shows that there are technical concepts built upon technical concepts – but at the level that Arthur C. Clarke would have described as “indistinguishable from magic.” I have no idea how the Channel Network could be set up, and building planet-size structures is clearly fantastical. (And none of you know yet what’s in The Cathedral!) But I made sure that the story was relevant to us Earthdwellers, and I lean strongly on plausible concepts to describe things like astronomical bodies or planetary orbits.

Great Galactic Map, showing major markers and the Channel Network

For example, take Heliast, the resort world on which much of “Conference” takes place. Here’s the description that conference-goers got of the world:

The tour guide explains how Heliast is an ancient world with a single moon nearly half its own size, and how that has dominated the history of the planet and made it ideal for resort paradises. A billion or so years ago, the planet spun many times under one orbit of the moon, and the energy input of ocean tides among all the planet’s archipelagoes – Heliast is over eighty percent water – gave rise to life. But nowadays, the moon orbits in tidal lockstep with one Heliast day, the prime factor contributing to the perpetual calm of its seas. The small radius of Heliast’s solar orbit leaves the planet with a reasonable day length, while the dimness of its sun places it in the liquid-water zone. Without tides, with a massive moon helping to protect the planet from asteroid impacts, and with barely any eccentricity in its orbit to create seasons, there have been few selective pressures on Heliast’s life forms. Life on the planet thus failed to diversify much, and after millions of years of evolution with few external stressors, there are now only a few ecological niches on the world. Three or four avian species, eight or ten surface-level swimmers, two or three land animals, and about six land plants are all most tourists have the chance to interact with. The rest of the planet is geological beauty for visitors to enjoy.

So, the planet’s “month” equals its “day,” but there are still many days per year and there is much liquid water on the surface. The dynamics shaped the world’s evolution. That was fun to think of! But, more and more, I am completely amazed by the strange worlds that actually exist in our own universe. Many Earth- and space-based observatories keep returning data on new exoplanet candidates, and in the last few years, the galaxy seems a lot more planet-populous than it has in the past.

This past Monday, I went to a fascinating astronomy seminar on the potential climates of Gliese 581g given by Dr. Raymond Pierrehumbert from the University of Chicago. (He’s preparing these climate models for an arXiv preprint.) Besides tying the Gleise 581 system with 55 Cancri for most number of known exoplanets around the same star (5), this planet is interesting because it falls right smack in the middle of the traditional “habitable zone,” the range of orbital radii necessary for planet surface temperatures that could support liquid surface water. Now, of course, the discovery of Gliese 581g has to be confirmed to become official – and there’s some doubt about that! – but it’s at least got scientists thinking about these dwarf-star systems in interesting ways.

Gliese 581g, which the discovering astronomer apparently would like to name “Zarmina,” is a larger-than-Earth world that is most likely tidally locked to its star. “Tidally locked” means that the planet rotates once (one day) for every orbit around the star (one year), similar to the orbital situation of the Moon around the Earth. With a little visualization, it doesn’t take long to realize that a planet with a day equal to its year would always have the same point on its surface pointing towards its sun (the “subsolar point”). So one side of the planet would be hot, and one side would be cold, and the comfortable intermediate temperatures for liquid water – and life! – would be in the narrow band of longitudes around the terminator. (Star Wars fans: this is supposedly the case with Ryloth.) Right?

Well, not really, as I found out yesterday. In turns out that global climate models of a world like Zarmina give some interesting results in terms of how water behaves on its surface.

For instance, consider a case in which the planet has water on the surface, but has no atmosphere. Water vapor is a very inefficient greenhouse gas, so the planet surface would radiate energy away into space over almost all of its surface – and any surface ocean would be frozen on top. If the ocean is global, this would be like a super-size version of Europa. However, the surface around the sub-solar point would still receive enough incident radiation that the surface temperature there might be about 270 K. So, the ice layer there might be pretty thin (meters to kilometers), and a relatively small fluctuation in the local temperature could melt the ice and expose liquid water to the surface.

So, the sub-solar point might be more slushy than icy. Now, without an atmosphere above, the exposed water would immediately boil. There might be Enceladus-style geysers blasting water up out of the slush periodically. This slushy area provides a conduit for interactions between the surface and the ocean: a way for energy from sunlight to get down into the ocean, and a way for fresh materials from the ocean to get thrown up onto the ice. Those sorts of exchanges are very important when considering habitability! And it’s pretty cool to think about anyway. What a wild world that would be!

The result is worse for habitability if Gliese 581g has an atmosphere composed mostly on non-greenhouse gases, like N₂. The atmospheric dynamics from Dr. Pierrehumbert’s climate models are such that fast high-altitude winds would efficiently distribute the solar heat flux over the entire world. (Surface winds would actually be fairly slow.) But this thermal mixing would actually even out the surface temperatures, so the planet wouldn’t get a slushy sub-solar point. Any ocean would be covered with a kilometers-thick layer of solid ice.

But here’s the totally awesome hypothetical case:

If Gliese 581g has an atmosphere with about 20% greenhouse gas content, CO₂ in Dr. Pierrehumbert’s model (which is apparently not unreasonable for known chemistry with silicate rocks), then there could be a circular region around the sub-solar point that supports liquid water. He showed us pictures with a global ocean over the planet, with a thick ice layer over most of the planet surface but exposed liquid water inside a circle extending to about ±45° in latitude and longitude. He called this case “Eyeball Earth.”

"Eyeball Earth:" White ice and blue water under a red sun

The liquid water ocean within the “iris” of the eye would have a surface area about equivalent to the area of Earth’s oceans, since Zarmina is larger than Earth. There could be islands or continents within the ocean, all under a red sun that never moves from its zenith.

Now I feel like my world-building capabilities have been completely put to shame by the laws of physics! Just imagine the mythologies that would arise among cultures on a planet in which the known surface world – with about the same area as our own – would actually be an almost-flat disc bordered by ice. If any adventurous Zarminan heroes pushed out into the ice, they would find themselves trekking across a barren wilderness that gets progressively colder…and darker…until they finally lose sight of their sun entirely and start to freeze. What a world!

Of course, if the surface water distribution isn’t global, then there might not be an ocean completely covering the sub-solar “iris” and there might not be an unbroken icy wall around the ocean. Nor would there be ice over the entire rest of the surface. There might be visible rock. But it would be true that any water more than ~45° away from the sub-solar point would be covered by a thick ice layer. (There’s also a complication in that a completely iced-over world is apparently also a solution to this case. The planet might alternate between an “iceball” state and an “eyeball” state.)

See? The universe is much better at building cool worlds and sci-fi settings that I am. (Makes me wish I was more like Robert Forward!) Even if the detection of Gliese 581g turns out to be spurious, that doesn’t change the fact that the physical models give these results for the same parameters. With all the stars out there, it is very plausible that many worlds like the ones I described exist!

And Zarmina isn’t the only case of the observable universe out-world-building me. Some other interesting dynamical cases arise for tidally locked planets for which the planet’s orbit is a little bit eccentric, as I found out in another astronomy talk last semester. With that slightly eccentric orbit, a person standing on the planet would see the sun rise from the horizon in the morning, reach a low-angle zenith at noon, and then retrace its steps to sink back down to a sunset in the same direction as sunrise!

Then there’s HD 209458 b – a gas planet with “superstorm” winds as fast as 2 kilometers per second rush from the 1000 C side of the planet facing its sun to the other hemisphere. Just to give you an idea how outrageously fast that is, the International Space Station orbits at a velocity of 7 km/s. Escape velocity from the surface of the Earth’s moon is 2.4 km/s. These winds are really flying, and they are driven by the simple physics of a thermal gradient.

And check out this article on Centauri Dreams about brown dwarfs and the concept of the “habitable zone,” in systems where the fading light of the star over time may have implications for evolution – and the evolution of intelligence. In short: it’s possible that, since these planets start off habitable and get progressively less comfortable as time passes, there would be a selective pressure that favors species able to mitigate their planet’s slow death – or even colonize worlds. Woah!

George Lucas couldn’t make these things up. And they can, or do, actually exist.

Edit: Check out Phil Plait’s similar comments about planets orbiting eclipsing binaries!

RATS is a program in which NASA engineers, scientists, and astronauts take prototype equipment into remote locations on Earth and practice the procedures and operations that they would use if they were actually on another planet. It’s an opportunity for the engineers to see what their creations are capable of, scientists to see how much work astronauts can get done and teach them basic skills like field geology, and the astronauts to get some experience using the equipment so they can provide feedback.

Not only is RATS showing off the best capabilities of the most successful part of the Constellation Program – the Lunar Electric Rover Concept, or LERC – but they have gone to an especially cool site, a well-preserved but little-known cinder cone volcano known as SP Mountain! As that video played, I kept thinking to myself: “that looks familiar…” Here’s my view of SP and the lava flow coming out of the base of the mountain:

SP Cone

SP flow

When I was there, with a class of planetary geology grad students led by Cornell Mars scientist Jim Bell, I couldn’t help but picture the rugged a’a terrain of SP flow with astronauts picking their way along. What a tremendous place to practice exploration operations!

Grad students exploring the flow

Thanks to magnetometer measurements and images from the Galileo mission, it’s pretty much established at this point that Europa has an icy outer shell over a global liquid ocean, with a rocky core on the inside.^* The only question is how thick that ice shell is – I’ve read estimates ranging from 10 meters to 100 kilometers, with a pretty high confidence of ones to tens of kilometers. The ice shell gives rise to a number of interesting surface features. A particularly cool sort of feature, found with global extent across Europa, is the double ridge.

A prominent double-ridge feature on Europa, most likely a crack in the icy shell

Planetary scientists have a number of models for how these double ridges form, and they generally seem to agree that the ridges mark the locations of cracks in the ice crust. One especially well-established model suggests that these cracks occur when Jupiter raises tides in Europa’s ocean – just like how the Moon raises tides in terrestrial oceans, but much stronger, because Jupiter is frakking huge compared to Earth’s moon. Europa’s ice crust bulges out over the ocean’s tidal swell and then cracks under the incredible stress. (I like to take a moment to think about the mindbogglingness of that statement: the whole moon’s surface cracks. I’ve stood on a frozen pond when a crack pings through the foot or so of ice on top of the water – Just imagine standing on Europa when this happens!) Once a crack forms, the tides don’t go away. As Europa rotates, about once every three and a half Earth days, the tides periodically lever these cracks apart and squeeze them back together again. In this model, every time the cracks gape open the subsurface ocean gets exposed to space. The surface water boils and rapidly crusts over with ice, and when the cracks get smushed closed, all this ice gets crushed up and forced to the top and bottom of the crack, forming the ridges. The ridges appear in pairs because the crack opens up again after that. These double-ridge features are mounds of crushed ice flanking passages into Europa’s ocean!^†

Dr. Richard Greenberg is a planetary scientist who thinks that these cracks in the ice shell might be potential sites for life to take hold. Unlike the rest of the subsurface ocean, they get exposed to sunlight, which means that photosynthesis could take place. The periodic in-and-out forcing of the crack would also drive strong currents, which is another energy source Europan life could use. (Those aren’t the only energy sources: other possibilities include thermal gradients in the water, volcanic vents on the ocean floor, or even induction as Europa travels through the Jovian magnetic field.) Of course, that life would also have to adapt to the crack opening and closing once every 3 1/2 Earth days!^‡

Europa's possible ice-fissure biosphere (from New Scientist; click for full article)

We do at least know, from the Galileo mission, that these cracks often have accompanying veneers of organic (e.g. carbon-based) molecules and salts splashed onto the ice surface. This is why the cracks appear as brown stripes in large-scale context images. The crack/veneer combination suggests that there are organic molecules and salts in the Europan ocean, and that those compounds get pumped to the surface through these cracks.

So, let’s take stock: Europa is the only extraterrestrial world with a global liquid water ocean, there is a definite possibility for life in that ocean, and these double-ridged cracks are a possible gateway into the alien biosphere.

Well, then, let’s go diving! Read on for my concept system architecture for an ambitious Europan ocean-exploring mission, which I call the Ice Fracture Explorer.

The Ice Fracture Explorer, or IFE, would be a combination lander/penetrator vehicle that I imagine to be a little smaller than the size of one of the MER rovers. Ideally, several IFEs would accompany an orbiter to Europa. The orbiter component of the mission would contain instruments designed to give the planetary scientists on the mission enough information to select a few double-ridged cracks that are actively being worked open and shut by tides. The flight controllers would then dispatch an IFE to each of those cracks.

Soft landing on a double ridge interior

The IFE includes thrusters and landing pads to make a soft landing on the surface. Europa’s surface gravity is less than half of Mars’, so a soft landing should be easier than the successful Viking or Pheonix missions, even without the benefit of a parachute. The IFE landers would target the inside-facing slopes of the double ridges, as shown above. Each vehicle also includes cameras and instruments, and the IFE will pause on the inside of the double ridges to relay data back to Earth.

The goal of these images are to verify that the crack is opening and closing each tidal cycle, and to establish the timing of the cycle. Once the cycle is known, flight controllers will uplink commands to the IFE to begin the second phase of the mission.

The IFE will wait until the crack is closed, and then separate form the landing legs and inflate some gas-bladder cushions, causing the vehicle to roll down towards the center of the double ridge. Using its thrusters for attitude adjustment, the IFE will right itself, centered over the crack.

Bouncing on air bags

At this point, the gas cushions would deflate. Since the IFE has to operate in the middle of the double ridge, I imagine it will need to run off an RTG power supply instead of solar panels.

After righting itself, the spacecraft deflates the air bags

Next, the IFE would fire projectiles into the crushed-ice ridges on either side of the vehicle. These projectiles could be barbed, contain chemical flash heaters, or anything else design to make them really stick into the ice, because they would be the anchors for twin tether lines that unreel from the spacecraft. The IFE would also deploy a high-gain antenna for communicating with the orbiter overhead, since the mission will have to happen very quickly from this point on.

Anchored to the walls, and antenna deployed

Now, the spacecraft waits. Inside the IFE, the tethers would be mounted so that they unreel in unison. When the ice crack opens up again, the tether cables will support the IFE above the center of the crack.

The mechanism pays out both cables at the same rate

As Jupiter rises overhead wobbles around zenith, its tides will pull apart the two sides of the ice fracture. The IFE will be suspended in the middle as the crack opens, with nothing below it until the ocean 1-10 km down! At this point, the IFE will drop its deflated cushions and begin to deploy a smaller penetrator vehicle from its underside. The penetrator is a small, two-stage vehicle with two instrument packages, a hard-shell body, and a data line connecting it to the IFE’s main bus.

Hanging over the abyss

The IFE would drop the penetrator, letting gravity take it down into the fracture. As it falls, the penetrator will photograph the fracture walls in visible and infrared light. Data will be rapidly carried up the line to the main bus hanging overhead, which will relay the images to the orbiter

Eventually, the penetrator would hit the ocean surface. The water would have iced over, but the weighted penetrator with its reinforced lower body would smash through the ice and reach the liquid water below. At that point, a buoyant surface instrument package would separate from the lower penetrator, which would continue down into the water. The surface instruments would try to identify any interesting chemistry or biology occurring at the water surface, where photosynthesis might take place. The lower body of the penetrator would simply try to go as far down as it can, illuminating the depths and taking pictures.

The IFE drops two instrument packages, one that stops just under the surface and one that goes as far down as possible

The IFE will have perhaps less than one quarter of a Europa day to operate. When the crack closes, it will be crushed inside unless something exceptionally lucky happens or the ice shell turns out to be very, very thin. One possible mission design might call for the IFE main bus to reel in the tethers when the crack closes in an attempt to survive for a second day, but the penetrators will likely be lost.

If the Europan ice shell is indeed hundreds of kilometers thick, then a data line might not be the best solution for penetrator-to-main-bus communication. I don’t think radio or laser communication would work well across the water/vacuum interface, though. In that case, one possible mission profile might be to wait for the crack to start closing, when the water level inside the crack might be squeezed up towards the surface. However, there would likely be much less data available.

Finally, since the IFE would be crushed inside the fracture as it closes, it will have to relay all its data to the orbiter before it loses communications capability. The orbiter would have to be timed to be overhead for the IFE to relay all its data – perhaps in an eccentric orbit that spends the entire quarter-day over the crack, or maybe just coming into communications range of the IFE during its last moments, so the IFE can dump its entire data store and then beam images out as it can before it gets squished.

All this discussion highlights one fact: Europa is a unique challenge. Though it is one of the very few places in the Solar System that we can imagine harboring life, the mission design to explore the Europan biosphere is very difficult and requires many stretches of space technology. The requirements analysis and detailed design of such a mission would take a great deal of innovation and effort. Those are challenges that I would love to see the space program address, though – because the discovery of extraterrestrial life would have a profound impact on our science and society. Diving into the cracks in Europa’s icy shell may be our best bet at making such a discovery!

* See, e.g.: Zimmer, C., Khurana, K. K., and Kivelson, M. G. Subsurface oceans on Europa and Callisto: constraints from Galileo magnetometer observations. Icarus vol. 147, p. 329–347 (2000)

† For more information on cracks in Europa’s icy surface, check out:

• Greenberg, R., and Geissler, P., Europa’s dynamic icy crust. Meteoritics and Planetary Science, vol. 33, p. 1685-1710. 2002.
• Greenberg, R., Hoppa, G., Bart, G., and Hurford, T.,Tidal stress patterns on Europa’s crust. Celestial Mechanics & Dynamical Astronomy, vol. 87, p. 171-188. 2003.
• Greenberg, R., Geissler, P., Hoppa, G., and Tufts, B. Tidal-tectonic processes and their implications for the character of Europa’s icy crust. Reviews of Geophysics, vol. 40, Art. No. 1004. 2002.
• Aydin A. Failure modes of the lineaments on Jupiter’s moon, Europa: Implications for the evolution of its icy crust. Journal of Structural Geology, vol. 28, p. 2222-2236. 2006.

‡ A couple cool articles on potential Europan biospheres are:

• Greenberg, R., Geissler, P., Tufts, B.R., and Hoppa, G.V., Habiltability of Europa’s crust: The role of tidal-tectonic processes. Journal of Geophysical Research, vol. 105, p. 17,551-17,562. 2000.
• Greenberg, R. Tides and the biosphere of Europa. American Scientist, vol. 89, p. 48-55. 2002.
• Chyba, C.F. and Hand, K.P. Life without photosynthesis. Science, vol. 292, p. 2026-2027. 2001.
• Schulze-Makuch, D. and Irwin, L.N. Energy cycling and hypothetical organisms in Europa’s ocean. Astrobiology, vol. 2, p. 105-121. 2002

Mysterious craters on Mars

I’m shamelessly bouncing all you readers over to the Bad Astronomy blog for this post, which is a great outline of the detective process that is planetary geology. It’s also a great illustration of how much context matters and how leaping to conclusions is…bad. AND it’s a good demonstration that, when there are several hypotheses in consideration, elements of each could be synthesized into the proper conclusion.

All things for us to keep in mind, in science and in everyday life!

(Also, way cool pictures that are reminders of TOTALLY AWESOME events in the past!)

Gatlinburg

They don’t call them the Smoky Mountains for nothing. It was pretty hazy most of the time I was there, so we got a lot of views of faded ridgelines marching off into the distance, covered by lush deciduous forests. Of course, as this was a family event, we spent most of our time in that cabin and generally had a great time. But my girlfriend and I managed to make two highly successful jaunts into Smoky Mountains NP.

The first was a bike around the 11-mile Cades Cove Loop Road. This loop starts at a visitor center an hour’s drive into the park and circumnavigates a flat expanse of farmland in the middle of the mountains. Even the drive in was fun – it reminded me quite a bit of the drive from my home to Williams on Rt. 2, on the part around the Mohawk Trail. Makes sense – the Berkshires and Appalachians formed in similar orogenies, though the scales were far different. Anyway, a farming community has existed in Cades Cove since the first settlers made it that far west and persists today. The loop road winds along what were – to an Ithaca biker – gentle hills and afforded us a lot of panoramic views as well as some brief visits to historic buildings. (The Park Service calls the Loop Road a “moderately difficult” bike. That was definitely on account of the condition of the rental bikes, and that they tell you not to use the front derailleur – I think the one on my girlfriend’s bike was actually disabled. Fortunately, I had my multitool….)

View from the Cades Cove Loop Road

There were quite a few panoramic views from the road. Cars and cyclists share the road, and despite the many pullouts, traffic was slow as the people in cars paused to take photos. I was happy to be a bit more mobile and flexible!

Cades Cove panorama

At the far end of the loop was another visitor center built near the old farming community’s mill – which looked very picturesque among all the trees! I had a fun time playing with the CDHK high-dynamic-range script on my little Canon point-and-shoot to get a picture of the half-shaded, half-sunlit mill building:

Cades Cove mill

Inside the mill, they sell corn meal ground at the site. Of course, we weren’t going to hump any of that out the remaining 5 miles on our clunker rental bikes! After pausing awhile for lunch – which gave us an opportunity to improve my blood sugar, the water level in our bottles, and the worst of the derailleur problems on our bikes – we set off again. I was most interested in the scenery of Cades Cove, as after seeing one or two of the historic old houses, you’ve pretty much seen ‘em all. However, there were still interesting historical tidbits to be had. Here’s a pretty cool grave we found in the cemetery around the Primitive Baptist Church:

Murdered by Rebs!

Our bike tour finished with a close encounter. On the way back, all the cars on the road suddenly jammed up, with occasional people pointing out of rolled-down windows. I cast a look off to one side and spotted a BEAR. I skidded to a stop, and it turned out to be a mother black bear with two cubs, rooting around in the shrubbery. They had warned us about bears at the visitor center, but I took those warning in the same way I take any warning about animals in parks – yeah, yeah, okay, if I see a bear I’ll be sure to keep that stuff in mind! Little did I know that they give these warning in the Smoky Mountains because you will probably run into bears. I took some grainy movies before they got too close for us to do anything but get back on the bikes and get going. We had no windows to roll up!

After the Cades Cove bike, we collapsed at the lodge. The next day, though, we were feeling intrepid enough to be looking at hikes in the park, and based on the description alone we picked out Chimney Tops trail. This was a big win.

Chimney Tops is a 2-mile trail to the summit of a mountain right next to LeConte Peak, the highest point in the park. The last mile of the two gets steeper and steeper, ending with a bare-rock climb. I, my girlfriend, and her cousin were very excited as we set off.

One of the first things to strike my about this hike was how lush everything was. I’m used to forests that consist large of trees and ground cover, like those in New England. Whenever I see a different forest ecosystem things seem a little funny to me. So far, the weirdest to me has been the ponderosa pine forest around the Grand Canyon, which consists of huge ponderosas and nothing else. Well, Smoky Mountains National Park is at the other extreme: solid green growing things from ground level up to the canopy.

Chimney Tops Trail

(Other great examples here and here.) We even managed to spot a Jordan’s Red-Cheeked Salamander in all that foliage – a salamander species found only in this National Park! Shortly thereafter, we almost had another encounter with a black bear, as we saw some fellow hikers hoof it down the trail to us and tell us that they were doubling back a bit to avoid a bear that had burst out of the undergrowth right in front of them. We didn’t see the bear – only some wet footprints a few minutes later.

Chimney Tops Trail reminded me a little bit of Angel’s Landing in Zion National Park – though the climates, geology, and trails had plenty of differences, of course – in that it ends with hikers climbing out onto a spur of rock that sticks out into a valley. So, as the hike got steeper and steeper, eventually it turned into this!

Scaling the summit

Pausing at a convenient stopping point partway up those rocks, I turned to one side and snapped the following panorama, a preview of what we saw at the very top.

Panorama near the top

Finally, after a bit of exciting scrabbling, we got to wedge ourselves into some crevices at the top and have a good look around. Scenic! (Click to panoramify.)

Chimney Tops summit panorama

Have I mentioned that I love my little Canon SD1000, which fits in my pocket, has a nice panorama mode, and lets me take HDR photos with CHDK? In fact, the HDR tricks I’ve been playing with were wonderful up on Chimney Tops, because they let me combine exposures to get some good shots of the progressively faded mountain ridges staggering off into the distance over the near hills. One such example:

HDR of mountain ridges

And another, capturing the tip of the mountain spur forming the Chimney Tops:

HDR mountain rock spur

Under the shade of the trees it had been nice and cool, but as we came out onto the bare rocks it warmed up. As we snacked, sitting on the mountain, the sun was flirting with the edges of some clouds. Coupled with the haze and humidity, this meant that we got a pretty nice optics show of sunbeams blazing down on the distant mountains. I tried to capture some of that with my camera, too, but found it about as difficult as the one time I was in a position to photograph the aurora borealis. Still, I got a few nice images!

Smoky Mountain sunbeams

I even figured out how to postprocess the living heck out of one panorama to bring out the sunbeams without totally destroying the rest of the image. I quite like the result, below! You’ll want to click on this one.

Smoky Mountains sunbeams

Maybe next time, I’ll have to try and time that hike for sunset or sunrise!

After a good deal of gawking, it was time to head back. Of course, climbing up the rocks and climbing down the rocks are two different problems, and we were a bit slower picking our way down the steep surfaces. This is the best picture I got that gives a real sense of going down. Notice all the deformation in the tilted stratigraphy, with girlfriend and her cousin for scale. (The runner-up is this photo.)

Going down!

I’ve gotta say that the Smoky Mountains were definitely worth a visit, and I’d happily recommend Chimney Tops as a good morning or afternoon hike. I like hiking, and I like our National Parks, so I was happy for the chance to get to see another in an area of the country I haven’t been to.That’s one of the things I like best about the United States: we’ve got so much stuff within our borders, but everyone uses the same money, understands English, and follows the same road signs. And sometimes we even feel like protecting what we have, so we can go into these spectacular places!

Gatlinburg ridges at sunrise

See all my pictures on Picasa - I had fun tweaking them all on the plane back to Ithaca!

An image-of-the-day gadget on my iGoogle home page showed me this picture, which I subsequently spent about a half hour trying to locate at a primary-source web site. It is wicked cool.

Possible Cyclic Bedding in Arabia Terra (HiRISE/MRO)

Click to go to this image’s description page on the University of Arizona HiRISE site. (Be sure to bookmark the 2560×1600 wallpaper version!!!)

I really want to know how these terraced buttes got to be the way they are…it looks like they must have been eroded in stages, with each layer from the top getting peeled back successively, but somehow the individual layers hold together – those are some pretty steep walls. I can see in the southwestern portion of this image that some of the terrace walls are eroding away in chunks; there are a couple good fallen boulders over there. The layers might be some kind of sandstone, because they haven’t eroded away in lots of rocks and boulders, so they don’t seem very friable, but there’s obviously a lot of source material for dunes in this area so the butte walls might be getting ground down into very small grains. I’m not sure what the fluvial history of Arabia Terra is – on Earth, that would be bound to play an important role in creating landforms like this.

I also really love the expression of the more recent aeolian features in this area. Looks like there are prevailing north-south winds on the east side of this image (I’m going to say the wind blows to the north because the north sides of the dunes look more like slip faces to me), but from the east-moving dunes in the terraced valley-like feature at center bottom and the east-west oriented ripples on the larger dune field, the winds are apparently going in rather circuitous routes around these buttes. There are also some confusingly-oriented dunes and ripples in the southwest portion of this image, probably from the wind winding around all the rocky towers. (In my mind, I can hear it whistling.)

Looks like the valley from which the east-going dunes have traveled is an exposed outcrop of one of the terrace layers. This image can resolve objects less than a meter in size, so the various crisscrossing dark lines in the light-toned outcrop might be joints or something.

Anyway, this is not a new image and I haven’t studied or researched this stuff…I just saw it today and wrote a little stream of consciousness of geological ideas. I just think this image looks beautiful and I want to send some rovers/people there. Any planetary science guys want to comment?

Last, and just for grins, here are some goodies I turned up in my search for that image on the UA HiRISE site. Here we have some dramatic contrast between dunes and some lighter, rockier topographically high areas:

Pitted Layers Northeast of Hellas Region

Here’s some great layer exposures around some hills – and if you zoom into the large version of this one, you can find some wild and interesting ripple patterns:

Light-Toned Rock Exposures in Noctis Labyrinthus

Thus ends today’s amateur geology geekage.