The memory metal remembered and recoiled to its rest state. The Frog Prince leapt, pulled along by its own tongue. When it landed, it would tug itself loose and take another lick.
Donovan turned to the door.
And Olafsdottr was crowding in, blocking his escape.
His cry emerged as high-pitched as a bat’s, so far into overdrive was he. Olafsdottr brushed him aside with her right arm. The Frog’s tongue lanced again. She seized the ribbon with her left hand, pushing it aside, as she had seized the flying shim during their workout, even as she fired the teaser with her right. She screamed.
“Serrated!” She released the tongue of steel, which with a lick swiped her across the side as it rewound.
But the teaser had found its target. A teaser fires a coherent electromagnetic pulse. At certain settings and focuses, it can play havoc with a man’s nervous system. Other settings can fry electronic devices. The Frog Prince flashed and sparked as the induced currents ran along its body and internal circuitry. Its head turned toward Donovan. The mouth opened . . .
. . . and smoke came out.
The Brute threw the wrench and it spun into the frog’s visual sensors, shattering them. But by then this was mere grace, for the bright blue of the Frog’s body was fading with its power source. Donovan found the wrench and used it to beat the machine into scrap.
When Olafsdottr awoke, she was lying on a pallet in the infirmary. Both hands were encased in restoration gloves while regressed cells rebuilt the torn flesh and snapped bones. Her side, where the tongue had swiped it, was likewise bandaged. To inhale sent a stabbing pain through her.
Donovan sat by the pallet reading a book screen. He looked up when she moved.
“Rib?” she said.
He nodded. “Two. And a deep laceration. What possessed you to grab the tongue like that?”
“I thought only to knock it aside. I did not expect a saw blade.” She raised the two gloves. “My hands?”
“The left one was badly sliced up. You must have grabbed at it with your right after you dropped the teaser.”
“I promised Gidula I would deliver you in one piece to Henrietta. Could not let Froggie punch holes through you.” She took another experimental breath. “I must praise your medical skills, sweet.”
“The meshinospida l did all the work. I just zipped you in the basket and followed the instructions. The automatics took cell samples, regressed them, and applied them in the proper course.”
“Ooh, but you had noo oobligation to deliver me whole. Or to deliver me at all. Foortunate, then . . .”
Donovan shrugged, studied his hands. “Look,” he said, “can we drop the Alabaster accent? We’re past that, I think.”
“Fortunate, then,” she said more quietly, “that you spied the Frog Prince in time, or we would both be dead.”
“Inner Child is paranoid. Makes a good sentry.”
Olafsdottr sighed. “It must be a wonderful thing to divide your attentions that way. I was told it had incapacitated you.”
“It does have its drawbacks sometimes.”
“How do you plan to explain the corpse to the Megranomese authorities?” she asked. “Or how you came by this ship?”
“It was his ship. He was giving us a ride. This thing broke out of its box. Missy, if between the two of us we can’t concoct a story to fool a Megranomic copper, we should both of us quit the Long Game.”
Olafsdottr cocked her head sideways. “I thought you h a d quit.”
“You know what I mean.”
“Almost, you tempt me, sweet. But I am unaccustomed to asking for help.”
Donovan grinned. “I’ve had practice. I’ll teach you.”
The answering smile was almost sad. “Sweet, between the two of us, we defeated a Foreganger killing machine. Tell me you are not the man we need for the struggle.”
Donovan sat back so that his head rested upon the wall of the little galley. He closed his eyes and his breath slowly gusted from him. “I’m not the man you need.”
“Sorry I am to hear that, for it would have been entertaining to watch developments. How long before we reach the Megranome way station?”
The scarred man shrugged. “The ’o spidal had you in suspension for five days. We’re out of Megranome space.”
“Ah. You take me direct to Dangchao, then. Perhaps Bridget ban keep me in clean cage.”
Donovan rose, wiped his palms on his trousers. “You sleep now, ‘sweet.’ Your hands are too badly cut up to pilot the ship. I don’t have a certificate myself, except as a chartsman; but every chartsman is a pilot in training, and certificates are only for officials. We’ll be on the Tightrope in another two days.”
Olafsdottr struggled to sit up, winced at the pain, and slid back prone. “On the Tightrope?”
The scarred man, at the infirmary door, shrugged. “And don’t ask us why, because there’s not a single one of us knows the answer.”
(EDITOR’S NOTE: The scarred man appeared earlier in “On Rickety Thistlewaite” [January/February 2010].)
Copyright © 2010 Michael F. Flynn
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Science Fact
Other Earths in Space and Time
Kevin Walsh
Right now, we don’t know of any other Earths, but we will soon. Worlds only slightly more massive than Earth have already been detected, and smaller planets will soon be found, orbs the size of Earth with similar temperatures and atmospheres. In the meantime, it is useful to consider what kind of...
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Science Fact
Other Earths in Space and Time
Kevin Walsh
Right now, we don’t know of any other Earths, but we will soon. Worlds only slightly more massive than Earth have already been detected, and smaller planets will soon be found, orbs the size of Earth with similar temperatures and atmospheres. In the meantime, it is useful to consider what kind of places these other Earths could be, partly out of sheer scientific curiosity but also for more practical reasons. A clearer definition of the characteristics of Earth-like worlds will better enable them to be detected, as it would help astronomers narrow down the stellar systems where they should search for them. This is important, as the number of potential stellar systems to be examined is very large. Even if we confine ourselves to the relatively small region within 100 parsecs of the Sun, in this volume of the galaxy there are still about 40,000 Sun-like G stars, as well as several hundred thousand fainter stars that could host an Earth-like planet under certain circumstances. This is a lot of territory by anyone’s standards, and the quicker it can be explored, the better.
Of course, the term “Earth-like planet” is almost a misnomer. Here, we restrict the discussion to planets with approximately the same amount of incident solar radiation as Earth, planets within the so-called habitable zone. Outside of the habitable zone, there are certain to be plenty of Earth-sized planets, as the recent discovery of super-hot super-Earths has shown. But despite the huge number of possible planets even in the nearby part of the galaxy, we will probably not find a very close equivalent to the Earth for some time, because the number of variables that can modify or restrict planetary habitability is just too great. And even the term “habitable” means different things, depending on its application.
One definition would require an evaluation of a planet on the basis of the level of technology required to maintain human life on its surface. For instance, Earth is people-friendly, with a large portion of its surface having temperatures that do not depart too far from the human optimum of 22°C (72°F). It also possesses an eminently breathable atmosphere at sea level. Remove the oxygen from the atmosphere and Earth would still be relatively friendly, but we would need oxygen masks to survive and so the planet would be less habitable for humans. Many microbes, though, wouldn’t notice the difference.
Increase the rate of incoming meteor strike by a factor of 1000 and impacts big
enough to take out a city would occur 10 times a year, and unless the planet were surrounded by a very effective anti-asteroid system, it would probably be too risky for humans to consider living there. Increase the average temperature by 40°C and the planet would be impossibly warm for humans and survivable conditions could only be maintained by significant energy expenditure on climate-controlled living spaces. Some organisms, though, would love the extra warmth.
There will be lots of planets that are Earth-sized, but if we are talking about planets with breathable atmospheres and equable temperatures, worlds that are free from nasty problems like frequent multi-megaton asteroid strikes, these will be a very low percentage of the total—although there will still be many of them because there are just so darned many stars.
Oddly enough, we are reasonably confident that all of the types of Earth-type worlds mentioned in the preceding paragraphs actually exist today. This is because they have existed already, on Earth itself, billions of years ago. An argument is often made that because we only have one known example of an “Earth-like” planet, it is impossible to generalize about what other Earth-type worlds might be like, as this would be like trying to make conclusions about urban life in the U.S. by studying, say, Carmel or Santa Barbara. But we already have before us the example of Earth and its wildly different habitability conditions over geologic time. We can also study the other planets of the solar system, where many of the geological and meteorological processes known on Earth also operate. For instance, on Mars there are glaciers, landslides, gullies, fog and frozen seas, just like on Earth. True, Mars is currently much less habitable than Earth, but there is plenty of evidence to suggest that billions of years ago, Mars was warmer, wetter, had a thicker atmosphere, and was considerably more habitable than it is today. By examining the evolution of the terrestrial planets in our system, including that of Earth, we can learn a lot about the kinds of planets we are likely to find in other systems.
Previous Earths
Like Mars, Earth’s habitability has varied. It has been both slightly more habitable and much less habitable in the past than it is today. Thus there is nothing typical about Earth’s current habitability. In fact, it would be easier to argue that Earth’s current conditions are anomalous. At present, there are large polar ice caps, and this situation is not usual in the past few hundred million years. In addition, we are currently undergoing large climatic oscillations—Ice Ages—that only began in earnest about three million years ago. About 100 million years ago, the planet was more habitable for human beings than it is today. There were no large areas of permanent ice, the polar regions had genuine summers, and the atmosphere was just as breathable as it is today. But for most of its geologic history, Earth has been less welcoming: about 700 million years ago, it almost froze over completely, while two billion years ago the atmosphere was unbreathable, and conditions were even worse earlier than that. This is relevant to the discovery of other Earths because other solar systems will have different ages. Also, because we believe the processes that form solar systems operate in a similar fashion throughout the galaxy, the evolution of other Earths will be analogous in many respects to that of our own Earth. Thus we can learn about the kinds of worlds we are likely to encounter in our part of the galaxy from the type of world that Earth has been in the past, and the kind of world that it is likely to be in the future.
The human habitability of the early Earth at the time of its formation was very poor. The formation of Earth from rocks of various sizes was largely complete by 4.54 billion years ago. This date is reasonably precise, as it has been determined by analyzing the decay rates of radioactive rocks. Some tens of millions of years later, a catastrophe occurred that appears to have been a blessing in disguise. Many readers will know that at that time a Mars-sized world gave the Earth a glancing blow, almost but not quite smashing our planet to bits. Some of the resulting debris stayed in orbit around the Earth and became the Moon. Now such collisions may occur in other systems, although not always with the same outcome. Other probable outcomes include both planets being so smashed up that they can never reassemble themselves again, thus creating an asteroid belt instead of the Earth-Moon system. Alternatively, both planets could be blown apart but eventually one of them could accrete again, creating a large Moon-like world from the burnt debris. Then again, the blow could so glancing that not enough mass was tossed off to create a large satellite at all, but instead a short-lived ring system was created around the larger world. Or, even more likely, such a collision would never have happened at all, leaving an Earth-like world without a large satellite. The result is that many Earth-type worlds could have small or no satellites. We are lucky that we have a large satellite, as it helps stabilize the axial inclination of the Earth, which would otherwise oscillate towards large values every few million years. Earth’s current axial inclination of about 23.5° gives substantial seasonal variations in most parts of the planet. Larger values would cause more extreme seasons. Such temperature variations would not be fatal to the development of complex life, but they would certainly be unpleasant for humans, and thus the planet would be less habitable.
Unfortunately, observational work by Nadya Gorlova and colleagues has suggested that double planets like the Earth-Moon system are not very common. They examined numerous young stellar systems to see if they exhibited any telltale excess infrared emission caused by large amounts of dust of the type that would be generated by a catastrophic collision of the kind that formed the Moon. They found very few, suggesting that considerably less than 10 percent of Earth-like planets would have satellites like the Moon. More recent work suggests that this fraction could easily be as low as one in a hundred.
Leaving aside the minor issue of the near-destruction of our planet early in its life, the environment of our world at the time of the formation of the Moon was anything but pleasant. For the first 150 million years of its existence, Earth was hell, and in fact this period lies within the so-called Hadean era. While geological records from the Hadean era are sparse, it is clear that the early Earth was much hotter than it is today, possibly several hundred degrees hotter. Monster volcanoes belched gases and huge debris fell from the sky, the junk left over from the formation of the solar system. Quite a few extrasolar Earths will be in this stage of their development today. There is even a nearby system that is a prime candidate to host such a planet: Epsilon Eridani, only five parsecs distant, a star slightly smaller than the Sun and one that has featured widely in speculative fiction. It is a young star with an age of less than one billion years, and its system has about 1000 times as much dust in it as in our own solar system today—in this context, “dust” can mean anything in size from actual dust to asteroids. In fact, planets have been identified in this system solely on the basis of the gaps they create in the dust as they sweep out their orbits. So any Earth-type planets in this system are likely to be undergoing a fierce bombardment that is incompatible with habitability, just like Earth in the early years of its existence. For this reason, it is unlikely that the Epsilon Eridani system is still a high priority for future searches designed to detect signs of life.
After a few hundred million years of Earth’s existence, the number of impacts dramatically reduced. The surface temperature dropped below the boiling point of water and the steam ocean surrounding the Earth condensed into a water ocean, although a very hot one: there is good evidence to suggest that ocean temperatures were up to 40°C (70°F) higher than today as late as 3.5 billion years ago. Just after the ocean condensed on Earth, dry land was hard to find, as the continents had not yet formed. The presence of an ocean on Earth-type worlds cannot be taken for granted: simulations of planetary formation suggest that the amount of water on an Earthlike planet is very variable, depending largely on the number of icy asteroids and comets that were slung closer to the Sun and were swallowed up by the forming Earth. Given a different arrangement of planets, a likely alternative outcome would be a water world, a planet whose surface consi
sts entirely of water. This would be a fine location for sea life if the water were at a reasonable temperature, but it would be a below-average habitat for human beings, even for the most avid sailor. The recently-discovered hot super-Earth GJ1214b is of this kind. At the other extreme, there will likely be some worlds with less than one thousandth of the total amount of water on Earth, genuine desert worlds with occasional oases where scarce subterranean water bubbles to the surface. We don’t know for sure whether it is possible for such dry worlds to have breathable atmospheres or not, but given the obvious links between water, life, and oxygen, life is bound to be less abundant on dry worlds than on water-rich ones, dooming desert Earths to the slow lane of evolution.
On Earth, shortly after the formation of the hot ocean, it is believed that the first life forms developed. The relative promptness of this event, occurring almost as soon as conditions permitted, implies that it is likely to occur on other Earths as well. Of course, we don’t know this for sure. An enormous boost to this argument would occur if life were discovered on Mars, that is, life that is clearly indigenous to Mars and not simply Earth life that had hitched a ride on a meteorite blasted off Earth’s surface billions of years ago. In making general inferences from the single example of the origin of life that we have, we are almost in the position of those Carmel-based urban sociologists that I mentioned before.
While the Hadean Earth was by now at least not entirely inhospitable, it wasn’t out of the woods yet. About 3.8 billion years ago, the asteroid bombardment rate suddenly increased, as indicated by the cratering rates observed on the Moon and other bodies of the solar system. This late heavy bombardment (LHB) may have been caused by a gravitational instability in the orbits of the outer planets that caused them to migrate in their orbits and start tossing surrounding asteroids in all directions. The LHB caused havoc but some evidence suggests that life may have survived it without having to start again from scratch. On other Earths, though, there is no guarantee that life would survive LHB events, as these could easily be more extreme than Earth’s was, nor even that LHB events would be confined to the early period of the evolution of the planet, as the timing of the onset of orbital instability depends on the size and arrangement of the planets and so would be different in every system.
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