Similar chaos zones occur on the Moon, opposite its largest craters (although the one associated with Caloris is much more dramatic, perhaps due to the seismic properties of Mercury's large core). But other features of the Caloris Basin are unique.
In its outer portions, it has circumferential ridges, much like those in the smaller Rembrandt Basin. But further in, a series of troughs, more than 100 of them, radiate almost exactly from the Caloris Basin's center. These features were initially dubbed “the spider,” but are now formally known as Pantheon Fossae. (Fossae, in planetary geology, are trenches; these reminded somebody of groove-like architectural features in the Roman Pantheon.)
The circumferential ridges and Pantheon Fossae appear to have been formed by a two-stage process: subsidence of the crater floor followed by uplift of its center. The subsidence produced the ring-shaped outer cracks. The uplift produced the inner, radial ones.
But what caused the uplift? The default theory would normally be volcanism. In fact, Head thinks the Fossae aren't really cracks, but are instead dikes, where lava intruded into older rock, producing a splay of spoke-like rays. Similar features, he adds, have been seen on Venus.
Solomon disagrees.
To him, the key is a crater called Apollodorus (named for the architect credited with designing the Pantheon).
Apollodorus is a forty-kilometer crater—by no means the result of a planet-busting impact, but nothing to be sneered at, either. What's interesting is that it sits smack-dab in the middle of the fossae. Coincidence? Maybe. But maybe not. Drawing on work by Andrew Freed at Purdue University, Solomon suggests that the impact, also directly in the middle of the Caloris Basin, might have hit just right to release a lot of leftover stress from the earlier, basin-forming impact, allowing the basin floor to rise quickly.[14] “It's a bit like when a pebble hits your [car] window,” he says, noting that car windshields are under similar stress.
If so, it must have been dramatic. Some parts of the fossae are filled with ejecta from Apollodorus, indicating that they would have had to form faster than the impact debris could rain back to the surface. That isn't impossible, says Solomon, but determining if it happened is going to take careful measurements of Mercury's gravity field and surface topography.
Head agrees. “It's pretty much a stalemate at the present time, because we don't have sufficient [data] to distinguish between these hypotheses,” he says. “But we will when we get into orbit."
Getting into orbit is the Holy Grail of the MESSENGER mission. If all goes well, it will happen on March 18, 2011, after which MESSENGER will begin systematic mapping, eventually bring its full instrumentation to play on most of the planet's surface. In addition to photos, it's expected to conduct spectrometry, laser altimetry, gravity mapping, and use multiple images from different angles to produce 3-D pictures.
But already Mercury is emerging as more than a distant dot so close to the Sun that most people never see it. “We're seeing a planet that's much more interesting than even we suspected,” says Solomon.
"Stay tuned,” he adds.
Copyright © 2010 Richard A. Lovett
* * * *
1 See en.wikipedia.org/wiki/Mercuryinfiction.
2 Because Mercury, like Earth and most other bodies in the Solar System, spins in the same direction in which it orbits, this means it takes two complete Mercury “years” to rack up an extra rotation on its spin axis. In other words, a Mercurian “day,” from sunrise to sunset, is two Mercury years long, or 176 Earth days.
3 For updates, see www.nasa.gov/missionpages/messenger/main/.
4 Technically, it's the densest “uncompressed” planet. The Earth is slightly denser, but only because it's enough bigger that gravity increases its density enough to slightly edge out Mercury.
5 Its high orbital eccentricity, however, means its distance from the Sun varies from 46 million to 70 million kilometers, enough to produce seasons of a different sort.
6 Quotes and data, unless otherwise cited, come from this meeting, October 18-21, 2009.
7 William E. McClintock, et al., “MESSENGER Observations of Mercury's Exosphere: Detection of Magnesium and Distribution of Constituents,” Science, Vol. 324, 1 May 2009, pp. 610-613.
8 Some of this evidence is also in Brett W. Denevi, et al., “The Evolution of Mercury's Crust: A Global Perspective from MESSENGER,” Science, Vol. 324, 1 May 2009, pp. 613-618.
9 For an example, see www.solarviews.com/eng/mercury.htm.
10 See Holly Hight, “Jupiter Shift Pelted Inner Planets with Asteroids,” Cosmos Online, 21 October 2009, www.cosmosmagazine.com/news/3085/jupiter-shift-pelted-inner-planets-with-asteroids.
11 “Pattern of lobate scarps on Mercury's surface reproduced by a model of mantle convection,” Nature Geoscience, Vol. 1, 229-232 (2008).
12 See Thomas R. Watters et al., “Evolution of the Rembrandt Impact Basin on Mercury,” Science, Vol. 324, 1 May 2009, pp. 618-21.
13 The reason these are believed to have occurred in two stages is that some of these features cross each other, indicating that they were produced by two types of stress, presumably at different times. There could, of course, have been more than two stages.
14 See Freed, Solomon, Watters, et al, “Could Pantheon Fossae be the result of the Apollodorus crater-forming impact within the Caloris basin, Mercury?", Earth and Planetary Science Letters, vol. 285, 15 August 2009, pp. 320-327.
[Back to Table of Contents]
Novelette: MIDWIFE CRISIS by Dave Creek
Old job descriptions can change drastically under new circumstances....
So let me get this straight,” Carrie Molina said. This was only about five minutes after landing on the water world called Welkin. She stood on a landing pad where her small shuttle barely fit next to a submersible craft. The pad stood next to a small Earth Unity base perched upon a motile island. She heard waves splashing ashore and caught a whiff of salt spray. “You brought me here to inject me into a creature called a Levi-athan so I can treat its unborn child? What am I, some sort of antivirus or something?"
Carrie saw Matt Christian's grimace and knew she wasn't making a good first impression, but she didn't care. You opened the door with this crazy idea for a mission, she thought, and you take whatever comes through it.
"Not at all,” Matt said. He was a tall, slender man in his late twenties, just a little younger than Carrie. “It's all fairly straightforward. The Leviathan's not quite fifty meters long—"
"That's about twice as big as a blue whale back on Earth!"
"Exactly the analogy I was about to use."
"And the medical problem it's having is . . . what?"
Carrie watched Matt take a deep breath. “Why don't you come see her for yourself? And as soon as he gets here, I can introduce you to your partner."
"Partner?"
"He's someone I work with closely,” Matt said. “And he should complement your own unique abilities."
* * * *
Carrie followed Matt down to the shoreline and a dock. No boats were tied up there, but her eyes widened as she spotted a dark presence floating just at the surface of the water. Matt said, “Meet the Leviathan—Varis. She has datalink access, but she doesn't care to speak to Humans much. She's a little prejudiced against landside lifeforms."
He didn't exaggerate the size, Carrie thought. No wonder they're called Leviathans. I've traveled aboard ships that were smaller.
A closer look, and she found herself staring into eyes the size of bowling balls. Eyes with an amazing intelligence behind them, she thought. And I don't think I'm anthropomorphizing. Just behind those eyes was a pair of blowholes—Varis was, like an Earthly whale, an air breather, not a fish. A mouth the width of a small shuttlecraft opened and Varis chomped down on a clump of vegetation provided for her at dockside. The chewing sounds were prodigious.
Matt said, “Your partner should be here soon.” He shed his clothing except for swim trunks and jumped into the water. He placed both his hands upo
n the dark form of the Leviathan. Carrie didn't hesitate, and removed her own clothing—she wasn't wearing a swimsuit, but was accustomed to casual nudity—and moved toward the edge of the dock.
Even as she stepped off, Carrie was conscious of the differences between Welkin and Earth. Its .85 grav meant she fell just a bit longer than she would have on the homeworld, and when she dove beneath its waters, she was aware that the water didn't press against her as much as she was accustomed to.
She took a moment for what Matt called her “unique abilities” to assert themselves. They were, after all, why she was here. As her bioengineered body adapted to her environment, her heart rate sped up to pump blood furiously through her body to keep it warm, and her lungs expanded to half-again their usual size.
Carrie didn't breathe water, didn't have gills; the often-used term “Human fish” was a misnomer. Water didn't retain enough oxygen absorbed in it for the physical exertion she re- quired, and it didn't transfer oxygen into the bloodstream as efficiently. There were reasons many of the largest sea creatures were mammals.
She shivered slightly as the micro-dermal ridges of her skin, a trait she shared with dolphins, opened up—a goose-bumply feeling. Though barely visible, they trapped a thin layer of water molecules against her skin. That let her glide through the water with less resistance, since liquid flows against another liquid more smoothly than against the Human body.
Before heading to the surface, Carrie took this opportunity to check out the rest of the Leviathan's gigantic form. She saw what must be an incredibly strong fluke at the Leviathan's rear. She wondered just how fast it could propel itself through the ocean's waters, despite its massive bulk. Varis's sleek underside was interrupted by a round bulge of considerable proportion. That has to be one big baby, Carrie thought.
Toward the front of Varis's body, just behind those wise-looking eyes, were appendages that looked more like hands than flippers. They were webbed, and she was impressed with the four digits that looked as if they could manipulate objects much as a Human hand would. Tool-using aquatic forms. How did they arise here?
Before she could consider that question further, the Leviathan's body began to shake violently, sending out strong underwater waves that pushed Carrie away. The surface of the Leviathan's rubbery skin rippled again and again until the seizures subsided.
Carrie barely had time to react to that when another aquatic form, about the size of a walrus, but much faster, zoomed past her. I never even saw it coming, she thought. What the hell is it? She kicked upward, breaking the surface next to Matt...
. . . who caught the slightest glimpse of her naked body, blushed, and looked away.
Damn, Carrie thought. One of those. I hate nudity taboos.
Carrie was still figuring out how to regain Matt's attention when the walrus-sized creature surfaced between them. Over her data-link, she heard, “You must be Carrie Molina. I'm Sarbin."
Matt turned back her way, though he seemed relieved that Sarbin mostly blocked his view of Carrie. Poor man, she thought. Can't even enjoy the sight of a good-looking woman.
"Sarbin,” Matt said, “is an Aquatile.” His broad body featured stubby arms, different in detail but apparently similar in function to the Leviathan's. His wide, bright eyes spoke of an intelligence at least equal to a Human's. His snout ended in a single nostril. “I've heard of your people,” Carrie said. “I'm pleased to meet you."
"And I'm pleased to be your new partner,” Sarbin said. Carrie heard clicks and low tones that she realized must be the Aquatile's true speech, which her datalink translated.
Matt said, “Your temporary partner, Sar-bin.” Carrie tried not to react to the firmness she heard in Matt's tone, or what she believed was a note of jealousy. To Carrie, he said, “Let's get back on shore, and I'll let you know what little we've figured out about Varis's seizures."
Carrie expected Matt to escort her to the Unity base for a briefing. Instead, he excused himself to go inside the submersible shuttle on the landing pad. She would've preferred to sun herself awhile and dry off before getting dressed, but decided not to shock Matt's sensibilities any more than she had to and put her clothes back on. They'll dry soon enough under this sun, she thought.
Matt's hand, holding a towel, thrust itself through the submersible's hatchway. His voice was muffled a bit since he spoke without sticking his head outside. “I thought you might want to dry off."
Carrie didn't try to suppress her grin. “That's okay, Matt. A little late now, but I appreciate the offer."
Matt's head moved cautiously from behind the hatchway. “Oh. Sorry."
"A little water's the last thing that bothers me. You were going to show me what's wrong with Varis?"
Matt came down the shuttle's steps with a holopad under his arm. “Let's go back down to the water. I don't want to leave Sarbin out."
That's why we didn't go up to the base, Carrie thought. She followed Matt down to a shallower area of the island's waterline, where Sarbin had beached himself. Matt sat down next to the Aquatile, and Carrie settled down on the other side. Matt made a couple of adjustments to the pad, and a cutaway image of the Leviathan Varis appeared among them, her internal organs clearly visible, along with the outline of the unborn child she bore.
"Damn,” Carrie said as she leaned forward to examine the Leviathan's insides more closely. “I've stayed in hotel rooms smaller than Varis's heart."
Matt said, “As large as she is, you've seen how her seizures affect her."
"And what causes them?"
Sarbin said, “The Leviathans believe they become ill because they're sinful."
"What do you believe?"
"Aquatiles don't believe in sin."
Matt said, “The Leviathans do, though, and they banish from the open ocean those who become ill. They make them come to these motile islands and follow them around awhile. Usually they get well within a few days."
"Which makes it seem as if the banishment actually works."
"And which is reinforced by the fact that sometimes they get sick again once they go back to their families."
Carrie said, “But Varis hasn't gotten better yet."
"Not quickly enough. We suspect the pregnancy is the problem."
"How close is she to delivering?"
"She's about sixteen months along—so about another three months."
"Damn,” Carrie said. “That makes my belly hurt just thinking about it."
Matt said, “We're not sure how to treat Varis herself—it's been difficult analyzing what's wrong with such a large being. But doctors and scientists here at the base have come up with tech they believe can protect the child from further infection, and strengthen her against Mom's seizures."
Carrie ran a hand through her dark hair, which was nearly dry. “So I'm the delivery system."
Matt pointed within the holo to the unborn child's position deep within Varis. “We don't know enough about Leviathan physiology to design a self-propelled delivery system."
"I read up on them as much as I could on the way here,” Carrie said. “I understand the difficulties. How will we even be able to see while we're traveling inside her veins?"
"You'll be wearing goggles that combine infrared imaging technology and sonography. Some things might be a little blurry or indistinct, but you'll be able to see where you are and where you're going—especially given your enhanced eyesight and echolocation abilities."
"But how wide will a needle have to be to inject me?"
Matt grinned mischievously. “That's been its own technical problem. But we think we have a solution."
* * * *
That solution started with Matt leading Carrie into his submersible shuttle, lifting off, and heading out just far enough over the ocean to settle into its waters just beyond the spot where Varis floated. Even from within the submersible's small cabin, the Leviathan's size was intimidating. Although, Carrie thought, the bigger the better if I'm taking a trip inside there. She told Matt, “
When the Unity recruited me for this mission, the briefer told me I'd be taking a fantastic voyage. I thought that meant some kind of ocean trip."
"At least you won't be alone. Sarbin's going in with you.” Matt turned and peered into the cargo bay behind them, which was filling up with water.
"With all respect to Sarbin, why?"
"Varis doesn't trust Humans. She's sentient, but she believes the superstitions about sin causing her illness. Sarbin, being a native and a fellow aquatic being, is the one who's tried to convince her otherwise."
"Except you don't have a good explanation."
"Which doesn't help our credibility much. But we can't wait for research breakthroughs here. Varis's child will die unless we can protect it against whatever is making his mother sick."
Carrie said, “Having a Human—an alien lifeform—crawling around inside your own body has to be a frightening proposition."
"Which is why Sarbin will be there to reassure Varis that everything's fine as you get this job done."
Carrie stared upward at the dark mass of the Leviathan. “Let's hope everything really is fine."
"The Unity asked for you because of your abilities in a liquid environment—plus you have plenty of endurance, and you've shown that you keep your head in a tough situation."
Carrie turned back toward Matt. “That sounds like you're quoting from my file."
Matt looked away from the submersible's controls just long enough to glance back at her. “Well—I did read it."
"You're worried about Sarbin."
Matt's kept his gaze forward. “He's my friend. I never expected I'd become this close to someone who can't even live on land. I've saved his life at least once. And he's risked his for me."
Carrie returned to the co-pilot's position. “I know what it's like to lose someone close to you. I'll take good care of him."
Matt's expression hardened. “If you don't mind—who was it you lost?"
"My sister. Adriana. A man named Malcolm Vicari hurt her badly. She died a few weeks ago."
Analog SFF, October 2010 Page 9