Category: Astronomy: Planetary Science

The Sky’s The Limit

On By rolcottIn Astronomy: Planetary Science, Space Travel, UncategorizedLeave a comment

Another meeting of the Acme Pizza and Science Society, at our usual big round table in Pizza Eddie’s place on the Acme Building’s second floor. (The table’s also used for after‑hours practical studies of applied statistics, “only don’t tell nobody, okay?“) It’s Eddie’s turn to announce the topic for the evening. “This one’s from my nephew, guys. How high up is the sky on Mars?”

General silence ensues, then Al throws in a chip. “Well, how high up is the sky on Earth?”

Being a pilot, Vinnie’s our aviation expert. “Depends on who’s defining ‘sky‘ and why they did that. I’m thinking ‘the sky’s the limit‘ and for me that’s the highest altitude I can get up to legal‑like. Private prop planes generally stay below 10,000 feet, commercial jets aren’t certified above 43,000 feet, private jets aren’t supposed to go above 51,000 feet.”

Eddie counters. “How about the Concorde? And those military high-flyers?”

“They’re special. The SST has, um, had unique engineering to let it go up to 60,000 feet ’cause they didn’t want sonic boom complaints from ground level. But it don’t fly no more anyhow. I’ve heard that the Air Force’s SR-71 could hit 85,000 feet but it got retired, too.”

Al’s not impressed. “All that’s legal stuff. There’s a helicopter flying on Mars but the FAA don’t make the rules there. What else we got?”

Geologist Kareem swallows his last bite of cheese melt. “How about the top of the troposphere? That’s the lowest layer of our atmosphere, the one where most of our weather and sunset colors happen. If you look at clouds in the sky, they’re inside the troposphere.”

“How high is that?”

“It expands with heating, so the top depends where you’re measuring. At the Equator it can be as high as 18½ kilometers; near a pole in local winter the top squeezes down to 6 kilometers or so. And to your next question — above the troposphere we’ve got the stratosphere that goes up to 50 kilometers. What’s that in feet, Sy?”

<drawing Old Reliable and screen-tapping…> “Says about 31.2 miles or 165,000 feet. Let’s keep things in kilometers from here on, okay?”

“Then you’ve got the mesosphere and the exosphere but the light scattering that gives us a blue sky happens below them so I’d say the sky stops at 50 kilometers.”

Al’s been rummaging through his astronomy magazines. “I read somewhere here that you’re not an astronaut unless you’ve gone past either 80 or 100 kilometers, which is weird with two cut‑offs. Who came up with those?”

Vinnie’s back in. “Who came up with the idea was a guy named von Kármán. One of the many Hungarians who came to the US in the 30s to get away from the Nazis. He did a bunch of advanced aircraft design work, helped found Aerojet and JPL. Anyway, he said the boundary between aeronautics and astronautics is how high you are when the atmosphere gets too thin for wings to keep you up with aerodynamic lift. Beyond that you need rockets or you’re in orbit or you fall down. He had equations and everything. For the Bell X‑2 he figured the threshold was around 52 miles up. What’s that in kilometers, Sy?”

“About 84.”

“So that’s where the 80 comes from. NASA liked that number for their astronauts but the Europeans rounded it up to 100. Politics, I suppose. Do von Kármán’s equations apply to Mars as well as Earth?”

“Now we’re getting somewhere, Vinnie. They do, sort of. It’s complicated, because there’s a four‑way tug‑of‑war going on. Your aircraft has gravity pulling you down, lift and centrifugal force pulling you up. Lift depends on the atmosphere’s density and your vehicle’s configuration. The fourth player is the kicker — frictional heat ruining the craft. Lift, centrifugal force and heating all get stronger with speed. Von Kármán based his calculations on the Bell X‑2’s configuration and heat‑management capabilities. Problem is, we’re not sending an X‑2 to Mars.”

“Can you re‑calibrate his equation to put a virtual X‑2 up there?”

“Hey, guys, I think someone did that. This magazine says the Karman line on Mars is 88 kilometers up.”

“Go tell your nephew, Eddie.”

~~ Rich Olcott

The Venetian Blind Problem

On By rolcottIn Astronomy: Planetary Science, Chemistry, Physics: Light and Relativity, UncategorizedLeave a comment

Susan Kim gives me the side‑eye. “Sy, I get real suspicious when someone shows me a graph with no axis markings. I’ve seen that ploy used too often by people pushing a bias — you don’t know what happens offstage either side and you don’t know whether an effect was large or small. Your animated chart was very impressive, how that big methane infrared absorption peak just happens to fill in the space between CO2 and H2O peaks. But how wide is the chart compared to the whole spectrum? Did you cherry‑pick a region that just happens to make your point?”

“Susan, how could you accuse me of such underhanded tactics? But I confess — you’re right, sort of. <more tapping on Old Reliable’s keyboard> The animation only covered the near‑IR wavelengths from 1.0 to 5.0 micrometers. Here’s the whole strip from 0.2 micrometers in the near UV, out to 70 micrometers in the far IR. Among other things, it explains the James Webb Space Telescope, right, Al?”

Spectrum of Earth’s atmosphere. Adapted
under the Creative Commons 3.0 license
from Robert Wohde’s work
with the HITRAN2004 spectroscopic database,

“I know the Webb’s set up for IR astronomy from space, Sy. Wait, does this graph say there’s too much water vapor blocking the galaxy’s IR and that’s why they’re putting the scope like millions of miles away out there?”

“Not quite. The mission designers’ problem was the Sun’s heat, not Earth’s water vapor. The solution was to use Earth itself to shield the device from the Sun’s IR emissions. The plan is to orbit the Webb around the Earth‑Sun L2 point, about a million miles further out along the Sun‑Earth line. Earth’s atmosphere being only 60 miles thick, most of it, the Webb will be quite safe from our water molecules. No, our steamy atmosphere’s only a problem for Earth‑based observatories that have to peer through a Venetian blind with a few missing slats at very specific wavelengths.”

“Don’t forget, guys, the water spectrum is a barrier in both directions. Wavelengths the astronomers want to look at can’t get in, but also Earth’s heat radiation at those wavelengths can’t get out. Our heat balance depends on the right amount of IR energy making it out through where those missing slats are. That’s where Sy’s chart comes in — it identifies the wavelengths under threat by trace gases that aren’t so trace any more.”

“And we’re back to your point, Susan. We have to look at the whole spectrum. I heard one pitch by a fossil fuel defender who based his whole argument on the 2.8‑micrometer CO2 peak. ‘It’s totally buried by water’s absorption,‘ he claimed. ‘Can’t possibly do us any further damage.’ True, so far as it goes, but he carefully ignored CO2‘s other absorption wavelengths. Pseudoscience charlatan, ought to be ashamed of himself. Methane’s not as strong an absorber as CO2, but its peaks are mostly in the right places to do us wrong. Worse, both gas concentrations are going up — CO2 is 1½ times what it was in Newton’s day, and methane is 2½ times higher.”

Data from EPA Climate Indicators Explorer

“Funny how they both go up together. I thought the CO2 thing was about humanity burning fossil fuels but you said methane operations came late to that game.”

“Right on both counts, Al. Researchers are still debating why methane’s risen so bad but I think they’re zeroing in on cow gas — belches and farts. By and large, industry has made the world’s population richer over the past two centuries. People who used to subsist on a grain diet can now afford to buy meat so we’ve expanded our herds. Better off is good, but there’s an environmental cost.”

Al gets a far-away look. “Both those gases have carbon in them, right? How about we burn methane without the carbon in, just straight hydrogen?”

Susan gets excited. “Several groups in our lab are working on exactly that possibility, Al. The 2H2+O2→2H2O reaction yields 30% more energy per oxygen atom than burning methane. We just need to figure out how to use hydrogen economically.”

~~ Rich Olcott

It’s A Trap!

On By rolcottIn Astronomy: Planetary Science, Physics: Light and Relativity, UncategorizedLeave a comment

Late morning, no-one else in his coffee shop so Al pulls up a chair. “OK, Susan, so coal’s a mess for ash and air pollution but also each carbon from coal gives us less energy than a carbon from methane. So why the muttering against switching to natural gas?”

“Big-ticket reasons, Al. One, natural gas isn’t pure methane. Mostly methane, sure, but depending on the source you get a whole collection of other things in the mix — heavier hydrocarbons like propane and butane, stinky sulfides and amines, even helium and mercury. Gas from a well has to be purified before you’d want it piped to your house.”

“Piped. Oh, yeah, pipelines. Probably a lot more efficient than coal transport but I see how they get problems, too.”

“Indeed they do. Pipelines break and leak and some idiots even use them for target practice. The worst kind of waste.”

“Yeah, when the oil gets out and ruins the land or someone’s water supply.”

“That’s bad locally, all right, but it’s when methane leaks out that the global damage starts.”

“Global?”

“Mm-hm, because methane’s a gas and mixes in with the rest of the atmosphere. If a pipeline or a truck or anything springs a leak in, say, Chicago, the methane molecules can go anywhere.”

“So?”

“So a couple of things. A decade in the atmosphere oxidizes most methane molecules to, guess what, CO2, the same problematic CO2 we get from burning coal. But before it degrades, methane’s an even bigger heat‑trapper than CO2 is.”

“Whaddaya mean, heat‑trapper?”

“Do you want to take this, Sy? It’s more Physics than Chemistry and besides, my mocha latte’s getting cold.”

“Hmm, there’s a bunch of moving parts in this. Al, you owe Susan a warm-up while I think.”

“Here ya go, Susan.”

“Thanks, Al. I’ll get you guys started. Why did my coffee get cold?”

“Good one, Susan. Al, it’s a universal principle — left to itself, energy spreads out. Heat finds ways to travel from a concentrated, high‑temperature source to low‑temperature absorbers. The exceptions occur when some extra process expends energy to pump heat in the other direction. So, that coffee naturally lost heat to the table by conduction, to the air by convection and to the general environment by radiation. The only thing that can stop those processes is perfect insulation. That’s the thing about the atmosphere.”

“Whoa, that’s a jump or three too fast.”

“OK, let’s follow a sunbeam aimed in the Earth’s direction. Its photons carry a wide range of energies, ultraviolet down to far infrared. On the way in, a UV photon hits an atmospheric ozone molecule and gets absorbed. No more UV photon but now the molecule is in an excited state. It calms down by joggling its neighbor molecules, that’s heat transfer, and maybe emitting a longer wavelength photon or two. Ozone filters out incoming UV and in the process spreads out the photon’s concentrated energy. What’s left in the sunbeam is visible and infrared light that gets down to us. You with me?”

“Makes sense so far.”

“Good. Next stage is that the visible and IR light heat the Earth, which then re-radiates the energy as infrared light mostly at longer wavelengths. The problem is that not all the IR gets out. Water molecules absorb some wavelengths in that range. Every absorption event means more heat distribution into the atmosphere when the molecule relaxes. Ocean evaporation maintains a huge number of IR‑blocking water molecules in the atmosphere.”

“I heard that ‘some‘ weasel‑word. Other wavelengths still make it through, right?”

I unholster Old Reliable, tap a few keys. “Here’s water’s absorption pattern in the mid‑to‑far‑infrared. A high peak means absorption centered at that wavelength. This is scaled per molecule per unit area, so double the molecules gives you double the absorption.”

Spectrum profiles from M. Etminan, et al., doi:10.1002/2016GL071930

“Lots of blank space between the peaks, though.”

“Which is where CO2 and methane get into the game. It’s like putting green and blue filters in front of a red one. With enough of those insulating molecules up there there’s no blank space and lots of imbalance from trapped heat.”

“Methane’s worse.”

“Lots worse.”

~~ Rich Olcott

Going from Worse to Bad

On By rolcottIn Astronomy: Planetary Science, Chemistry, Uncategorized3 Comments

Al delivers coffees to our table, then pauses. “Why methane?”

Susan Kim looks up from her mocha latte. “Sorry?”

“Why all the fuss about methane all of a sudden? I thought carbon dioxide was the baddie. Everybody’s switching from coal to natural gas which they say is just methane and now that’s a bad thing, too. I’m confused. You’re a chemist, unconfuse me.”

“You’re right, there’s mixed message out there. Here’s the bottom line. Methane’s bad, but coal’s a worse bad.”

“OK, but why?”

“Pass me a paper napkin so I can write down the chemical reactions. When we look at them in detail there’s all kinds of complicated reaction paths, but the overall processes are pretty simple. The burnable part of coal is carbon. In an efficient coal‑fired process what happens is
  C + O2 → CO2 + energy.
The C is carbon, of course and O2 is an oxygen molecule, two atoms linked together. Carbon atoms weigh 12 and each oxygen atoms weighs 16, so 12 grams of carbon produces 12+(2×16)=44 grams of CO2. Scaling up, 12 tons of carbon produces 44 tons of CO2 and so on. The energy scales up, too. and that’s what heats the boilers that make the steam that spins the turbines that make electricity.”

“I heard a couple of weasel words but go on to methane.”

“You caught them, eh? They’re important weasels and we’ll get to them. OK, methane is CH4 and its overall burn equation is
  CH4 + 2O2 → CO2 + 2H2O + energy.
Oxidizing those hydrogens releases about twice as much energy per carbon as the coal reaction does.”

“Already I see one big advantage for methane — more bang per CO2. So about those weasels…”

“Right. Well, coal isn’t just pure burnable carbon. It’s 350‑million‑year‑old trees and ferns and animal carcasses and swamp muck and mineral sediments, all pressure‑baked together. There’s sulfur and nitrogen in there, mixed in with nasty elements like mercury and arsenic.”

“The extras go up the smokestack along with the CO2, huh? Bad, for sure.”

“The good news is that coal-burning power plants are under the gun to clean up those emissions. The bad news is that effective mitigation technologies themselves cost energy. That lowers the net yield. But the inefficiency gets worse. Think coal trains.”

“Yeah, half the time I get held up on the way home by one of those hundred‑car strings, either full-up heading to the power plant or empties going back for another load.”

“Mm-hm. Transporting coal takes energy, and so does mining it and crushing it and pre‑treating to get rid of dirt and then taking care of the ashes. Even less net energy output per ton of smokestack CO2, even worse inefficiency. See why coal’s on its way out?”

“I guess all that didn’t matter when it was cheap to dig up and there wasn’t much competition.”

“You put your finger on it, Al. Coal got its foot in the door with steam engines 300 years ago when about the only other things you could burn were wood and whale oil. Crude oil got big in the mid‑1800s but it had to be refined and that made it expensive. Cheap natural gas wasn’t really a thing until fracking came along 50 years ago, but that brought a different set of issues.”

“Yeah, I’ve seen videos of people lighting their kitchen sink water on fire. And wasn’t there an earthquake thing in Oklahoma?”

“That was an interesting situation. Oklahoma’s in the middle of the continent, not a place you’d expect earthquakes, but they began experiencing flurries of shallow ones in 2011. The fracking process starts with water pumped at high pressure into gas-bearing strata to loosen things up. People suspected fracking was connected to the earthquakes. It was, but only indirectly. When fracked gas comes out of a well, water does, too. The rig operators pump that expelled water down old oil wells. Among other things, the state’s Corporation Commission is in charge of their hydrocarbon production. When the Commission ordered a 60% cut in the waste‑water down‑pumping, the earthquake rate dropped by 90%. Sometimes regulations are good things, huh?”

~~ Rich Olcott

A Diamond in The Sky with Lucy

On By rolcottIn Astronomy: Planetary Science, Space Travel, UncategorizedLeave a comment

Mid-afternoon coffee-and-scone time. As I step into his coffee shop Al’s quizzing Cathleen about something in one of his Astronomy magazines. “This Lucy space mission they just sent up, how come it looks like they’re shooting at either side of Jupiter instead of hitting it straight-on? And it’s got this crazy butterfly orbit that crosses the whole Solar System a couple of times. What sense does that make?”

Planned path of Lucy‘s mission to study Trojan asteroids (black dots).
After diagrams by NASA and Southwest Research Institute

“It shoots to either side because there’s interesting stuff out there. We think the Solar System started as a whirling disk of dust that gradually clumped together. The gravity from Jupiter’s clump scarfed up the lion’s share of the leftovers after the Sun coalesced. The good news is, not all of Jupiter’s hoard wound up in the planet. Some pieces made it to Jupiter’s orbit but then collected in the Trojan regions ahead and behind it. Looking at that material may teach us about the early Solar System.”

“Way out there? Why not just fall into Jupiter like everything else did?”

I do Physics, I can’t help but cut in. “It’s the many‑body problem in its simplest case, just the Sun, Jupiter and an asteroid in a three‑body interaction—”

Cathleen gives me a look. “Inappropriate physicsplaining, Sy, we’re talking Astronomy here. Al’s magazine is about locating and identifying objects in space. These asteroids happen to cluster in special locations roughly sixty degrees away from Jupiter.”

“But Al’s question was, ‘Why?‘ You told him why we’re sending Lucy to the Trojans, but Physics is why they exist and why that mission map looks so weird.”

“Good point, go ahead. OK with you, Al?”

“Sure.”

I unholster Old Reliable, my tricked‑out tablet, and start sketching on its screen. “OK, orange dot’s Jupiter, yellow dot’s the Sun. Calculating their motion is a two-body problem. Gravity pulls them together but centrifugal force pulls them apart. The forces balance when the two bodies orbit in ellipses around their common center of gravity. Jupiter’s ellipse is nearly a circle but it wobbles because the Sun orbits their center of gravity. Naturally, once Newton solved that problem people turned to the next harder one.”

“That’s where Lucy comes in?”

“Not yet, Al, we’ve still got those Trojan asteroids to account for. Suppose the Jupiter‑Sun system’s gravity captures an asteroid flying in from somewhere. Where will it settle down? Most places, one body dominates the gravitational field so the asteroid orbits that one. But suppose the asteroid finds a point where the two fields are equal.”

“Oh, like halfway between, right?”

“Between, Al, but not halfway.”

“Right, Cathleen. The Sun/Jupiter mass ratio and Newton’s inverse‑square law put the equal‑pull point a lot closer to Jupiter than to the Sun. If the asteroid found that point it would hang around forever or until it got nudged away. That’s Lagrange’s L1 point. There are two other balance points along the Sun‑Jupiter line. L2 is beyond Jupiter where the Sun’s gravity is even weaker. L3 is way on the other side of the Sun, a bit inside Jupiter’s orbit.”

“Hey, so those 60° points on the orbit, those are two more balances because they’re each the same distance from Jupiter and the Sun, right?”

“There you go, Al. L4 leads Jupiter and L5 runs behind. Lagrange published his 5‑point solution to the three‑body problem in 1762, just 250 years ago. The asteroids found Jupiter’s Trojan regions billions of years earlier.”

“We astronomers call the L4 cluster the Trojan camp and the L5 cluster the Greek camp, but that’s always bothered me. It’d be OK if we called the planet Zeus, but Jupiter’s a Roman god. Roman times were a millennium after classical Greece’s Trojan War so the names are just wrong.”

“I hadn’t thought about that, Cathleen, but you’re right. Anyway, back to Al’s diagram of Lucy’s journey. <activating Old Reliable’s ‘Animate’ function> Sorry, Al, but you’ve been misled. The magazine’s butterfly chart has Jupiter standing still. Here’s a stars-eye view. It’s more like the Trojans will come to Lucy than the reverse.”

~~ Rich Olcott

Space Potatoes

On By rolcottIn Astronomy: Planetary Science, Physics: Newton and Gravity, UncategorizedLeave a comment

“Uncle Sy, what’s the name of the Moon face that’s just a sliver?”

“It’s called a crescent, Teena, and it’s ‘phase,’ not ‘face’. Hear the z-sound?”

“Ah-hah, one of those spelling things, huh?”

“I’m afraid so. What brought that question up?”

“I was telling Bratty Brian about the Moon shadows and he said he saw a cartoon about something that punched a hole in the Moon and left just the sliver.”

“Not going to happen, Sweetie. Anything as big as the Moon, Mr Newton’s Law of Gravity says that it’ll be round, mostly, except for mountains and things.”

“Cause there’s something really heavy in the center?”

“No, and that’s probably what shocked people the most back in those days. They had Kings and Emperors, remember, and a Pope who led all the Christians in Europe. People expected everything to have some central figure in charge. That’s why they argued about whether the center of the Universe was the Earth or the Sun. Mr Newton showed that you don’t need anything at all at the center of things.”

“But then what pulls the things together?”

“The things themselves and the rules they follow. Remember the bird murmuration rules?”

“That was a long time ago, Uncle Sy. Umm… wasn’t one rule that each bird in the flock tries to stay about the same distance from all its neighbors?”

“Good memory. That was one of the rules. The others were to fly in the same general direction as everybody else and to try stay near the middle of the flock. Those three rules pretty much kept the whole flock together and protected most of the birds from predators. Mr Newton had simpler rules for rocks and things floating in space. His first rule was. ‘Keep going in the direction you’ve been going unless something pulls you in another direction.’ We call that inertia. The second rule explained why rocks fly differently than birds do.”

“Rocks don’t fly, Uncle Sy, they fall down.”

“Better to think of it as flying towards other things. Instead of the safe‑distance rule, Mr Newton said, ‘The closer two things are, the harder they pull together.’ Simple, huh?”

“Oh, like my magnet doggies.”

“Yes, exactly like that, except gravity always attracts. There’s no pushing away like magnets do when you turn one around. Suppose that back when the Solar System was being formed, two big rocks got close. What would happen?”

“They’d bang together.”

“And then?”

“They’d attract other rocks and more and more. Bangbangbangbang!”

“Right. What do you suppose happens to the energy from those bangs? Remember, we’re out in space so there’s no air to carry the sound waves away.”

“It’d break the rocks into smaller rocks. But the energy’s still there, just in smaller pieces, right?”

“The most broken-up energy is heat. What does that tell you?”

“The rock jumble must get … does it get hot enough to melt?”

“It can So now suppose there’s a blob of melted rock floating in space, and every atom in the melted rock is attracted to every other atom. Pretend you’re an atom out at one end of the blob.”

“I see as many atoms to one side as to the other so I’m gonna pull in towards the middle.”

“And so will all the other atoms. What shape is that going to make the blob?”

“Ooooh. Round like a planet. Or the Sun. Or the Moon!”

“So now tell me what would happen if someone punched a hole in the Moon?”

“All the crumbles at the crescent points would get pulled in towards the middle. It wouldn’t be a crescent any more!”

“Exactly. Mind you, if it doesn’t melt it may not be spherical. Melted stuff can only get round because molten atoms are free to move.”

“Are there not-round things in space?”

“Lots and lots. Small blobs couldn’t pull themselves spherical before freezing solid. They could be potato‑shaped, like the Martian moons Phobos and Deimos. Some rocks came together so gently that they didn’t melt. They just stuck together, like Asteroid Bennu where our OSIRIS-REx spacecraft sampled.”

“Space has surprising shapes, huh?”

“Space always surprises.”

~~ Rich Olcott

  • Thanks to Xander and Alex who asked the question.

Shadow Play

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

“Uncle Sy! Uncle Sy! You’re back! Didja see the red moon?”

“Hi, Teena. Good to be home. No, I didn’t get to see the red moon. Where I was it didn’t even get red.”

“I saw it! I saw it! Mommie put me to bed early so I could wake up to see it earrrly in the morning. I saw the red part but the Moon looked smaller than it does coming up from behind the houses and they said it was going to be sooo big but it wasn’t. Anyway, I didn’t stay awake. Why was it red?”

“Was it really red red like your favorite crayon?”

“Mm-no, more like orange-y red.”

“Sunset color, right?”

“Uh-huh. Was it sunset on the Moon?”

“Sort of. The sunsets we see on Earth are red mostly because our air absorbs the Sun’s blue light when we’re looking across the atmosphere. Only the red light gets through to our eyes. Remember the solar eclipse we saw, when the Moon came exactly between us and the Sun? Moon eclipses are inside out from that. We come between the Moon and the Sun. The only light getting past us has gone across our atmosphere just like sunset light does so it’s orange‑y red like a sunset.”

“Oooh … does the Sun ever get between us and the Moon?”

“Don’t worry, Sweetie. We’re far, far from the Sun. Mr Newton’s Laws of Motion say that we and the Moon will be waltzing out here for a long, long time.”

“Whee, we’re dancing around the Sun! MOMmie, Uncle Sy’s here!”

“Hi, Sis. You saw the eclipse, then.”

“Mm-hm. I realized while I was watching it that lunar phase shadows work differently from eclipses.”

“Oh? How so?”

“The shadow shapes are different, for one. The edge of the lunar phase shadow always passes through both poles. In a solar eclipse the shadow only reaches the poles at totality, and in a lunar eclipse there’s this almost straight shadow arc that marches across the whole face.”

“Interesting. You said ‘for one,’ so what else?”

“Eclipse shadows move in the wrong direction. Starting from a full moon, the shadow comes in from the right until you get to new moon, then it falls away to the left until you get back to full moon. Agreed?”

“I always get confused. I’ll take your word for it.”

“I looked it up. In two places. Anyhow, in both kinds of eclipse the shadow creeps from left to right. Just backwards from the lunar phases. I wonder if that has anything to do with ancient societies thinking that an eclipse is somehow evil.”

“Mommie, you know you’re not supposed to use words I don’t know unless you’re keeping secrets. What’s lunar faces?”

“Sorry, Teena, not secret. Lunar means Moon. Sy, can you show her phases on Old Reliable?”

“Sure. Here’s a quick sketch, Teena. Pretend that the little ball is the Moon going around the Earth. The Sun is off to the right. You know the Moon goes around the Earth and it always keeps the same side towards us, right?”

“That’s the Man In The Moon except it’s really mountains and stuff pointing at us.”

“That’s what the little triangle shows, like it’s his nose. See how sometimes it’s in the light and sometimes it’s in shadow? The big ball is what we see when the Moon is in each position. When the Man is facing straight towards the Sun we call that the Full Moon phase. When he’s completely in shadow that’s the New Moon phase. There’s names for other special positions, and all of the special positions are phases, OK?”

“I suppose you have a logical explanation for the shadows?”

“Sure, Sis. It’s all about where the shadow’s being cast and how the shadow caster is moving at the time. This diagram tells the story. Nearly everything in the Solar System runs counterclockwise—”

“Widdershins.”

“… Right. Every orbit runs left‑to‑right half the time, right‑to‑left the other half. The two kinds of eclipse happen in opposite halves. The geometry works out that we see both eclipse shadows move left‑to‑right. See?”

“Cool.”

~~ Rich Olcott

  • Thanks to Alex for the question, and to Lori for the shadow observation, which I hadn’t seen discussed before.

Listen to The Rock Music

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

“Kareem, how did we learn this stuff about the Earth’s insides? I mean, clouds and winds hundreds of miles down?”

“Fair question, Eddie. Jules Verne’s Voyage to The Center of The Earth couldn’t happen, because hollow volcanic tubes don’t go near far enough down. Drilling’s not useful for exploring the mantle — we’ve only gotten about six miles through the seafloor crust and that’s still probably a dozen miles up from where the mantle starts. Forget what you’ve seen in the comics or a movie, we won’t in our lifetimes have a sub‑like vehicle that can melt through rock, withstand million‑atmosphere pressures and swim through superheated lava. So what we do is oscillate, triangulate and calulate.”

“I’ll bite. Oscillate? Triangulate?”

“How we do earthquake chasing, Sy. For thousands of years, humanity experienced a quake as a local jolt. It wasn’t until the 1850s that we realized each quake incident has multiple components: a sudden rupture somewhere, the resulting shock that travels through the Earth to other locations, and maybe aftershocks from follow‑on ruptures. The shock is a whole train of waves. We used to record them on those big cylindrical seismograph drums with oscillating pens, but most stations have gone digital since the early 90s. More accurate data, easier to handle but less picturesque.”

“True. The TV weather guys love pics of the big cylinder with all the wiggly lines. How about the triangulations?”

“Suppose you feel an earthquake shock. How do you find out where the rupture occurred and how big it was?”

“Hard to do from one location. A really big one far away would give you the same blip as a small one close by. And you probably wouldn’t know how deep it was or what direction it came from. I guess you’d need to compare notes with some far‑away observers. The one closest to the rupture would have received the strongest signal.”

“Yeah, Sy, and if everybody kept track of when they felt the jolt then you could draw a map with the different times and that’d zero in on it. Uhh … three places and you’ve got it.”

The IRIS Global Seismic Network as of 2021.

“Three points makes a triangle, Eddie, you’ve just described triangulation. It’s a general principle — the more points of view you have to work with, the better the image. Seismic tomography is all about merging well‑characterized data from lots of stations. That’s why we built an international Global Seismic Network, 152 identically‑equipped stations. Here’s a map.”

“How ’bout that, Sy? Lotsa triangles, all over the world.”

“Reminds me of Feynman’s insight that an electron doesn’t take just one path from A to B, it takes all possible paths. Earthquake shocks must go around the Earth and through the Earth, so each of those stations could hear multiple wave trains from a strong‑enough earthquake. These days it’s all digital, I suppose, and tied together with high‑precision time‑ticks. Kareem, they must be able to localize within a millimeter.”

“Not really, Sy. There’s a complication the early seismologists discovered even with primitive timing and recording equipment. The waves don’t all travel at the same speed. Depending on what’s in the way some of them even stop.”

“Wait, these shocks are basically sound waves. Does sound go fast or slow or stop depending on where it is in the Earth?”

“Sonic physics, Sy. The stiffer the material the faster sound travels. About 1½ kilometer/second in water, 3 in stone and 6 in metals but those numbers vary with composition, temperature and pressure. Especially pressure, like millions of atmospheres near the center. In the early 1900s Mohorovičić saw two signals from the same quake. One P‑wave/S‑wave pair came direct through the crust, the second followed a bent path through some different material. That was our first clue that crust and mantle are distinct but they’re both solid.”

“P‑wave? S‑wave?”

“Like Push‑wave and Shake‑wave, Eddie. S‑waves shake side‑to‑side but fluids don’t shake so they block S‑waves. P‑waves pass right through. S‑waves traversing the LLSVP ‘clouds’ mean the regions are probably solid, but the waves don’t go as fast as a solid should carry them. It’s a strange world down there.”

~~ Rich Olcott

Mineral Winds

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

“Hey, if you guys are gonna use one of my tables at lunchtime, you oughtta order pizza.”

“Eddie, Eddie, you’re the one asking the questions that kept Kareem here into lunch hour. You owe him, seems to me.”

“Mmm, okayyy, but Sy, you can ante up. What can I get you, Kareem?”

“Nothing, thanks, unless you’ve got a halal oven.”

“Matter of fact I do, sort of. There’s a hotspot on the top left I only use for cheese melts so it should be OK for you. No pork spatters up there ever, that’s for sure.”

“A cheese melt would be fine, thanks.”

“Same for me, Eddie.”

<a few minutes later> “Here ya go, guys, straight outta the hotspot, lightly browned on top. Better let them sit a minute, you don’t wanna burn your mouth.”

“Thanks for the warning, Eddie.”

“Whatcha got there, a map?”

“Mm-hm, red dots for Earth’s sixty confirmed or proposed hotspots. Sy wanted to know more about the one that did a number on India.”

“What’s a hotspot? It’s like a big volcano, right?”

“Related but not quite. Most volcanoes are near where two plates are colliding. The classic case is the volcanoes along the western coastlines of the Americas The continents push westward and ride over Pacific seafloor plates, even break off slabs they shove down into the mantle where the heat melts them. The molten material squeezes up through cracks and escapes through volcanoes. Look where the dots are, though.”

 BOW Bowie  COB Cobb
 HAW Hawai’i
 ANA Anahim YEL Yellowstone

“Most of them aren’t anywhere near the edge of anything. Yellowstone and those guys in Africa are as far from an edge as you can get. And I don’t see any red dots near Japan or the Philippines which are both really active for volcanoes and earthquakes.”

“Right, Sy. The primary criterion for a hot spot is vulcanism far from plate edges. But there’s another characteristic that many share. It’s easiest to see in this close‑up. Start with the Hawai’i, Cobb and Bowie hotspots. Each one is at the head of a straight‑line chain of volcanoes, older to younger as you get closer to the hotspot. The chains even run parallel with each other. The Anahim and Yellowstone hotspots also have parallel chains but they go west‑to‑east which makes sense if the continents are moving westward. It all fits with the idea that hotspots have stable locations in the mantle, and they scribble volcanoes on the plates that move over them. That’s the basis for much of what we know about ocean‑plate motion. But.”

“But?”

“There’s controversy, of course. Magnetism surveys and isotope data seem to show that some hotspots may move or even flutter slowly in some geology‑timescale wind. I just read—”

“Hey, Kareem, I’ve decorated so many pizzas with pepperoni slices I see red‑dot patterns everywhere. Your world map looks like there’s a ring of red dots around Africa and a stripe across the south Pacific. Does that mean anything?”

“We think it does, Eddie, but we’re still figuring out what. A technique called seismic tomography has given us evidence for a pair of huge somethings called LLSVPs deep into the mantle and on opposite sides of the Earth. One, unofficially known as TUZO, underlies much of Africa and that hotspot ring you noticed. The other one, JASON, is below your hotspot stripe in the South Pacific. We know very little about them so far, just that they stick out in the tomograms and they’ve probably been more‑or‑less where they are for a billion years. And no, we have no idea why hotspots appear around the edge of TUZO but along the center of JASON.”

“What else is lurking down there?”

“Who knows? The textbook diagrams show the mantle as this inert homogeneous shell sitting between core and crust. But its upper part is fluid and six times deeper than our atmosphere. The new tech is showing us currents something like winds and objects something like clouds, all at geological sizes and timescales. Classical Geophysics down there has been like doing weather science but ignoring clouds, mountains and oceans. There’s weather beneath us and we’re just beginning to see it.”

~~ Rich Olcott

The Bad, Sad, Rad Red Dot

On By rolcottIn Astronomy: Planetary ScienceLeave a comment

“Was it just my imagination, Kareem, or was there some side action going on in that Africa‑Eurasia nutcracker video?”

“Always the trained observer, eh, Sy? You’re right, India had an interesting life in the same era. Here, let me bring up another Gplates video on Old Reliable. I need to show both sides of the world so I’ll switch from orthographic to Mollweide projection. Aannd I don’t need to go quite as far back, only to about 120 million years. Mmm, yeah, I’ll squeeze in some special markings, give me a sec… There. This slick enough for you?”

India’s 120-million-year journey
rendered using the GPlates system
and configuration data from Müller, et al., 2019, doi.org/10.1029/2018TC005462

“Busy, indeed. Care to read out what‑all is happening?”

“Sure. The big thing, of course, is the new ocean opening up around the Mid-Atlantic Rift. Further south, by 120 million years ago Gondwanaland had already calved off South America and Africa so all it had left was Madagascar, India, Australia and the Antarctic.”

“Somehow I’d always thought that Madagascar was tied to southern Africa, but I guess not.”

“Hasn’t been for 175 million years, and back then it was up level with where Kenya and Somalia are now. OK, what caught your eye east of Africa was India zoomin’ on up there three times faster than South America was drifting away from Africa. What I’ve done here, I locked the display onto Antarctica so everything’s moving relative to that even though Antarctica wandered around a bit, too. Then I marked a spot in central India, dialed back to 120 million years ago and started scanning forward by three‑million-year increments. At each step I put an orange dot over my marked spot. The dot sequence shows the subcontinent’s motion up to today. You can see it’s not a straight line and the points aren’t evenly spaced.”

“The uneven spacing and wiggly line say that India didn’t move at constant velocity.”

“Spoken like a true physicist.”

“And like any physicist who sees a velocity change I wonder about the forces that make that happen. That red dot, for instance, why did it break the pattern?”

“The red dot is special because it marks 66 million years ago. Does that date ring a bell with you?”

“Umm … Ah-hah! That was the meteor that killed off the dinosaurs, right?”

“The Chicxulub impactor had a lot to do with it, but that wasn’t the whole story. The dot is already far ahead of where it should have been considering India’s previous vector. Something happened that sped that plate along a good three million years before the meteor hit. We’re pretty sure the something was related to massive continental volcanic activity on India just south of where my dots are. The lava covered half the continent, six hundred thousand square miles. All that molten discharge undoubtedly came along with toxic gases that would have fouled the planet’s atmosphere and troubled the dinosaurs and everything else trying to breathe,”

“And what caused the volcanoes?”

“Really bad luck. There’s an active hotspot, we call it Réunion after the French island that’s on top of it at the moment. India just happened to pass right over the hotspot between 69 and 63 million years ago. The spot’s rising magma punched through the subcontinent’s bedrock, ran all over the place and maybe lubricated the passage. Then along comes the meteor when India’s only halfway across the hotspot. The asteroid doesn’t hit India but where it hits is almost as bad — just off the Mexican coast, almost exactly on the other side of the planet from where India is at the time. Imagine a massive ring of violent earthquakes sweeping around the Earth’s surface and coming to a focus smack in the middle of the volcanoes. That’s my shooting red line, except the shakers really come at India from every direction. The magma outflow rate doubles. Altogether, the discharge finally lays over 1015 metric tons of lava on top of poor India and whatever’s living there at the time.”

“Wow. Talk about your perfect storm.”

“The only good thing to come out of it is all the minerals in the magma left India with incredibly fertile soil.”

“That’s something.”

~~ Rich Olcott