mindstalk: (Default)
Still watching his Coursera. He's often funny.

Asteroids are the rubber duckies of the Solar System.

Many acronyms are "lame!" As a Caltech professor, I wonder what he thinks about SURF (Summer Undergraduate Research Fellowship.)

He's shameless about "eight planets", and how Pluto is the second biggest Kuiper Belt object. (Would be third if Neptune hadn't captured Triton.)

Bunch more, but I have a poor memory for funny moments, my own or other people's.

Doppler water hack

2017-Jun-01, Thursday 20:29
mindstalk: (science)
Say you want to look at Mars and know if it has water.

Basic spectroscopy review: Perfect blackbodies radiate in all wavelengths. Real blackbodies are made of atoms and molecules whose electrons absorb some wavelengths of light. So, it's simple: look at Mars, and if the wavelengths absorbed by water are missing, it has water!

Problem: *our* atmosphere has lots of water, so we'd expect to not see those wavelengths no matter what we looked at, because they'd be absorbed by our atmosphere's water.

You could get around that by putting a telescope In Spaaaaace, which makes everything cooler. But that wasn't much of an option in 1963.

Earth and Mars usually have up to relative motion, up to 30 km/s (said Mike Brown in a popular lecture.) That means Doppler shift of the light from Mars, changing its wavelength (and frequency). Not by much, one part in 10,000, but that's apparently enough to shift narrow absorbtion bands into transparent regions of our own atmosphere.

So, new plan! You look at Mars at quadrature or something, hoping not to see certain wavelengths, which are water-like but shifted: if they're there, our air lets them in, so if they're not, Mars never emitted them in the first place.

And if you do see them, then Mars doesn't have much water.

(Spoiler: we mostly see them, and Mars doesn't have much water. Well, on the atmosphere, or emitting light from the surface. Shit ton underneath it, but those are different observations.)

I thought I knew basic spectroscopy. But using Doppler shift as an information hack against our own atmosphere? That's new to me.

Martian water

2017-May-31, Wednesday 22:02
mindstalk: (Default)
So I've been watching videos in this Mike Brown/Caltech Coursera on the solar system. I think it's free for anyone to audit. Lots of cool stuff, even for this PlSc major; I wasn't that deep in it, and haven't kept up.

Like, apparently the top meter of Mars, above 60 degrees latitude, is like 30% water. How do we know? Cosmic rays knock neutrons out of nuclei, which hit more nuclei, either causing gamma rays or escaping on their own. Gamma ray frequencies tell us what nuclei they're from. Thermal as opposed to fast neutrons indicate the presence of something able to slow neutrons down, and that something is pretty much hydrogen. (No one expects Mars to be covered in ammonia...) So between those signals... we see lots of water signs.

More visually, the Phoenix lander dug. Ice! And we've observed some fresh meteor craters, which start out with a shiny white center, that goes way due to either sublimation or dust covering the ice, I'm not sure which.
mindstalk: (science)
Something I've been brooding about recently, especially after re-reading Neptune's Brood. It's rather amazing how much we think we know about the visible universe, just from peeking out at it from our little planet. Astronomers tell us the structure of the Sun, it's lifecycle, that of other rather different stars, explain novae and supernovae... I'm not saying there aren't any mysteries, but if you stop and think about it, the amount of stellar detail is absurd. We barely know that much about Earth, and the rest of our system's planets -- or planemos, planetary mass objects, big enough to force themselves to be a sphere and at least briefly geologically interesting -- are an endless wellspring of surprises, with more to learn every time we peek closer. What we can guess of exoplanets has not changed that.

But it does make sense in a way. Stars be definition are shining light and therefore information at us. (The Earth intercepts 2 kilograms of sunlight every second, for a gratuitous figure.) That very fact constrains them a lot -- they're big balls of hydrogen and helium and contaminants, under great forces of gravity and heat and light pressure and magnetism. If they weren't just so, they wouldn't fuse; if they weren't balanced between gravity and fusion, they wouldn't be stars. Like a high-speed fish or a rocket engine, there's not a lot of leeway. THey're plasma, so fluid, and tending to homogeneity. Finally, there's a lot of them, all shining at us; if we've populated a periodic table of stars types and age categories, it's probably because we've seen multiple examples of each. In short, it's a data-rich and highly constrained field.

I've just started reading the 1999 edition (from the library) of The New Solar System (Beatty et al.), rather younger (and much thicker) than the one I had as a planetary science major in college. It alerted me to one specific way in which stars fall over themselves to tell us about themselves: helioseismology is a thing. How can we track sound waves in the Sun? Not by listening to them, but by simply looking at them: sound waves in the Sun move the photosphere, the light-emitting 'surface', which causes Doppler shifts in the light. Voila! Your telescope is also a seismometer -- a global seismic detector net, even -- and we can apply seismological techniques to infer the structure and composition of the Sun. Heck, we can apparently even do this with other stars, which I don't remember hearing about before.

By contrast, planets are dark and opaque. They're potentially a lot more complicated: much more diverse in chemical composition, able to be gaseious, liquid, or solid, or all three (solid is the big one, allowing really wacky differentiation); able to support actual chemistry. The Himalayas probably causing long-term cooling and eventually the Ice Ages, by scrubbing CO2 out of the air via excessive weathering, *as well as* causing the monsoon cycle, is my favorite "who would have expected that?" example. And finally, there's not that many observable planets: 32 planemos, including all the large moons, Ceres, and recently named Kuiper belt objects; more like 27 within range of telescopes or fly-by craft so far, though New Horizons will add Pluto and Charon. So even if there is a regular pattern to planets, we could easily lack the data to perceive it.

Still, I'm impressed by how *nothing* closely resembles anything else. Moon and Mercury are different, all the gas giants are different, even Ganymede and Callisto are different. There are some similarities and discernable forces, but AFAIK nowhere can we say "yeah, given A, B is no surprise."

***

As for Kant, the book reminded me that he came up with the nebular hypothesis, that the solar system was formed from the collapse of a rotating nebula of gas. That wasn't his only work in physics. WP is more intriguing than informative, but Kant was cited by Lord Kelvin and Thomas Huxley -- not bad!

***

It seems odd to be reading a thick book from 1999, when there's been so much new work (e.g.), but this seems to be the latest edition from these authors, and I don't know what's similarly lay-comprehensive now. Wikipedia would be more up to date, but this has more depth and photos. And some stuff wouldn't go out of date, like history I hadn't appreciated: just how dead study of the planet was before the space program. NASA had trouble finding astronomers to get on board with them, Kuiper being one of the exceptions.

Also amusing to read the Sun description. Paraphrasing: "we think we know how the fusion works, but 2/3 of the neutrinos are missing. So we're either really wrong about the Sun, or about neutrinos. Leaning toward the latter, because helioseismology supports what we infer about the Sun." Ding!

Wow, I've never needed a geology or planetary science tag on LJ before. Astronomy, yes. Lame of me.

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