#paper tag

[ask-fantublings]

Wait who are the kids god parents??? Red 40 mentioned having a favourite one

the kids and their godparents!! ❤️💚🧡

[mewrnn]

Paper

I bought myself a cheap blue pencil on money that I found on the road lol

[timberwind]

It's a little sad to me just how limited the topographic map data we have of Titan is. The strips of yellow-brown across the map pictured above are the only areas where we have real radar altimetry, assembled as the Cassini spacecraft made fly-by measurements - everything in between is a best-guess interpolation. There's plenty of unanswered questions about the in-depth specifics of tectonism and cryovolcanism and the possibility that Titan's currently in a dry period and may have had bona-fide seas of liquid ethane (as opposed to the lakes we see today covering ~1% of the surface at the north pole) in the not too distant past here that we can barely even begin to tackle because our dataset is such a patchwork...

Of course, the Cassini data is of orders of magnitude better than being completely in the dark as to what things are like under Titan's global haze layer, as we were before the Cassini fly-bys and Huygens probe landing. The paper linked above applies a similar dataset sampling to Earth and successfully reconstructs the existence of all the continents, but does miss the Tibetan plateau and derive continental shapes that are... wonky. I really hope we send an orbiter with a suite of mapping instruments (2-micron IR LIDAR might actually work through the haze fairly well, plausibly with at least MOLA-level performance) sooner rather than later, so we can get data back before like 2050.

[poprockspillage]

hair paper

hair paper

[halebop-s-art]

The diagram for the classic Origami Goat, with a twist to make its horns curve ! It's from this video by by Origami Word, but it seems to be a pretty old/intemporal design that a lot of people have made. I could not find a diagram online so I made one.

(If you make her in yellow/orangy-yellow, then decorate her with red ribbons, you can make your own origami gävlebocken ! )

[hellspawnmotel]

girls day out

[timberwind]

You don't get hydrogen atmospheres around Sunlike stars because near a star solar radiation is enough to cause hydrogen to escape the upper atmosphere

I'm actually not quite sure this is true, even if you discount gas and ice giants! For worlds more massive than the Earth but less massive than Neptune we do see a significant number of close-in exoplanets on the far side of the radius gap (>2 R🜨) in the datasets of Kepler and TESS and the other exoplanet-finder missions - these worlds probably formed beyond the icelines of their parent stars, where they could accumulate significant (~1% bulk mass fraction) hydrogen envelopes and then later migrate inwards to more easily detectable orbits around their parent stars.

It's possible that worlds that form further out from their parent stars and then don't migrate inwards can retain a significant hydrogen envelope even at relatively low masses, although at the moment it's not entirely certain where that lower boundary might lie - it's even plausible that there's a smooth transition between bare icy cores and hydrogen-rich worlds. The great unconstrained factor here is to what extent hydrogen escape is driven by photoevaporation versus core driven envelope loss or impact erosion. If the former is more dominant as the erosion mechanism in most cases, low mass hydrogen-rich planets might be fairly common in the outer systems of many stars.

Notably there's a fairly respected school of thought in the study of the Martian palaeoclimate that claims Mars might have had some ~100 mbar of hydrogen in its otherwise CO2-dominated atmosphere in the Noachian era; this would constitute the missing-link greenhouse gas which reconciles the faintness of the young Sun ~4 Gyr ago with the clear geological evidence of large-scale liquid water flows in that period. A big issue there though is that there's distinct evidence from Perseverance's measurements at Gale Crater that at least some of the palaeolakes on Mars were oxidizing, which is difficult to reconcile with a reducing H2-rich atmosphere...

This is a fun paper on the arXiv: Towards the Minimum Inner Edge Distance of the Habitable Zone. In other words, given a Sunlike star, how close can we possibly get a habitable planet while gerrymandering all the other variables in our favor? The answer is about 0.4 AU, which corresponds to around 600% the insolation that the Earth receives. Toasty! You need a very dry planet, though, not only to prevent water vapor acting as a greenhouse gas, but to prevent a runaway moist greenhouse scenario. They even explore very hot atmospheres (with high pressure, to keep what water there is liquid), but note that DNA and amino acids become unstable above about 500 K.

I would have thought you would need a very thin atmosphere to reduce heat retention, but apparently if the atmosphere is too thin (<0.1 bar), the planet loses all its water in about a billion years. If pressure is too high, on the other hand, you don't get a proper water cycle (heat is too evenly distributed for precipitation to occur). But the dominant variable affecting where the inner edge of the CHZ is is really the amount of water in the atmosphere. Humidity would have to be around 1% (Earth averages 70% at sea level), and the albedo would still have to be decently high so that a good portion of solar energy was reflected back out into space. Clouds would help with that--but by the time you got enough moisture in the atmosphere to form clouds, you'd be getting enough to significantly heat the planet from water vapor acting as a greenhouse gas.

They only look at the inner edge of the CHZ because, as they point out in the introduction, in principle the outer edge can extend to infinity--a planet with sufficient internal heating from, say, the decay of radioactive elements, or tidal heating from a gas giant primary, could remain habitable even in deep space, if it had a sufficiently thick hydrogen envelope. You don't get hydrogen atmospheres around Sunlike stars because near a star solar radiation is enough to cause hydrogen to escape the upper atmosphere--which is obviously not an issue for a rogue planet.

I think in practice the hard limit for smaller stars would be further out than this, because of the tidal locking issue--slow rotators seem to be a bad fit for this kind of extremal climate. Maybe if it was a really small star, so the inner edge of the CHZ had one of those single-digit-day orbits? I kind of like that mental image: an enormous blood red sun that occupies like six and a half degrees of sky, thirteen times the size of the Sun in our own. A totally cloudless atmosphere, water confined to small patches here and there, and mostly near the poles. Because there's not enough water to properly hydrate the upper mantle, you have drip-and-plume tectonics with enormous mountainous uplands surrounded by flat sandy plains. Or even massive Mars-like uplands and huge shield volcanoes, heavily weathered by the thicker atmosphere, but still towering over the landscape. Eventually the interior of the planet may get so cold the carbon-silicate cycle stops and the atmosphere slowly leaks away, driven by the stellar wind of the close parent star.

But there are other issues with the habitability of red dwarf systems, so maybe not.

[linkedin-offficial]

im cooking chat

[ptewie]

he's learning

[turtleblogatlast]

Leo learns something about himself 🏳️‍⚧️

Based roughly on this old post.

Bonus:

[Leo is taking the fact that he was born biologically female simultaneously very well and also not so well but overall he’s mostly coping with the fact that it was Draxum that just essentially gave him the turtle equivalent of ‘The Talk’.]

[ask-fantublings]

Fan and Test Tube, who has been the worst baby sitter by far?

worst babysitters 🧪🪭

[mewrnn]

paper

[timberwind]

High obliquity (axial tilt) worlds are really very strange. The ice caps of terrestrial planets with substantial water inventories creep down from their high polar latitudes as you increase tilt, blanketing more and more of the surface in ice sheets right up until you reach a tipping point found somewhere around forty to fifty degrees. There, due to the way sunlight is distributed across the planet seasonally - like a sunbather turning over just twice a year - the ice caps suddenly migrate into a new configuration, hot polar summers driving the formation of an ice belt around the equator. We know Earth likely can't enter such a state (although a few papers claim otherwise) - the tidal influence of the Moon, coming in at just over one percent of the Earth's mass, is thought to damp down the spin-orbit resonances that drive axial wobble.

But then there's Mars with its two pebble-like asteroid-moons, axial tilt relatively free to wander compared to the Earth. From <20 to >50 degrees it lurches around on, as far as we can tell, a timescale of hundreds of kiloyears to a few megayears - really surprisingly short in geological time. Interestingly, there's evidence* in the paper above that those periods of higher 30-50+ degree obliquity have, taken cumulatively over the whole post-Noachian history of Mars, lasted for up to a billion years. It'd have been interesting if we got to see Mars during these relative ice ages, a Mars almost more white than red, or a Mars with its lowlands bisected by a great wall of ice... The (orographic?) snow west of the four big Tharsis volcanoes in the high-obliquity present-atmosphere model (bottom right) below is kind of fun I think.

Also kind of interesting that so much snow piles up on the high-altitude uplands in scenarios with more Earthlike atmospheric pressures (if not compositions), mostly independent of obliquity. Seems fairly plausible that you'd get a similar kind of scenario when terraforming Mars or Marslike exoplanets, ending up with deeply glaciated highlands feeding glacial rivers down into lakes and seas in the more temperate lowlands. It does things just a little more difficult though - ice is more reflective than dirt, and all that increased sunlight bounced back into space means you'd need to ramp up the heat you inject into the climate as you get into the pressure ranges where polar ice can sublimate into the atmosphere and fall as snow or risk atmospheric collapse... realistically of course you're doing that anyway with your soletta mirrors and all that. Food for thought, though.

(*)The paper looks for a specific kind of crater that probably only forms when a meteor strikes an ice sheet, and finds a fair chunk of them down in the midlatitudes and equatorial regions.

[ashwii]

I love this stupid game and its stupid dialog options, it's the best part

[tawnysoup]

CW DEATH, DEAD BODY, STRANGULATION, BLOOD, IMPLIED SELF-HARM (in case my tags aren't enough!!!! stay safe)

(Above you, you hear Loop trying to take a breath.)