Zarmina (Gliese 581g) - habitable planet tidally locked to a red dwarf

[jshoer]

I promise I won't be so rapid-fire with the finished maps in the future, but I need to get this one out of my queue so I can make a related WIP thread!

This is "Zarmina," a map inspired by the (unconfirmed) habitable exoplanet Gliese 581g. The planet orbits its red dwarf star so closely that we know it must be tidally locked, showing the same face towards its star at all times - like the Moon to the Earth. Rather than having liquid water on the surface along the twilight band at the terminator, though (a situation very familiar to Star Wars expanded universe fans!), this planet is in what's known as the "eyeball Earth" scenario. The circular area right under the star is a liquid ocean with continents and islands, and the rest of the planet is encased in ice.

Gravitational tides from the star also have the effect of pulling the rocky surface of the planet into an oblong shape, like a rugby ball. The planet has a tiny orbital eccentricity - from perturbations by the other planets in the system - which causes a periodic shift in the gravity force on the planet: slightly east to slightly west, and back again, every Zarminan day (about 37 Earth days). The combination of the periodic variation in stellar tide and the fact that the ocean is more mobile than rock makes dry land much more common in the center of the disc than near the edge, as we see in the map. (This is the decision about the map design about which I have the least confidence. But I just decided to go with it.)

There are five major geologic processes active on Zarmina:

In areas predominantly located to the west and east of the substellar point, the crust can stretch and crack open into concentric ring-like shapes as the planet rocks east and west – like parentheses around the center of the disc. Blocks of rock surrounded by these faults drop downward slightly into arc-shaped rifts called graben. These graben are the primary tectonic features on the planet.

You might notice, however, that there are a number of graben with orientations that don’t match the distribution described above. Instead of being centered to the east and west sides of the disc, and curving around the middle of the circle, they have a center point somewhere to the north. An explanation for these features involves true polar wander. Due to gravitational tides, volcanos, or other forces, the mass distribution of the planet’s crust changed enough that the entire world shifted. Its spin axis flipped around, over a geologically short period of time. Since Zarmina is predisposed to have land in the center of its disc, and ocean towards the edge, the land mass to the north of the disc must be the ancient substellar point. This instance of polar wander divides Zarminan geology into two major epochs.

Though Zarmina shows no evidence of plate tectonics, it does have a liquid core and mantle. Magma from the mantle can ooze its way to the surface wherever a “hot spot” coincides with a weak point in the crust, eventually building up a shield volcano or spatter cone and extruding lava flows. In the absence of tectonic plate collisions, volcanism is the only major mountain-building process on Zarmina. A few active volcanoes are even gradually building Hawai’ian-style islands in the ocean.

Unlike on Earth, many well-preserved impact craters are visible on Zarmina. Earth’s plate tectonics warp and change surface features fast enough to eliminate craters quickly. But on Zarmina, without plate tectonics, craters last a long time. Surface erosion from water and wind eventually smooths out the craters, though, especially the older ones from the first epoch. In addition, the thick atmosphere prevents bolides (meteors) below a critical size from reaching the surface, so impact craters on Zarmina are all larger than a certain minimum diameter.

Finally, there is erosion. The wind patterns on Zarmina move generally from the ice line in toward the center of the planet, where the red dwarf heats the surface and the air rises. Intervening terrain bends the major air currents to follow the topography. Air picks up moisture as it passes over the oceans; this moisture rains out when the air moves to higher altitudes over land. So, the coastal areas of the planet receive most of Zarmina’s rainfall – and endure most of the wind- and water-caused erosion.

I had a tremendous amount of fun developing this map. My usual process is more imaginative and artistic than scientific, though I do occasionally consider such phenomena as rain shadows and fault lines. This time, I came up with the basic structure of the world (land in the middle, sea at the edge) and a list of a couple geologic processes: graben formation, impact cratering, volcanism, erosion, and polar wander. Then I went through a couple iterations of applying them in sequence. Here is the geologic map I constructed and then worked from to create the hand-drawn pen-and-watercolor product:

The really awesome thing, from an artistic point of view, is that this process took a lot of the control out of my hands. I was watching the world develop, and I had only very general things to say about how that happened. It became harder and harder to decide, “I want a mountain there and a canyon there!” Another effect I observed was that sometimes I ended up with really cool-looking landforms…and then as soon as I applied my next set of geologic processes, I’d see those locations obliterated. Of course, equally interesting features would pop up somewhere else on the planet. I will have to do some more maps this way, and see how they come out. And I'll have to remember to save off the geologic map every few eons!

I also wrote a myth told by the people of this world.

Stay tuned. I'm going to work on a political map of a region on this planet next!

[a.coldyham]

That myth was, frankly, beautiful to read. The sense of wonder and fear of the unknown really work well gj

[jshoer]

Thanks very much! I'm glad someone appreciates my writing. :-)

[su_liam]

I don't think there will be any great tendency for land to concentrate under the subsolar point. The solid mass of the planet will be stretched out into an ellipsoid with a long axis between the subsolar point and, I guess the antisolar point, but the water will be even more easily stretched. Larry Niven's Jinx has the same problem only more so. If water is willing to stretch, air is positively clamoring to.

That said, if a large continent were to form near the subsolar point and if such a continent represented a mass concentration, tidal forces would tend to try to center it over the subsolar point and maintain it in that position over time. So a configuration like this could form and persist for a significant period of time, but given continental drift, it's likely to be a geologically fairly short-lived situation. As soon as the supercontinent breaks up, as they cyclically do, the net tidal forces from the various mass concentrations will no longer act as predictably. Another problem is that I'm not sure that continents are actually mascons. That's debatable. While continental crust is a thicker spot it's also less dense than oceanic crust. I'm not sure how that works out.

I'm sorry to poop on the party not just 'cause it makes me look quite the jerk hole, but because the same problem ruins Niven's very interesting planet. A heavy gee world with a thick atmosphere and nearly moonlike environment at the (east and west) poles, what's not to like?

This also ruins the iceless antisolar pole that would have resulted if your assumptions has worked out. Ice should be more resistant to flow than water, but unfortunately not so much as rock.

The bare antisolar pole may be saved by climatology. The amount of rainfall fetch that makes it all the way to the deepest cold side might be limited. Don't know…

One thing added by this is the possibility that continental drift might tend to add a bit of chaos to the planet's rotation, helping to keep it nodding and librating.

[jshoer]

I'm sorry to poop on the party

Nah, I love it! I'm happy someone wanted to engage with me about the science.

I don't think there will be any great tendency for land to concentrate under the subsolar point. The solid mass of the planet will be stretched out into an ellipsoid with a long axis between the subsolar point and, I guess the antisolar point, but the water will be even more easily stretched.

Looking back on my thought process to construct the Zarmina map, I tend to agree with you. Either there would tend to be more land near the circumference of the ice boundary, because the water deforms into an even more oblate ellipsoid than the bedrock, or the land would be relatively evenly distributed. But, back when I formulated the world, I decided for whatever reason to go with a propensity for more land toward the center and I proceeded from there. I'll own up to it now as more of an assumption than a logical consequence.

I do want to address a couple points you made:

This also ruins the iceless antisolar pole that would have resulted if your assumptions has worked out.

I don't have an iceless antisolar point, just an iceless subsolar point. The entire rest of the planet, covering the terminator through the dark side, is icy.

It wasn't my idea - it came from a climate model of Gliese 581g by Raymond Pierrehumbert. If you're curious, here's the scientific paper, and here's a general article. I saw Dr. Pierrehumbert give a colloquium in grad school, an idea that inspired my map.

but given continental drift

While continental crust is a thicker spot it's also less dense than oceanic crust.

Zarmina has no plate tectonics! This means no midocean spreading ridges and subduction zones to drive plate motion, and it means no particular distinction between continental and oceanic crust. My hypothesis is that all the bedrock is basically basaltic, and most locally raised topography comes from volcano-building.

There is a limestone-like unit that formed on top of the basal when an area that was sitting under an ocean moved above sea level. That can only happen on this world during (geologically infrequent) episodes of true polar wander. In the geologic map above, a polar wander event happens exactly once. Polar wander also doesn't require any decoupling between the inner and outer layers of the planet, like happens on Europa - it could be a gross motion of the body. Some planetary scientists think this happened in Mars' history, when the Tharsis bulge formed. The Tharsis feature is just such a big lump on the side of the planet that it changed the principal axes of the planet's inertia, and Mars eventually tipped over until it was spinning about its major axis again. That could be exactly what happened on Zarmina. Come to think of it, that could be a mechanism to make land more concentrated in the center of the substellar ocean, after all, since tides would tend to align such a bulge towards the star!

Another problem is that I'm not sure that continents are actually mascons.

This issue is an aside, since Zarmina doesn't have continents in the terrestrial sense, but we could arrive at an answer by looking at a gravity map of the Earth.

Thanks for thinking about hypothetical exoplanets with me!