Space: Desert Planets

[Mailanka]

Here's my take on Desert planets.

You have to make some concessions in GURPS space to have a breathable atmosphere, presuming that's what you want.

[Anthony]

The problem with desert planets is mostly that the only means we know of for creating or sustaining a breathable atmosphere involves quite substantial amounts of water. Other than that, dry planets are perfectly possible, though they'd probably resemble Mars more than Arrakis.

[David Johnston2]

Originally Posted by Mailanka

Here's my take on Desert planets.

You have to make some concessions in GURPS space to have a breathable atmosphere, presuming that's what you want.

You overlooked the way to get a natural garden-world's hydrosphere lower than 45%. Tide lock it.

[Mailanka]

Originally Posted by Agemegos

I suggest adding a brief mention of Barsoom and of Mars as in Stanley Weinbaum's A Martian Odyssey, Heinlein's Red Planet, Stranger in a Strange Land etc. That Mars is not a small rock planet in Space terms.

You seem to assume that desert worlds have to be hot, but there's no reason that they can't be mild or even rather cold. Cold desert worlds won't turn into ice worlds because there is not enough water to freeze into enough ice to cover them over.

Right, so, okay, my intention here: This is for a GURPS Space Opera... concept that I'm working on, and the idea is that it emulates the sort of Captain-and-Crew sci-f that tends to populate TV. One element of that is that you have many "single biome" planets, and that those biomes imitate what we expect. You are completely correct that in a strict definition of "desert," we mean "lacking water," but to most people, when you say "Desert," they picture a hot, sweltering landscape of sand and stone. When you load up a video game or turn on a TV show and it features a planet with such a landscape, people would cal it "a desert planet."

Hence why my desert planet is hot. It's not because that's the only way to get a world with low hydrography, but because I'm trying to show how to emulate the sort of planets I'm talking about that you might find in such sci-fi. That means the two defining characteristics for the average space opera fan of a "desert planet" would be "It's hot" and "It's dry"

(The link I gave, of course, lacked that context).

In your GURPS Space example you don't mention the fact that with very little water vapour in their atmospheres desert planets have a greenhouse factor that is significantly lower than that of an Ocean or Garden world.

There is an abiotic process that produces oxygen and depletes water, which might lead to a desert world with a breathable atmosphere if you fuzz the edges of the science a bit. Water in the upper atmosphere gets dissociated into hydrogen and oxygen by UV light from the star. The hydrogen is lost to Jeans escape, but the oxygen is retained. In fact this process is too slow to account for the breathable atmosphere of Earth, but if you make the star a it bluer and the system a bit older, and confiscate everyone's pencils and envelopes you might get away with it.

The purpose of my GURPS Space examples are to show how you might go about creating such a world "by the book," following the GURPS Space rules. On the softer-science side, I have no problem with someone simply declaring a world a desert world, and on the harder science side, I don't have a problem with someone coming up with their own planet-generation system. The purpose of that side-bar is primarily to discuss such a world in the context of the GURPS rules.

I'm completely open to the idea of expanding a new section that details the scientific mechanics of other ways to plausibly have a world (I want this particular site to cater to a sliding scale of scientific accuracy), but I honestly lack the scientific acumen to expand such a section on my other biome-worlds.

EDIT: And I believe my side-bar actually accounts for all of the stuff you mention regarding the lack of moisture, at least inasmuch as the climate rules of GURPS space covers that, which is why the black-body temperature is so high. The lower hydrography does impact those numbers, IIRC, though it's been some months since I looked at the numbers. Though, as I said, that sidebar is about staying in the lines painted by GURPS Space, rather than necessarily digging into the actual mechanics of real planets.

[Mailanka]

Originally Posted by Agemegos

Sorry. I thought this thread was about the actual mechanics of real, or at least physically possible, planets. My mistake.

The thread itself can be about those things. I was just offering the context for my particular article. Someone is asking about desert planets, I'm showing some of the work I've done on it. It doesn't have to be exhaustive or all-encompassing. Many of the points you made are valid. But you also made them in regards to my specific article, so I was answering your points in regards to my specific article.

For the purposes of the OP and this thread, I suspect your points are perfectly valid, and a nice contribution in addition to and in parallel with my contribution.

[Flyndaran]

Deserts are defined as getting fewer than 10 inches of rainfall a year, I believe. I feel people here define it as hot and dry.
The problem is that truly dry regions on earth like the heart of the Atacama, where rain hasn't fallen for centuries, there is zero life, not even lichen or native bacteria.
If you want a true desert planet with water based life, then you need to throw realism out the window entirely.

[Not]

Can you get a water cycle with something other than oceans and rain? A shifting fog, perhaps?

[whswhs]

Originally Posted by Flyndaran

Deserts are defined as getting fewer than 10 inches of rainfall a year, I believe. I feel people here define it as hot and dry.
The problem is that truly dry regions on earth like the heart of the Atacama, where rain hasn't fallen for centuries, there is zero life, not even lichen or native bacteria.
If you want a true desert planet with water based life, then you need to throw realism out the window entirely.

How did we get from "less than 250 mm" to "zero"?

[Flyndaran]

Originally Posted by whswhs

How did we get from "less than 250 mm" to "zero"?

Depends on what one means when they say, "Desert Planet". Usually it means no oceans, but maybe a few lakes.
You can have life only so far from bodies of water rendering most of the planet dead.

[Nymdok]

Originally Posted by Flyndaran

Depends on what one means when they say, "Desert Planet". Usually it means no oceans, but maybe a few lakes.
You can have life only so far from bodies of water rendering most of the planet dead.

Well, a hotter planet would have a more vigorous atmosphere I would think which might stretch the range of rain clouds and allow for more spotty lakes?

Nymdok