Planets and some questions

[Zixinus]

I have a tendency for making planets in my setting and I like adding senseless details.

My questions are as follows:

1. Is planet pressure proportional to gravity, that is, the planet's mass? Questions goes whether the atmosphere's composition is the same or different then Earth's.

2. Is there a handy planet generation software, preferably freeware, out there that I can use? I want to create planets, and having a map already gives me building blocks I can use.

3. How does air pressure (weight of atmosphere) effect life?

4. Does the ration of oxygen to nitrogen effect life? Does more oxygen mean richer or bigger lifeforms?

[Darth Raptor]

Atmospheric density and composition affects air pressure way more than planetary mass does... to a point. Small planetoids like Mercury or Pluto don't have enough gravity to hold an atmosphere, but I presume you're talking about terrestrial planets with a gravity of around 1.

For example, the air pressure on Venus implodes the probes we send. This, despite the fact that Venus has an almost identical mass and gravity to that of Terra. So obviously, it's a function of density and chemical composition. Life forms found on planets with very high atmospheric pressure would have peculiar anatomical and physiological adaptations. I'd assume they would be similar to creatures found in the greatest depths of the ocean: With bodies that are either almost insubstantial or extremely rigid. And passive respiratory systems. I don't imagine they'd get very big.

Higher concentrations of oxygen does allow animals to get larger. Vertebrates don't have to breathe as much and animals that don't actively breathe (like insects and spiders) can grow much larger than they do today. However, more oxygen in the atmosphere comes with its own unique set of problems: Oxidation damage to organic molecules, for one. But the biggest hazard will be the literally explosive atmosphere. A single lightning strike and the whole forest goes up in flames, regardless of how green or wet it is.

Our own planet went through a phase like that though, so it's not unrealistic or anything. Read up on the Carboniferous period for an idea of what a high-oxygen world would look like. Eagle-sized dragonflies and human head-sized proto-spiders. It's great.

[Admiral Valdemar]

I have a tendency for making planets in my setting and I like adding senseless details.

My questions are as follows:

1. Is planet pressure proportional to gravity, that is, the planet's mass? Questions goes whether the atmosphere's composition is the same or different then Earth's.

Depending on the make-up of the atmosphere and planet mass, yes. It should have a larger atmosphere if enough material is about and the gravity well strong enough.

2. Is there a handy planet generation software, preferably freeware, out there that I can use? I want to create planets, and having a map already gives me building blocks I can use.

Not that I know off the top of my head.

3. How does air pressure (weight of atmosphere) effect life?

Yes.

4. Does the ration of oxygen to nitrogen effect life? Does more oxygen mean richer or bigger lifeforms?

Yes. The Paleozoic era had dragonflies larger than eagles thanks to the high O2 content. This did also have the added side-effect of more fires in forests thanks to the explosive threat during thunderstorms.

[Junghalli]

3. How does air pressure (weight of atmosphere) effect life?

The higher the air pressure the less trouble an animal has getting oxygen from the air. Low atmosphere pressure is basically what you have on a high mountain. On a thin atmosphere planet you'd probably have critters with huge lungs and other such adaptations. A high pressure planet would make creatures with less efficient respiratory systems viable. High pressure would also make flight easier, as it's easier to get lift.

4. Does the ration of oxygen to nitrogen effect life? Does more oxygen mean richer or bigger lifeforms?

The more oxygen in the atmosphere the less efficient the respiratory systems of animals have to be for them to survive.

[Erik von Nein]

Higher concentrations of oxygen does allow animals to get larger. Vertebrates don't have to breathe as much and animals that don't actively breathe (like insects and spiders) can grow much larger than they do today.

Buh? They actively breath. In fact, their respitory system is part of the reason why they can't grow as large as vertebrates (as well as the diminish returns problem their exoskeletons have).

[Junghalli]

All other factors being equal, less massive planets will be less dense, will have thinner atmospheres, and will have less water to land (because water percentage increases faster than surface area with mass). I once made a table for what Earthlike planets of different masses might be like. Note that atmosphere here means atmosphere mass NOT necessarily sea level pressure. Also, I started with a 1 Earth mass planet with 15% more water and atmosphere than Earth, because according to Neil F. Comins's estimate that's about the amount of volatiles that was probably lost in the impact that formed the moon.

Mass: 1 Earth mass
Radius: 100% Earth's
Surface Area: 100% Earth's
Atmosphere: 115% Earth's
Sea Level: 755 meters higher

Mass: .9 Earth mass
Radius: 98% Earth's
Surface Area: 96.3% Earth's
Atmosphere: 103.5% Earth's
Sea Level: 262 meters higher

Mass: .8 Earth mass
Radius: 94.3% Earth's
Surface Area: 89.13% Earth's
Atmosphere: 92% Earth's
Sea Level: 230 meters higher

Mass: .7 Earth mass
Radius: 90% Earth's
Surface Area: 81.2% Earth's
Atmosphere: 80.5% Earth's
Sea Level: 40 meters higher

Mass: .6 Earth mass
Radius: 86% Earth's
Surface Area: 74% Earth's
Atmosphere: 69% Earth's
Sea Level: 180 meters lower

Mass: .5 Earth mass
Radius: 80% Earth's
Surface Area: 64% Earth's
Atmosphere: 57.5% Earth's
Sea Level: 258 meters lower

Mass: .4 Earth mass
Radius: 73% Earth's
Surface Area: 53.4% Earth's
Atmosphere: 46% Earth's
Sea Level: 305 meters lower

You can get a feel for what the alterations in sea level would do to by playing around with this site, though of course the underlying shape of the continental plateaus and ocean basins would matter more than anything else.

http://merkel.zoneo.net/Topo/Applet/

[Junghalli]

Edit: that should be sea level "higher/lower than Earth's".

[Darth Yoshi]

Invertebrate respiration doesn't involve muscles, as opposed to lungs or gills, both of which require pumping to cycle oxygen and CO2.

[Darth Raptor]

Buh? They actively breath. In fact, their respitory system is part of the reason why they can't grow as large as vertebrates (as well as the diminish returns problem their exoskeletons have).

When I think of "active breathing" I think of a vertebrate expanding and contracting their lungs to forcibly move air in and out, not just passive ventilation that exposes tissues to the open air directly (insects) or modified gills that do gas exchange as the air passes over them (arachnids).

[Johonebesus]

A denser atmosphere also makes flight easier.

[Sikon]

1. Is planet pressure proportional to gravity, that is, the planet's mass? Questions goes whether the atmosphere's composition is the same or different then Earth's.

If there is given mass of atmosphere per unit area of the planet's surface, the resulting pressure at ground level is proportional to the planet's gravity.

How much mass of atmosphere exists in the first place depends on far more.

Escape velocity as determined by size and density determines whether or not an extraterrestrial body can hold onto an atmosphere, for particular molecular weight gases at a given temperature range. However, generally a planet does have enough mass, enough gravity, and sufficient escape velocity, making that not the issue. Rather, whether a planet actually has an atmosphere or the particular mass of its atmosphere depends on additional factors, such as the planet's history.

For example, Venus has an atmosphere two orders of magnitude more massive than Earth's atmosphere, despite both planets having almost the same mass, but that's because hot Venus is closer to the sun and had huge quantities of carbonates in rocks broken down by temperature to produce its thick mostly carbon dioxide atmosphere.

As another example, the planet Mercury masses more than Saturn's moon Titan. Yet the moon Titan has even greater atmospheric pressure at ground level than Earth's atmosphere, while meanwhile the planet Mercury has zero atmosphere aside from trace deviations from the vacuum. The mass and escape velocity of Mercury could hold an atmosphere providing greater atmospheric pressure than Titan, if not for Mercury's proximity to the sun, which resulted in gases being blown away. Titan is just 2% of earth's mass.

Too high temperature dissipates an atmosphere over time. Too low temperature freezes it out.

One general rule is that an excessively low mass extraterrestrial body can't hold onto an atmosphere, although that's unlikely to be the issue for an object massive enough to be called a planet.

The size and density of an extraterrestrial body determines its escape velocity. If that is too low, gas can escape into space over too short of a timeframe. For example, a too small moon has too little gravity.

Temperature and the molecular weight of the gas of concern are other factors. For example, with an escape velocity of 11 km/s, earth's gravity is sufficient to hold onto gases such as oxygen and nitrogen practically forever. Yet helium with its much lower molecular weight is such that not even earth's escape velocity can keep it so well in the geological long-term (aside from the slight rate of replenishment from underground radioactive decay historically maintaining trace amounts at around the part per million level).

Different molecules in a gas have different velocities at a given time, following an exponential distribution where nearly all have velocities within a typical range (affected by temperature and molecular weight) but a statistical few have unusually high velocities at a given moment.

For example, there have been calculations in studies relating that to earth's Moon, whether in regard to how long oxygen gas leaked from hypothetical large lunar settlements would stay around disrupting the vacuum (e.g. almost forever in human terms but not in geological terms), or in regard to what slight degree of lunar atmosphere would be created if terraforming comets were crashed into the Moon.

Any body which should truly be called a planet masses at least a comparable amount to bodies such as Saturn's moon Titan, if not far more.* As a result, basically any planet could hold onto a massive atmosphere resulting in high atmospheric pressure. Whether or not it actually does so depends on factors like its distance from its star, past history, etc.

* The "planet" Pluto masses several times less than Earth's Moon or an order of magnitude less than Saturn's moon Titan. Being actually smaller than the recently discovered trans-Neptunian object Eris, Pluto has been reclassified by the International Astronomical Union. (And there are believed to be more objects in the little-known reaches of the outer solar system of comparable mass to Pluto, even beyond those recently discovered). Let's assume the term planet refers to more massive bodies, not small objects like Pluto but rather large bodies more like Mercury, Mars, etc.

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Incidentally, creatures the size of dragons could actually fly on a planetoid if it had both high atmospheric pressure and low gravity (like Titan in that one regard), e.g. helped by high oxygen concentration. For gases of a given molecular weight, high pressure provides high density, not needing to be nearly as dense as water but still denser than terrestrial air. As has been pointed out by other posts above, higher atmospheric pressure makes flight easier. To add to that, if there was simultaneously both a few times greater air density and a few times lesser gravity, the net effect would be even far greater than either factor by itself. A large creature with a relatively high ratio of mass to wing surface area could fly.

[Winston Blake]

Regarding high-pressure life: Venus' surface pressure is roughly that found at 900m ocean depth. The bathyal zone starts at 1000m, yet life has no trouble surviving there.

Because of the lack of light, some species do not have eyes, but one of the species that does is the viperfish. Many forms of nekton live in the bathyal zone, such as squid, large whales, and octopuses, but this zone is difficult for fish to live in. Sponges, brachiopods, sea stars, and echinoids are also common in the bathyal zone.

Further, even though the mean surface temperature is over 460ºC, there may be small cold pockets that life could exist in. So it's not out of the question for a Venus-like planet to sustain life.

[OmegaGuy]

For software, there's an old Maxis game called SimEarth which is pretty good

[Zixinus]

Thanks for the answers, it does make more sense now.

Another question: does buoyancy (if I am using the right word) effect what gases are in a planet? Sikon mentioned that Earth can't keep helium in its atmosphere, but can keep the heavier stuff like oxygen and nitrogen. I understand that this is partly because lighter materials need less energy to escape the planet. But doesn't the fact that denser gases pushes away less dense gas also effect what atmosphere a planet can hold?

Also, again, is there a random planet generator I could use? I'm mostly interested in terrastrial planets, but non-terrastrial rocky planet may also interest me.

[Sikon]

Another question: does buoyancy (if I am using the right word) effect what gases are in a planet? Sikon mentioned that Earth can't keep helium in its atmosphere, but can keep the heavier stuff like oxygen and nitrogen. I understand that this is partly because lighter materials need less energy to escape the planet. But doesn't the fact that denser gases pushes away less dense gas also effect what atmosphere a planet can hold?

Trace amounts of light gases like hydrogen and helium do rise more than other, heavier gases to the top of earth's atmosphere, where the hydrogen tends to escape over time, but, for example, it is of note that there would be hydrogen at the top of the atmosphere whether denser gases were around or not. A way in which the higher molecular weight gases indirectly influence the escape of hydrogen is through their radiative properties, considering that temperature is influenced both by a planet's distance from its star and by its atmospheric composition.

Here are excerpts from one discussion:

Berkner and Marshall (1964, 1965, 1966, 1967) tried to estimate prebiotic O2 concentrations. They recognized that the net source of O2 was photolysis of H2O followed by escape of H to space [...]

Hydrogen escape can be limited either at the exobase (~500 km altitude) or at the homopause (~100 km altitude) [...]
Exobase—the altitude at which the atmosphere becomes collisionless [...]



Hydrogen escape from the exobase:

Earth’s upper atmosphere is rich in O2 (a good EUV absorber) and poor in CO2 (a good IR radiator)
-> the exosphere is hot [a high temperature because of that and earth's distance from the sun]

T =~ 1000 K (solar min)
=~ 2500 K (solar max)

Furthermore, H2 is broken apart into H atoms by reaction with hot O atoms

H2 + O -> H + OH
OH + O -> O2 + H

Escape of light H atoms is therefore relatively easy [low molecular weight] [...]

For Earth, there are 3 important H escape mechanisms:

• Jeans escape: thermal escape from the high-energy tail of the Maxwellian velocity distribution
• Charge exchange with hot H+ ions in the magnetosphere
• The polar wind

[...]

In order to escape, the kinetic energy of an escaping molecule must exceed its gravitational potential energy and it must be headed upwards and not suffer any collisions that would slow it down [...]

Most probable velocity: v_s = (2kT/m)^1/2
Evaluate for atomic H at T = 1000 K
v_s = 4.07 km/s
Compare with escape velocity [at this altitude]
v_esc = 10.8 km/s
These numbers are not too different
-> an appreciable number of H atoms can escape [those which obtain unusually high velocity compared to the average]

If the exospheric temperature is high [such as it is for earth], then Jeans’ escape is efficient and hydrogen is easily lost

If the exospheric temperature is low [such as for a planet more distant from its star], then hydrogen escape may be bottled up at the exobase

There are also other nonthermal H escape processes that can operate [...]

[Lightweight hydrogen rises more to the top of the atmosphere than other gases:]

From here

The above illustrates major factors: atmospheric temperature (aside from nonthermal mechanisms like the solar wind which also depend on energy supplied), the molecular weight of the gas of concern, and the planet's escape velocity.

There's a tendency for a planet to either have an atmosphere mostly of hydrogen & helium (Jupiter, Saturn, etc) or to have an atmosphere with almost none of those two gases (Earth, Mars, etc), depending on whether the planet is capable of holding onto them.

Example:

A massive planet with a high escape velocity, the atmosphere of the gas giant Jupiter has both heavier gases such as nitrogen and the lighter gases of hydrogen & helium. However, since hydrogen and helium are by far the most common elements in the gas cloud forming a star system, Jupiter's ability to capture all gases made its massive atmosphere have far more of them than other gases. In principle, Jupiter could have held onto an atmosphere with as much nitrogen as hydrogen, but, in practice, that didn't happen since hydrogen is a far more common element. Hydrogen is multiple orders of magnitude more common in the universe, the sun, and, most specifically, the cloud that formed the sun and the rest of the solar system. Jupiter's atmosphere has a huge amount of nitrogen, a total mass far more than in earth's atmosphere, but it isn't as noticeable since Jupiter has managed to hold onto an even far greater amount of hydrogen.

[Zixinus]

Thanks Sikon, as always, you leave no detail unanswered.

[Jadeite]

Thanks for the answers, it does make more sense now.

Another question: does buoyancy (if I am using the right word) effect what gases are in a planet? Sikon mentioned that Earth can't keep helium in its atmosphere, but can keep the heavier stuff like oxygen and nitrogen. I understand that this is partly because lighter materials need less energy to escape the planet. But doesn't the fact that denser gases pushes away less dense gas also effect what atmosphere a planet can hold?

Also, again, is there a random planet generator I could use? I'm mostly interested in terrastrial planets, but non-terrastrial rocky planet may also interest me.

You could try AstroSynthesis. It's used mostly for creating star systems, but it does have planet generation. This screenshot shows what variables there are for planets.