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44 votes

Why is Uranus called a "green planet"?

I think your issue is linguistic not astronomical. The way a language cuts up the space of colours into discrete names is varies strongly from one language to another, and even from one dialect to ...
James K's user avatar
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41 votes
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Can a tectonically inactive planet retain a long-term atmosphere?

Yes, a tectonically inactive planet can retain a long-term atmosphere. You make the connection that a lack of plate tectonics on a planet indicates a "dead" core and thus said planet has no ...
zephyr's user avatar
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41 votes

Can a planet with no atmosphere be orbited at extremely low altitudes?

Yes, but. Firstly the 400km orbits of the ISS are already extremely low, in comparison to the 6400km radius of the Earth. See https://what-if.xkcd.com/58/ for pictures of orbits. So if you rephrase ...
James K's user avatar
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36 votes
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Why is there so little nitrogen in the Martian and Venusian atmospheres?

Nitrogen, with a molecular mass of 28 atomic mass units, is too light to have remained in Mars's atmosphere. Carbon dioxide, with a molecular mass of 44 amu, could (and does) exist on Mars, but it is ...
David Hammen's user avatar
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32 votes
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Why is Venus's atmospheric pressure 75 times that of earth when carbon dioxide is only 1.5 times heavier than air?

Venus's atmosphere is very dense at the surface because Venus's atmosphere is very massive. The composition is nearly irrelevant. The pressure at Venus's surface is proportional to the mass of Venus's ...
David Hammen's user avatar
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32 votes
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Why is it impossible to infer the surface temperature of Venus by spectroscopy observation from earth?

The important difference is that the word "surface" has a different meaning depending on whether we're talking about the Sun or Venus. When we say "the surface of the Sun," we ...
Tanner Swett's user avatar
30 votes
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How do we get to know the total mass of an atmosphere?

There is a simple$^*$ way to know the total mass of the atmosphere: measuring the pressure it exerts on the surface, which necessarily integrate all of the atmosphere above ground level. If you take ...
Jean-Marie Prival's user avatar
29 votes
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Why do gas giants have clearly delineated surfaces, whereas the Earth's atmosphere fades into space?

In an isothermal atmosphere, the exponential scale height of the atmosphere is $$ h \sim \frac{k_\mathrm B T}{\mu g},$$ where $g$ is the gravitational field, $\mu$ is the mean mass of a particle and $...
ProfRob's user avatar
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25 votes
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Is Earth's Atmospheric Thickness Related to Cosmic Ray Radiation?

It won't be (and isn't) zero. To see that, you want the plot on a logarithmic scale. Thus there isn't really a question to answer. The attenuation of cosmic rays will follow some exponential decay ...
ProfRob's user avatar
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22 votes
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Why doesn't Titan have a carbon dioxide atmosphere?

The chemistry of Titan's atmosphere is complex, with reactions occurring between carbon dioxide, oxygen, carbon monoxide, hydroxl, and other compounds. This means that carbon dioxide production and ...
HDE 226868's user avatar
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18 votes

Why is Uranus called a "green planet"?

This answer looks into why Uranus is "green" or at least greener compared with Neptune's blue. Both planets have similar atmospheric compositions and would ordinarily be expected to show ...
Oscar Lanzi's user avatar
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18 votes

Why is it impossible to infer the surface temperature of Venus by spectroscopy observation from earth?

I think the problem is that spectroscopy at visible and infrared wavelengths simply can't penetrate the thick atmosphere of Venus. Thus the information comes from much higher in the atmosphere and not ...
ProfRob's user avatar
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15 votes
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What does forward modeling mean?

There are different ways to model something. From what you're asking, there are two main types of modeling: forward modeling and inverse modeling. Forward Modeling In this type of modeling, you have ...
zephyr's user avatar
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14 votes
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Do brown dwarfs have stripes?

A number of brown dwarfs have had 'surface maps' created using the light from those stars. In 2013, observations of 2MASS J22282889–4310262, a brown dwarf 35 light years away, were published. These ...
Dave Gremlin's user avatar
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14 votes

Can a planet with no atmosphere be orbited at extremely low altitudes?

An example of a planetary mass object or planemo that is almost airless is the Moon. It does have an atmosphere, but very, very thin. The Moon has an atmosphere so tenuous as to be nearly vacuum, ...
M. A. Golding's user avatar
13 votes

Why is there so little nitrogen in the Martian and Venusian atmospheres?

3.5% of all atmosphere in Venus still accounts for more partial pressure of nitrogen than on Earth. Venus has ~90bar pressure at the surface, 3.5% of them are ~3.2 bar nitrogen. Earth has only 0.8 bar ...
fraxinus's user avatar
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11 votes
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How does gravity affect pressure under water?

For a liquid, hydrostatic pressure is $\rho g h$ where $\rho$ is density (this is always the same for all water) g is gravitational acceleration and h is depth. The gravitational acceleration on ...
James K's user avatar
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11 votes

How does the Earth not lose its atmosphere to space?

Your assertion that our atmosphere doesn't escape is wrong. Helium and Hydrogen atoms have a low enough mass that they do have an escape velocity at the temperatures on the edge of our atmosphere. ...
UKMonkey's user avatar
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11 votes
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Why do the upper-atmosphere clouds of Venus appear to have that V shape?

This is supplementary to antispinward's excellent answer and provides additional sources and a visualization from the JAXA spacecraft Atasuki. It has been shamelessly borrowed from Would it be ...
uhoh's user avatar
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11 votes

How do we get to know the total mass of an atmosphere?

Suppose the atmosphere has a density that decays exponentially with height. e.g. $$ \rho = \rho_0 \exp[-h/h_0]\ ,$$ where $\rho_0$ is the density at some surface and $h_0$ is a characteristic height ...
ProfRob's user avatar
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11 votes
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Is there a formula for calculating surface temperature of a rocky world based on atmosphere and solar input?

Guillot et al. 2010 give a comparatively simple solution (their Eqn. 27) for a double-gray purely radiative atmosphere. It reproduces the equilibrium temperature in the optically thin limit. Robinson &...
AtmosphericPrisonEscape's user avatar
10 votes

How can clouds form in Jupiter's atmosphere of Hydrogen and Helium?

First, it's a great question. Mostly the answer is straight forward, so I can answer it, but it's still a great question. and I'll add a similar, but slightly more detailed picture to the one you ...
userLTK's user avatar
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10 votes
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At what depth on Mars would the atmosphere have equal pressure of that on Earth?

Nasa has a atmospheric model of mars: $$0.699 *e^{-0.00009 h} $$ A naive application of this model, solving for a pressure of 101 kPa, gives a depth of -55 km. The Armstrong limit depth (at which ...
James K's user avatar
  • 125k
10 votes
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What is the underwater temperature of Europa?

No, nothing on Europa could possibly be photosynthesizing as we know it. Jupiter doesn't emit light, and what it reflects from the sun is not enough, plus there's no significant amount of carbon ...
Nathanael Vetters's user avatar
10 votes

What was the first astronomical measurement which demonstrated that "the Earth is surrounded by vacuum"?

Torricelli, the inventor of the Mercury Barometer (~1644) argued that the height of the column of mercury was governed by atmospheric pressure (the "weight of the atmosphere" as he would have put it). ...
JonesTheAstronomer's user avatar
9 votes
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Why are Saturn bands much fainter than Jupiter's?

I'll give this one a shot. Correction is welcome. Upper atmosphere temperature. It's not just elements that give a planet color, but the temperature of elements. When we examine what a planet ...
userLTK's user avatar
  • 24.1k
9 votes

What does forward modeling mean?

Forward modeling is the use of a model in order to simulate an outcome. The problem of getting the model to produce data from the input is called the forward problem. The forward model takes certain ...
pablodf76's user avatar
  • 502
9 votes

How does the Earth not lose its atmosphere to space?

The underlying reason that the molecules of Earth's atmosphere do not fly away into the surrounding vacuum is that they are slower than the escape velocity, which would be 11200 m/s. The typical ...
Peter - Reinstate Monica's user avatar
9 votes

Acetylene at Uranus? What's JWST's evidence and current thinking of how it's possible?

I think part of the issue here is that you seem surprised to learn that JWST detected acetylene in the atmosphere of Uranus, as though this is both unexpected and merits a certain amount of skepticism ...
Peter Erwin's user avatar
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