# Descending Maxwell Montes

The tallest mountain on Venus is Maxwell Montes, soaring 11 kilometers above the mean ground level. There have been some thoughts that if we were to return to Venus with a new probe, this would be a good location, as the pressure and temperature are much lower than that at ground level.

Venus’s atmosphere is composed primarily of CO[sub]2[/sub], which means at the surface, at 90 atmospheres of pressure, it is supercritical. But at the mountain top is only half that, at 44 atmospheres. With the transition point at 73 atmospheres, at some point along the side of the mountain, you would go from a supercritical pressure to just really dense.

Something I have not seen is what this transition would be like. I’ve seen supercritical CO[sub]2[/sub] in laboratory conditions, it looks like a liquid.

So, my question is, after you have landed on the peak of this mountain, and you begin your descent (I suggest some warm weather clothing), as you pass the point where the pressure tops 73 atmospheres, what would the phenomen you encounter be like? Would it be something not really noticeable, as it goes from dense gas to supercritical liquid smoothly? Would it be subtle, but noticeable, like the distortions over a hot road? Or would it be very distinct, where it is very noticeable that you are going from a gas to a “liquid” state, as noticeable as if you went from land to water on earth?

I think this belongs in GQ, as there should be a definitive answer to this, but if there is not, and it makes more sense in IMHO or somewhere else, I have no objection.

The critical point of a fluid is not just a pressure, but also a temperature; for CO[sub]2[/sub], it’s around 31 °C. The phase transitions for a particular substance are usually thought of in terms of a so-called P-T diagram, which shows the pressures and temperatures at which the phase transitions occur. For carbon dioxide, you can find such a diagram here.

As you descend Maxwell Montes, the pressure and temperature will change (both increasing), tracing out a curve in that P-T diagram. If that curve happened to cross one of the phase boundaries in the P-T diagram, you would experience a sudden phase transition during your descent of the type you’re imagining. But given that the temperatures even at the top of Maxwell Montes are comfortably above 31 °C (I’m pretty sure), you would never cross one of these phase boundaries. So there would never actually be a distinct transition in the bulk properties of the carbon dioxide as you descended the mountain, unless somehow the temperature fell below 31°C at some point.

I’m not actually sure how pressure and temperature would be related to each other for the Venusian atmosphere. On Earth, you can frequently get away with using the adiabatic atmosphere model, but the power-law relationship between pressure and temperature relies on the ideal gas law, and I wouldn’t necessarily think that the dense Venusian atmosphere obeys the ideal gas law. Regardless of the exact form of this relationship, though, it seems pretty likely that you won’t be crossing a boundary into a CO[sub]2[/sub] “ocean” as you descend.

It’s not liquid CO[sub]2[/sub], it is supercritical CO[sub]2[/sub] that I am talking about. You are correct that you only get a liquid at lower temperatures, but if the temperature stays high, and the pressure increases, that is when you get supercritical CO[sub]2[/sub]. The phase diagram that you cited stops at the critical point. This one does not, and shows the pressure/temperature where supercritical CO[sub]2[/sub] exists, which is pressures above 72.9 bar and temperatures above 31 C.

Supercritical CO[sub]2[/sub] is like a liquid, in that it will have a density similar to liquid, but will still be able to expand or be compressed like a gas. Here is a video of heating up liquid CO[sub]2[/sub] at high pressure until it reaches the critical point, and phase transitions into supercritical CO[sub]2[/sub]. (There are many other videos on the right side there that are fascinating to watch.)

The reason I am interested is because I run a Sci-fi RPG about once a month, and I thought a Venus visit may be fun, and was looking for to see if my description of what I was thinking would be an interesting and unique phenomena is at all correct. I have read and watched quite a bit about Venus, and have never heard anyone refer to this phenomenon, though, so I do have to wonder if it something that hasn’t been thought of, or if there is something that I am missing.

The way I visualize it, is that because CO[sub]2[/sub] as a gas has a different refractive index than supercritical CO[sub]2[/sub], there would be a rather visually noticeable transition between them, even if there isn’t much of a density difference. My current description also includes gravity waves, slowly lapping the supercritical CO[sub]2[/sub] upon the “shores” of the mountain rising up above the “ocean.” Supercritical CO[sub]2[/sub] is also much more efficient at transferring heat than when it is in its gas phase, so I would think that there would be a severe temperature change at the phase transition as well.

My only experience with supercritical stuff was in Thermo lab where we took a tube of toluene and heated it to go supercritical.

It didn’t look like the YouTube videos of CO2 going supercritical. Starts with liquid on bottom, gas on top, normal boundary between them. As it heats up the boundary gets weird. Words like “foamy”, “boiling” don’t do it justice. It just got a weird airy look to it. This boundary expanded until the whole tube was like that.

I don’t see this boundary state in the CO2 videos. Just the liquid CO2 “evaporating”.

Perhaps in a large scale situation with a larger variation of temp/pressure this boundary zone would exist for CO2 and you’d have a sort of “foam” zone walking down a mountain.

That would actually be even cooler than waves.

I’m not planning on taking a trip there myself, so it’s not all that important that the description be accurate, but I do try to run my campaign nights as fairly hard sci-fi, so anything I can do to make it more accurate and realistic, while still literally “out of this world” is my goal.

I should preface this by saying that thermodynamics isn’t my area of expertise in physics (I’m more of a general relativity guy).

However, to the best of my understanding, that phase diagram is deeply misleading, which is why I had to search through Google for a while to find one that didn’t use that type of coloration. The one I posted doesn’t end at 31 C (as can be seen from the solid-liquid phase coexistence curve above it); but the boundary curve between the liquid and gaseous phases does in fact end at 31 C. See also this Q&A over on Chemistry StackExchange, which says much the same thing.

If one is well above the critical temperature and pressure, there are no sudden changes in the material properties of the CO[sub]2[/sub] when one crosses one of the dotted lines in your diagram. As you descend the mountain, and the temperature and pressure increase, the density of the fluid will change smoothly. There would not be a particular altitude at which you would go from “gas phase” to “supercritical phase”, because there isn’t really a change between the two in the same way there’s a change between “gas” and “liquid” are when you cross the phase boundary between them at lower temperatures. Moreover, properties of a particular substance in a pure fluid phase, such as its refractive index, are largely dependent on the density alone; so you wouldn’t actually “see” any boundary like you’re imagining.

Notice, by the way, that I never said “liquid” in my original answer; I said “fluid”, which was intentional. In science lingo, a “fluid” is any phase of matter where the molecules have no fixed structure, and includes both liquids and gases. If one stays above the critical temperature (or the critical pressure), one can’t really talk about a “phase transition” between liquid or gas or “supercritical” phases. One can only really talk about a fluid, whose properties vary continuously as the pressure and temperature change, without any sudden transitions corresponding to phase changes.

As an addendum to this: since the density does vary continuously as you descend, the speed of light will also vary continuously. This could give rise to effects like mirages, which, if the density gradients are large enough, would be visible to the naked eye.

Indeed; I’ve heard that, from the surface, Venus appears to be a bowl, with distant land curving upwards, and the “horizon” an infinite distance away.

To expand on that, every fluid has what’s called an “equation of state”, a relationship between density and pressure. A liquid is, by definition, a fluid whose equation of state is that density is a constant. A gas is a fluid whose equation of state is that density is proportional to pressure divided by temperature. Now, it happens that, in our ordinary experience, one or the other of those two equations of state is an excellent approximation for most of the substances we’re familiar with, and so we think of liquids and gases as being the only kinds of fluids possible. But there exist substances in the Universe with all sorts of other equations of state, some of which are almost like a liquid or almost like a gas, and some which are completely different from either.

I appreciate the replies, though I have to admit, I am a bit disappointed by the answers. I feel like I had deduced the existence of rainbows from hearing about the properties of water, and finding that it just doesn’t work that way. Would have been a cool scene.

Just to clarify, to make sure I have it right, that means that there is no discontinuous difference between a volume of CO[sub]2[/sub] at 72.8 atm, and one at 73 atm at temperatures well above the critical point?

Still could be a fun trip, and the odd atmospheric distortions could still make for a good scene. Visibility on Venus would be a bit shorter than that on earth due to the thickness of the atmosphere, but you should still be able to see a decent ways. It’s hard to get a good gauge on the few pictures that made it back from the surface of the planet, kinda hazy, but you can see to some sort of horizon.