When googling the cause for Venus’s high surface temperature I find only one explanation: solar energy trapped by the greenhouse effect from Venus’s thick, CO2 atmosphere.
But the surface of Venus also receives heat from below - heat from the 5000K core working its way outwards, by conduction at least, and maybe convection and volcanism. That heat is also trapped by the atmosphere. If fact, if the atmosphere was a perfect heat insulator the surface would eventually become molten, even with zero solar input.
What is the contribution of internal heating relative to solar heating? Is it orders of magnitude smaller - or some significant fraction? Suppose we install a magic shell just below the surface of Venus that blocks all internal heat escaping; the surface temperature would necessarily drop - but by how much? A fraction of a degree, or some measurable amount?
Most of the surface heating of Venus comes from the runaway greenhouse effect from its thick atmosphere.
There is also a theory (I’m not sure how widely accepted) that since Venus doesn’t have plate tectonics, that internal heat doesn’t escape much. Eventually all of that trapped heat causes the surface to melt, all of the heat escapes, the surface freezes again, and the cycle repeats. This theory is used to explain why Venus doesn’t have enough impact craters for its age.
But while the internal heat may or may not eventually cause the surface to melt, that is a very slow process and most of the surface heat comes from the greenhouse effect. And since the internal heat is trapped deeper within the planet, that further reduces its effect on the surface.
Although there is evidence of recent (and likely active) tectonics and vulcanism on the surface of Venus, virtually all of the atmospheric heating is due to solar insolation despite the high albedo (0.59) of Venus reflecting more sunlight as compared to Earth. The ‘greenhouse’ trapping of incoming radiation is sufficient to explain the persistent surface temperature of 460 °C, and although we have no subsurface observations or temperature measurements, it is clear that the Venus has a geological composition which is at least roughly similar to that of Earth, and thus would have a comparable interior heat transfer.
Stranger
Thanks for the replies. I does seem reasonable that most, or virtually all of Venus’s surface heating is due to solar heating, but I’m still curious about what contribution internal heat makes to the equation. With a more-than-4000C difference between Venus’s core and surface, there must be some heat flow - a value that could possibly be calculated or estimated. I’ve never seen a value mentioned in casual reading, so I can guess that it must be tiny. Still, it’s not intuitively obvious to me that it has to be insignificant; it would be interesting to know whether the Watts-per-square-meter is, say, one-tenth of solar input - or closer to one-billionth.
The temperature of the Earth’s core ranges from 4400 - 6000 degrees Celsius. I’m not clear on the details of Earth’s global energy budget, but it does appear to permit vast 2 km thick ice sheets in Antarctica.
We have a paucity of data from the surface of Venus, and none from even the immediate subsurface much less at any depth. Even if we had such data, the amount of thermal energy coming from the core and mantle to the surface would be in the noise in terms of measurement. Regardless of the (inferred) temperature of the core, that thermal energy has to be mediated through the mantle and the crust with much of it driving what tectonics and vulcanism are occurring, whereas the solar radiation being emitted by the Sun’s photosphere at 5,772 Kelvin and at an intensity of nearly twice of that at Earth is entering the atmosphere directly, and the ~40% that isn’t reflected accumulates in Venus’s supercritical fluid atmosphere. Even on Earth, where it is so cold that we have large zones of permanently frozen ice on both land and in the ocean (at least, now and for the next few decades), the thermal budget is driven by solar radiance with ~173,000 TW from sunlight and only <50 TW from radiogenic and primordial heating, so less than 0.03% is from internal heating. On Venus, with its much greater surface temperature, the percentage would be four or five orders of magnitude less.
Stranger
Thank you. Stranger’s numbers show quite convincingly the insignificance of internal heating.
The point about Antarctic ice sheets is well taken. It got me thinking about Earth’s internal temperature profile. I see that the temperature at the top of the mantle - under miles of insulating rock - is still cooler than the surface of Venus.
You may find this interesting. It doesn’t address the surface temperature of Venus (so it does nothing to answer the main point of this thread) but it does discuss the surface of Venus in general.
The article discusses things like even though Venus is in many ways similar to Earth, it is very different. For example, volcanos on Earth are formed primarily by plate tectonics. Venus has no moving plates, and possibly never did, and yet evidence for quite a bit of volcanism exists on the surface of Venus. So how did all of these volcanos form?
Another big question is impact craters, or more specifically a notable lack of old impact craters. The impact crates that we currently find on Venus are fairly pristine, and not worn down by erosion and volcanic activity, meaning that most of them formed after most of the volcanic activity had ceased. Many scientist think the lack of old impact craters is evidence of a massive resurfacing event. However, scientists disagree on what that event could have been or if it even happened. Theories range from a global melting of the entire surface to regular volcanism erasing the older craters, with all kinds of theories in between.
The long and short of it is that there is just a lot that we don’t know about Venus.
The harsh surface of Venus makes it difficult to send probes to the surface. So far, the longest lasting probe sent to the surface was the Soviet Vanera 13 probe from the 1980s, which lasted a whopping 127 minutes before the harsh environment killed it. Its twin, Vanera 14, only managed to survive for 57 minutes.
We know less about the surface of Venus than any other solid planetary-sized body in the Solar System. We arguably know more about the surface of Pluto and have better hypotheses for the processes which shaped the surface than we know of Venus.
Stranger