Could Uranus have a solid surface due to its internal composition and lack of internal heat?

Question

Uranus is the seventh planet from the Sun. It is classified as an Ice Giant, which, unlike a Gas Giant, such as Jupiter or Saturn, is composed mainly of elements heavier than Hydrogen or Helium.

Uranus is considered to have a mantle made of "ices" as supercritical fluids (Water, Ammonia and Methane), beneath an atmosphere made of molecular Hydrogen, Helium and Methane.

One of the striking particularities of Uranus, besides its axial tilt, is its significant lack of internal heat, to the point that it doesn't give off more heat than it receives from the sun. The temperature recorded at the tropopause is 49 K, making Uranus the coldest planet in the solar system.

This made me wonder, what if (since we never sent a probe inside Uranus' atmosphere) the temperature at the point where the atmosphere gives way to the icy mantle only rises to, say, 200-230 K? (Or do we actually have evidence ruling out this possibility?)

At those temperatures, water remains in a solid state regardless of the pressure. So I started to think about whether it is possible that Uranus could have a solid crust made of water ice, that would float on a mantle made of supercritical fluids (in the case of water, ice has a lesser density than supercritical water if I'm not wrong?). This crust could have liquid ammonia on it that would have its own cycle like water on Earth.

I am looking to create an Artist Impression and was wondering whether this environment I was thinking about is actually possible given the knowledge we have about planetary science, or is my imagination just going way too far?

Answer 1

Deep snow.

Close to the surface, the hydrogen atmosphere of Uranus is under high pressure, and so even though it is very cold, the dense atmosphere can hold a fair amount of gaseous water, ammonia, alkanes and hydrogen sulfide.

The amount that the atmosphere can hold varies with the season and temperature. When winter comes, the cooling atmosphere has less capacity to hold "dissolved" substances and these precipitate out as the solid: snow.

The onset of winter means heavy snows on Uranus, and the high winds produce spectacular shifting drifts. In summer the drifts regress, sublimating back into the warming atmosphere.

These drifts of particulate ice are themselves colorless but it is not black and white on the surface of Uranus. The drifts glow with electrical charge. The action of the wind on the snow produces immense electrical charges in the bone dry atmosphere but the poor conductivity of the various pure ices means that finding a path to ground for these charges is just about impossible. Instead, the charge re-equilibrates with the atmosphere. The pointed tops of snow formations produce St Elmo's fire, which in the hydrogen atmosphere produces shades of lavender, pink and magenta.

Answer 2

Possibly it might be a good idea to have only few small solid floating islands in the atmosphere of Uranus, that a space probe coud try to land one, instead of a solid surface over the entire planet.

In E.E. Smith's Triplanetary, Amazing Stories, Jan.- April 1934, and First Lensman (1950) Four wars with JOvians are mentioned. Mention of the adepts of North Polar Jupiter made me wonder whether there was supposed to be a giant floating Island in the Jovian atmosphere over the north pole in the Lensman series.

http://www.isfdb.org/cgi-bin/title.cgi?1918374[1]

http://www.isfdb.org/cgi-bin/title.cgi?1198[2]

I believe that the great red spot on Jupiter was once theorized to be a giant solid island floating in the atmosphere. And possibly it might be more plausible for Uranus to have an atmospheric feature which is smaller than any of those which have been studied and shown to be storms, that is some sort of solid floating island, than to be entirely covered by a solid crust. The Uranus Dark Spot is believed to be a cyclone about 1,300 kilometers by 2,700 kilometers. So any feature on Uranus which has not been proven to be a storm would probably be much smaller than the Uranus Dark Spot, but possibly large enough to have interesting stuff on it.

Part Two: Some Science Ficiton Stories with Solid Surfaces on the Giant Planets.

There have been a few early science fiction stories in which the giant planets Jupiter, Saturn, Uranus, and Neptune, had solid surfaces.

For example, "The Planet of Doubt" by Stanley G. Weinbaum, Astounding stories, October, 1935. Which is set on Uranus.

http://www.isfdb.org/cgi-bin/title.cgi?47540[3]

Another story is "We also Walk Dogs" by Robert Heinlein, Astounding Science Fiction, July, 1941, in which intelligent jovians who apparently live on a solid surface are mentioned.

http://www.isfdb.org/cgi-bin/title.cgi?46427[4]

In Isaac Asimov's "Not Final!", Astounding Science Fiction, October 1941, and "Victory Unintentional", Super Science Stories, August, 1942,

http://www.isfdb.org/cgi-bin/title.cgi?46442[5]

http://www.isfdb.org/cgi-bin/title.cgi?44199[6]

Jovians living on a solid surface were depicted.

In James Blish's "Bridge" Astounding Science Fiction, February 1952, a bridge to no where was built on the solid surface of Jupiter. Curiously, Blish coined the phrase "gas giant" in 1952, and so should have been aware of the lack of solid surface on Jupiter.

http://www.isfdb.org/cgi-bin/title.cgi?55942[7]

In Poul Anderson's "Call Me Joe", Astounding Science Fiction, April 1957, a solid surface on Jupiter is depicted.

http://www.isfdb.org/cgi-bin/title.cgi?55693[8]

And I can't help thinking that those were among the last good science fiction stories depicting solid surfaces on the giant planets. Certainly more and more popular astronomy books began to descriped the giant planets as without any solid surfaces as the 20th cnetury progressed, making it harder and harder for science fiction fans to accept such solid surfaces in stories.