Sciency Words: Frost Line (An A to Z Challenge Post)

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, F is for:


They say it’s cold in space. That’s not quite true. First off, how do you define what temperature means in a vacuum? That’s a much harder question that you might think.

But secondly—and more importantly for today’s post—a lot depends on where you are in space, because if you happen to be anywhere near a star, I guarantee you will feel the heat.

If you read enough scientific literature about space, you’ll eventually encounter the term “frost line,” and you’ll probably be able to guess from context what it means. Objects on one side of the line are close enough to the Sun for ice to melt (or more likely, sublimate), while objects on the other side are far enough away that ice remains frozen.

In our Solar System, the frost line is usually placed somewhere in the middle of the asteroid belt.

But there’s a lot of disagreement about where specifically the frost line is, in large part because there’s a lot of disagreement about how, specifically, the term should be defined.

Some astrophysicists define the frost line based on temperature conditions in the Solar System today. Others define it based on conditions from back when the Solar System was still forming. Also, there can be different frost lines for different chemicals, because the freezing point of water is different than that of methane or nitrogen or carbon dioxide.

This is a case of how some scientific terms are more clearly and precisely defined than others. And yet despite all the ambiguity about the frost line (or lines), it is still an incredibly useful term to help describe the layout of the Solar System. Which is why, if you read enough scientific literature about space, you are bound to come across this term eventually.

Next time on Sciency Words: A to Z Challenge, where did the word gravity come from?

16 thoughts on “Sciency Words: Frost Line (An A to Z Challenge Post)

  1. Maybe a good definition of the frost line is the distance from the sun where the energy received from the sun is too low to cause water ice to melt (or sublimate). Of course, the sun’s intensity is supposed to be gradually increasing, so the frost line is probably getting progressively further out over the evolution of the solar system.

    Liked by 1 person

    1. Yes, which is why I think it’s kind of pointless to define the frost line based on where it was when the solar system formed. As Sol starts to die, the frost line is going to be pushed out past Jupiter’s present position, and by then there’s not much point in saying the frost line is just past Mars simply because that’s where it was when the system formed.

      Liked by 2 people

      1. So the logic (as I understand it) is that objects that formed on one side of the frost line are geologically different from objects that formed on the other. I’m guessing that even today, a good planetary geologist would be able to tell the difference between a planetoid that formed as a rocky body verses a planetoid that formed as an icy/rocky mix and has since melted/sublimated. I think that’s why some people are more interested in where the frost line was originally rather than where it is now.


    1. That is an awesome question! I have no idea, but with some binary star systems, this could get really complicated, because you sometimes have stars moving in a figure-eight pattern around each other.

      When this A to Z thing is over, I’m going to have to look into that and see what I can find!

      Liked by 1 person

  2. Speaking of Frost lines,


    Most think the universe will Rip,
    some few think Crunch.
    Dark Energy is gaining grip.
    It’s most unlikely things will flip.
    Gravity’s always lacked the punch
    to stop the matter expanding,
    but whether it goes crunch or rip,
    there’s no such thing
    as a free lunch.

    * my update of “Fire and Ice.”

    Liked by 1 person

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