What is the “Frost Line” of a Solar System?

A writer friend of mine asked if she could barrage me with astrophysics questions for a story she was writing. Of course, I said “fire away!” I will always help out a fellow writer! Her first question was “What is the Snow line?” (Which is also known as the Frost Line). The answer is pretty interesting, so I thought I would share it with you guys!

But before that, just in case you are unaware, I will cover a related topic: The Habitable Zone. From here, the Frost Line will make more sense.

Every star in our known universe has an area a specific distance from the star where water can exist as liquid, solid and a gas. This varies from star to star as it depends on the star itself: the size of the star, how hot the star is and which stage of life the star is currently in. This area is known as the “Habitable” zone, or the “Goldilocks Zone”, named because the planet is not too far away from the star, and not too close, but the perfect distance away for the only known prerequisite for life as we know it: Liquid water. Earth, very obviously, is in this zone, and no other planet is. Sort of.

Mars is on the very outskirts of the “optimistic” habitable zone. When I say “optimistic”, I mean that there is no definite distance for the inner edge and outer edge of the habitable zone. We can’t say the inner edge of the Habitable Zone is (for example) exactly 2.23560283 thousand miles from the Sun (totally wrong distance by the way, just making a point). The edges are blurry, for this reason. So, if we are optimistic in the size of the Habitable Zone, Mars is in it. For this reason, there are lakes of water ice (and I say water ice for a reason) on the poles , and during the Martian summer season, temperatures can reach highs of 27 degrees Celsius (pack your bags, England, Mars has hotter summers than we do!). This, of course, means that any ice in direct sunlight in the warmer areas will melt on Mars. So, why can’t Mars support life? I will cover that exact question in a future post but we are getting off topic here.

So, in conclusion, the habitable zone is an area around a star where a planet can retain liquid water on its surface, thus giving it the ability to have life.

So, what’s the Frost Line?

We know there is a point from the Sun where water ice will melt called the habitable zone, which has a central distance from the Sun of roughly 1.34 AU (1 AU, or astronomical unit, is the distance between the Sun and the Earth, it is often used to measure large distances in our solar system), but there is a distance from the Sun where it will not. For our Sun, it’s about 5 AU. That’s around the distance of Jupiter from the Sun! At this point water ice, even if it’s in direct sunlight, WILL NOT melt!

Well, I say “WILL NOT”, by that I mean it will not melt if you just leave it alone in direct sunlight. There are lots of things that could cause water ice to melt at 5AU, but that’s for another time.

The Frost Line and habitable zone distance will naturally change depending on the star. If it is a small star, the habitable zone and Frost Line will be a lot closer, and for really big stars, they will be further away from the star. The distances mentioned in this post only related to our little Sun. And yes, I said “little” because, astronomically speaking, our Sun is a little on the small side, believe it or not!

 

Well, I hope this post has been interesting. If something doesn’t make sense, leave a comment and I will help clear things up and also edit this post to make myself clearer.

Thanks for reading!