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Frost line (astrophysics)
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<p>In <a href="/facts/Astronomy/0ArJVM1p">astronomy</a> or <a href="/facts/Planetary_science/ocX6rp0d">planetary science</a>, the frost line, also known as the snow line or ice line, is the minimum distance from the central <a href="/facts/Protostar/fhQYMSBA">protostar</a> of a <a href="/facts/Protoplanetary_disk/FtfSbP61">solar nebula</a> where the <a href="/facts/Temperature/QyGe4gmj">temperature</a> is low enough for <a href="/facts/Volatile_(astrogeology)/yYCe49qX">volatile</a> compounds such as <a href="/facts/Water/0lkXnJPK">water</a>, <a href="/facts/Ammonia/MtLtJFtz">ammonia</a>, <a href="/facts/Methane/kBHneYnh">methane</a>, <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a> and <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a> to <a href="/facts/Condense/yvmkI08A">condense</a> into <a href="/facts/Solid/hj5UsXno">solid</a> grains, which will allow their <a href="/facts/Accretion_(astrophysics)/atcWRsDP">accretion</a> into <a href="/facts/Planetesimal/PK60dofs">planetesimals</a>. Beyond the line, otherwise <a href="/facts/Gas/hVVAoodV">gaseous</a> compounds (which are much more abundant) can be quite easily condensed to allow formation of <a href="/facts/Gas_giant/pmF6KztG">gas</a> and <a href="/facts/Ice_giant/mJ4YTuup">ice</a> <a href="/facts/Giant_planet/PjSfFuUF">giants</a>; while within it, only heavier compounds can be accreted to form the typically much smaller <a href="/facts/Rocky_planet/jpMC1ULt">rocky planets</a>. </p><p>The term itself is <a href="/facts/Loanword/PWGYczgY">borrowed</a> from the notion of "<a href="/facts/Frost_line/PxJ2vdxI">frost line</a>" in <a href="/facts/Soil_science/nJ9SxpEE">soil science</a>, which describes the maximum depth from the surface that <a href="/facts/Groundwater/JYM2IDS2">groundwater</a> can freeze. </p><p>Each volatile substance has its own frost line (e.g. carbon monoxide, <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a>, and <a href="/facts/Argon/QX6NRH6d">argon</a>), so it is important to always specify which material's frost line is referred to, though omission is common, especially for the water frost line. A <a href="/facts/Tracer_gas/nY7qYC18">tracer gas</a> may be used for materials that are otherwise difficult to detect; for example <a href="/facts/Diazenylium/EkVIF1uJ">diazenylium</a> for carbon monoxide. </p>