A possible surface type that may form in the environments of M-dwarf planets
is sodium chloride dihydrate, or "hydrohalite" (NaCl β 2H2βO), which
can precipitate in bare sea ice at low temperatures. Unlike salt-free water
ice, hydrohalite is highly reflective in the near-infrared, where M-dwarf stars
emit strongly, making the effect of the interaction between hydrohalite and the
M-dwarf SED necessary to quantify. We carried out the first exploration of the
climatic effect of hydrohalite-induced salt-albedo feedback on extrasolar
planets, using a three-dimensional global climate model. Under fixed CO2β
conditions, rapidly-rotating habitable-zone M-dwarf planets receiving 65% or
less of the modern solar constant from their host stars exhibit cooler
temperatures when an albedo parameterization for hydrohalite is included in
climate simulations, compared to simulations without such a parameterization.
Differences in global mean surface temperature with and without this
parameterization increase as the instellation is lowered, which may increase
CO2β build-up requirements for habitable conditions on planets with active
carbon cycles. Synchronously-rotating habitable-zone M-dwarf planets appear
susceptible to salt-albedo feedback at higher levels of instellation (90% or
less of the modern solar constant) than planets with Earth-like rotation
periods, due to their cooler minimum day-side temperatures. These instellation
levels where hydrohalite seems most relevant correspond to several
recently-discovered potentially habitable M-dwarf planets, including Proxima
Centauri b, TRAPPIST-1e, and LHS 1140b, making an albedo parameterization for
hydrohalite of immediate importance in future climate simulations.Comment: 12 pages, 4 figures, 1 table, accepted for publication in the
Astrophysical Journa