Why are Mountaintops Cold? The Decorrelation of Surface Temperature and
Elevation Due to the Greenhouse Effect Weakening on Early Mars
Abstract
\justifying The wet-to-dry transition of Mars is recorded
by a changing distribution of rivers, which was interpreted as the
result of a decorrelation between surface temperature and elevation with
a thinner atmosphere. Here we use a climate model to re-examine the
interpretation. We find that the weakening of the greenhouse effect,
rather than atmospheric pressure, accounts for the decorrelation. The
decorrelation happens near surface pressure $ \sim 0.1$
bar for a pure $\mathrm{CO_2}$ atmosphere, or
longwave optical depth $\sim$ 1 for a gray gas. Under
the weak greenhouse limit, the surface is mainly heated by insolation
and thus surface temperature decorrelates with elevation. Under the
strong greenhouse limit, the surface is mainly heated by the atmosphere,
whose temperature is controlled by advection and convection over the
lowlands. Our results suggest that the correlation between river
distribution and elevation should be re-examined with different
greenhouse forcings and a low atmospheric pressure.