The ebb of rivers -- the paleo-river elevation transition due to the
decline of greenhouse effect on early Mars
Abstract
Long term climate change on early Mars is characterized by a shift in
the spatial distribution of rivers and lakes. Geological datasets
suggest earlier paleo-rivers prefer higher surface elevations compared
to rivers that formed later (Kite, 2019). On the other hand, modeling
work also suggest a transition of surface lapse rate that comes with
atmospheric escape throughout the Martian history (Wordsworth, 2016).
The surface lapse rate follows the atmospheric lapse rate, which is
close to dry adiabatic, when the CO2 atmosphere is
thick, but decouples when the atmosphere is thin. Figuring out the
surface temperature distribution on early Mars is critical, because it
tells us where the water sources from ice/snowmelt would have been
during warming episodes. We use the MarsWRF GCM to explore the
transition of river-forming climates. We assume the atmosphere is
CO2-only, but allow additional greenhouse warming by a
gray gas scheme. To simplify the relation between elevation and surface
temperature, we set 0 obliquity and include simulations with both
idealized topography and real topography. The range of surface pressure
is between 0.01 bar and 2 bar. We use a surface energy budget framework
to analyze outputs (Fig. 2). Under the framework, variations in surface
emission LWs correspond to surface temperature
variations. We find greenhouse heating LWa is the only
term that scales with surface temperature under high
PCO2, in contrast to predictions from the previous
literature that sensible heat SH was the cause of the regime transition
(Wordsworth, 2016). This conclusion does not change with switching to
realistic topography or switching CO2 radiation to a
gray gas scheme. Under the low Ps but high-optical-depth
κ gray gas case, the surface lapse rate still follows the atmosphere, so
the regime transition can be attributed to the evolution of greenhouse
gases other than CO2. In future, we will add a surface
liquid water potential algorithm to link the surface energy balance to
paleo-river observations. Assuming surface liquid water is formed during
transient ice-melting period, surface liquid water potential can be
calculated from the Ts and Ps
distributions. The output will be compared with different historical
epochs to find the best-fit scenario with both CO2 and
non-CO2 greenhouse forcing.