Abstract
We are using radio occultation (RO) measurements from Mars Global
Surveyor to investigate the nighttime structure and dynamics in the
lower atmosphere of Mars. High-resolution temperature profiles retrieved
from the RO data contain unique information about nocturnal mixed layers
(NMLs) – detached layers of neutral stability that form at night in
response to radiative cooling by a water-ice cloud layer. Basic
properties of the NMLs and constraints on their spatial distribution and
seasonal evolution can be obtained through analysis of the RO profiles.
We have examined more than 3000 RO profiles in a latitude band centered
on the Phoenix landing site (234°E, 68°N), where nighttime water-ice
clouds were observed by the LIDAR instrument (Whiteway et al., Science
325, 68-70, 2009). NMLs appear routinely in the western hemisphere in RO
observations at 5 h local time from early summer of MY27. There is a
close resemblance in both thickness (a few km) and altitude (about 4 km
above the surface) to the cloud layer observed at the same local time by
the Phoenix LIDAR in MY29. The NMLs confirm that radiative cooling by
the Phoenix cloud is sufficient to trigger convective instability, as
predicted by a Large Eddy Simulation (Spiga et al., Nat. Geosci. 10,
652-657, 2017). We have also analyzed more than 800 RO profiles from the
northern tropics near summer solstice of MY28. Tropical NMLs are largely
confined to regions of elevated terrain, where the daytime convective
boundary layer is deep. At 4 h local time, the top of the NML is about
10 km below the peak of Olympus Mons. The spatial distribution of the
NMLs appears to be influenced by diverse processes ranging from
topographic circulations to planetary-scale thermal tides. In addition,
we are using a Mars Global Circulation Model and Large Eddy Simulations
to interpret the RO results. Goals of the modeling effort include: to
identify the atmospheric processes that control the formation of
nocturnal water ice clouds; to understand the spatial distribution of
the clouds and their evolution with time of day and season; and to
assess the impact of NMLs on the nighttime weather and water transport
in the lowest scale height above the surface.