Mars Perihelion Cloud Trails as revealed by MARCI: Mesoscale Topographically Focused Updrafts and Gravity Wave Forcing of High Altitude Clouds
R. T. Clancy, M. J. Wolff, N. G. Heavens, P. B. James, S. W. Lee, B. J. Sandor, Space Science Institute,
Boulder, CO USA (clancy@spacescience.org), B. A. Cantor, M. C. Malin, Malin Space Science Systems, San
Diego, CA USA, D. Tyler, Jr., Oregon State University, Corvallis, OR USA, and A. Spiga, LMD/IPS, Sorbonne
University, Paris FR.
Published

in

Clancy

et

al.,

Introduction:
The study of high altitude Mars clouds is stimulated by the wide variety of properties and behaviors
of mesospheric (z > 50km) aerosols (e.g., Clancy et
al., 2019). Mars mesospheric aerosols reflect distinctive processes such as large-scale dust storm injection of dust (e.g.,
et al, 2019), saturation/condensation of the bulk atmosphere (i.e., CO2;
e.g., Listowski et al., 2014), and gravity-wave generation of discrete ice clouds (CO2 and H2O; e.g.,
Spiga et al., 2012). Here, we employ MARCI wide
angle global imaging (Malin et al, 2008) to reveal
another class of Mars water ice clouds at the base of
the mesosphere (40-50 km), with distinctive EW
linear morphologies and mesoscale, plume-like formation regions tied to southern perihelion topographic updrafts. These mesoscale updrafts generate
vigorous gravity wave forcing, which leads to cloud
formation 30-40 km above these topographic convergence updrafts, where the cloud ice particles are
then rapidly transported westward in the rapid zonal
circulation of the lower mesosphere.

Icarus,
362,
2021.
trails (PCT). PCT are a class of high altitude (40-50
km), horizontally extended (200-1000 km) water ice
clouds that are aligned with the W to WSW

Figure 2: The global occurrence frequency of PCT in
MY 28 (lower, global dust storm) and MY29 (upper,
regional dust storm). Enhanced dust solar heating
during dust storm activity suppresses water vapor
saturation at the 40-50km altitudes of PCT formation.

direction of strong winds associated with the low-tomid latitude lower mesosphere during Mars southern
summer. The eastern cloud-forming mesoscale (N-S
scales ∼50 km) origins of PCT appear over specific
southern low-to-mid latitude (5S-40S) locations in
association with significant topographic gradients.
Such mesoscale cloud trails were first reported in
association with northern and southern rim regions
of Valles Marineris (figure 1, Clancy et al., 2009).
The

Figure 1: MARCI color image projection of perihelion cloud trails (PCT) in association with Valles
Perihelion
Cloud
Trails:
Marineris (from
Clancy
et al, 2009).

Daily, global imaging of Mars clouds in MARCI
(MARs Color Imager, Malin et al., 2008) ultraviolet
and visible bands yields the spatial/seasonal distributions and physical characteristics of perihelion cloud

Figure 3: PCT eastern margin (formation regions)
identified in MARCI 2007-2011 imaging in the
Valles Marineris region (on USGS topography map).
The wider range of PCT occurrence appear in figure
6.

current study employs MARCI 2007-2011 imaging
of PCT; and indicates several distinct locations of
PCT occurrences (figure 7), including S W Arsia
Mons, elevated regions of Syria, Solis, and
Thaumasia Planitia, along Valles Marineris margins
(figure 3), and the NE rim of Hellas Basin. PCT
appear daily on a global basis (but not at a single
location, figure 2) over Mars solar longitudes (Ls) of
210-310º, in late morning to mid-afternoon hours
(10am-3pm), they are among the brightest and most
distinctive clouds exhibited during the perihelion
portion of the Mars orbit. Their locations (i.e., eastern margin origins) correspond to strong local elevation gradients, and their timing to peak solar heating
conditions (perihelion, subsolar latitudes and midday
local times). Based on cloud surface shadow analyses, PCT form at 40-50 km aeroid altitudes, where
water vapor is generally at near-saturation conditions
in this perihelion period (e.g. Millour et al., 2014).
PCT were notable absent during periods of planet
encircling and regional dust storm activity in 2007
and 2009, respectively, presumably due to reduced
water saturation conditions above 35-40 km altitudes
associated with increased dust heating over the vertically extended atmosphere (e.g., Neary et al., 2019).
PCT exhibit smaller particle sizes (Reff = 0.2-0.5μm,
0.5μm, figure 4) than are typical in the lower atmosphere, but quite are optically bright ( vis ~ 0.1)
as they incorporate significant fractions (20-50%) of
the available water vapor at these altitudes.

Figure 4: PCT Ice particle sizes from MARCI uv-vis
imaging of cloud surface shadow extinctions.

PCT ice particles are inferred to form continuously
(over ∼4 hours, figure 5) at their PCT eastern origins, associated with localized updrafts, and are
entrained in upper level zonal/meridional winds
(towards W or WSW with ∼50 m/sec speeds at 4050 km altitudes) to create long, linear cloud trails.
PCT cloud formation is apparently forced in the
lower atmosphere (≤10-15 km) by strong updrafts
associated with distinctive topographic gradients,

Figure 5: Observed lengths (upper) and local times of
origin (bottom, grey) for PCT are limited by the
MARCI fov in longitude and local time (LT). For a
50 m/sec growth in the PCT westward extension,
MARCI length and eastern margin LT imply a wide
range of daytime (10am-3pm, black) initiation LT.

such as simulated in mesoscale studies (e.g., Tyler
and Barnes, 2015) and indicated by the surface specific PCT locations (figure 3, 7). These lower scale
height updrafts are proposed to generate vertically
propagating gravity waves (GW), leading to PCT
formation above ∼40 km altitudes where water vapor saturation conditions promote vigorous cloud ice
formation (figure 6).

Figure 6: A schematic of PCT formation and evolution. The 4 key factors are topographic updrafts,
gravity wave generation and propagation to 40-50 km
region of near-saturation water vapor conditions, and
resulting ice clouds entrained in westerly circulation.

Recent mapping of GW amplitudes at ∼25 km altitud s, from Mars C mat Sou d r 15 μm rad a c
variations (Heavens et al., 2020), in fact demonstrates close correspondences to the detailed spatial
distributions of observed PCT (figure 7), relative to
other potential factors such as surface albedo and
surface elevation (or related boundary layer depths).

We note that a one-to-one correspondence in GW
amplitudes at ~25 km and PCT at 40-50km is not
expected, given spatial variations in GW propagation
to >40 km and saturation conditions at 40-50km that
lead to PCT formation. The implied correspondence
of PCT (and MCS GW amplitudes at ~25 km) to
topographic updrafts suggests such updrafts are a
potentially significant mesoscale contribution to GW
forcing in the current Mars atmosphere.

Figure 7: Apparent spatial correspondence of MCSderived gravity wave amplitudes at ~25 km (Heavens
et al., 2020; upper panel) to the spatial distribution of
PCT at 40-50 km from MARCI imaging (Clancy et
al, 2021).

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