OPTICAL ATMOSPHERIC EFFECTS AS VIEWED BY THE MSL CHEMCAM REMOTE MICRO-IMAGER.
S. A. Los, H. E. Newsom, L. A. Scuderi, University of New Mexico, Department of Earth & Planetary Sciences,
Albuquerque, New Mexico, USA.

Introduction

The Mars Science Laboratory (MSL) Remote MicroImager (RMI) has a high-resolution and highly limited field-of-view designed for close-up observations of
ChemCam sample targets [1]. Yet with its long focallength, RMI has proven increasingly useful for capturing long-distance (100s m - 10s km) imagery of geomorphological features [2]. In the time since MSL’s
landing in Gale Crater, significant improvements have
been made to RMI image processing procedures using
masking to diminish light scattering and image stacking
to resample to a higher resolution [3]. However, image aberrations of unknown cause persist. Given long
light paths through the atmosphere at high magnification, RMI long-distance imagery are particularly sensitive, compared to other MSL imagers, to various atmospheric optical effects. Highly variable line-of-sight
opacity from dust aerosols can often affect viewing conditions, in extreme cases obscuring the crater rim and
the majority of nearby Aeolis Palus during global dust
events [4]. Though fogs are unlikely given relative humidities observed at Gale Crater, enhancement of dust
haze by water-ice coating of dust aerosols may be possible as well [5]. In addition to opacity variations, a less
explored impact on imagery is that of thermal effects
from strong daytime temperature gradients present near
the surface [6]. Though theoretically possible, it remains
unclear if optical phenomena such as heat shimmer or
mirages exist on present day Mars and if they could have
a noticeable effect on high resolution images over long
distances [7]. With a growing knowledge of the longterm variability of dust and meteorological conditions
in Gale Crater [8][9], along with the intense mission
scheduling demands for MSL instruments, there is a
need to better characterize typical long-distance viewing conditions to guide image acquisition strategy. Also,
the sensitivity of long-distance imagery to these effects
may provide a novel way to study near-surface thermal
and dust conditions. The work presented here seeks to
better understand how daily and seasonal changes in the
near-surface environment impact RMI long-distance imagery. We focus on methods to detect possible thermal
effects and estimate their spatial and temporal variability.

Thermal Effects
The thin, dry, and dusty Martian atmosphere yields
near-surface temperature and turbulence profiles largely
unique from those typical on Earth [10]. In particular,
the low air density limits the cooling (or heating) of the
surface by turbulence, often supporting super-adiabatic
daytime temperature gradients in the lowest meters to
10s of meters above the surface [11][12]. Theoretically
such strong temperature gradients are capable of producing inferior mirages at distances of ∼1-2 km from
imagers depending on terrain and viewing geometry [7].
Gradient driven refraction might produce a full mirage,
where features are shifted from true position, or it may
simply distort portions of a landscape scene slightly,
such as distant background features viewed over the
top of a closer foreground. On Earth such temperature
gradients may produce scintillation or "heat shimmer"
from coherent convective structures and turbulent eddies, but is unclear if this is the case on present day
Mars. The atmosphere’s low density, dust aerosols, and
CO2 -rich composition result in near-surface turbulence
being driven less by surface sensible heat and more by
internal longwave radiative heating such that turbulent
heat fluxes peak a few hundred meters above the surface (as opposed to a few meters such as on Earth)[13].
This is still in the line-of-sight of some long distance
observations, though it is unknown if such turbulence
produces a significant optical effect again given the density. Furthermore, 3-D large-eddy simulations of the
martian boundary layer suggest that thermal convection
in concert with radiative heating of dust may work to aggregate dust aerosols in convective plumes affecting the
intensity and heterogeneity of dust opacity under certain
conditions [14]. Large-scale increases in dust opacity,
however, likely work to shade the surface limiting temperature gradients and associated thermal effects. In
short, less explored thermal phenomena could influence
long-distance viewing conditions through both static and
dynamic distortions.

Methodology
The analyses presented rely on reviewing RMI longdistance imagery previously captured during the mission. Focus is placed on stacks of successive RMI images taken over short time intervals (seconds to minutes)
from the same viewing geometry, often these stacks hav-

REFERENCES

ing been captured in efforts to increase image resolution.
A circular mask is applied to address blurring and overexposure issues near the outer rim of RMI images. A
number of image processing techniques are tested to analyze pixel variability between images, comparable to
other cloud, dust devil, and dune movement detection
methods. Image-to-image differencing and mean image
subtraction are used to assess time variations across entire images/scenes and also portions of scenes of varying
distance (foreground, crater floor, crater rim). Particular
attention is given to viewing geometries that yield a distinct near foreground and distant backgound separated
by a sharp horizon edge in order to examine the line-ofsight passing above the foreground for distortions that
vary with time. In the case of sharp nearby horizons
or prominent individual features such as a high contrast
boulder, the edges of the horizon or feature are defined
via an algorithm in the first image of a stack and then
variations of this edge are tracked through successive
images. Consideration is given in the above mentioned
methods to distinguish secular movement of the entire
scene, which may suggest motion of the rover and imager, versus other variations. Finally, we compare image
quality and image analysis results for sets of RMI longdistance imagery to the time of day and season, available
dust opacity observations [5][9], and temperature and
humidity measurements from the Rover Environmental
Monitoring Station (REMS) [15] to give context of the
environmental conditions.

Acknowledgements
This work has been supported by a New Mexico Space
Grant Consortium Graduate Fellowship
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