EXPANDING THE DATASET OF MARS ATMOSPHERIC RETRIEVALS
FROM
MGS
TES
USING
LIMB-GEOMETRY
BOLOMETER
OBSERVATIONS
E. L. Mason, Center for Space Science and Technology, University of Maryland Baltimore County,
NASA/GSFC, Center for Research and Exploration in Space Science and Technology, Baltimore, MD, USA,
(emily.mason-1@nasa.gov), M. D. Smith, NASA Goddard Space Flight Center, Greenbelt, MD, USA, M. J.
Wolff, Space Science Institute, Boulder, CO, USA.

Introduction:
The Thermal Emission Spectrometer (TES) onboard the Mars Global Surveyor (MGS) was a thermal infrared spectrometer (6-50 µm) with additional
broadband infrared (5-100 µm) and visible (0.3-3.0
µm) wavelength bolometers (Christensen et al.,
1992; 2001). Beginning in March 1999, the MGS
TES operated nearly continuously for almost three
Martian Years (MY) serving as the primary source
for information on the state of the Martian atmosphere. Throughout the MGS mission, TES bolometer
data were taken simultaneously.
Over time, the neon lamp used for producing the
interferogram, the primary raw output of the spectrometer, began to degrade, and by September 2004,
the lamp could no longer provide sufficient signal to
collect usable spectrometer data. However, TES continued to take observations using its visible and infrared band bolometers for nearly a full Martian Year
(MY) after the spectrometer ceased observations.
The extended bolometer-only dataset covers a period
that includes the entire MY 27 dust storm season and
most of the MY 28 aphelion cloud belt season (see
Figure 1).

size when combined with the visible bolometer. The
thermal bolometer signal (right panel of Figure 1)
clearly shows the seasonal patterns of temperature
and aerosol optical depth. The visible bolometer (left
panel of Figure 1) is sensitive to the scattering geometry of aerosols and requires radiative transfer modeling for interpretation.
Bolometer Dataset:
TES extended bolometer observations span the
visible (0.3-3.0 µm) and infrared (5-100 µm) wavelength ranges and are available from MY 27, Ls =
180˚ until MY 28, Ls = 120˚. MGS was in a sunsynchronous, nearly polar orbit. For a single day,
TES collected two sets of twelve strips of observations running from the north to south pole and spaced
29˚ apart in longitude. One set was taken near a local
time of 2:00 PM (dayside) and the other was taken
near 2:00 AM (nightside). During the period of bolometer-only operation, the observational sequence
consisted of a pattern that alternated two orbits of
nadir-only observations with two orbits of limb-only
observations. This provided six complete pole-topole surveys of the limb each day at a wide range of
longitudes with each individual limb scan spaced
every 1.25° of latitude (Figure 2, left panel).

Figure 1: TES bolometer visible (left) and IR (right) brightness temperatures over the span of the bolometer-only observations

Figure 2: Bolometer observations in on a latitude-longitude
map for six sols (left) and vertical distribution of observations
(right).

Previous studies have used the bolometer data to
determine aerosol sizes (Clancy et al., 2003; 2010;
Wolff and Pankine, 2016) and retrieve atmospheric
temperatures between 60 and 100 km in altitude
(Kutepov et al., 2008). These applications were limited to where coincident spectrometer data are available. Like the spectrometer data, the bolometer data
contain important information about the state of the
lower atmosphere. Limb-geometry infrared bolometer allows atmospheric retrievals of temperature and
provides additional constraints on aerosol particle

Each orbit of bolometer-only observations devoted to limb scanning produced a set of observations
like that shown in Figure 2, right panel. There are
roughly 140 scans of the limb from pole to pole for
both the daytime (~2:00 PM) and nighttime (~2:00
AM) portions of the orbit. The red points in Figure 2
are from one individual scan of the limb centered
near 10° N latitude, and consist of roughly 35 measurements between the surface and a tangent height of
80 km. The projected size of a TES pixel at the limb

is 13 km.
Retrieval Algorithm: The retrieval algorithm
combines limb-geometry visible and infrared bolometer observations with supplemental information obtained from nadir-geometry visible and infrared bolometer observations taken nearby in location and
time. In the limb-viewing geometry, the signal from
the visible bolometer is dependent on scattering into
the line of sight. This signal is dependent on sunlight
reflected from the surface, the viewing geometry, and
the aerosol scattering properties, which include contributions from dust and ice particles. Only dayside
observations are used in the retrieval algorithm. The
signal observed by the thermal infrared bolometer is
dependent on thermal emission from the atmosphere,
surface, and aerosols as well as scattering. Night and
day observations in the infrared are used.
Full multiple scattering is included in the
radiative transfer model (Smith et al., 2013;
McConnochie & Smith, 2008). The model uses a
discrete ordinates method to treat scattering with 64
streams and 50 atmospheric layers set at 0.2 scale
heights. Aerosol properties are taken from modeling
results of Wolff et al. (2009), and gas absorption is
included using the correlated-k approximation (Lacis
& Oinas, 1991) with gas coefficients take from the
HITRAN database (Gordon et al., 2017). Spherical
geometry is treated within the code by a ‘psuedospherical’ approach where the curved path is defined
by computing the correct limb-geometry emission
angle for the path at the boundary of each layer.
The retrieval uses limb geometry from a single
scan, which consists of 35 individual observations
between the surface and 80 km), in conjunction with
the nearest nadir-geometry bolometer observations in
time and location. Aerosol optical depth and atmospheric and surface temperature are taken from TES
climatology nadir observations as a first guess. Visible and infrared nadir observations are first used to
retrieve surface albedo and temperature, respectively.
Limb-geometry observations are used with the resulting surface albedo to derive vertical distributions of
aerosol optical depth (from the visible bolometer),
atmospheric temperatures, and column-integrated
aerosol optical depth (from the infrared bolometer).

retrieval results for one dusty spectrum taken at MY
27, Ls=246°, 29° S latitude, 82° W longitude. The
retrieval shows an excellent fit to the observation
with the aerosol having a nearly uniform mixing up
to a height of 40 km and declining rapidly above that
level. The column extinction optical depth of the
aerosol is 0.63 for dust at a wavelength of 9 μm.
Validation and Testing: Retrievals of atmospheric thermal and aerosol properties can be validated against concurrent MGS TES spectrometer limb
and nadir observations during times when the spectrometer was operational. For nearly three Martian
years, limb profiles were acquired every 10˚ or 20˚
degrees of latitude, and both bolometer and spectrometer were taken simultaneously. Accurate profiles from the TES spectrometer are available. In
addition, The Thermal Emission Imaging System
(THEMIS) (Christensen et al., 2004) onboard Mars
Odyssey, which has been operational since 2001,
provides retrieved column-integrated optical depth
and atmospheric temperature (Smith, 2009) as validation during the bolometer-only observations.
Summary: Atmospheric aerosol and thermal
properties can be retrieved from the extended bolometer-only dataset. These properties include aerosol optical depth and atmospheric temperature from
limb-viewing geometry as well as surface temperature and albedo from nadir-viewing geometry. We
intend to present retrievals during times when the
spectrometer is operational to validate the results.
This work will be extended to the TES bolometeronly observations to derive a systematic view of aerosol and thermal properties across Mars.
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Figure 3: Example fit from MY 27, Ls = 246˚ at 29˚S latitude
and 82˚ W longitude (Solis Planum).

The retrieval uses a chi-squared best fit metric to
minimize radiance computed from the RT model as a
function of height above the limb. Figure 3 shows the

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