Introduction:
Good knowledge about the aerosol distribution and
compositions is essential for the understanding of
thermodynamic processes in the Martian atmosphere,
which in turn is important for the understanding of the
Martian climate and the altitude of the upper boundary
of the atmosphere. The last point is of special interest
for spacecraft aerobreaking manoeuvres.
The Martian atmosphere often shows horizontal
layers of haze up to altitudes of about 80 km. These
have been described and analysed e.g. by Jaquin et al.,
1986, usingViking Orbiter images and by Montmessin
et al., 2006, who used SPICAM stellar occultation
data. Both showed seasonal and latitudinal changes
in the vertical structure of the aerosol distribution and
composition. Apart from SPICAM, the High Resolution
Stereo Camera (HRSC) is also on board ESA’s
robotic spacecraft Mars Express. HRSC was build and
is operated by the German Aerospace Center (Neukum
et al. 2004; Jaumann et al. 2007). Mars Express is orbiting
Mars in an elliptical orbit, with HRSC scanning
the surface of Mars, primarily for geological research.
In addition to that, HRSC has been used to sample the
planetary limb.
We examine the HRSC planetary limb data and
analyse the seasonal and latitudinal variations of the
maximum altitude of the haze layer and of the occurrence
of high altitude detached hazes. We make
some comparisons with earlier work. In contrast to the
SPICAM instrument, HRSC observes the atmosphere
during daytime, which makes it possible to compare
night and daytime observations.
The HRSC Limb Data:
HRSC is a push broom scanner with nine line sensors
pointing in different directions to facilitate stereoscopic
imaging. Four of the sensors have colour filters
at 440 nm, 530 nm, 750 nm and 970 nm, respectively.
The five other sensors all have filters centred at 650
nm. These panchromatic filters have a much wider
bandpass than the four colour filters.
The surface observations which are HRSC’s main purpose,
are usually take while the spacecraft is nadirtracking
near pericentre. Limb observations, however,
are mostly made with a pointing of the spacecraft being
inertially fixxed in celestial space. This leaves
only a small time window to make observations of
the limb during descent or ascent. Therefore, usually
only a few of the nine sensors can be used for the
limb observation. Due to the motion of the spacecraft,
the individual image lines are taken at different
geographical locations and altitudes. The position of
each image pixel above the limb has to be calculated
from the spacecraft positioning information (Scholten,
pers. comm.). The typical difference in altitude between
two neighbouring pixels is between a couple
of dozen metres and 150 m. HRSC has been observing
the limb occasionally throughout the mission since
2004. So far the northern hemisphere and especially
the north polar region, were particularly well covered
(Figure 1 and 2).
In Figure 2, we give an overview of the available
data, sorted by season (LS) and latitude. The
channel in which the observations have been made is colourcoded.
Most observations were made with the
panchromatic channels. There are also many observations
with the blue and green sensors and only a
few were made in the red and infra red channels. We
find the best data coverage in northern spring in the
northern most latitudes. For obvious reasons, we do
not have any data during polar nights. For most of
our actual analysis we sample the five central pixels of
the sensor lines. This allows for minimal horizontal
averaging.
Analysis:
As an example, Fig. 3 shows images and profiles for
the blue, nadir, and green channels from orbit 6104.
Al three images show a continuously bright limb haze
until an altitude of about 20 km. At higher altitudes
the limb haze becomes darker and stratified consistent
with the limb profiles described by Jacquin et al., 1986.
As Mars Express progresses along its orbit, the limb
observations are made at different locations above the
surface. The locations of the three profiles in Fig. 3
are still in close proximity of each other, in fact they
overlap, but none the less they show different vertical
aerosol distributions. Beginning above the North Polar
cap and going southward, we observe less reflectivity
above 20 km and more reflectivity below 20 km, hinting
at different compositions or amounts of aerosols.
It is not possible to obtain and compare profiles at
the same location and at the same time with different
sensors, but still, averages of profiles over place and
season can provide us with information about typical
atmospheric conditions. In Fig. 4 we show spectra
from the average profiles at three different latitudinal
bands between 70�N–90�N, 30�S–30�N, and 90�S–
70�S, on the left, centre, and right, respectively. The
different symbols and colours represent the different
altitudes at which the spectra were sampled. The size
of the symbol increases with the number of averaged
profiles. There are very few observations above the
South Polar region (compare Fig. 1). In the North (and
South) Polar region there is almost no signal above
30 km altitude, while around the equator the limb haze
remains bright until altitudes of about 60 km. At the
poles, the spectrum at 10 km is reddish. At higher
altitudes the spectrum gets whiter, indicating smaller
particles or higher ice content. At the low latitudes
the spectra are reddish up to 40 km. At 60 km we see
a more or less white spectrum. Figure 5 shows the
maximum altitude of the aerosols as seen by HRSC,
depending on season. During aphelion (LS � 70�)
the maximum altitude of the aerosols that are visible
with HRSC is around 40 km. During perihelion (LS �
250�) the maximum altitude is around 70 km.
Discussion:
Figure 1 and 2 show that there are plenty of visual
and near infra red HRSC observations of the Martian
limb available. These show aerosol distributions that
change with season and latitude (Fig. 3 and 4). The
plots in Fig. 4 show the spectra of the average limb
profiles at several altitudes for three latitudinal bands.
Two important distinctions can be made between the
equatorial and the polar regions. First, the altitude at
which aerosol occur is higher in the equatorial region
and second, the composition of the aerosols at different
altitudes is different. While the spectrum is white
around 20 km altitude above the north pole, it is red at
the low latitudes.
The seasonal variations of maximum altitude of
the aerosols is in good agreement with Jaquin et al.
(1986) and with Montmessin et al. (2006). The similarity
between Montmessin’s results and ours is likely
to be due to the large annual variation of atmospheric
dust load compared to the diurnal cycle. A much
closer look at the data, is forseen to analyse the daily
variation of aerosols in the Martian atmosphere. The
CO2 and waterice aerosols are more likely to change
their vertical distribution (above the planetary boundary
layer) between day and night than the mineral
(dust) aerosols. Spectral information would help to
discriminate between these components. HRSC can
not provide it, because the observation for the different
filters take place at different locations and times
(see Fig. 3). An alternative is to fit aerosol models to
the inverted profiles. Currently, we are preparing this
work.
Mars Express’ HRSC limb data present a valuable
opportunity to analyse Mars daytime atmospheric dust
at a high vertical resolution. This work gives a short
overview of the available data and analyses some seasonal
and latitudinal properties