CaSSIS: A Swiss Camera Goes To Mars

INTERNATIONAL
SPACE
SCIENCE
INSTITUTE
SPATIUM
Published by the Association Pro ISSI No. 40, November 2017

Editorial
When I am in Rome, a visit to
Campo dei Fiori is one of my priorities.
The place is a charming
venue today where farmers offer
fragrant flowers, vegetables and the
like, while some 400 years ago, one
of humankind’s most courageous
thinkers payed for intellectual freedom
with his life.
On a cold winter day in February
1600, a handful of menials pile
firewood on Campo dei Fiori.
They prepare the stake for a haggard
Dominican priest, who has
passed the last seven years in nearby
Castel Sant’Angelo’s muggy
prisons. His misdoing is to fancy
there could be an infinite Universe
and uncountable civilizations out
there. To the Roman inquisition,
this is a clear case of heresy. The
next day, on 17 February, Giordano
Bruno dies in the flames. His bravery,
however, grants him immortality
in humanity’s history.
Times have changed a little bit:
those, who dream of extra-terrestrial
life and participate in its discovery,
enjoy the great public’s admiration
today.
When it comes to the search for
alien life, Mars plays a prominent
role. The current view is that this
planet was very life-friendly in its
early days, and that life may have
emerged there more or less simultaneously,
as it did here on Earth.
Unfortunately, however, the Red
Planet took another way in its evolution
than our Blue Planet: Mars
became cold and dry, hostile to any
life on its surface, while on Earth,
every corner brims over with animate
beings. Why that difference?
Could our home planet eventually
take the same route as Mars? The
search for life on Mars helps understand
its history and no less that
of our home planet.
The current issue of Spatium pays
homage to one of the teams rushing
after some of the elusive signs
of Martian life that research could
get hold of so far. It was merely a
little trace gas in its atmosphere.
Yet, it convinced the European
Space Agency to implement a
dedicated mission to Mars and the
scientists at the University of Bern
to bring forward a cutting-edge
camera to find out where the trace
gas comes from. We are proud to
present our readers herewith the
summary of Professor Nicolas
Thomas’ fascinating report for the
Pro ISSI audience on 8 June 2016.
Meanwhile, their camera has made
the first fly-by around Mars confirming
the anxiously waiting
team in Bern its perfect functionality
to start the scientific mission
in March 2018. If there is any life
there, they should find it!
Hansjörg Schlaepfer
Brissago, October 2017
Impressum
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SPATIUM 39 2

CaSSIS: A Swiss Camera Goes
To Mars 1
By Professor Nicolas Thomas, University of Bern
Introduction
This issue of Spatium is devoted to
Mars, Fig. 1. The careful reader
might remark to his surprise that
it is not the first time that Spatium
is about the Red Planet 2 . Yet, there
are valuable reasons for doing so:
firstly, amongst all cosmic objects,
Mars stands out as being the nearest
planet to Earth. This facilitates
its exploration; it may even allow
exploration by humans in the near
future. Secondly, its surface conditions
are the most similar to those
on our home planet and they may
have been even more so in the distant
past. Last but perhaps most importantly,
Mars is also the nearest
celestial body that might harbour
some forms of life 3 (or may have
done so in the past).
Other hot spots in the solar system
that might offer suitable conditions
for life are in the oceans under the
thick ice sheet on Jupiter’s moon
Europa, or even more speculatively,
under the ice cover of Enceladus, a
Saturnian satellite. Much, much
farther out, some of the innumerable
planets circling stars other than
the Sun might equally be capable
of harbouring life. Yet, the exploration
of those distant worlds poses
technical challenges that are far
greater than exploring the Red
Planet.
When it comes to searching for
present or extinct life, the first step
is the detailed mapping of the planetary
surface. This applies not only
to the topography, but also to the
detection and location of any possible
traces of life, such as for instance
waste products. To this end,
one looks for the outcome of the
terrestrial organisms’ metabolism
in the hope that in an alien world
similar forms of life would produce
similar waste products. Here, trace
gases come into play; these are gaseous
components of the atmosphere
in minute concentrations. If
such gases of potentially biologic
origin can be found, the next step
is to pinpoint their sources to further
direct research to these specific
parts of the planetary surface.
This is exactly the mission objective
of the European Space Agency’s
Trace Gas Orbiter, of which one
of the instruments is the Colour
and Stereo Surface Imaging System,
CaSSIS.
Much progress has been made in
the last few years; so it is no wonder
that Spatium addresses Mars
again. The reader will gladly notice
that there are many further
good reasons to do so, be it only
the fact that Swiss scientists and engineers
are proudly contributing
systems that are currently investigating
the Red Planet …
Fig. 1: Earth and Mars compared: The Red Planet is roughly half the size of our
home planet with a mass of about 12% of the Earth’s. It circles the Sun on an orbit
about 50% larger, which translates into a solar irradiation of some 43% as compared
to Earth. The Red Planet has practically no atmosphere; the iron oxide on its surface
provides it a tawny tint. In contrast, Earth has a relatively dense atmosphere
and oceans giving a bluish shading. (Credit: NASA, Jet Propulsion Laboratory)
1
The text constitutes a free interpretation of Prof. Thomas’ lecture for the Pro ISSI audience on 8 June 2016.
It was drafted by Dr. Hansjörg Schlaepfer and revised by Prof. Thomas.
2
See for instance Spatium no. 5: Earth, Moon and Mars by Johannes Geiss, June 2000.
3
When talking about life in the present context, we have complex organisms in mind that are similar to what we find on
Earth in that their cells contain water as the key solvent. This definition is by far not conclusive; indeed, nothing is
trickier than the definition of life including possible extra-terrestrial forms (see also Spatium no. 16: Astrobiology by
Oliver Botta, July 2006).
SPATIUM 39 3

A Brief History
of Mars
In order to understand the scientific
rationale for assuming present or
extinct life on Mars, one has to look
back at the planet’s early history.
Like all the planets in the solar system,
Mars evolved 4.6 billion years
ago from a huge disk of dust and
gas 4 from which, in its very centre,
the young Sun emerged. This huge
central body acquired some 98% of
the disk’s mass while leaving the remaining
debris to the emerging
planets and all the other bodies
making up the solar system. The
Sun acquired sufficient mass to produce
a gravity field strong enough
to ignite nuclear fusion processes in
its core, in which, initially, two
hydrogen nuclei combine to one
helium nucleus thereby releasing
tremendous amounts of energy.
This made the Sun shine.
While Earth circles the Sun at a
distance of 150 million km, the
Red Planet’s orbit is some 50%
larger than Earth’s. This greater
orbit leads to lower solar irradiation
and a climate generally colder
than Earth’s. Mars is about half the
diameter of the Earth, which translates
into a mass ratio of about 1:8.
This smaller mass of Mars is a key
to understanding its dissimilar evolution
as compared to Earth’s. The
planets formed by collisions with
and incorporation of increasingly
larger chunks of material. This
process caused the emerging planets
to absorb much kinetic energy,
which as heat energy made them
melt. Later, when the bombardment
ceased, the bodies began
cooling down. Mars, due to its
smaller mass, cooled faster than
Earth. Therefore, from a geologic
point of view, Mars is probably a
dead planet today: there are no
plate tectonics and even the solar
system’s largest volcano, Olympus
Mons, ceased erupting millions of
years ago. This is an essential aspect
as tectonic activity with its
volcanism and the consequent atmosphere
appears to be an indispensable
requisite for the evolution
and subsistence of life on a planet 5 .
The CO₂ atmosphere of Mars is
relatively low pressure (around
1/150th of the Earth’s atmospheric
pressure) and consequently wide
surface temperature swings from
–133 °C lows up to 25 °C highs are
evident precluding water-based life
on its surface most of the time.
In contrast, in its early days, when
Mars still had enough heat energy,
it must have possessed a denser atmosphere
allowing liquid water to
exist on its surface, probably in
great quantities as some topologic
features (river beds, gullies, etc.)
suggest.
To summarize, Mars has passed
through different evolutionary
phases: some areas show strong
pockmarks by impact craters,
while others are smooth and plane.
Since the estimation of the age of
a surface usually takes into account
the density of impacts on a particular
area, the diversity in crater density
suggests different ages of the
various parts of the planetary surface.
Consequently, Martian history
can be divided into four distinct
epochs as described below:
The Pre-Noachian 6
(4.6–4.1 billion years ago)
Mars emerged from the protosolar
nebula 4.6 billion years ago. It continued
acquiring mass as a result of
impacts with bodies crossing its orbit
during the first ~500 million
years. Even at this time, Mars probably
began developing an atmosphere
by impact-related outgassing
from the planet’s mantle. As impact
rates decreased, the temperature
fell, allowing atmospheric water
vapour to condensate and rain
out to form vast oceans. With further
cooling, a first window opened
for the possible emergence of life
in the Martian oceans around 4.4
to 4.3 billion years ago.
4
See Spatium no. 6: From Dust to Planets by Willy Benz, October 2000.
5
See Spatium no. 30: Planets and Life by Tilman Spohn, October 2012.
6
The name Noachian stems from Noachis Terra (lit. “Land of Noah”), a heavily cratered highland region west of the
Hellas basin.
SPATIUM 39 4

The Noachian
(4.1–3.7 billion years ago)
The Noachian includes the period
of the Late Heavy Bombardment
once again with numerous asteroid
and comet impacts, Fig. 2. The Hellas,
Isidis and Argyre basins, the
largest impact structures still visible
on the planet, are the result of
these events, as well as many of the
craters that characterize the southern
highlands.
At the same time, large-scale volcanic
eruptions poured ash and
gases into the atmosphere including
water vapour, which rained out
to erode the valley networks we
still can see today in many basins
and craters.
Because the planet cooled rapidly,
the magnetic dynamo shut down 7 :
Mars lost its global magnetic field.
This paved the way for a thinning
of the atmosphere by high energetic
particles from space that
could now strip away atmospheric
molecules. Habitable environments
gradually became smaller
and more localized, but Noachian
Fig. 2: Colorized relief map of Noachis Terra. Colours indicate elevation, with
red highest and blue-violet lowest. The blue feature at bottom right is the northwestern
portion of the giant Hellas impact basin. (Credit: NASA, Arizona State
University)
surface conditions continued to be
favourable for the emergence of
life.
The Hesperian
(3.7–2.9 billion years ago)
The Hesperian 8 period was relatively
calm with fewer impacts
compared to the Noachian. It constitutes
the interim phase between
the humid, warm climate of the
Noachian and the subsequent cold
and dry phase we see on Mars
today.
During the Hesperian, there was
still considerable volcanism, albeit
at a slowing pace. Volcanoes released
large amounts of sulphur
dioxide and water; the gases reacted
to make sulphuric acid,
which then rained onto the surface.
As a result, the Hesperian is
characterized by extensive sulphate
deposits.
Valley network formation waned
as the climate became colder and
much of the water was probably
locked up as permafrost or subsurface
ice. Episodically, impacts
hit the surface thereby heating the
ground ice and subsurface water.
This gave rise to catastrophic, yet
short-lived, floods creating huge
outflow channels along with the
formation of so-called chaotic
terrain (See Fig. 3 on the next
page).
7
Like Earth, Mars is differentiated, that is, it has a dense metallic (iron) core overlain by a rocky (silicate) mantle.
The spinning hot and magnetized iron core creates a magnetic dynamo. This gives rise to electrical currents within the
core that generate the planet’s global magnetic field. This field deviates energetic particles from space preventing them
to hit the planet’s atmosphere and surface.
8
The Hesperian period is named after Hesperia Planum, a moderately cratered highland region northeast of the Hellas
basin.
SPATIUM 39 5

Fig. 3: Kasei Valles. Kasei, the Japanese name for Mars, constitute the longest valleys on the Red Planet. They start in a chaotic
region and divide into several arms. This system probably formed because of episodic floods with huge amounts of water
masses seeking their way down to the Chryse Planitia lowlands. The waters are thought to have been released by melting subsurface
processes heated by tectonic activities or asteroid impacts. (Credit: NASA, JPL-CalTech, Arizona State University)
Amazonian
(2.9 billion years ago to
present)
The Amazonian 9 period is characterized
by the absence of largescale
geological and climatic
changes. For much of the period,
the planet’s surface has been dry
and cold.
Late-stage volcanism included the
last eruptions of Olympus Mons
and widespread lava flows elsewhere.
Meanwhile, aeolian (wind)
erosion and deposition shaped large
areas of Mars, notably the broad
plains and sand dunes near the
poles.
There is increasing evidence that
Mars experiences long-term climate
cycles that have a significant
influence on the distribution of
ices. Such long-term climate cycles
may take place over thousands to
millions of years as the axial tilt of
the planet and its distance from the
Sun undergo cyclical changes.
9
The Amazonian period has its name from Amazonis Planitia, which has a sparse crater density over a wide area.
SPATIUM 39 6

The Red Planet
Today
Even though Mars is very different
from our world, it is nevertheless
the planet resembling our
Earth the most. Its present climate,
however, makes it more than questionable
whether there is still active
life on the Martian surface. Indeed,
none of the rovers inspecting
sites could identify any trace of
current life to date. However, life
could still be active below the surface
where warmer aquifers might
allow water to remain liquid the
entire Martian year round and offer
shelter against the deadly highenergy
radiation from space 10 .
The Atmosphere of Mars
Compared to Earth, the actual atmosphere
of Mars is extremely
thin. Atmospheric pressure on the
surface ranges from a low of 30 Pa
on Olympus Mons to over 1,155
Pa in Hellas Planitia, with a mean
pressure of 600 Pa. The resulting
mean surface pressure is only about
0.6% of that on Earth.
The chemical composition of the
thin Martian atmosphere is also
quite different to the Earth’s. It is
composed mostly of carbon dioxide,
whereas the Earth’s atmosphere
holds some 78% nitrogen. Its
atmosphere is dusty giving the
Martian sky a tawny reddish hue
when seen from the surface. It
would also be noticeable to someone
on the surface that the brightness
of the sky is highly non-uniform
– unlike on the Earth where
a cloudless sky is uniformly blue to
a first approximation. This highlights
a fundamental difference between
the two planets. On Mars,
dust scatters light in the atmosphere
whereas on Earth it is mostly
the gas molecules that do the
scattering.
Methane
Methane is one of the many trace
gases in the Martian atmosphere.
Yet, it causes a disproportionate interest
amongst scientists, as on
Earth, this gas is mostly the product
of biological and anthropogenic
processes. (Since the industrial
revolution Man has had a
significant impact on atmospheric
methane concentrations, increasing
them by roughly 250%.) It is
also well-known as an important
greenhouse gas. The methane
found on Mars, so the speculation
goes, could therefore be of biological
origin as well indicating the
presence of life beneath the
surface.
Several observations underpin this
hypothesis.
Firstly, methane occurs in the Martian
atmosphere in extended
plumes only, the profiles of which
imply that it is released from discrete
regions. Currently, there are
three such areas, of which the most
important plume features some
19,000 metric tons of methane,
with an estimated source strength
of 0.6 kilograms per second.
Secondly, observational data suggest
that water once flowed over these
regions, which could have supported
the emergence of life there.
Thirdly, most challenging is the fact
that in the Martian atmosphere
methane has a life expectancy of
50–200 years. Thus, its presence
requires an active source that compensates
for the losses incurred by
dissociation.
One of the hypotheses to explain
these observations is that underground
liquid water areas would
be able to provide a habitat for microorganisms,
which release methane
as a waste product. If the methane
is of biological origin, two
scenarios may apply:
1. Long-extinct microbes, which
disappeared millions of years
ago, have left the methane frozen
in the Martian upper subsurface,
and this gas is released
into the atmosphere today as
temperatures and pressure near
the surface change, or
2. Some very resistant methaneproducing
organisms still survive.
One way to confirm the
biological origin of methane
would be to measure the isotope
10
Recently, scientists found complex multi-cellular organisms in depths down to 3.6 km in the deep mines in South
Africa, see Nature 474, 79–82, (02 June 2011). These may be analogous habitats as those subsurface aquifers on Mars.
SPATIUM 39 7

atios of carbon and hydrogen,
the two elements of methane.
Life on Earth tends to use lighter
isotopes, for example, more 12 C
than 13 C, because this requires
less energy for bonding. Yet to
answer this question, scientists
must first get hold of some samples
of the Martian methane.
While the hypothesis of a connection
to active biology is highly intriguing,
it is important to note
that other sources (e.g. volcanism)
might also contribute to the methane
budget.
Summary
The European
Space Agency’s
ExoMars
Programme
In the framework of the ESA Cosmic
Vision 11 programme, there is a
theme devoted to planets and life
entitled “Life and Habitability in
the Solar System”. Under this title,
ESA runs the ExoMars Programme
2016–2020 addressing the question
of whether life ever existed on
Mars and preparing new technologies
paving the way for a future
Mars sample return mission in the
2020’s.
The ExoMars programme contains
two missions carried out in cooperation
with Roscosmos 12 :
––
The 2016 mission consists of the
Trace Gas Orbiter (TGO) including
a landing demonstrator
module, and
––
The 2020 mission includes a
rover that will carry a drill and a
suite of instruments dedicated to
exobiology and geochemistry
research.
Evidence suggests that Mars was
significantly more habitable during
early epochs than it is today.
Life may have emerged in the distant
past as it did on our own
planet. Yet, whilst on Earth conditions
for life remained more or
less favourable for the subsistence
of life, dramatic climate changes
followed on Mars that make the
surface a hostile environment today.
Yet, by analogy to findings on
Earth, life may still be active below
the surface for which the
methane plumes could act as precious
signposts. Advancing our
knowledge regarding these potential
sites of life is the key goal of
the European Space Agency’s Exo-
Mars programme, which we are
going to address now.
Fig. 4: The Trace Gas Orbiter in an artist’s impression. Clearly visible are the
very large solar panels required for providing the spacecraft with enough electrical
energy under the reduced solar illumination conditions at the Martian orbit. (Credit:
ESA)
11
Cosmic Vision is the name of the current phase of ESA’s Science Programme.
12
Roscosmos is the space agency of the Russian Federation.
SPATIUM 39 8

The ExoMars Trace Gas
Orbiter
The Trace Gas Orbiter, Fig. 4, carries
a scientific payload capable of
detecting and characterizing various
trace gases in the Martian atmosphere.
It will also investigate
the location and nature of the
sources that produce these gases.
To cope with these objectives, the
TGO carries the following four instruments
aboard:
––
NOMAD: The Nadir and Occultation
for Mars Discovery combines
three spectrometers (two in the
infrared and one in the ultraviolet
spectrum) to perform highsensitivity
orbital identification
of atmospheric components via
both solar occultation 13 and direct
reflected-light nadir observations
14 . The Belgian Institute
for Space Aeronomy is responsible
for this instrument.
––
CaSSIS: The Colour and Stereo
Surface Imaging System is a
high-resolution, 4.5 m per pixel
colour stereo camera for taking
images in natural colours and for
the production of accurate digital
elevation models of the Martian
surface. This camera is a
contribution by the University of
Bern together with Italian and
Polish partners.
––
FREND: The Fine-Resolution Epithermal
Neutron Detector is a
neutron detector that can provide
information on the presence
of hydrogen, in the form of water
or hydrated minerals, in the
top one meter of the Martian
surface. This package is a contribution
from the Space Research
Institute (IKI) in Moscow.
FREND measures the flux of neutrons
from the Martian surface.
These neutrons are produced by
the continuous cosmic ray bombardment
that interacts with the
Fig. 5: Artist’s impression of the ExoMars 2016 Trace Gas Orbiter seen from
the planet-facing side. The four instruments (CaSSIS, NOMAD, FREND and
ACS) are mounted on the exterior of the spacecraft to facilitate undisturbed observation
of the planet’s surface. To the right is Schiaparelli, the entry, descent and
landing demonstrator module. The large solar arrays are partially cut away to highlight
the spacecraft’s body. (Credit: ESA)
––
ACS: The Atmospheric Chemistry
Suite consists of three infrared
instruments that help investigate
the chemistry and structure of
the Martian atmosphere. ACS
complements NOMAD by extending
the coverage at infrared
wavelengths and by taking images
of the Sun to better analyse
the solar occultation data. This
package is a contribution from
the Space Research Institute
(IKI) in Moscow.
13
The term solar occultation refers to an operational mode, whereby the instrument looks toward the Sun and examines
the composition of the sunlight that passes through the atmosphere. The atmospheric gases partially occult the sunlight
at specific wavelengths thus allowing the instrument to determine the local chemical composition of the atmosphere.
14
The term reflected-light nadir designates a mode whereby the instrument looks directly downward toward the surface
and observes the sunlight scattered by the atmospheric constituents. This is a complementary way of analysing the composition
of the atmosphere.
SPATIUM 39 9

first few metres of rock. The cosmic
rays are sufficiently energetic
to break apart atoms, releasing
high-energy neutrons that are then
slowed down and absorbed by the
nuclei of elements in the surrounding
material. Not all the neutrons
are captured though, many escape,
creating a leakage flux of neutrons
that the FREND instrument will
observe. The distribution of neutron
velocities, which depends
upon how much they were slowed
down before escaping, can reveal
much about the surface material
since it depends on the composition
of that material, primarily on
its hydrogen content. The hydrogen
serves as an indicator of the
presence of water.
TGO Timeline
The ExoMars programme will follow
with the Surface Science Platform
and the ExoMars Rover in
2020. For these probes as well as
for NASA spacecraft, the TGO
will operate as a communication
link with Earth until 2022.
The Colour and Stereo
Surface Imaging System
(CaSSIS)
In order to locate potential sources
of trace gases on the planetary surface
as well as to qualify potential
sites for future landes missions, a
high-resolution camera is required.
This is the scope of CaSSIS, proposed
by Prof. Nicolas Thomas and
his team at the University of Bern.
He has overall responsibility for the
programme (the Principal Investigator
in the space argot) together with
the Co-Principal Investigator Gabriele
Cremonese of the Astronomical
Observatory of Padua,
Italy.
Specifically, CaSSIS serves to:
––
Characterize the sites, which
have been identified as potential
sources of trace gases.
––
Investigate dynamic surface processes
(e. g. sublimation, erosional
processes, volcanism)
which may contribute to the atmospheric
gas inventory.
––
Certify potential future landing
sites by characterizing local
slopes, rocks, and other latent
hazards.
The ExoMars Trace Gas Orbiter
was launched on 14 March 2016 by
a Russian Proton-M rocket from
the Baikonur Cosmodrome in Kazakhstan
and injected into Mars
orbit on 19 October 2016. The
subsequent aero-braking phase
aims at reducing its speed to a value
suitable for the nominal 400 km
circular orbit. The science activities
start in March 2018 and run for
almost two years.
Fig. 6: The CaSSIS flight model on a bench in the University of Bern laboratory.
The yellow/red body on the left is the electronics unit. On the right, the
telescope structure with the four mirrors is displayed. The telescope is cantilevered
off the gold coloured support structure. (Credit: University of Bern)
The TGO delivered the Schiaparelli
lander on 16 October 2016, which
successfully entered the Martian atmosphere
returning significant
amounts of science data. Unfortunately,
however, the radio signal was
lost during descent and the lander
crashed onto the surface.
SPATIUM 39 10

The subsequent chapters will provide
a brief overview of the CaS-
SIS camera which is specifically
designed to obtain colour and stereo
images. The stereo capability
facilitates construction of digital
elevation models – effectively providing
the topography in addition
to the 2-dimensional image.
System Overview
CaSSIS sits on the surface-facing
side of the orbiter, see Fig. 5. The
orbiter will rotate about an axis
that will maintain its solar panels
oriented towards the Sun to generate
enough electrical energy
while avoiding solar illumination
of its thermal radiators. This requires
CaSSIS to compensate for
the spacecraft’s yaw rotation with
its own mechanism.
The rotation mechanism is able to
turn the entire telescope system by
180° in 15 seconds while its support
structure remains fixed, see
Fig. 6. In order to gather the stereo
pair of images required for the production
of elevation models, the
imager looks 10° ahead of the
spacecraft, as outlined in Fig. 7, to
acquire the first image, then it
turns by 180° to look 10° backwards
to acquire the second picture
of the same surface element.
CaSSIS will also deliver high-resolution
imagery of the surface that
allows scientists to investigate
whether specific types of geological
processes might be associated
with trace gas sources and sinks.
The horizontal resolution is of
about five metres per pixel when
Fig. 7: The CaSSIS stereo image acquisition principle. During the first part
of the orbit (to the left in this sketch), the optical system acquires swaths perpendicular
to the direction of motion at an angle of 10° to the nadir. Then, the camera
is turned by 180° allowing the same region to be observed from a second direction
inclined -10° to the nadir. This procedure allows the system to obtain
quasi-simultaneous stereo pairs over the full swath width for high-resolution digital
terrain models. The resulting images are transferred to the processing computers
on Earth, which produce the digital elevation model of the terrain along with
the colour images. (Credit: ESA).
the spacecraft is in the nominal orbit
(400 km above the surface).
This resolution equals seeing a
Swiss franc at a distance of 2 km!
The digital elevation model of the
terrain will have a resolution of
about six metres in the vertical
direction.
The camera is composed of the following
subsystems:
The Telescope Unit
The telescope unit projects an accurate
image of the Martian surface
on the focal plane assembly. It
was originally conceived as a threemirror
anastigmatic system (offaxis)
with a fold mirror. The tight
time schedule in the TGO programme
required the adaption of
an existing system for a laser communication
terminal to satisfy the
requirements for CaSSIS.
The mirror structure is made from
carbon fibre reinforced plastic assuring
excellent stability over the
expected spectrum of thermal and
mechanical loads. This subsystem
was designed and fabricated by
RUAG of Zurich, Switzerland.
SPATIUM 39 11

The Focal Plane Assembly
Fig. 8: The CaSSIS telescope unit. The telescope delivers a precise image of the
Martian surface to the focal plane assembly. To reach the challenging thermal and
mechanical stability requirements, the unit is made of a carbon fibre reinforced
poly mer structure. The primary mirror (to the right) is 13.5 cm in diameter. (Credit:
University of Bern)
Fig. 9: The focal plane assembly. It serves to convert the incoming light
from the telescope into electrical signals. As CaSSIS is intended to provide
colour images, there are four optical filters directly deposited on a silica substrate
that covers the detector unit. The numeric combination of the four data channels
delivers imagery in natural colours. (Credit: University of Bern)
The Focal Plane Assembly converts
the incoming light into electrical
signals. It is a heritage from the BepiColombo
mission – a slightly
adapted flight spare unit from the
SIMBIO-SYS experiment. As can
be seen in Fig. 9, the detector is
covered by a single substrate on
which four colour filters are deposited.
With these four filters, the
detector produces images in four
different colour bands which,
when composed to one single
image, permit the reproduction of
natural colours and scientifically
relevant colour ratios. To avoid
blurring by the speed of the spacecraft,
the detector is read-out extremely
quickly with 14-bit digital
resolution.
The Rotation Mechanism
The rotation mechanism rotates
the camera’s optical axis, see Fig. 10.
It connects the telescope unit and
the focal plane assembly to the
spacecraft. This solves two problems:
Firstly, the rotation of the
spacecraft about the nadir direction
can be compensated for. Prior to
image acquisition, the imager can
be rotated, so that the lines are orthogonal
to the direction of motion.
Secondly, the rotation mechanism
can be swivelled by ~180°
to acquire stereo images as outlined
in Fig. 7.
The rotation mechanism’s gear is
made of high-strength titanium alloys,
which are hard-coated to provide
durability. A stepper motor,
connected to the torus shaft via a
SPATIUM 39 12

Fig. 10: The CaSSIS Rotation Mechanism. The rotation mechanism holds the
telescope unit and the focal plane assembly on the basic instrument structure, which
in turn is fixed to the spacecraft. It serves to adjust the instrument’s optical axis in
the direction needed, for instance to create the stereo images. (Credit: University
of Bern)
bellow coupling creates the necessary
torque.
The Electronics Unit
The electronics unit provides the
necessary electrical energy to the
various consumers in the camera.
In addition, it performs the first
step in the digital processing chain
of the image data before it is conveyed
to the spacecraft system bus
for transmission to Earth. The
electronics unit comprises three
modules, which assembled with
board-to-board connectors to generate
a complete and compact box.
When the TGO arrived at the Red
Planet on 22 November 2016, the
aero-braking phase began decelerating
the spacecraft to reach its final
400 km orbit. On the occasion
of its first encounter with the
planet, the spacecraft and its instruments
were briefly turned-on to
test the systems. CaSSIS took a first
glimpse of the Martian surface and
the imagery returned was absolutely
spectacular (original quote
of the principal investigator Nicolas
Thomas), see Fig. 11 and 12. Most
importantly, they confirmed also
the instrument’s full functionality.
The upper panel of Fig. 11 is composed
of a large number of individual
small framelets. Upon pre-processing
by the electronics unit,
these framelets were transferred to
the spacecraft’s telecom system,
which in turn sent the data to the
ESA’s ground station on Earth.
From here, it went to the University
of Bern. With the help of experts
from the Astronomical Observatory
in Padua, the 3D image
shown in the lower table of Fig. 11
was generated.The colour capability
of the instrument was also
tested and the remarkable picture
of lava flows near Arsia Mons was
returned, see Fig. 12.
Fig. 11: Noctis Labyrinthus: a preliminary reconstruction of a detail of the
Martian surface. The image in the top panel was gathered by CaSSIS during a
first fly-by at Mars before slowing down to the final orbit. The table below displays
the reconstructed 3D image from the digital elevation model. (Credit: University
of Bern)
CaSSIS’s First Pictures from
Mars
SPATIUM 39 13

Fig. 12: Lava flow near Arsia Mons. This image is particularly interesting because it does not merely show dark streaks
arising from the motion of dust but also a bright region to the upper left which is almost certainly a cloud feature in the lower
atmosphere. The detail evident in the image is exquisite. (Credit: University of Bern)
SPATIUM 39 14

Outlook
The Italian astronomer Giovanni
Schiaparelli 15 became world-famous
in 1877 with his maps of the
Martian surface, which, however,
the space age debunked as fantastic
interpretations of what his eyes
may have seen through his (too)
simple lens telescope. Nevertheless,
his publications stirred up the public’s
fantasy on a large scale and set
the stage for the small green Martians,
the possible artificers of the
long water channels Schiaparelli
was thought to have detected.
The technologies have changed
since Schiaparelli’s time but the
three ingredients of the story have
excitingly remained the same:
Mars, water and life. Thanks to an
armada of spacecraft and rovers exploring
the Red Planet in the last
five decades, our understanding
has grown immensely; and all this
tremendous amount of knowledge
points towards the possibility of life
on Mars either actual or extinct.
In the meantime, scientists explore
the Red Planet with the currently
available technology of which the
CaSSIS camera is an excellent example.
This wonderful piece of
hardware will provide crucial information
about the Martian surface
not least by qualifying landing
sites for future human space
missions.
As Carl Sagan 16 said, “The cosmos is
full beyond measure of elegant truths,
of exquisite interrelationships, of the
awesome machinery of nature.” This
is certainly true for Mars in particular
and CaSSIS will help to
shed light on that awesome machinery
of nature that is at work
on Mars.
This vague option continues
prompting space agencies to implement
missions to Mars. Within
perhaps two decades, this planet
may even come into the reach of
human space flight, which then
will give rise to a new chapter of
the Mars exploration story.
15
Giovanni Virginio Schiaparelli, 1835, Cuneo, Italy – 1910, Milan,
Italian astronomer.
16
Carl Edward Sagan, 1934, Brooklyn, New York – 1996, Seattle, Washington,
American astronomer, cosmologist, and science communicator.
SPATIUM 39 15

SPATIUM
The Author
Nicolas Thomas gave his first physics
lecture at the tender age of eight.
Like his colleagues, he was asked
to present a topic of interest; and as
at that time, Neil Armstrong had
just set foot on the lunar surface,
young Nicolas decided to give a
presentation on the solar system.
This laudable attempt, however,
was stopped at Jupiter since his time
allowance of two minutes was over
at the giant planet. Notwithstanding
this setback, planetary science
continued to fascinate the youngster
who took up where he left off
by writing a review about the Jovian
satellite, Io, when completing
a Master’s degree in Experimental
Space Physics at the University of
Leicester. This encouraged him to
continue his studies at the University
of York with a thesis about Io’s
atmosphere, which earned him the
Stott prize for the best physics thesis
at the University of York in 1986.
His career then led to the Max
Planck Institute for Aeronomy
(MPAe) of Lindau, where he
worked on the analysis of data
from the Halley Multicolour
Camera aboard the Giotto spacecraft.
A further post-doctoral
fellowship allowed him to join
the Space Science Department of
ESA, and then he returned to
MPAe. During that time, he was
also engaged as a visiting scientist
at the Lunar and Planetary Laboratory
at the University of Arizona,
Tucson, and at the Queen’s University
of Belfast. In March 2003,
the University of Bern elected
him as Professor of Experimental
Physics.
Nicolas Thomas is the Principle
Investigator for the Microscope on
the Beagle 2 Lander and the Co-
Principle Investigator of ESA’s
BepiColombo Laser Altimeter
Experiment to Mercury. In addition,
Nicolas Thomas has held
leading responsibilities with numerous
other science instruments
and served in various international
space science teams. Specifically,
he was a Member of the Scientific
Board of the International Space
Sciences Institute 2003–2006 and
later a Member of the ISSI Board
of Trustees (2011-present).
Over the years, Nicolas Thomas
has published over 100 publications
covering the fields of cometary,
Jupiter and Mars research. He is
deeply fascinated by the beauties
and miracles of our solar system,
even, as he underlines, as a nonbeliever
that there is life elsewhere
in our solar system. But further out
there? That is another story …
Scientists should maintain skepticism,
he confesses, and base arguments
on facts. One does not have
to “spice up” the stories in our solar
system to fascinate the public.
Just tell them the truth. That is
beautiful enough.