Mars

2.21April 2004 Vol 45

Mars as neverseen before

Images from the High Resolution

Stereo Camera on Mars Express

reveal the surface of the Red Planet

in unprecedented detail – and should

have the whole planet mapped at

high resolution in just two years.

Over the next six pages, Ernst

Hauber and Gerhard Neukum set

the scene and reveal some of the

spectacular images already captured.

(All images are courtesy of ESA/

DLR/FU Berlin/G Neukum unless

otherwise credited.)

1: The HRSC camera (DLR/FU Berlin).

2 (main): The HRSC is a multipleline-scanner with nine CCD linesensors and the additionalSuper Resolution Channel (DLR).

On 25 December 2003 the first European

planetary spacecraft mission ever, Mars

Express, was successfully inserted into

orbit around Mars. A few days before, its lan-

der module Beagle 2 had been separated.

Unfortunately, Beagle 2 has failed to communi-

cate since its first radio contact was missed

shortly after it was due to land on Mars on

Christmas Day. The Beagle 2 Management

Board met in London on 6 February and, follow-

ing an assessment of the situation, declared the

lander lost. The orbiter module Mars Express is

more fortunate, however. The spacecraft reached

its near-polar mapping orbit on 28 January

2004, after several manoeuvres to reduce the

eccentricity of the orbit, which was initially very

high. Meanwhile, the instruments performed

their first checks and began to acquire scientific

data. All its instruments are working well. The

40 m antenna of the ground-penetrating radar

MARSIS (Mars Advanced Radar for Subsurface

and Ionospheric Sounding) will be deployed

no earlier than April. This activity and the

subsequent test of this last instrument will mark

the end of the commissioning phase, during

which the science observations are restricted

by constraints from the spacecraft side.

Even within this commissioning phase, sig-

nificant scientific results have been reported.

For example, the French-built visible and

infrared spectrometer Omega (Observatoire

pour la Mineralogie, l’Eau, la Glace et

l’Activite) is the first instrument in history to see

firm and direct evidence for water molecules on

the martian surface. These findings were sup-

ported by data from the other two spectrom-

eters, the Italian PFS (Planetary Fourier

Spectrometer), a new high-resolution spec-

trometer of unprecedented accuracy, and the

French SPICAM (SPectroscopy for the

Investigation of the Characteristics of the

Atmosphere of Mars), a lightweight UV–IR

instrument on board the Mars Express orbiter

dedicated to the study of the martian atmo-

sphere and ionosphere. Both instruments

observed signatures from water in their spectra.

Mars

2.22 April 2004 Vol 45

Mars Express is Europe’s first planetary

explorer, a collaboration among 10

countries that carries instruments to

examine the surface, atmosphere and

space environment of Mars. Among the

instruments is the High Resolution Stereo

Camera, which has already produced

spectacular images of scientific value.

Here we present a selection of these first

images, together with a summary of the

capabilities and aims of this instrument.

The images so far show details of surfaces

shaped by volcanic, tectonic and fluid

flow processes during the history of the

planet. HRSC has a list of targets for

observation that should throw light on

some problematic aspects of martian

evolution, such as the existence of bodies

of surface water, and the influence of

tectonic and volcanic processes.

Abstract

3: The HRSC images acquired from January through early March 2004(orbits 1–160), are superposed on a background topographic map of Mars(derived from Mars Orbiter Laser Altimeter data; simple cylindricalprojection). (DLR/FU Berlin.)

The first PFS data also show that the carbon

oxide distribution is different in the northern

and southern hemispheres of Mars. ASPERA

(Analyzer of Space Plasma and Energetic

Atoms), a plasma and energetic neutral atoms

analyser built in Sweden, is aiming at answer-

ing the fundamental question of whether solar-

wind erosion led to the present lack of water on

Mars. Preliminary results show a difference in

the characteristics between the area of impact

of the solar wind and measurements made in

the tail of Mars’ magnetosphere.

On 9 January 2004 the German High

Resolution Stereo Camera (HRSC) began to

acquire multispectral stereo images of the mar-

tian surface with a lateral resolution of between

10 and 20 m/pixel. Imaging and mapping the

martian surface by the HRSC is one of the main

goals of Mars Express. The camera is a mult-

iple line-scanner and works in the pushbroom

mode. Nine CCD line sensors, each with 5184

pixels, are operated in parallel (figure 1). Five

panchromatic lines provide high-resolution

images for stereo and photometric analyses. The

stereo information is derived from the pan-

chromatic sensors, which observe the surface

with different imaging geometries as the space-

craft moves along the orbit. Four lines are

equipped with colour filters for multispectral

investigations. An additional Super Resolution

Channel (SRC) is equipped with a CCD frame

sensor and provides very high-resolution images

(from 2–3 m/pixel) that are embedded within

the HRSC context (figure 2). Such imagery is

ideal to study the variety of known (and possi-

bly unknown) surface processes that have acted

on Mars (see “What will HSRC do?” below).

In particular, the combination of broad areal

coverage with high lateral resolution is an

important step forward from previous imaging

experiments from Mars orbit. Due to the

improvement in lateral resolution of HRSC

(10–100 m) as compared to the Mars Orbiter

Laser Altimeter (MOLA; 100–1000 m), it is

also an ideal tool to investigate the topography

of small-scale surface features. While the

number of colour channels is restricted to four

(blue, green, red and near infrared), their high

lateral resolution complements the capacities of

spectrometers such as the Thermal Emission

Imaging System (Themis) on Mars Odyssey or

Omega on Mars Express, which have high spec-

tral, but limited lateral resolution.

What will HSRC do?

The scientific objectives and measurement goals

of the HRSC experiment on Mars Express have

been formulated by the international team of 45

co-investigators from 10 countries under the

leadership of one of the authors (Neukum), who

is the Principal Investigator of the instrument.

The major scientific objective of the HRSC is the

study of the evolution of the martian surface

structure and morphology through time and the

geological processes involved. Another impor-

tant topic is the investigation of atmospheric

characteristics and dynamic atmospheric phe-

nomena. When imaging conditions are

favourable, we will also take images of the two

Mars

2.23April 2004 Vol 45

5: Erosion shaped the chaoticterrains east of the giant troughs of VallesMarineris near the martian equator. Millions ofcubic kilometres of rock have been removed by theaction of water. The elevation difference in this image is about3000 m. The detail shown in this perspective view is 60 km across.The image was taken during orbit 18 on 14 January 2004 (centred at 5.5°Sand 37°W, resolution 11 m/pixel, altitude 270 km, north is towards the upper right).

4: A perspective view of a mesa in the chaotic terrains in the true colours of Mars. The summit plateaustands about 3 km above the surrounding terrain. The original surface was dissected by erosion, onlyisolated mesas remained intact. The large crater in the upper left background has a diameter of7.6 km. This picture was taken in orbit 18 on 14 January 2004 (centred at 9°N and 37°W, resolution12 m/pixel, altitude ~270 km, north is at the top).

small martian moons, Phobos and Deimos. In

particular, HRSC images will help to reveal the

past climate and the role of water throughout

martian history, the evolution of volcanism and

tectonism on Mars, and the interactions between

the atmosphere and the surface. An additional

objective is to determine the characteristics of

past, present, and future landing sites.

In order to meet these scientific objectives, the

following imaging capabilities have been imple-

mented in the HRSC experiment:

� Characterization of surface features and mor-

phology at high spatial resolution (~10 m/pixel)

� Characterization of surface topography at

high spatial and vertical resolution with dedi-

cated stereo imaging

� Characterization of surface features and mor-

phology at “super-resolution” (~2 m/pixel)

� Terrain classification at high spatial resolu-

tion by multispectral image data

� Scattering properties of the regolith and the

atmosphere by multiphase angle observations

� Characterization of atmospheric properties

and phenomena by limb sounding and nadir

observations

� Imaging quasi-simultaneously at high-resolu-

tion with stereo, in colours and at different

phase angles in order to avoid differences in

atmospheric and illumination conditions that

up to now have caused severe problems in the

photogrammetric data evaluation

� Acquisition of nested-in images at super-

resolution for exact georeferencing.

Orbital imagery in the 10 m/pixel range and

in the 2 m/pixel range as obtained by the HRSC

experiment is an essential prerequisite for

detailed surface exploration and for the solution

of open questions such as the role of water

throughout martian history, or volcanic evolu-

tion. The ability to study morphologic surface

features in more detail by photogeological

analysis is complemented by the possibility of

deriving ages even for small features such as

valley floors or surfaces of former lakes. The

HRSC high-resolution imagery also makes it

possible to count craters much smaller than the

features themselves – such data will be essential

for the reconstruction of the geological history.

The stereo and colour capabilities of the HRSC

significantly enhance the interpretation of the

imaging data. For instance, the accurate deter-

mination of erosion rates and modelling of var-

ious geologic processes such as water flow and

emplacement of lava flows, is presently limited

by the lack of information on local elevation dif-

ferences. The colour information will be impor-

tant for terrain classification, for the detection

of compositional layering and variations in the

composition of surface materials, and for the

recognition of different surface processes. The

multiphase imagery, finally, will allow the study

of physical properties of the martian soil and

will support the photogrammetric data evalua-

tion by providing a second stereo angle triplet

(in combination with the nadir channel).

If Mars Express fulfills its mission goals over

the entire planned lifetime of one Mars year

(two Earth years), HRSC will achieve one of the

largest extraterrestrial mapping efforts yet:

Mars

2.24 April 2004 Vol 45

6: Reull Vallis is a channel once formed by flowing water. The dark materialis probably either wind-blown material or sediments deposited by water.The image was taken in orbit 22 on 15 January 2004 (the location is east ofthe Hellas basin at 41°S and 101°E, image area is 100 km across,resolution 24 m/pixel, from a height of 540 km, north is at the top).

7: Kasei Vallis is the largest outflow channel on Mars. It wasformed by erosion involving liquid water or glaciers. Thisperspective view in false colours shows the depression ofEchus Chasma, the head of Kasei Vallis. The floor is coveredby lava flows (in the false-colour image showing up as bluishmaterial), obscuring any previous traces of water. The imagewas taken on orbit 71 on 1 February 2004 (centred at 0°Nand 79.5°W, resolution 12 m/pixel, altitude about 270 km,image is about 50 km across, south is at the top).

50% of the surface will be mapped at resolu-

tions better than 20 m/pixel, 70% at better than

40 m/pixel, and the entire surface at better than

100 m/pixel. If the mission lasts four years we

will be able to cover nearly 100% of the surface

of Mars at a resolution better than 20 m/pixel.

For comparison, the surface of Mars is equal to

the total land surface of all continents on Earth.

Scientific targets

The HRSC team has compiled a list of more than

1500 geoscientific targets by mapping the sites

and regions over the whole of Mars that hold the

greatest scientific interest. This huge list contains

all information with respect to geographic loca-

tion, extent, preferred imaging mode and reso-

lution, preferred season and illumination

conditions, as well as information on already

existing image coverage, classification of geo-

logic processes involved in the formation of each

target area and others. The list also covers the

field of surface/atmosphere interactions and of

variable features. Targets of specific interest for

cartographic and photogrammetric research

include the crater Airy, which defines the 0° lon-

gitude, and landing sites in order to refine the

geodetic control network. These targets require

more than one observation during the mission.

We also compiled a list of atmospheric targets for

limb sounding observations, which are particu-

larly interesting for atmospheric science. At least

two to three limb measurements for each Earth

month are envisaged throughout the mission.

Due to favourable conditions such as high

telemetry rates, large areas could be imaged in

the early stages of the mission. Some of the first

images cover areas as much as 60 to 200 km

wide and up to 4000 km long (figure 3). The

total area covered in the first month of imaging

is about 4 million km2, about half the area of

Europe. Several major physiographic provinces

have been sampled already, including the south

polar cap, Hellas, Tharsis with several shield

volcanoes, the Valles Marineris, the chaotic

regions (figures 4 and 5, and on the cover of this

issue), the outflow channels Reull and Kasei

Valles (figures 6 and 7), and different parts of

the southern highlands. We also took long

image traverses across some of the most promi-

nent volcanoes on Mars. Spectacular images of

the calderas of Olympus and Ascraeus Montes

in Tharsis (figures 8 and 9), and of Albor and

Hecates Tholi (figure 10) in Elysium, allow for

detailed investigations of the topography and

morphology of their floors. Tectonic surface

features and the aureole of Olympus Mons were

also shown in great detail (figures 11 and 12).

While the total data volume transmitted so far

is high, other constraints prevented us achieving

some of our initial imaging objectives. A hard-

ware problem meant that the power perfor-

mance of the spacecraft’s solar panels was lower

than expected. As a consequence, the spacecraft

could make fewer pointing manoeuvres and the

flexibility of operations was reduced. Therefore

we have acquired only two adjacent image strips

so far, which make up our only image mosaic

existing at the time of writing (March 2004).

The high dust content of the atmosphere in

Mars

2.25April 2004 Vol 45

8: Olympus Mons is the largest shield volcano in the solar system. Its caldera has a diameter of60 × 90 km. The image was taken during orbit 37 on 21 January 2004 (centred at 18.3°N and133°W, resolution 18 m/pixel, image is about 102 km across, altitude ~400 km, south is at the top).

9: The summitcaldera of AscraeusMons has experiencedseveral stages of collapse.The diameter of the calderacomplex is between 50 km and60 km. It is up to 4000 m deep. Theimage was taken on orbit 68 on 31January 2004 (centred at 11.3°N and104.4°W, resolution 16 m/pixel, altitudeabout 360 km, north is towards the upper right).

early 2004, also noted by the two Mars

Exploration Rovers and other orbiting instru-

ments, meant that images of some regions with

low elevations have low contrast. For example,

the supposed landing site of Beagle 2 in Isidis

Planitia was imaged by HRSC in search of clues

to the descent (such as the parachute). The

dusty atmosphere, however, made any success-

ful search in the images impossible.

The dust also caused another problem early in

the mission. It is not easy presently to arrive at

the true colours of the martian landscape in the

reproduction of images because Mars is very

dusty and the scenes were taken when the Sun

was high. Under these atmospheric conditions

the dust in the atmosphere acts as tiny red fil-

ters and one looks at the surface as if in a dif-

fuse reddish glow with a somewhat fuzzy

appearance. Imagine that you had to judge the

colour of a cloth in a room with artificial red-

dish light – it would be very difficult. The

colours in most pictures released so far are close

to true colours, but a slight tint in the reddish

or bluish direction from non-perfect adjust-

ments of the colour histograms cannot be

excluded due to the somewhat abnormal “see-

ing” conditions.

While the focus of HRSC observations is on

surface observations of Mars, additional objec-

tives are the investigation of the planet’s atmo-

sphere. On two occasions, on 16 January and

1 February 2004, Mars Express together with

the NASA rover Spirit in Gusev crater made the

first joint observations of surface and atmo-

sphere. HRSC and other instruments on Mars

Express observed the rover’s landing site from

orbit, while simultaneously some of the rover

instruments looked upwards into the sky.

Looking to the future

The HRSC on Mars Express will be able to

close the existing gap between medium to low-

resolution coverage on the one hand and the

very high-resolution images of Mars Global

Surveyor as well as the in-situ observations and

measurements by landers on the other hand.

The HRSC on Mars Express will make a major

contribution to the areas of geosciences, atmo-

spheric sciences, photogrammetry/cartography,

and spectrophotometry of Mars. Special

emphasis will be put on the evolution of the

martian surface due to volcanic processes and

the role of water throughout martian history.

The instrument will obtain images containing

morphologic and topographic information at

high spatial and vertical resolution of unique

photogrammetric quality allowing the improve-

ment of the martian cartographic database

down to scales of 1:50 000.

The HRSC team will present its first major

scientific discoveries at the Lunar and Planetary

Science Conference, Houston, and at the 1st

General Assembly of the European Geosciences

Union, Nice, France, in late April 2004. �

Ernst Hauber, Inst. of Planetary Research, GermanAerospace Centre (DLR), Berlin, Germany;Gerhard Neukum, Inst. of Geosciences, FreieUniversität Berlin, Berlin, Germany.

Mars

2.26 April 2004 Vol 45

10: The summit caldera of Hecates Tholus,the northernmost volcano of the Elysiumvolcano group. The volcano reveals multiplecaldera collapses. On the flanks of HecatesTholus, several flow features related towater (lines radiating outwards) and pitchains related to lava can be observed. Thevolcano has an elevation of 5300 m, thecaldera has a diameter of maximum 10 kmand a depth of 600 m. The image was takenon orbit 32 on 19 January 2004 (centred at150°E and 31.7°N, resolution 25 m/pixel,altitude about 560 km, north is at the top).

11: The AcheronFossae are a large

system of tectonic featuresnorth of Olympus Mons, near the

Tharsis region. They were probably formedby large-scale lithospheric extension. The

largest grabens are more than 2 km deep. The imagewas taken from a height of 675 km during orbit 37 on 21

January 2004 (centred at 37°N and 132°W, resolution 30 m/pixel,the image width is about 150 km, north is towards the upper right).

12 (right): The eastern flankof the giant shield volcano

Alba Patera is dissected bynumerous tectonic grabens.

They reflect the complexevolution of Alba Patera andthe entire Tharsis region of

Mars. The image was takenon orbit 68 on 31 January2004 (centred at at 40°N

and 103°W, resolution32 m/pixel, altitude about

720 km, image width about112 km, north is at the top).

Mars

2.27April 2004 Vol 45