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Last Updated: Tuesday, 3 February, 2004, 16:34 GMT
Wish you were here: Imaging Mars
By Paul Rincon
BBC News Online science staff

The spectacular images of Mars being sent back by European and US spacecraft give us a thrilling insight into what it must be like to travel to the Red Planet.

A channel probably cut by water

While the camera aboard Europe's Mars Express orbiter has captured the breathtaking scale of the planet's mesas, channels and calderas, those on the US space agency's (Nasa) rovers have caught the exhilarating strangeness of the Martian surface.

But these vistas are more than mere eye candy. Imaging technology is crucial to both current Mars missions.

The main camera on Nasa's Mars rovers Spirit and Opportunity is the panoramic camera, or Pancam. It sits atop a long mast that can move up or down and rotate through 360 degrees.

The Pancam itself actually consists of two cameras, to give it stereo vision. This allows scientists to measure sizes and depths on the Martian surface.

Each Pancam camera has a revolving wheel with a range of colour filters, including red, green, blue and infrared.

True colours

We try to get as close as possible by using a combination of what comes out of the camera and experience
Dr Jim Bell, Nasa
Pictures taken through red, green and blue (RGB) filters can be combined on Earth to form a composite colour image - a process Dr Jim Bell, head of Nasa's Pancam team, describes as a mixture of science and art.

Colours in these images can never match what we would see if we were standing on Mars observing the same scene. There are two principal reasons for this.

Firstly, the RGB filters see much narrower parts of the electromagnetic spectrum than do our own eyes. Secondly, people differ in the way they perceive colours.

Nasa scientists use software to re-process the image once it has been transmitted to Earth. But the end result can never be more than a close approximation to the colours as viewed by an average person.

"We try to get as close as possible by using a combination of what comes out of the camera and experience," Dr Bell explains.

View 360 degree panorama from the Opportunity probe's landing site


"But true colour as a scientific definition is not something you can get everybody to agree on."

Speed of light

When the rover takes a picture, it stores it in either its Ram or flash memory. The image is put into binary code, allowing it to be sent back to Earth as a data packet during a communication session.

These communication sessions are conducted three or four times a day, either through "direct-to-Earth" links using the rover's high-gain antenna, or via the two US probes in orbit around the planet: Mars Odyssey and Mars Global Surveyor.

The image data is transmitted back to Earth at the speed of light and usually at 12 bits per pixel. It is received by one of three radio telescopes around the world: at Goldstone in the US, at Tidbinbilla in Australia, and at Madrid in Spain.

"It takes 10 or 11 minutes to get the signal at the speed of light from Mars and then a matter of seconds for it to be sent to the Jet Propulsion Laboratory," says Dr Bell.

You'll be able pick out any point on the picture and get an altitude for it
Dr John Murray, Open University
In order to construct the amazing colour panoramic images of Spirit and Opportunity's landing sites, the rovers slowly rotate the Pancam on its mast, taking pictures as they go.

Mosaic image

The images must then be pieced together as a mosaic by scientists on Earth.

"It can take many days to acquire all those pictures because we're taking them a little tiny piece at a time and then stitching them together," Dr Bell explains.

"Over the time that it takes to build them up, the lighting changes on Mars and dust levels change in the atmosphere. You end up with a patchwork of different intensity levels: bright areas and lighter areas because the Sun is brighter or lower. Shadows are different.

"It's somewhat of a painstaking activity to normalise or blend all that out."

While the rovers inspect the fine detail of the Martian landscape, Europe's Mars Express probe orbits high above the surface, building up a 3D map of the planet.

EUROPE'S VISION OF MARS
Mars Express, Esa

The European orbiter is the first probe to capture images of the Red Planet using a high-resolution stereo camera (HRSC).

The stereo facility will enable scientists to precisely measure the heights of features on Mars. In the past they had to rely on indirect ways of doing the same thing.

This data will then be used to construct the 3D digital model of Martian surface terrain.

3D model

"We will have very accurate topography. You'll be able pick out any point on the picture and get an altitude for it," explains Dr John Murray, of the Open University and a co-investigator on the HRSC.

This will help scientists understand the forces responsible for shaping the Martian surface.

View colour 360 degree panorama from Spirit's landing site


"We'll be able to get slopes, depths of craters, depths of canyons, volumes of lava flows, thicknesses of geological layers or polar ice layers," adds Dr Murray.

The HRSC uses the principle of a line-scan camera. It exposes nine "lines", each consisting of 5,184 light-sensitive cells or pixels, creating nine independent image strips.

The camera is capable of taking pictures at a resolution of up to 10 metres per pixel with its normal camera head and up to 2.3 m per pixel with its super resolution channel.

Forward-looking

There are three stereo channels, one that looks forwards, one that looks backwards and one that looks down.

To get a stereo reconstruction of the Martian topography, the camera takes pictures using the forward and down channels and the backward and down channels.

The High Resolution Stereo Camera aboard Mars Express

Software then correlates patterns from the pictures to find corresponding points, and matches them automatically. Pictures from the different channels are then combined to produce a virtual model which can be viewed from any point.

Commands for taking images are sent from Earth to the onboard computer that controls the camera. An internal clock tells the camera when to start capturing an image.

The resulting pictures are recorded on solid-state memory and squashed in size using a preset compression factor for transmission to Earth.

Seamless journey

Compression of the images is necessary to ensure the amount of data sent back does not exceed the bandwidth of the transmitter.

Mars Express views the caldera of the volcano Albor Tholus

"Clearly, there's competition between all the different instruments. They can't all be capturing the highest quality data at the same time," says Professor Jan-Peter Muller of University College London and another co-investigator on the HRSC.

When the orbiting spacecraft swings away from Mars, it starts transmitting the pictures back to Earth at the speed of light. The data is received either at Nasa's radio telescope in Madrid, Spain, or at another dish at New Norcia in Australia.

The project has a special group working on calibrating the camera's photometry channels to get as close to the true colour of Mars.

One of the most exciting prospects for the future is a possible collaboration between the European Space Agency and Nasa to combine image data from Mars Express, Mars Global Surveyor, Mars Odyssey and the two rovers to produce a seamless 3D "journey" around Spirit and Opportunity's landing sites.




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