Mars Express HRSC images Frequently Asked Questions (FAQs)

About images and the High Resolution Stereo Camera (HRSC)

What can the images from the Mars Express HRSC tell us?

The HRSC is the first fully functional stereo camera in the history of planetary research. Computer animations enable its images to be turned and tilted, so that the observer has a real feeling of dashing about over the surface of the planet.

The 3D pictures enable exact reproductions of the relief. This, in turn, enables scientists, for the first time, to make a detailed examination of the altimetry of the Martian surface, to calculate the slope of the valleys, study the layer formation and calculate the masses of mountains.

In short, the images provide important data for the geomorphological and geological interpretation of the surface of Mars.

How do the Mars Express HRSC pictures get back to Earth?

Before the HRSC team can lay their hands on the pictures, the data have to make the long journey back home. After the pictures are taken and stored on board Mars Express, the data is transmitted to ESA’s New Norcia Station in Australia, or to NASA’s Deep Space Network Station in Madrid. From there, the data are collected at the European Space Operations Centre (ESOC) at Darmstadt in Germany. They are then transmitted to the German Aerospace Research Establishment (DLR) in Berlin, where most of them will be processed into the pictures we see.

How long does it take to process pictures, and why?

It usually takes between four and seven days for a picture to be ‘ready’. Depending on the picture size, analysis objective and the amount of computer capacity available, this period can easily stretch to ten to fourteen days. These techniques are still very new and recently gained knowledge and experience has often meant that more-expensive picture processing is required.

From the start of January to the middle of February, Mars Express produced a total of 18 strips of pictures in 100 orbits of Mars. In general, one orbit produces an image with a length of over 250 000 lines, sometimes more, sometimes less. The calculations required and combination of the channels takes a lot of computing time. All new images have to be released by the Principle Investigator to the Co-Investigators, and then are prepared for release to the media and the public - this can take up to a week per image.

What are the green/blue patches we see in some of the images?

The colour of the greenish patches in Gusev crater in the first image released of this site is not correct. Unfortunately, an early version of Gusev image was released before the colour had been adjusted finally to be closer to true colour as the human eye would see it in a non-dusty atmosphere of Mars.

The early version of this image was replaced by a newly computed version with proper colours where the patches are grey to black with a blue tint. This is close to what a human eye would see under normal atmospheric conditions. Currently it is not easy to get true colours of the Martian landscape because Mars is very dusty and the scenes were taken with high sun angles. The scattering of light caused in these atmospheric and lighting conditions, by the dust in the atmosphere acting as tiny red filters, means that you see the surface with a diffuse reddish glow with somewhat fuzzy appearance.

Are the colour photographs processed?

Yes, the images have been processed but that is quite normal. We are not taking colour photographs, we have to combine the different colour channels which requires processing time. Each of the four colour channels operate with a filter of different wavelength (red, green, blue and infrared) and produce data sets which have to be combined and calculated on to a digital elevation model.

The colour channels are absolutely real, but they do not reflect the true colour as we would see it with our eyes. These views can be obtained by processing the data, which does not mean ‘faking’ the colours, but fitting them to standard spectral curves which we know. We adjust the obtained colour image data to a standard spectral curve derived from Earth-based observations in terms of wavelength and intensity.

If we have dust, haze or other atmospheric conditions, various wavelengths get partially filtered. Obviously, images get more blurry and some colours become more dominant. Sun-exposed slopes reflect or absorb light in a different way to dark slopes or dark material (such as dark spots of possible sediments seen in some images).

The main limitation, of course, is that from orbit we do not have any colour adjustment possibilities as for example, the NASA rovers. They have colour references mounted on their rovers and even they have problems matching their colours.

Are the HRSC images ‘doctored’ in any way?

No, the images are not ‘doctored’. We use standard techniques for radiometric and geometric calibration but this is not ‘doctoring’. Nobody wants to see the raw image data as they are received, but even these will become accessible to the public, normally after six months. If, by chance, we received images suffering from, for example, ‘salt and pepper’ effects, or oversaturated pixels, they are ‘recovered’ using filter techniques from standard systematic processing procedures. These techniques have been used since the early days of image processing.

Did Mars Express HRSC take pictures of the so-called ‘face’ in the Cydonia region?

Yes.

The Mars Express orbiter was not in a suitable position to collect images of Cydonia area until late in 2004. Until then, Mars Express was able to observe closely only southern latitude features. Planetary scientists are interested in Cydonia because it is part of the boundary between highland and lowland areas.

Mars Express did cover this area on 1 April 2004 (orbit 262), 29 December 2004 (orbit 1216), 1 January 2006 (orbit 2533) and 6 April 2006 (orbit 2872), but it was not possible to obtain images of high enough quality due to factors such as the swath width at that location, resulting in poor ground resolution and atmospheric distortion during data acquisition, heavily reducing data quality.

The next encounter was on 22 July 2006 (orbit 3253). HRSC parameters were set to get the best possible data quality for nadir and both stereo channels. Luckily, unpredictable atmospheric phenomena such as clouds or dust over the target area did not limit or prevent acquisition of high quality data for Cydonia, now public.

To view the Cydonia images obtained by Mars Express click here.

When the orbiter arrived over a suitable position in the northern hemisphere, it used its High Resolution Stereo Camera in the course of its planned and routine mapping of the surface, but not specifically to study any ‘face-like’ features. The mesas of Cydonia are of interest to planetary scientists, but they were not a priority for observations. These images are being used to look at the geological formations in this region to see where water once flowed, as some scientists think the northern plains are all that is left of an ancient Martian ocean.

Twenty five years ago, NASA's Viking 1 spacecraft was circling Mars, taking photographs of possible landing sites for its twin, Viking 2, when it spotted the shadowy likeness of a human face in the region of the Red Planet called Cydonia. The ‘Face on Mars’ became a popular icon. It has starred in films and appeared in books and magazines for 25 years! Some people thought the ‘face’ is real evidence of life on Mars.

Although most scientists did not believe the face was an alien artifact, photographing Cydonia became a priority for NASA’s Mars Global Surveyor when it arrived at the Red Planet in 1997. In April 1998, Mars Global Surveyor flew over Cydonia for the first time and took a high-resolution photograph, ten times sharper than the original Viking pictures. It revealed a natural landform, there was no alien monument after all.

But not everyone was satisfied, because the camera on board Mars Global Surveyor had to peer through wispy clouds to see the surface. Perhaps, the sceptics said, alien markings were hidden by haze. In 2001, Mars Global Surveyor drew close enough for a second look and captured an extraordinary photograph using maximum resolution. Each pixel in the 2001 image represented 1.5 metres, compared to 43 metres per pixel in the best Viking image from 1976.

The picture showed a feature called a ‘mesa’, like the landforms common in western USA. The Cydonia region is littered with many mesas like this, but which do not look like human heads and they attract little popular attention. Subsequent laser altimetry scans, with height measurements made to within 20 to 30 centimetres, confirm that its shape and all of its dimensions are similar to the other mesas and not exotic in any way.

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