1. March 2018
Mars Express mission

New Mars Ex­press im­ages of the Mar­tian moons Pho­bos and Deimos

As­tro­met­ric ob­ser­va­tion of Pho­bos
Image 1/4, Credit: ESA/DLR/FU Berlin CC BY-SA 3.0 IGO.

Astrometric observation of Phobos

As­tro­met­ric ob­ser­va­tion per­formed us­ing the Su­per Res­o­lu­tion Chan­nel (SRC) of the High Res­o­lu­tion Stereo Cam­era (HRSC). In the fore­ground, the be­tween 18.2 and 26.0-kilo­me­tre Mar­tian moon, Pho­bos; in the back­ground, a ref­er­ence star (cir­cled in red) used to ac­cu­rate­ly de­ter­mine the po­si­tion. (Or­bit 17,746, 8 Jan­uary 2018).
Deimos and Sat­urn
Image 2/4, Credit: ESA/DLR/FU Berlin CC BY-SA 3.0 IGO.

Deimos and Saturn

As­tro­met­ric ob­ser­va­tion per­formed us­ing the Su­per Res­o­lu­tion Chan­nel (SRC) of the High Res­o­lu­tion Stereo Cam­era (HRSC). In the fore­ground is the on­ly 6.2-kilo­me­tre Mar­tian moon, Deimos; in the back­ground, Sat­urn is vis­i­ble, clear­ly recog­nis­able by its rings. (Or­bit 17,770, 15 Jan­uary 2018).
Im­prove­ment of the sur­face shape mod­el of Pho­bos
Image 3/4, Credit: ESA/DLR/FU Berlin CC BY-SA 3.0 IGO.

Improvement of the surface shape model of Phobos

These slight­ly dif­fer­ent im­ages were ac­quired by five dif­fer­ent chan­nels of the High Res­o­lu­tion Stereo Cam­era (HRSC) dur­ing Or­bit 17,342 on 12 Septem­ber 2017 – the nadir chan­nel (cen­tre im­age), the two pho­tom­e­try chan­nels (left and right of the cen­tre im­age) and the stereo chan­nels (on the far left and right). The fact that an­oth­er area of the moon’s sur­face can be seen in ev­ery im­age at a slight­ly dif­fer­ent view­ing an­gle al­lows the mod­el of its sur­face shape to be im­proved.
Image of Phobos acquired by the nadir channel of HRSC
Im­age of Pho­bos ac­quired by the nadir chan­nel of HRSC
Image 4/4, Credit: ESA/DLR/FU Berlin CC BY-SA 3.0 IGO.

Image of Phobos acquired by the nadir channel of HRSC

Im­age of the Mars moon Pho­bos ac­quired by the High Res­o­lu­tion Stereo Cam­era (HRSC) nadir chan­nel at full res­o­lu­tion in the course of a fly­by that took place on 12 Septem­ber 2017 dur­ing or­bit 17,342 of ESA’s Mars Ex­press space­craft.
  • New, high-resolution images of the HRSC camera on board Mars Express show the surface of the Martian moon Phobos with a resolution of nine metres per pixel.
  • Such data improves the calculation of positional data and enables more accurate models of the moon's surface.
  • Phobos would be an ideal location for a permanent 'observation post' to study the Martian surface, and the HRSC data could help in the selection of a landing site.
  • Focus points: space, planetary research

Mars Express is currently the only satellite exploring Mars from an elliptical orbit. This allows regular, close flybys of Phobos, the larger of the two Martian moons. In summer 2017, the High Resolution Stereo Camera (HRSC) observed the moon from a distance of approximately 115 kilometres. The HRSC, developed and operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), acquired images of Phobos at a resolution of up to four metres per pixel. During the ESA Mars Express mission, which has been operating for more than 14 years, Phobos has already been observed several times from distances of less than 150 kilometres.

A prerequisite for such spectacular images from space is a very accurate knowledge of the position of Phobos on its orbit around Mars. As the spacecraft passes the object at approximately three kilometres per second, it only has a few seconds to observe the irregularly-shaped body, which has a maximum diameter of just 26 kilometres. Since the start of the mission in 2003, measurements of the positions of the Martian moons in relation to the stars have repeatedly been obtained by examining images acquired by the Super Resolution Channel (SRC), which is part of the HRSC and has four times the resolution delivered by the main HRSC channels (Image 1). These are used to continuously determine the positions of the Martian satellites and improve position forecasting.

Detailed models of Phobos’ surface using HRSC images

The methods for calculating positional data have continuously improved during the course of the Mars Express mission as a result of on-going development. At first, only simple models of the body were used, known as triaxial ellipsoids. Unlike on Earth, for example, the equator of such bodies is not described by a circle with radius a, but with an ellipse with radii a and b, in addition to the smallest radius c along the polar axis. Such a triaxial ellipsoid allows the position of Phobos to be determined to within around one to two kilometres in the images. We can now use accurate surface shape models to simulate the anticipated position and appearance of Phobos in the images. The observations are therefore much more precise. However, the uncertainty in determining the position remains between 100 and 200 metres.

Astrometric observation of Phobos and Saturn performed using the Super Resolution Channel (SRC) of the High Resolution Stereo Camera (HRSC). The animation consists of 30 separate images acquired during Mars Express Orbit 16,346 on 26 November 2016. The slight up and down movement of Saturn and Phobos in these images is caused by oscillation of the spaceship orientation after completing the turn towards the moon.
Credit: DLR (CC-BY 3.0)

Last year, special images were analysed that show several Solar System bodies in one image. Phobos and Deimos were imaged together or individually, with Jupiter or Saturn (Image 2, Video) in the background. The two planets are located several hundred million to one billion kilometres away from Mars Express. From these observations, we can determine the position of a body in the Solar System in relation to other bodies, without the camera's inevitable orientation errors affecting the result. We are also able to better determine the position of the two Martian moons.

The above-mentioned surface shape models were obtained using HRSC image data from close flybys (Image 3). High-resolution images show the surface of Phobos at nine metres per pixel. The HRSC was specifically designed to provide data for the creation of digital surface models during one flyby. For Phobos, many flybys are necessary to be able to record the small satellite from all sides, so that gradually a reconstruction of the moon’s entire surface is possible. The images from summer 2017 provided scientists with another piece of the puzzle, enabling the surface shape models to be further refined. Such spatial models are of enormous help when answering scientific questions.

Martian moon Phobos a possible destination for a landing mission

It is still not entirely clear how the two Martian moons formed. There is discussion as to whether Phobos and Deimos, like Earth's Moon, were created from the debris of a major impact in Mars' early history, for example, and have been in orbit ever since. An alternative possibility is that they originate from another region of the Solar System (such as the asteroid belt) and have been captured by the planet's gravity. Not least because of this question, Phobos is attracting attention as the destination of a future landing mission. The moon would also be an ideal location for a permanent 'observation post' to study the Martian surface, due to it being tidally locked (it always points the same part of its surface towards Mars, because its rotation period is equal to its orbital period around the planet). When selecting a landing site, the HRSC terrain models will be an important basis for decision-making.

Contact
  • Elke Heinemann
    Ger­man Aerospace Cen­ter (DLR)
    Pub­lic Af­fairs and Com­mu­ni­ca­tions
    Telephone: +49 2203 601-2867
    Fax: +49 2203 601-3249

    Contact
  • Prof.Dr. Ralf Jaumann
    Freie Uni­ver­sität Berlin
    In­sti­tute of Ge­o­log­i­cal Sci­ences
    Plan­e­tary Sci­ences and Re­mote Sens­ing
    Telephone: +49-172-2355864
    Malteserstr. 74-100
    12249 Berlin
    Contact
  • Ulrich Köhler
    Pub­lic re­la­tions co­or­di­na­tor
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Telephone: +49 30 67055-215
    Fax: +49 30 67055-402
    Rutherfordstraße 2
    12489 Berlin
    Contact
  • Daniela Tirsch
    Ger­man Aerospace Cen­ter (DLR)

    In­sti­tute of Plan­e­tary Re­search
    Telephone: +49 30 67055-488
    Fax: +49 30 67055-402
    Linder Höhe
    51147 Köln
    Contact
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