Since January 2004, the High Resolution Stereo Camera (HRSC) operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board ESA's Mars Express spacecraft has been transmitting images of both the Red Planet and its two moons, Phobos and Deimos. On particularly close fly-bys – at distances of less than 250 kilometres from the larger of the two Martian moons, Phobos – the Super Resolution Channel (SRC) can acquire images with resolutions down to 2.5 metres per pixel. The animation shown here was created from 41 SRC images acquired at a slightly greater distance on 17 November 2019, during orbit 20,076. Phobos flew past the spacecraft and the camera 'tracked' the moon throughout the brief rendezvous between Phobos and Mars Express. The slight up-and-down motion of the moon is caused by the spacecraft’s slight oscillation, having rotated from its normal Mars pointing towards Phobos to acquire the images. On this fly-by, the distance between spacecraft and moon was approximately 2400 kilometres. The images therefore have a resolution of around 21 metres per pixel. Nevertheless, numerous details can be seen, such as impact craters and kilometre-long linear markings and furrows on the irregularly shaped body. Stickney, a crater on Phobos measuring 10 kilometres across, can be seen near the centre of the images.
At first sight, the animation seems unremarkable. The 26-kilometre-long body can be seen from different angles, so that the moon first becomes lighter and then darker again. For scientists, however, such images acquired at different phase angles (as the angle between the Sun and the observer is known [see Image 1]) are of particular interest. Because of the different shadows that are cast and the varying amounts of reflected sunlight at different angles, it is possible to draw valuable conclusions about the surface material properties, particularly its roughness and porosity.
Even planetary bodies can be observed in 'opposition'
Of particular interest is observation at a phase angle of exactly zero degrees; that is, when the Sun is directly behind the observer (Part B of Image 1). At this point, the Sun shines vertically on to the surface and all shadows disappear. This leads to an increased surface brightness at the centre of the image. If, for example, the observer’s shadow is captured in the image, a kind of halo around is created around it in the centre of the image. As this only occurs if the Sun, as the light source, is exactly in opposition to the imaged object, this phenomenon is referred to as the opposition effect.
This can even be observed from Earth to some extent during a full moon, as not only is a large area of the Moon illuminated, but the Sun, Earth and Moon are in a relatively straight line (that is, in opposition), thus creating no visible shadows on the Moon's surface. This is why a full moon appears up to 10 times brighter than a half moon. Many people may already have noticed that when looking down at the ground from an aircraft, a light halo can be observed around the shadow of the aircraft. Other well-known examples include photographs taken on the surface of the Moon. When the Apollo astronauts taking the photographs had the Sun directly behind them, a halo would appear around the shadow of their head. All of these phenomena are due to the opposition effect. Conclusions about the nature of the surface material can be derived from the strength of this effect.
What can we learn from 'opposition'?
The reason for this lies in the fact that if light illuminates a surface at an oblique angle, a large proportion of it is reflected multiple times by the surface unevenness before either being reflected back into space with a small loss in brightness or partially absorbed by cavities from which it can no longer radiate. In the case of opposition, however, the entire amount of light that hits the surface vertically and is not absorbed by the surface material is reflected back into space. This leads to a significant brightening of the surface and partially reveals structures that are not visible under oblique illumination, such as the radiating ejecta patterns around impact craters (see Image 2). For scientists, studying the exact reflectivity of a planetary surface at different phase angles – deriving its phase curve – allows them to make statements about its material properties, such as the degree of weathering of the regolith, which darkens over many millions of years due to bombardment by micrometeorites.
Planetary surfaces reflect sunlight in very different ways. At full moon, the Moon is actually reflecting only 12 percent of the incident sunlight. Scientists specify this value – referred to as the 'geometric albedo' – as the proportion of sunlight reflected at a phase angle of zero degrees. The lunar disc illuminated by the Sun is in sharp contrast to the black of space. In reality, the 'mirror' of the lunar disc provides us with only one millionth of the sunlight that reaches Earth directly from the Sun. Earth’s average albedo is 36.7 percent. On Mars it is 15 percent and on the Martian moon Phobos it is just seven percent. Scientists have been pondering the cause of this for decades. Perhaps the difference in brightness is an indication that Phobos is not debris from a gigantic impact on Mars, but that the misshapen body is an asteroid captured by the planet’s gravity.
Rare opportunities with many benefits
As positional arrangements of the Sun, Mars Express and Phobos where the latter is observed at a phase angle of zero degrees are very rare (arrangements with phase angles of less than one degree occur approximately three times a year), the HRSC imaging planners take every opportunity to acquire such images, regardless of the distance from Phobos. They also make the most of phase angles near to (but not quite) zero degrees. In the image with the lowest phase angle here (the brightest picture in the middle of the animation), it is 0.92 degrees. The next opportunities will not arise until April and September 2020. On the latter date, it will be possible to perform an observation at exactly 0.0 degrees, at a distance of 2900 kilometres and with a resolution of 120 metres per pixel.
In addition to the characterisation of the surface and exploration of their origins, the images of the Martian moons also serve to accurately determine their ephemerides – which give their trajectories around Mars. For this purpose, the scientists primarily use images in which one of the moons is shown together with another object, such as a star or planet. Determining the exact location of these bodies is important, not least for the purpose of missions with the moons as their destination. At present, a Japanese mission (Martian Moons eXploration; MMX) is at the planning stage. Not only will it observe the moons, but will also transport a sample of one of them back to Earth. To this end, a rover jointly developed by DLR and the French space agency CNES is set to land on one of the moons and perform a detailed characterisation of its surface, in preparation for the subsequent acquisition of samples. The mission will be carried out by the Japanese Aerospace Exploration Agency (JAXA) and is currently scheduled for launch in 2024. Knowledge about the moons acquired with HRSC and SRC is indispensable for preparing this mission.