ESA’s ExoMars Trace Gas Orbiter (TGO) has sent back its first color image of Phobos, the larger and inner of the two natural satellites of Mars.
Color composite of Phobos taken with TGO’s CaSSIS camera on November 26, 2016. The observation was made at a distance of 4,785 miles (7,700 km). To create the final color image, two images were taken through each of the four color filters of the camera — panchromatic, blue-green, red and infrared — and then stitched together and combined to produce the high-resolution composite. Two of the color filters used by CaSSIS lie outside the wavelength response of the human eye, so this is not a ‘true’ color image. However, showing the data as a color representation can reveal details of the surface mineralogy. Different colors are clearly seen, with the bluest part in the direction of the large crater Stickney, which is out of view over the limb to the left. Although the exact composition of the material is unknown, the color differences are thought to be caused by compositional variations on scales of hundreds of feet to several miles. Image credit: ESA.
Phobos was discovered along with its smaller companion, Deimos, by the American astronomer Asaph Hall at the Naval Observatory in 1877.
The moon measures 16.2 x 13.7 x 11.2 miles (26 x 22 x 18 km) and has a very lumpy appearance. It also has craters and grooves on its surface.
It orbits Mars at a distance of 3,721 miles (5,989 km) every 7 hours and 39 minutes.
TGO’s Colour and Stereo Surface Imaging System (CaSSIS) instrument imaged the moon on November 26, 2016, from a distance of 4,785 miles (7,700 km).
The spacecraft’s elliptical orbit currently takes it to within 143-193 miles (230-310 km) of the Martian surface at its closest point and 60,895 miles (98,000 km) at its furthest every 4.2 days.
CaSSIS’s filters are optimized to reveal differences in mineralogical composition, seen as ‘bluer’ or ‘redder’ colors in the processed image.
“Although higher-resolution images of Phobos have been returned by other missions, such as ESA’s Mars Express and NASA’s Mars Reconnaissance Orbiter, this provided a good test of what can be done with our data in a very short time,” said CaSSIS principal investigator Dr. Nick Thomas, from the University of Bern in Switzerland.
Two other instruments on board the orbiter also made calibration measurements of Phobos, and the teams are analyzing their data.
“We’re very happy with the results of both test science orbits and will be using these calibration data to improve our measurements once we begin the main science mission later next year,” said Dr. Håkan Svedhem, ESA’s TGO project scientist.
The focus of the mission now returns to preparations for aerobraking required to bring the spacecraft towards its near-circular science orbit by the end of 2017. More details on the upcoming operations will be provided soon.
