Phobos

Introduction

First sighting of the Martian moons, Deimos and Phobos, was reported in 1877 by Asaph Hall, an astronomer of the Naval Observatory in Washington. The increasing interest in Phobos, the larger of the two Martian moons, is not only due to the steadily increasing number of space based and earth based observations but also to its low orbit about Mars. Its motion and rotation are thus effected by the Martian gravity field and variation may lead to further clues of its origin and evolution. Furthermore, Phobos was identified as potential target for sample return missions by different mission planning groups.

The German Aerospace Center (DLR) was involved in an in-depth study of this natural satellite. Based on images of the multi-spectral line scanning High Resolution Stereo Camera (HRSC) and the Super Resolution Channel (SRC) – a frame camera – the department of Planetary Geodesy

  • observed Phobos in its orbit
  • determined shape parameters
  • computed global digital elevation models
  • prepared a first low scale maps based on image data only
  • derived new estimates on its rotational parameters
  • and could derive mass distribution models agreeing to all other constraining parameters

Phobos is a very irregular body (Fig. 1) often approximated by a tri-axial ellipsoid for simplicity (Batson et al., 1992). With a mean distance of only 9375 km to the center of Mars – which relates to approximately 6000 km above the Martian surface – it is orbiting deep in the Martian gravity field. Its orbit is in a nearly equatorial plane and almost circular.

Figure 1: Phobos observed by the HRSC on Mars Express in August 2004.

Facts

Property Value
Mean Distance to Mars 9375 km
Radii a 13.0 x 11.39 x 9.07 km
GM b 0.0007158 ± 0.0000005 km3/sec2
Orbital period 7 h 39 min 14 sec
Eccentricity e 0.01515
Mean velocity (absolute) 2.14 km/sec
Mean velocity w.r.t. Mars 1.47 km/sec
Inclination i referred to Mars equator 1.075°

a Willner et al., 2009

b Konopliv et al., 2006

Image Data

Figure 2: Phobos observed by the HRSC during until January 2010 closest flyby during the orbit 5851. Close flyby images of the HRSC were used to prepare the global Digital Terrain Model of Phobos (Wählisch et al., 2009). Credits: ESA/ DLR/ FU Berlin (G. Neukum)

The HRSC and SRC are instruments on board the European Mars Express mission. The spacecraft is in a highly elliptical nearly polar orbit about Mars with an apoapsis height of about 11,000 km above the planets surface, reaching well beyond the orbit of Phobos. Hence, regular flyby observations are possible through both instruments. At flyby distances below 1000 km it is anticipated to obtain stereo information through the multiple channels of the HRSC (see www.dlr.de/mars/en/Desktopdefault.aspx/tabid-290/944_read-1409/ (or the description on europlanet.dlr.de/HRSC) for an introduction on the HRSC camera). An example of a flyby sequence obtained by the HRSC is shown in Fig. 2. The SRC acts as a magnifying glass compared to the HRSC as it has an almost 6 times larger focal length compared to the HRSC leading to a magnification factor of approximately 4.3. During a flyby the SRC obtains 8 images. Fig 3. depicts examples of SRC images from different flybys.

Figure 3: SRC images of different orbits in comparison to their HRSC counterparts. Credits: ESA/ DLR/ FU Berlin (G. Neukum)

Digital Terrain Model

HRSC images were used to obtain 3-dimensional coordinates of surface points of Phobos. As currently the HRSC could not gain global coverage yet, the data was complimented by Viking orbiter image data to be able to provide the digital terrain model globally. (Willner et al., 2008, 2009)

Mosaic and Map

26 SRC images and 8 Viking images were used to produce a global mosaic in a resolution of 16 pixel/ degree. This global mosaic was processed to form three map sheets, which were combined into a complete Phobos atlas in a scale of 1: 50,000. The printed map sheets have the size of 1000 mm in width by 800 mm in height showing contour lines and the nomenclature of surface features. The north/south pole maps were named "Grildrig" and "Hall", respectively, according to the largest surface features that are depicted. Likewise, the second sheet is named "Stickney", centered at 0° latitude, 90° West longitude and sheet 3 is called "Roche centered at 0° latitude, 270° West longitude. The Viking team chose the names of the surface features on Phobos after scientists involved in the discovery, dynamics, or properties of the Martian satellites, as well as people and places from Jonathan Swift's "Gulliver's Travels" [http://planetarynames.wr.usgs.gov/].

Further readings

Mars Express is continuing its mission, and further Phobos flybys and acquisition of more

high-resolution image data are planned. It is anticipated to further improve and update the terrain model, the preliminary image mosaics, and maps with more image data available.

  • Batson, R.M., Edwards, K. & Duxbury, T.C., 1992. Geodesy and cartography of the Martian satellites. In Mars. pp. 1249-1256.
  • Konopliv, A.S. et al., 2006. A global solution for the Mars static and seasonal gravity, Mars orientation, Phobos and Deimos masses, and Mars ephemeris. Icarus, 182, 23-50.
  • Wählisch, M., Willner, K., Oberst, J., Matz, K.-D., Scholten, F., Roatsch, T., Hoffmann, H., Semm, S., and Neukum, G.: A new topographic image atlas of Phobos, Earth and Planetary Science Letters, Vol. 294 (3-4), pp.541-546, 2010 www.sciencedirect.com/science
  • Willner, K., Oberst, J., Hussmann, H., Giese, B., Hoffmann, H., Matz, K., Roatsch, T., and Duxbury, T.: Phobos control point network, rotation, and shape, Earth and Planetary Science Letters, Vol. 294 (3-4), pp.547-553, 2010 www.sciencedirect.com/science
  • Willner, K., Oberst, J., Wählisch, M., Matz, K., Hoffmann, H., Roatsch, T., Jaumann, R., and Mertens, V.: New astrometric observations of Phobos with the SRC on Mars Express, Astronomy and Astrophysics, 488, 361-364, 2008. www.aanda.org/index.php

Downloads

  • Data
Data set:
DTM and mosaic
PDS format		 Data set:
DTM and mosaic
Tiff/Jpeg format		 Updated Version
base_dtm_phobos.pds DTM_MAP_colour_coded.jpg
DTM_MAP_colour_coded_legend.jpg		 01. June 2009 0.1
base_map_phobos.pds base_map_phobos.tif 01. June 2009 0.1
  • Maps
Data set:
PDF format		 Data set:
Mosaics
Tiff format		 Updated Version
Mercator1.pdf merc1.tif 01. June 2009 0.1
Mercator2.pdf merc2.tif 01. June 2009 0.1
Stereographic_1_2.pdf stereo1.tif
stereo2.tif		 01. June 2009 0.1

Acknowledgement

The research leading to the data products has received funding from the European Community's Seventh Framework Programme ([FP7/2007-2013]) under grant agreement n° 263466 for the FP7-ESPaCE program.

Last update: 28/01/2015 08:43