When Hubble Saw Mars

Mars is only half the diameter of the Earth, yet it never approaches closer than around 140 times the distance of the Moon from the Earth. While Mars is at its closest approach until early September, what a small telescope, or even binoculars, can discern of its surface depends on the local atmosphere and weather, along with the weather on Mars. In late spring and early summer, global dust storms on Mars –usually beginning in the southern hemisphere–can obscure many surface features. The size of the disk is less important than the transparency of the Martian atmosphere in determining the visibility of minor features. But since Mars has no oceans, its land area is equal to that of the Earth, and thus gives a broad and rich viewing landscape for remote geologists, astronomers and cartographers.

A telescope of at least 5 inches (7.5 cm) is perhaps the minimum necessary for a refractor, and 9 inches (22.5 cm) for a reflector—in the latter case, the mirror must possess a perfect figure, and preferably should be of long focus, say f/9 or f/10. The Hubble Space Telescope, while unobscured in its view by the Earth’s weather, is still highly dependent on the martian weather. The image captured by Hubble during late August shown below, shows clear martian weather and gives an annotated look at the major surface features on the red planet. Even closer views have been catalogued by the Mars Orbital Camera (MOC) which is part of the Mars Global Surveyor: from such a vantage point, features as detailed as the size of a bus can be imaged.

Mars as seen near opposition late August 2003, by the Hubble Space Telescope
Credit: NASA/STSci/Hubble; Captioning credit MSSS/ ASU Themis/ NASA/ JPL

While Mars mission planners have selected two particular sites–Gusev Crater and Terra Meridiani –for their January 2004 landing attempts, the motto adopted by site selectors to ‘follow the water’ underscores the rich, erosion patterns that point to an ancient watery past.


  • Arabia Terra: Erosion has exposed hundreds of layers of the same thickness and pattern on the floors of craters throughout western Arabia Terra (first named in 1979). Layers of repeated thickness and physical expression such as these indicate repeated (cyclic or episodic) patterns of change in the deposition of the sediments that formed these rocks billions of years ago. The fact that they tend to occur within martian impact craters might be an indicator that these layers formed as sediments in a lake. Alternatively, they formed from dust falling out of the air in an early martian environment that was dynamic and capable of transporting more dust than today’s thin atmosphere (which is about 100 times thinner than Earth’s at the surface).
  • Schiaparelli Crater: The impact crater Schiparelli near the center left was likely caused by a collision with an object the size of an asteroid. One of the earliest results of the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) investigation shortly after the spacecraft began to orbit Mars in 1997 was the discovery of layered rock outcrops reaching deep down into the martian crust in the walls of the Valles Marineris. Since that time, thousands of MOC images have revealed layered rock in a variety of settings–crater floors, canyon interiors, and scarps exposed by faulting and pitting. Spectacular layered examples are found on the floor of an impact crater located near the equator in northwestern Schiaparelli Basin (0.15°N, 345.6°W). Layers of uniform thickness and appearance suggest that these materials are ancient sediments, perhaps deposited in water, or perhaps deposited by wind. Wind has subsequently eroded and exposed the layers. The crater was considered as a potential landing site for the Mars Exploration Rovers.
  • Terra Meridiani: This general region is one of the more complex on Mars, with a rich array of sedimentary, volcanic, and impact surfaces that span a wide range of martian history. It is part of a geologic unit that was discovered by the Mars Global Surveyor Thermal Emission Spectrometer (TES) Science Team to have high concentrations of a unique mineral called grey (crystalline) hematite. This mineral typically forms by processes associated with water, and this region appears to have undergone alteration by hydrothermal (hot water) or other water-related processes. As a result of this evidence for water activity, this region is a leading candidate for further exploration by one of NASA’s upcoming Mars Exploration Rovers. The brightness and texture of the surface varies remarkably, and these differences are associated with different rock layers. The number of layers indicates that extensive deposition by volcanic and sedimentary processes has occurred in this region. Since that time, however, extensive erosion has occurred to produce the patchwork of different layers exposed across the surface. THEMIS infrared images of this region show that many of these rock layers have distinctly different temperatures, indicating that the physical properties vary from layer to layer. These differences suggest that the environment and the conditions under which these layers were deposited or solidified varied through time as these layers were formed. The two northward-pointing forks (middle left) are still sometimes referred to as Dawes’ forked bay, and under good observing conditions can sometimes be seen by amateur six-inch telescopes.
  • Margaritifer Terra: To the northwest is the plain of Chryse Terra, in the middle of which lies the Viking 1 landing site. Margaritifer Terra has a beaklike extension which sometimes appears broken off at the end. The Ares Valles, one of the largest Martian outflow channels, courses through the region on its way to Xanthe Terra to the northwest; Xanthe Terra is also the site of the great Tiu, Simud, and Shalbatana channels, in which spacecraft photographs have shown lemniscate islands and alluvial plains suggestive of massive flooding. These features originate in western Margaritifer Terra in the rough-and-tumble region known as "chaotic terrain." The flooding that took place here was on a catastrophic scale—much greater than anything ever seen on Earth. Some of the valley heads appear to terminate abruptly in rounded headwalls, that have been cited as evidence that erosion (sapping) by groundwater emerging to the surface has created the valley networks, because similar "theater-headed" valleys are found where groundwater sapping occurs on Earth. With good seeing, one can see even in modest telescopes that this is a region of complex formation.
  • South Polar Cap: In the south, the seasonal carbon dioxide frost cap generally fails to disappear completely—thus, unlike the northern cap, the residual cap consists mostly of carbon dioxide frost rather than water, and is always much smaller than its northern counterpart.
  • Hellas Basin: The great basins of Hellas, Argyre, and Elysium are usually frost covered and often appear brilliant white, and there are also many smaller patches of frost. In winter the Hellas basin is often partly or wholly filled with frost. Strange ovoid landforms are present here that give the appearance of flow. It is likely that water ice or even liquid water is present in the deposits and is somehow responsible for the observed landscape. The floor of Hellas remains a poorly understood portion of the planet.
  • Huygens Crater: The great crater Huygens is partly filled by dark material and can sometimes be glimpsed from Earth. The crater was named after Dutch astronomer Christian Huygens (1629-1695) and shows an unusual texture: smooth-topped mesas are scattered across a more rugged surface. The mesas are testament to a former smooth layer of material that is in the process of eroding away.
  • Syrtis Major: The major dark area visible with a telescope is Syrtis Major. The prominent, dark Syrtis Major is clearly shown in a 1659 drawing by Christiaan Huygens. A low-relief shield volcano has been identified within Syrtis Major whose eruptions were the source of the dark materials covering the region; and indeed, the whole region is an elevated volcanic plateau. The southern part of Syrtis Major is streaked and mottled, owing to wind action, and there are large dune fields within its great expanse. These lava flows are partly bounded to the east by a large depression, Isidis basin, which contains smooth plains, and to the west and north by heavily cratered and moderately faulted highlands.
  • North Polar Hood: If the northern polar cap is tilted towards the earth, then a white, often cloud and dust-covered top, can be seen to extend to around 65 degrees North. . In most years the seasonal carbon dioxide frost cap evaporates off completely, leaving a residual water ice remnant. As it retreats, the north polar cap appears to be surrounded with a dark collar, sometimes known as the Lowell band, which was once regarded as a shallow sea but coincides in position with a wide swath of sand dunes.

    A new batch of high resolution photos from the Mars Orbital Camera, taken between February and July 2002, were added online in April and they bring the total number of images in the online gallery to more than 123,800. The images are available from the Mars Orbiter Camera Gallery.

    What’s Next

    Twin NASA rovers and one European Space Agency probe are planned for Mars landing in late 2003 and early 2004. The first Mars Exploration Rover, or MER, will arrive at Mars on Jan. 4, 2004, the second, Jan. 25.

    Plans call for each to operate for at least three months. These missions continue NASA’s quest to understand the role of water on Mars.