Scientists are releasing a picture each weekday from the Mars Odyssey spacecraft. Odyssey's Thermal Emission Imaging System (THEMIS) captures the images as the craft orbits Mars. The photos are not yet fully calibrated for scientific use, and so no formal science findings are being discussed, said researchers who operate the camera from Arizona State University.

Today's picture ...

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Although the largest volcanoes on Mars (and the solar system) are located in the geologically young Tharsis region, there are many other martian volcanoes that display equally interesting features, such as Hadriaca Patera, shown in this image. Hadriaca Patera is located to the northeast of the Hellas Planitia impact basin in the southern hemisphere.

Unlike the Tharsis volcanoes, Hadriaca Patera has very low relief, standing only about 1-2 km above the surrounding plains. Many scientists believe that Hadriaca Patera and other patera volcanoes (e.g., Tyrrhena) had significant interaction with subsurface water that produced mostly explosive ash deposits (pyroclastic flows), rather than just lava flows. Nearby sources of water might have included Dao Vallis on the southern flank of the volcano.

The upper portion of this image shows relatively smooth terrain located in the central caldera, which has been nearly filled in with late-stage lava flows. The lower half of the image shows lobate flows as well as furrows in the ash deposits that make up the volcano's southern flank; these erosional furrows may have formed by surface runoff or sapping by groundwater. Just below the center of the image, a few small sinuous troughs are visible, and may be collapsed lava tubes or collapse features related to subsurface water.

The number of impact craters on a planetary surface is commonly used as a proxy for the age of the surface -- an old surface has had time to accumulate more craters than a young surface. The relatively small number of large craters in the image indicates that the surface in this area is younger than the nearby heavily cratered ancient terrains outside the Hellas basin, but there are more craters on this surface than would be found on the average volcanic surface in Tharsis (there are some very large old craters on the volcano's flank to the southeast of this image).

Paterae in general are older than the Tharsis volcanoes. At the far right edge of the central portion of the image, an ovoid-shaped crater is visible. Such craters are believed to form by extremely low-angle impact events.

[Source: ASU THEMIS Science Team]