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Stop 4 at Hellas

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Colorized shaded-relief perspective view of Hadriaca Patera, Mars
Colorized shaded-relief perspective view of Hadriaca Patera, Mars
Image: Color-coded elevation draped over perspective shaded-relief imagery from the Mars Orbiter Laser Altimeter
Location: Southern highlands northeast of the Hellas impact structure; view is to the northeast
Scale: Diameter of Hadriaca is approximately 77 kilometers; Vertical exaggeration is 10 X

The image above is a perspective view of Hadriaca Patera. A "patera" is a term used to describe a volcano on Mars. Hadriaca, along with Tyrrhena Patera that we will visit next, are among the oldest volcanoes with a central vent (lava source) on Mars and the only large volcanoes located in the southern hemipshere of Mars. These two volcanoes are near one another in the southern highlands northeast of the Hellas impact structure. They are both low-relief (summit is not very high compared to the surrounding plains), areally extensive features with calderas at the top, and radial channels and ridges along their flanks. Low-relief volcanoes on Earth are typically associated with effusive volcanic styles where flows are very fluid and explosive eruptions with billowing ash is not expected.

Volcanoes are found on all the rocky planets of the solar system and even on the satellites of some outer gaseous planets. Their occurrence throughout the solar system stresses the importance of volcanism in the creation and evolution of planets. Planets are born hot and continue to produce heat, albeit at a declined rate over time. Volcanism allows the internal heat of the planet to be released to the surface, which is then radiated out to space. Although the association of volcanoes and their destructive effects upon human activity is what garners attention, they are simply part of the natural evolution of a planet and by studying their eruptive styles, features, and flow materials we can learn a great deal about the planet's interior.

Channel and mesa surfaces along the flanks of Hadriaca Patera
Channel and mesa surfaces along the flanks of Hadriaca Patera
Image: Portion of THEMIS visible spectrum image V17097003
Location: Along the southern flanks of Hadriaca Patera
Scale: Image width (excluding black no-data areas) is approximately 20.8 kilometers

The flanks of Hadriaca have very well-defined channels. Many of the channels cover almost the entire length of the volcano slope with the head starting just below the caldera. These channels are well-developed and it is hypothesized that while Hadriaca may have started out as an explosive-erupting volcano with ash, it later transitioned to a hydro-magmatic style involving a near-surface supply of groundwater to produce pyroclastic flows that carved into the earlier volcanic ash, creating the initial channels.

What is a pyroclastic flow? Its a specific type of flow involving a dense cloud of hot gas and ash that travels downslope at high speeds. A great example of this type of flow occurred at Mount St. Helens in the 1980 eruption at Washington state. After the eruptions stopped, groundwater may have reached the surface at the head of the channels causing further erosion of the volcanic materials and deepening of the channel.

If you were to draw out the shape across one of these channels, you would find that it is V-shaped. This is a common shape for channels that are fluvially-eroded, that is, a channel formed by water flow. In the image to the left, the areas between the channels appear like flat mesa-like ("tabletop") surfaces that have been less modified compared to the channels. Planetary scientists at PSI are now studying the surfaces of these mesas and the channels using high-resolution images from the Mars Orbiter Camera (MOC) and Context Imager (CTX) to count the number of small impact craters to age-date the surface. Variations (if any) in the crater density of these surfaces may tell us something about the timing and deposition of volcanic materials at Hadriaca Patera.

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