Explosive volcanic eruptions on an earlier Mars with a thicker atmosphere would have scattered fine ash (pyroclastic debris) mainly east or west of the volcano, a new study finds. Also, a denser ancient atmosphere supports the addition of Arsia and Pavonis Montes to possible source volcanos for the puzzling Medusae Fossae formation, which scientists think may be made of thick deposits of volcanic ash.
TO THE WINDS. Volcanic ash erupted into Martian atmospheres thicker than that of today tends to drift mainly east and west. And as atmospheres become thicker (left to right) the plume of ash is less prone to drift north and south. For the purposes of the modeling, each eruption was set to last one Martian year. (Image taken from Figure 3 in the paper.)
These results appear in a paper published in Icarus by a team of scientists led by Laura Kerber (Laboratoire de Météorologie Dynamique, CNRS, France). The researchers used a global atmospheric circulation model developed at the Laboratoire de Météorologie Dynamique.
Earlier work using this same computer model, which assumed a 0.006 bar (6 millibar) atmospheric pressure like that of today, pointed to Apollinaris Mons as the source of the Medusae Fossae formation. The new results come from modeling with thicker Martian atmospheres of 0.05 bar (50 mb), 0.5 bar (500 mb), 1 bar (1,000 mb), and 2 bars (2,000 mb). (Earth’s atmospheric pressure is 1 bar.)
Higher atmospheric pressures altered the way Martian volcanic ash plumes behave, the researchers found. The initial eruption blast is more muted, but the plume may rise higher as the denser atmosphere becomes entrained and heated. In today’s Martian atmosphere, an erupting plume would be limited to about 20 kilometers’ (12 miles) altitude; with a 1 bar atmosphere that height could rise to 30 km (20 mi) or more.
The team also found that with thicker atmospheres, an explosive plume of ash is more closely confined to the latitude of the source vent and it tends to drift mostly east or west rather than north or south. With thin-atmosphere models, ash tends to drift more significantly in latitude as well as in longitude.
The researchers note, “Using these results it was possible to determine that some of the friable layered deposits (such as those in the basin of Argyre or those in Arabia Terra) would have been difficult or impossible to emplace from known Martian volcanoes, whereas others (such as the Medusae Fossae Formation) showed good matches under a variety of atmospheric and volcanic scenarios.”
While the giant Tharsis volcanos Arsia Mons and Pavonis Mons have been eyed as potential sources for the Medusae Fossae formation, scientists have thought that this could occur only when Mars’ axis was tipped at a greater angle to its orbit than today.
“But these recent simulations demonstrate that higher atmospheric pressures at the time of eruption could also increase the likelihood that these two volcanoes contributed material to this deposit,” the team concludes.
Nonetheless, they say, “The source most favorably located to produce the Medusae Fossae formation under the widest variety of conditions remains Apollinaris Mons.”