Geologist have long puzzled over the broad regions of Mars that are littered with shattered, tilted rocks and large-scale remnant massifs (or mountain groups) that have been moved and weathered as a block. According to a new study, these "chaotic terrains" were formed by a catastrophic ice lake collapse.
This transitional time period of Martian history around 3 billion years ago is called the Hesperian. Its characteristic chaotic terrains have been recognized as the source of catastrophic floods. A few scenarios have been proposed for the formation of chaotic terrain -- and they each involve a different volume of water and a different number of outflow events (ranging from one to many). Maybe ice reservoirs melted when molten material rose up, maybe groundwater in aquifers erupted, maybe massive subterranean ice lakes suddenly collapsed.
So, a team led by Manuel Roda from Universiteit Utrecht in the Netherlands tested these scenarios by analyzing imagery and elevation data for Aram Chaos, a 280-meters-wide impact crater located between the outflow channel Ares Vallis and Aureum Chaos. (It’s one of many regions characterized by chaotic terrain.) The team also combined those morphological and structural analyses with models of the evolution of the outflow valley.
They found a large-scale collapse and subsidence (or sinking) of the entire area. We’re talking about 1500 m (that’s nearly a mile) -- which is consistent with a massive expulsion of liquid water from the subsurface in one single event. Specifically, a single, rapid and catastrophic event -- with a flood volume of 93,000 cubic kilometers within tens of days, likely less than a month -- carved the 5-km-wide, 2.5-km-deep Aram Valley.
Their results suggest that a buried, sub-ice lake collapse model best explains the features of the Aram Chaos Valley system as well as the time scale required for its formation. It started with the outflow of water from two small channels; the 4.2-km-deep crater started collecting water soon after it was formed during the Hesperian. When the lake was about 1.5 km deep, it froze, and windblown sediment started to accumulate on top. When that reached a thickness of about 2 km, heat transfer from deep within the planet became blocked and large layers of the buried ice began to melt.
By around 2.5 Ga (for gigaannus, or billions years ago), the weight of the sediment caused the ice ceiling to collapse, sending immense amounts of water to the surface. This ended in a catastrophic lake rim collapse, thanks to a weak spot in the crater rim created during periods of the continuous groundwater sapping and headward erosion. Within about 30 days, the Aram Valley was carved by a volume of water equivalent to nearly 80 percent of Earth’s free-flowing fresh water.
The work was published in Icarus this month.
[Via Science]
