Underground water aquifers under thick permafrost, groundwater breaking through the surface, catastrophically flooding the ground, and forming an ocean in the lowlands.
This is the setup for the Hesperian era on Mars, around 3.71 to 3.37 billion years ago. The groundwater would have broken through near the end of the era, around 3.4 billion years ago, giving life to an ocean in the late Hesperian. The issue with this proposed ocean is that there is no sign of a past shoreline. The transition to highland rocks and lowland sediment is abrupt, with some lobe-shaped features on the southern edge. The geological features seen at some of the borders don’t match glacial movement, magma, or flooding. It almost seems that it formed from something moving upwards. It was not from sustained upwards wind, either, though. The features resemble more of something moving up and spreading side to side. As much as this may sound like an impossible feature, there is a logical explanation: tsunamis.
This brings us to today’s article, “Tsunami waves extensively resurfaced the shorelines of an early Martian ocean” written by J. Alexis P. Rodriguez et al and published Nature’s Scientific Reports in 2016. Using newer, higher resolution images, the authors note there are two sets of tsunami evidence features, one older than the other. This implies two separate events.
Looking at the features as evidence of tsunamis, a lot starts making sense. Some troughs cut through the some of the older features but none of the younger, indicating they formed between the two features. These troughs were caused by geological activity that took place over millions of years. While the two tsunamis were both in the ‘late Hesperian’, they could have still been several million years apart. Deposits of boulders appear much as they would on Earth, flowing around preexisting landscape features. There are also evidence of backwash channels, formed when gravity forced the water back downhill, which, like Earth-bound examples of the same, are perpendicular to the shoreline. Other who have noted these features also marked them as likely tsunami backwash channels.
The main difference seems to be the size; simulations run gave estimates for the area inundated by the older tsunami to be 800,000 square kilometers (308882 square miles) while the younger one inundated 1,000,000 square kilometers (386102 square miles). These are significantly larger than tsunami inundation areas on Earth, and the length of the backwash channels were around a hundred times larger than their Earth-based counterparts.
Simulations ran suggested that the cause of these tsunamis was a bolide impact (impact where the nature of the body, i.e. meteor or comet, is unknown). The impacting body would have left a crater (possibly no longer visible) about 30 kilometers (19 miles) across. Statistics agreed that two impacts of this size could have impacted the ocean in the timeframe needed to create these tsunamis. It is quite possible that there were older, smaller tsunami caused by marsquakes, massive landslides, or smaller bolide impacts.
While the end of the backwash channels are not visible on present-day Mars, estimates put the ocean at 4100 meters (2.55 miles) in elevation, and 3795 meters (2.36 miles). The loss of about 300 meters (984 feet) can be explained by evaporation or sublimation (ice immediately to vapor) in the several million years between the events. It has been suggested that the Martian ocean would have had a layer of ice across it, but at the time of the authors’ writing, there was no simulations of impact-driven tsunamis on such oceans. As always in science, another item to investigate.
National Aeronautics and Space Administration. NASA’s Mars Exploration Program: Global Views of Mars. NASA/JPL. 2001. Web.
The image of Mars used in today’s post.
Rodriguez, J. A. P. et al. Tsunami waves extensively resurfaced the shorelines of an early Martian ocean. Sci. Rep. 6, 25106; doi: 10.1038/srep25106 (2016)
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