Tadpole craters date from the time when Mars was drying out to become the arid world we know it as today.
You may have heard about the strange spiders on Mars, but what about tadpoles on Mars?
Tadpole-like craters could tell geologists and planetary scientists a lot about how Mars lost its water.
Liquid water has been found underneath the surface of Mars, and in its ancient past the Red Planet was much warmer and wetter.
Water on Mars, a brief history
Over its planetary history, Mars has suffered an environmental collapse.
During its early era, over 3.6 billion years ago, there was clearly a lot of liquid water moulding its landscape, carving out deep canyons and filling large lakes.
Today, however, the Red Planet is a freeze-dried desert.
In general, water in its liquid state is simply not stable on the planet’s surface and exists only as ice near the poles or as wispy vapour in the thin atmosphere.
There has also been evidence found of water ice near Mars’s equator.
This transformation from a wetter world to an arid planet occurred during a period known as the Hesperian and is recorded in the landscape by water-formed features that become increasingly few and far between.
The details of this climate catastrophe, including the exact timing and spatial pattern of the great drying of the planet, aren’t very well understood, however.
Alexandra Warren, in the department of the geophysical sciences, University of Chicago, and her colleagues studyied a particular kind of feature on Mars to try to understand this planetary aridification better: tadpole craters.
Mars tadpole craters
Tadpole craters are small craters on Mars typically between 0.5km and 15km in diameter, whose rims have been incised by one (or more) exit breaches.
This indicates that they once held liquid water, which burst out over the edge to flow away, the outflow channel making the whole structure look a bit like a tadpole.
These distinctive craters are believed to date to relatively late during the drying-out of the plane, the period that is the least well understood, and so they serve as valuable records of the climate at the time.
Hundreds of such tadpole craters are known to exist on Mars.
About a third of those that are found in the middle latitudes have not just one outflow, but multiple breaches in their rim.
These separate incisions appear around the rim of the crater at different heights. And this is pretty confusing to understand.
As with overspilling dams on Earth, you’d expect only the lowest point of the barrier to be overtopped by the water flow.
So how can these tadpole craters have routed liquid water to breach in several places with different elevations?
Solving the tadpole crater water mystery
Warren and her colleagues picked four tadpole craters on Mars with multiple breaches to study in detail.
They used high-resolution elevation models of the craters’ terrains produced by the HiRISE telescope aboard Mars Reconnaissance Orbiter to measure their profiles and the exit breaches on their rims.
The team considered four possible sources of liquid water in these tadpole craters: rainfall, surface melting of ice, discharge of groundwater from below and melting at the bottom of ice sheets.
They concluded that the best explanation for these tadpole craters with multiple outflows is that water formed beneath ice sheets filling the craters, which squeezed the meltwater to breach in different directions at different times.
This would require ice sheets at least hundreds of metres thick, and the formation of meltwater under them implies that the surface temperature must have been higher than 0°C for tens to hundreds of days at a time.
And this in turn, therefore, provides constraints on what the climate must have been like during this poorly understood late Hesperian period of Mars’s history.
Lewis Dartnell was reading Multiple Overspill Flood Channels from Young Craters require Surface Melting and Hundreds of Meters of Mid-latitude Ice Late in Mars History by Alexandra O Warren et al. Read it online at: arxiv.org/abs/2407.11395