Massive ancient lake on Mars is indicator of underground waterworks
The key to exposing past life on Mars may lie in watery, underground channels, a group of planetary geologists has proposed after studying a 92km crater on its surface.
The team used images and data captured by Nasa’s Mars Reconnaissance Orbiter to draw their conclusions, after identifying sedimentary rock layers at the base of McLaughlin Crater (located on the western side of Arabia Terra) containing clay and carbonate minerals. These two minerals are formed on Earth through an interaction with water, however, an analysis of the crater’s topography revealed a flat surface and no large inflow channels — indicators that any water present in the crater billions of years ago came from beneath the surface.
"Taken together, the observations in McLaughlin Crater provide the best evidence for carbonate forming within a lake environment instead of being washed into a crater from outside," said lead author on the Nature Geoscience paper and Natural History Museum planetary scientist Joseph Michalski.
The question of water — and consequently life — on Mars remains “a pendulum that has been swinging back and forth”, in the words of planetary scientist Jeff Andrews-Hanna, who investigated the possibility of groundwater on Mars in 2007.
In just the past year Nasa’s Curiosity Rover has provided us with evidence of an ancient river on Mars and evidence of water in the planet’s soil, while analysis of a Mars meteorite revealed water levels ten times that of any other Earth-bound Mars sample. On the other hand, atmospheric samples from Curiosity detected no traces of methane — produced by biological activity here on Earth — on Mars. Looking to the subsurface going forward could be the key to putting many of these arguments to rest, particularly since on Earth it’s predicted half of all life exists underground as microbes.
"If Mars’s climate was never stable, that would have been a challenge for life," Andrews-Hanna told Nature last year. "But as you go deeper in the subsurface, things become more stable."
Michalski and his team began their study by focusing only on the biggest, deepest craters on Mars’ northern hemisphere — the area Andrews-Hanna had already predicted could be a site of upwelling groundwater. Michalski had actually intended to disprove the existence of these lakes, but when it came to McLaughlin, the evidence began to turn his theory on its head.
The Mars Reconnaissance Orbiter is fitted out with a series of high resolution imaging tools, and HiRise helped pick out the surface topography while Crism (Compact Reconnaissance Imaging Spectrometer for Mars) did the investigative legwork. The latter’s visible and infrared spectrometers help researchers divide a surface image up according to what minerals are present, by highlighting colour graduations. Within the crater, it was possible to pick out evidence of carbonates and clay thanks to ancient meteorite hits on the planet’s surface, which carried clues from beneath Mars up to the surface in the form of these sedimentary rock formations. McLaughlin, being 2.2km deep, offered the best chance of finding evidence, and sure enough with the additional data captured by Nasa’s Thermal Emission Spectrometer and Imaging System the minerals were identified. Ancient water channels were also spotted around 500m above the crater’s base, indicating a former lake surface. The team estimates that the lake existed between 3.7 billion and four billion years ago.
Michalski has not, however, given himself over entirely to the idea of an ancient Mars dotted with upwelled lakes.
"We suggest that upwelling might have occurred but it was limited to only the deepest basins, and when it occurred, it produced sediments from alkaline fluids reflective of long transport distances in a basaltic crust," reads the paper.
He also does not predict that this will be the beginning of the discovery of ancient life on Mars, but rather a path to discovering more about the origins of life on Earth.
"We don’t know how life on Earth formed but it is conceivable that it originated underground, protected from harsh surface conditions that existed on early Earth," he said in a statement. "Due to plate tectonics, however, the early geological record of Earth is poorly preserved so we may never know what processes led to life’s origin and early evolution. Exploring these rocks on Mars, where the ancient geologic record is better preserved than on Earth, would be like finding a stack of pages that have been ripped out of Earth’s geological history book. Whether the Martian geologic record contains life or not, analysis of these types of rocks would certainly teach us a tremendous amount about early chemical processes in the solar system."
Michalski concludes that, going forward, future missions should focus on Mars’ subsurface, but that we shouldn’t be fitting out a potential Curiosity Rover 2 with giant drilling devices — everything we need to learn about the deep, should already be waiting for us on the surface.
"ExoMars will have a drill capable of penetrating meters below the surface — I am not saying that this is a mistake," Michalski told Wired.co.uk. "What I am saying is that the evidence of a deep biosphere would potentially lie much deeper than that drill could reach, kilometres down. So, the best way to investigate those environments would be to analyse rocks that have been uplifted naturally on Mars, to study rocks altered by subsurface fluids — like those in McLaughlin Crater."
Since Nasa’s orbiter has already identified a number of mounds at the crater’s base — that may have been caused by landslides which instantly buried mineral-rich sediments — it looks like there will be ample opportunity for discovery.
"That is really cool because rapid burial is the scenario that is most advantageous for preservation of organic material, if any was present at that time," Michalski told Space.com.