The idea of liquid water on Mars draws hopes and theories of life and possible habitation on Earth’s closest celestial neighbor. But while water does exist in ice and vapor on Mars, scientists from the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland have disproved the leading evidence that liquid water flows on the red planet. They recently published their research in Geophysical Research Letters.
Many scientists hypothesized that water flow was responsible for morphological changes that have been observed for several years in the Martian gullies in the Valles Marineris. Additionally, using images from NASA’s High Resolution Imaging Science Experiment (HiRISE) onboard the Mars Reconnaissance Orbiter, scientists discovered recurring slope lineae, or dark stains that appear seasonally along the slopes of the gully.
The white arrows point to some of Mars’ recurring slope lineae, or dark seasonal stains theorized to be caused by surface water. The image was taken by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. Image credit: NASA.
This recurring phenomenon led many observers to hypothesize that the lineae were resulting from the freezing and thawing of water, and seemed to support the theory of water flow in the gullies. But water would immediately boil and evaporate in Mars’ harsh atmosphere. So where would the water be coming from?
Planetary scientists began using HiRISE images to study the morphology of the gullies and look into emerging theories.
Jennifer Hanley, a planetary scientist at the Lowell Observatory in Arizona, worked on research into the role of hydrated salts in the recurring slope lineae. Hanley said one theory is that salt in the lineae “is pulling out water vapor from the atmosphere through deliquescence, [a process of becoming a liquid], and kind of seeping down the slope in that manner.”
However, Hanley suggested that the theory does not currently have much scientific support. “From what we know there is not that much water vapor to actually perform theses recurring slope lineae,” said Hanley, “We don’t have a clear answer as to how or where the water [would be] coming from yet.”
The APL team of six scientists took a different approach to the water question. A few years ago they began looking into the composition of the gullies using NASA’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument to see if the mineralogy even supported the theory of present, flowing, liquid water. And it turns out it doesn’t.
“With HIRISE you are looking at an image, and with that image you can do morphology and stuff. But with CRISM then you’re dealing with, instead of just one image, you’re dealing with basically hundreds of images to generate what is called a data cube, where you can extract a [composition] spectrum,” said Jorge Núñez, the APL project’s lead planetary scientist and principal author of the revealing research.
For their research into the gullies the scientists developed a product called Map Projected Targeted Reduced Data Records (MTRDRs) that allowed them to overlay CRISM data on top of HiRISE images. By doing this they were able to study the relationship between the morphology and the composition of the gullies and, as Núñez put it, “You can interpret what the squiggly lines mean.”
In this case, the squiggly lines did not mean current flowing water. If it was indeed liquid water forming the gullies and causing the lineae, there would be a presence of certain clays and other minerals associated with the amount of water flow needed to create them.
While the scientists did find some clays present, “Once you started correlating not only the compositional data with CRISM, as well as the location of these clays, but when you tie it in with the HiRISE data and the context imager, you can see pretty clearly that the clays were just being eroded out,” said Núñez.
The clays the scientist had found were ancient clays from water that had been in the area some millennia ago. The findings led the team to conclude that the gullies were “not something that had been formed as a result of liquid water flowing over it and carving” them, said Núñez.
Instead the scientists suggest other mechanisms for the creation of the gullies’ lineae, such as CO2 frost sublimation or “different mechanisms we just don’t have naturally here on Earth,” said Núñez.
For now, the search for liquid water continues on the red planet. While it has been confirmed that Mars harbors frozen water among and beneath the CO2 ice at the planet’s poles, some scientists believe more water may be hidden in underground aquafers.
Planetary scientists are currently using radar technology to investigate beneath the planet’s surface. But, to locate possible underground wells, “You either need to be lucky, or you need something like humans on Mars with a drilling rig,” said Núñez.
This scenario is not as far-fetched as it may seem. NASA is planning to send scientists into Mars’ orbit by the 2030s. “I would love to go,” said Núñez, “Ask any of the Mars scientists, and who wouldn’t?”