Mars gets its trademark red hue from the abundance of iron on its surface, but that's just what can be seen with the naked eye. The various minerals on the Red Planet emit unique signatures of light measurable through spectroscopy. Sulfur is particularly abundant and combines with different elements to make sulfate minerals, each with its own spectral signature that can be captured by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), an instrument orbiting the planet.
In 2010, an unusual spectral band was discovered at Aram Chaos, a heavily eroded impact crater, and the plateau above Juventae Chasma, a large canyonlike depression. Identifying the mineral that exhibits this particular spectral signature has eluded researchers because its unique shape and location is not consistent with any known mineral.
To identify the material, scientists needed more data. "The data that comes out of the spectrometer is not usable the way it is," explains Mario Parente, associate professor of electrical and computer engineering at UMass Amherst and one of the authors of the paper. "We have to calibrate the data, correct the data, remove the effect of the atmosphere," he says, highlighting that the light-which travels from the sun to the mineral to CRISM-has to go through the Martian atmosphere twice.
"There are scattering molecules and gases that absorb light in the atmosphere," he says. "For example, on Mars, there is an abundance of carbon dioxide, and that will distort the data." Parente has the most advanced atmospheric correction algorithm tailored to Mars. Using deep learning artificial intelligence approaches, his team can map the known and unknown minerals, automatically recognizing anomalies in the individual pixels of an image. Notably, Parente produced detailed maps of the Jezero Crater, landing site of the Perseverance rover because it is believed to have once contained water.
Using these methods, Parente and his team revealed additional locations on the planet with the same spectral band and clarified additional spectral features. From these newly refined characteristics, researchers at the SETI Institute and NASA Ames Research Center were able to reproduce the mineral in the lab and determined the mystery compound as ferric hydroxysulfate.
"The material formed in these lab experiments is likely a new mineral due to its unique crystal structure and thermal stability," lead author Janice Bishop, senior research scientist at the SETI Institute and NASA Ames Research Center said in a statement. "However, scientists must also find it on Earth to officially recognize it as a new mineral."
Ferric hydroxysulfate forms at high temperatures (50C to 100C) in an acidic environment and in the presence of oxygen and water. "Once you make a mineral attribution and you have good indications of a certain material, then you can start thinking about: When does this material occur? In what condition does it form?" says Parente.
The researchers concluded that ferric hydroxysulfate was formed at Aram Chaos via geothermal heat, while the same mineral was formed at Juventae through volcanic heating by ash or lava. They speculate that this likely happened during the Amazonian period, less than 3 billion years ago.
"Temperature, pressure and conditions such as pH are all very important indications of what the paleoclimate was," says Parente. He is excited about the new level of detail scientists have for understanding the Red Planet through this research. "The presence of this mineral puts a lot more nuance on what was going on. Parts of Mars have been chemically and thermally active more recently than we once believed-offering new insight into the planet's dynamic surface and its potential to have supported life."
Research Report:Characterization of ferric hydroxysulfate on Mars and implications of the geochemical environment supporting its formation
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University of Massachusetts Amherst
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