A combination of a once-debunked 19th-century identification of a
water-carrying iron mineral and the fact that these rocks are extremely common
on Earth, suggests the existence of a substantial water reservoir on Mars,
according to a team of geoscientists.
"One of my student's experiments was to crystalize hematite," said Peter J.
Heaney, professor of geosciences, Penn State. "She came up with an iron-poor
compound, so I went to Google Scholar and found two papers from the 1840s
where German mineralogists, using wet chemistry, proposed iron-poor versions
of hematite that contained water."
In 1844, Rudolf Hermann named his mineral turgite and in 1847 August
Breithaupt named his hydrohematite. According to Heaney, in 1920, other
mineralogists, using the then newly developed X-ray diffraction technique,
declared these two papers incorrect. But the nascent technique was too
primitive to see the difference between hematite and hydrohematite.
Si Athena Chen, Heaney's doctoral student in geosciences, began by acquiring
a variety of old samples of what had been labeled as containing water.
Heaney and Chen obtained a small piece of Breithaupt's original sample, a
sample labeled as turgite from the Smithsonian Institution, and,
surprisingly, five samples that were in Penn State's own Frederick Augustus
Genth collection.
After multiple examinations using a variety of instruments including
infrared spectroscopy and synchrotron X-ray diffraction, a more sensitive,
refined method than used in the mid-19th century, Chen showed that these
minerals were indeed light on iron and had hydroxyl — a hydrogen and oxygen
group — substituted for some of the iron atoms. The hydroxyl in the mineral
is stored water.
The researchers recently proposed in the journal Geology "that hydrohematite
is common in low-temperature occurrences of iron oxide on Earth, and by
extension it may inventory large quantities of water in apparently arid
planetary environments, such as the surface of Mars."
"I was trying to see what were the natural conditions to form iron oxides,"
said Chen. "What were the necessary temperatures and pH to crystallize these
hydrous phases and could I figure out a way to synthesize them."
She found that at temperatures lower than 300 degrees Fahrenheit, in a
watery, alkaline environment the hydrohematite can precipitate out, forming
sedimentary layers.
"Much of Mars' surface apparently originated when the surface was wetter and
iron oxides precipitated from that water," said Heaney. "But the existence
of hydrohematite on Mars is still speculative."
The "blueberries" found in 2004 by NASA's Opportunity rover are hematite.
Although the latest Mars rovers do have X-ray diffraction devices to
identify hematite, they are not sophisticated enough to differentiate
between hematite and hydrohematite.
"On Earth, these spherical structures are hydrohematite, so it seems
reasonable to me to speculate that the bright red pebbles on Mars are
hydrohematite," said Heaney.
The researchers note that anhydrous hematite — lacking water — and
hydrohematite — containing water — are two different colors, with
hydrohematite being redder or containing dark red streaks.
Chen's experiments found that naturally occurring hydrohematite contained
3.6% to 7.8% by weight of water and that goethite contained about 10% by
weight of water. Depending on the amount of hydrated iron minerals found on
Mars, the researchers believe there could be a substantial water reserve
there.
Mars is called the red planet because of its color, which comes from iron
compounds in the Martian dirt. According to the researchers, the presence of
hydrohematite on Mars would provide additional evidence that Mars was once a
watery planet, and water is the one compound necessary for all life forms on
Earth.
Reference:
Si Athena Chen, Peter J. Heaney, Jeffrey E. Post, Timothy B. Fischer, Peter
J. Eng, Joanne E. Stubbs. Superhydrous hematite and goethite: A potential
water reservoir in the red dust of Mars? Geology, 2021;
DOI: 10.1130/G48929.1