The discovery of the mineral jarosite in rocks analyzed by
the Mars Rover, Opportunity, on the Martian surface had special meaning for a
team of Syracuse University scientists who study the
mineral here on Earth. Jarosite can only form in the presence of water. Its
presence on Mars means that water had to exist at some point in the past. The
trick is in figuring out if jarosite can be used as a proxy for determining
when, and under what conditions, water was present on the planet.
The SU scientists have done just that. In a recent study
published in Earth and Planetary Science
Letters, Suzanne Baldwin, professor of Earth Sciences in SU’s College of Arts and Sciences; and Joseph Kula,
research associate and corresponding author for the study, established the “diffusion parameters” for argon in jarosite. In simpler terms, they discovered
a way to use the noble gas argon, which accumulates in jarosite over time, to
determine the age of the mineral and the surface conditions under which it
formed.
The new study is the first in a series of experiments
designed to provide a roadmap of sorts for scientists who may someday study
Martian samples brought back to Earth. “Our experiments indicate that over
billion-year timescales and at surface temperatures of 20 C (68 F) or colder,
jarosite will preserve the amount of argon that has accumulated since the
crystal formed,” Kula says, “which simply means that jarosite is a good marker
for measuring the amount of time that has passed since water was present on
Mars.”
Moreover, since the development of life requires water,
knowing when and for how long water was present on the Martian surface has
implications for the search for potential habitats harboring life, the
scientists say. “Jarosite requires water for its formation, but dry conditions
for its preservation,” Baldwin says. “We’d
like to know when water formed on the surface of Mars and how long it was
there. Studying jarosite may help answer some of these questions.”
Jarosite is a byproduct of the weathering of rocks exposed
at the surface of a planet (such as Earth and Mars). The mineral forms when the
right mixture of oxygen, iron, sulfur, potassium, and water is present. Once
formed, the crystals begin to accumulate argon, which is produced when certain
potassium isotopes in the crystals decay. Potassium decay is a radioactive
process that occurs at a known rate. By measuring the isotopes of argon trapped
within the crystals, scientists can determine the age of the crystals.
However, because argon is a gas, it can potentially escape
rapidly from the crystals under hot conditions or slowly over long durations at
cold conditions. In order to determine the reliability of the “argon clock” in
jarosite, the scientists had to determine the temperature limits to which the
crystals could be subjected and still retain the argon. Using a combination of
experiments and computer modeling, the team found that argon remains trapped
inside the crystals for long periods of time over a range of planetary surface
temperatures.
“Our results suggest that 4 billion-year-old jarosite will
preserve its argon and, along with it, a record of the climate conditions that
existed at the time it formed,” Baldwin says.
The scientists are in the process of conducting further studies on jarosite
that formed less than 50 million years ago in the Big
Horn Basin
in Wyoming,
which they hope will reveal when the minerals formed and how fast environmental
conditions changed from water-saturated to dry. The results can be used as a
context for interpreting findings on other planets.
