Researchers at Scripps Institution of Oceanography at UC San Diego and the University of Arizona published new findings that show that Mars likely received water from at least two vastly different sources early in its history.
The variability they found implies that there are distinct water reservoirs preserved in Mars that were never mixed together. The findings were published March 30 in the journal Nature Geoscience.
This work can teach us about Earth’s past climate and what it means for the future, said the researchers. While scientists think Earth had a magma ocean early in its history, these findings show that Mars may not have, but was likely hospitable to life at some stage in its past. Researchers are curious as to why it changed.
James Day, a geoscientist at Scripps Oceanography, worked with lead author Jessica Barnes, an assistant professor of planetary sciences in the University of Arizona Lunar and Planetary Laboratory. They and colleagues chemically analyzed meteorites to reconstruct the history of water on Mars and its planetary origins. These meteorites were found on Earth but known to be of martian origin because of their composition.
“Our goal was to understand what water within the interior of Mars reveals about how it formed,” said Day, an expert on martian meteorites. “The main idea to this point was that Mars had been completely molten early in its history, with a roiling, boiling magma ocean, much as Earth and the Moon were thought to be.”
The researchers used two types, or isotopes, of hydrogen to piece together Mars’s water history. One hydrogen contains one proton in its nucleus; this is sometimes called light hydrogen. The other is deuterium, which contains a proton and a neutron in the nucleus; this is sometimes referred to as heavy hydrogen. The ratio of these two hydrogen isotopes signals to a planetary scientist the processes and possible origins of water in the rocks they’re found in.
“We were able to use a key element in the water molecule to examine oceanography of a sort on Mars – magma oceanography,” said Day.
For about 20 years, researchers have been recording the isotopic ratios from martian meteorites, and bizarrely, the data were all over the place.
For reference, water locked in Earth rocks is called unfractionated, meaning it doesn’t deviate much from the standard reference value of ocean water – a 1:6420 ratio of heavy to light hydrogen. Mars’ atmosphere, on the other hand, is heavily fractionated – it is mostly populated by deuterium, or heavy hydrogen, likely because the solar wind stripped away the light hydrogen. Measurements from martian meteorites, many of which were excavated from deep within Mars by impact events, ran the gamut between Earth and Mars’ atmosphere.
With their study, Barnes’ team set out to investigate the hydrogen isotope composition of the martian crust specifically by studying samples they knew were of the crust: meteorites from North Africa – one famously known as Black Beauty – and meteorites from the Allan Hills region of Antarctica. Black Beauty was especially helpful for this study because it’s a mashup of surface material from many different points in Mars’ history. Those collected in Antarctica were part of the Antarctic Search for Meteorites program, which Day has contributed to.
“This allowed us to form an idea of what Mars’s crust looked like over several billions of years,” Barnes said.
The isotopic ratios of the crustal samples from Mars fell about midway between the value for Earth rocks and Mars’ atmosphere. When the findings of Barnes et al. were compared with previous studies, including results from the Curiosity Rover that is on Mars, it seems that this was the case for most of the past four billion years.
“The prevailing hypothesis before we started this work was that the interior of Mars was Earth-like and unfractionated, and so the variability in hydrogen isotope ratios within martian samples was due to either terrestrial contamination or atmospheric implantation as it made its way off Mars,” Barnes said.
The idea that Mars’ interior was Earthlike in composition came from one study of martian meteorite thought to have originated from the mantle, the interior between the planet’s core and its surface crust.
However, “Martian meteorites basically plot all over the place, and so trying to figure out what these samples are actually telling us about water in the mantle of Mars has historically been a challenge,” Barnes said. “The fact that our data for the crust was so different, prompted us to go back through the scientific literature and scrutinized the data.”
– Release adapted from the University of Arizona