Mars is dry—or at least it is now. In its distant past, the Red Planet may have had a liquid water ocean. Questions remain about the ultimate source of that water, however. (To be fair, such questions also persist about water on Earth.) A likely candidate is magma. It brings water from the interior of a planet to its surface through volcanism. Understanding how much water is in Martian magma is vital for understanding whether the planet had seas in its early history.
Most Martian surface rocks are billions of years old. This means that we can’t just pick up Martian basalt and measure the amount of water the magma initially contained. That water is long gone. There are some direct measurements of water in Martian volcanic rocks in meteorites from the Red Planet, but those values represent single points that also had to traverse the space between Mars and Earth.
Instead, planetary scientists need to use something else (a proxy) to convert to the potential starting water contents of the magma. Are there other elements that behave chemically like water? If so, can we measure them in Martian rocks to get at how much water was being brought to the surface?
Black, a volcanologist at Rutgers University, doesn’t usually work on Martian rocks. “I remember when I started thinking about this project,” said Black, “I was really thinking hard about how to understand gas for enormous flood basalt eruptions [on Earth].” His colleagues started talking about understanding water in Martian magma and realized that Black had a “whole set of tools” that could be applied to Mars. He also knew that there was a large set of gamma ray spectroscopy (GRS) data for the Martian surface from NASA’s Mars Odyssey orbiter.
Although the GRS data set doesn’t include information about water, it does include information on elements that behave like water. Black and his colleagues identified one of them, thorium, as a proxy. This heavy radioactive element behaves much like water in basalt magma because it doesn’t tend to like to go into minerals that crystallize out of basalt. As a result, both thorium and water are liberated early in the process of melting mantle rocks.
This new method using thorium allows for a truly global estimate for water in Martian magma—and how much water it could have fed to the surface.
Martian Magmas and Mighty Eruptions
What researchers found was that over the past 3.7 billion years, Martian lavas have released enough water to cover Mars with up to 40 meters of the liquid. The thorium data suggest that Martian lavas were more heterogeneous when it comes to water earlier in the planet’s history and have become more water rich across the past 2 billion years (what is known as the Amazonian period on Mars), in which volcanism has been focused at giant volcanoes such as Olympus Mons.
Masaki Ogawa, an associate professor of planetary physics at the University of Tokyo at Komaba, said that these findings agree with his own numerical models of Martian planetary dynamics. Ogawa was not part of this study.
The water content of magma is also a strong indicator of how explosive an eruption could be: more water, more explosivity. It turns out that Martian magma doesn’t appear to have enough water (100–3,000 parts per million) to explain the highly explosive nature of volcanic deposits seen on the Red Planet.
Black and his colleagues think that water on the Martian surface might have helped drive explosive volcanism, much like mixing of magma and ocean water on Earth adds power to events like the January 2022 eruption at Hunga Tonga-Hunga Haʻapai.
The thorium approach to estimating water contents of Martian lava is “very interesting,” according to Valerie Payré, a planetary scientist at the University of Iowa who was not part of this study. “[This work] provide[s] essential information regarding the amount of water in the primary melts,” said Payré, adding that Black’s estimates carry “significant implications regarding the eruption style and the evolution of composition, mineralogy, and physical properties of primary magmas on Mars.”
However, she added, there are some caveats: “The large spatial resolution of the GRS instrument (of several tens of kilometers per pixel)…might lead to overlooked regions.”
In the end, though, Payré thinks the new approach will be a big help in interpreting data from upcoming sample return missions from Mars.
—Erik Klemetti (@eruptionsblog), Science Writer