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In 2021, it seemed as if Mars had a surprisingly big heart. Scientists had been using InSight, a robotic lander to study the planet\u2019s insides. The spacecraft had listened to enough marsquakes to develop a picture of the layer-cake nature of the Martian underworld.<\/p>\n
The crust and mantle were not especially strange. The core, however, was too large, and not very dense, for such a small planet.<\/p>\n
For some researchers, that core measurement didn\u2019t ring true.<\/p>\n
\u201cWe missed out on something,\u201d said Amir Khan, a geophysicist at ETH Zurich in Switzerland who has studied InSight\u2019s data. \u201cBut what?\u201d<\/p>\n
It turns out Mars\u2019s core is small after all, Dr. Khan and other researchers have found.<\/p>\n
In two studies published Wednesday in the journal Nature, researchers re-evaluated InSight\u2019s seismic record. Both teams independently concluded that Mars\u2019s core is more like our own world\u2019s heavy metal heart than previously suspected. The initial higher-size estimate was a result of an undetected 90-to-125-mile-deep ocean of molten rock, which made the underlying core seem bigger than it is.<\/p>\n<\/div>\n<\/div>\n
But the deep sea of magma, hidden below Mars\u2019s solid mantle and kept molten by radioactive elements, is exotic. \u201cIt does not exist on Earth,\u201d Dr. Khan said, and its presence may require a rethink of the red planet\u2019s chaotic evolution.<\/p>\n
Scientists have studied Earth\u2019s geologic layers for more than a century using the illuminating power of quake-made seismic waves. InSight, which landed on Mars in November 2018, was sent to find if the rusty world\u2019s viscera were similar.<\/p>\n
But studying Mars with a single seismometer proved difficult. InSight\u2019s instruments detected only a few modest temblors that came mostly from a convulsing region close to the spacecraft, and only a small slice of the Martian pie was seismically imaged. For some time, marsquakes also seemed to bounce off but not plunge through the planet\u2019s innermost sanctum, revealing precious little information about the core.<\/p>\n
Researchers worked out that Mars\u2019s core had a radius of about 1,140 miles, suggesting it wasn\u2019t very dense. Terrestrial planet cores should be iron-rich, but the puffy Martian core \u2014 eventually confirmed to be fully liquid \u2014 seemed 27 percent lighter than one made of pure liquid iron. The implication was that Mars\u2019s core was strangely enriched in lighter elements like sulfur, carbon, oxygen and hydrogen \u2014 nebulous matter that the young sun should have blown away before Mars formed.<\/p>\n
Puzzled, scientists hoped a stronger seismic yawp would provide clarity. And on Sept. 18, 2021, the firmament delivered: A meteor careened into the hemisphere opposite to InSight, screeching out seismic waves that blasted through the core and ricocheted around its edges.<\/p>\n<\/div>\n<\/div>\n
\u201cThat was the turning point,\u201d said Henri Samuel, a geophysicist at Universit\u00e9 Paris Cit\u00e9 and an author of one of the new studies.<\/p>\n
Based on a model of Mars\u2019s thermal and chemical evolution, Dr. Samuel and his colleagues had proposed the existence of a core-straddling magma ocean in 2021. But \u201cwe had no seismological evidence,\u201d he said. With that meteor impact, his team confirmed the existence of this superhot radioactive soup.<\/p>\n
Dr. Khan\u2019s team also leveraged the impact to re-examine InSight\u2019s seismic data, combining it with computer simulations that explore how iron-rich alloys behave on a molecular level \u2014 and, in doing so, independently found Mars\u2019s hidden magma ocean.<\/p>\n
Its existence means the liquid core\u2019s radius is closer to 1,000 miles \u2014 a denser, iron-rich orb with fewer lighter elements, which is easier to explain.<\/p>\n
The discovery is \u201cvery cool,\u201d and the studies\u2019 shared conclusions are convincing, said Paula Koelemeijer, a seismologist at Oxford University, not involved in the research. \u201cBut they might open up a new problem.\u201d<\/p>\n<\/div>\n<\/div>\n
Before it collapsed 3.8 billion years ago, Mars had a magnetic field that shielded its atmosphere. Scientists thought the magnetic field was generated by a cooling, and thus vigorously churning, liquid iron core. But a radioactive, magmatic blanket swaddling it would have kept the core too toasty.<\/p>\n
So a new origin story for Mars\u2019s magnetic bubble is required. Dr. Samuel offered one suggestion: Perhaps long ago, Mars possessed moons more massive than its present-day Lilliputian pair, the sort whose strong gravity could stir up magnetism-making motions in the core. But for now, he said, that is just a hypothesis.<\/p>\n
After four years, InSight died in 2022. But the discovery of this magma ocean probably won\u2019t be the mission\u2019s final surprise. \u201cThis is just the beginning,\u201d Dr. Samuel said.<\/p>\n<\/div>\n<\/div>\n