Martian equator contains little or no ice, according to a new analysis of seismic data from NASA's Mars InSight mission.
The findings, published in the journal Geophysical Research Letters, describes the dry conditions in the top 300 metres of the subsurface beneath the landing site near the Martian equator.
"We find that Mars' crust is weak and porous. The sediments are not well-cemented. And there's no ice or not much ice filling the pore spaces," said geophysicist Vashan Wright of Scripps Institution of Oceanography at the University of California San Diego.
"These findings don't preclude that there could be grains of ice or small balls of ice that are not cementing other minerals together," said Wright. "The question is how likely is ice to be present in that form?"
Further, the team found that the red planet may have harboured oceans of water early in its history. Many experts suspected that much of the water became part of the minerals that make up underground cement.
"If you put water in contact with rocks, you produce a brand-new set of minerals, like clay, so the water's not a liquid. It's part of the mineral structure," said co-author Michael Manga of the University of California Berkeley. "There is some cement, but the rocks are not full of cement."
"Water may also go into minerals that do not act as cement. But the uncemented subsurface removes one way to preserve a record of life or biological activity," Wright said.
Cements by their very nature hold rocks and sediments together, protecting them from destructive erosion.
The lack of cemented sediments suggests a water scarcity in the 300 metres below InSight's landing site near the equator. The below-freezing average temperature at the Mars equator means that conditions would be cold enough to freeze water if it were there.
Many planetary scientists, including Manga, have long suspected that the Martian subsurface would be full of ice. Their suspicions have melted away. Still, big ice sheets and frozen ground ice remain at the Martian poles.
"As scientists, we're now confronted with the best data, the best observations. And our models predicted that there should still be frozen ground at that latitude with aquifers underneath," said Manga, professor and chair of Earth and planetary science at UC Berkeley.
The InSight spacecraft landed on Elysium Planitia, a flat, smooth, plain near the Martian equator, in 2018. Its instruments included a seismometer that measures vibrations caused by marsquakes and crashing meteorites.
Scientists want to probe the subsurface because if life exists on Mars, that is where it would be. There is no liquid water on the surface, and subsurface life would be protected from radiation.
Following a sample-return mission, a NASA priority for the next decade is the Mars Life Explorer mission concept. The goal is to drill two metres into the Martian crust at high latitude to search for life where ice, rock, and the atmosphere come together.