The boundary between water and rock on the exoplanets is not as solid as Earth

A team of researchers simulated conditions on water-rich exoplanets in the laboratory and learned something surprising about their geological composition.


Until now, scientists have had discovered worlds (exoplanets) that are much larger than Earth and completely covered in water. What kind of life could develop in such a world? Could a habitat like this even support life?

Scientists led by Arizona State University (ASU) recreated the conditions of those water worlds in the laboratory to explore the answers as they couldn’t travel there to bring the samples.

In this case, that laboratory was the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility at the DOE’s Argonne National Laboratory.

They found a new transitional phase between silica and water, indicating that the boundary between water and rock on these exoplanets is not as solid as it is here on Earth. This vital disclosure could change how astronomers and astrophysicists have been demonstrating these exoplanets and inform the way we consider life evolving on them.

Dan Shim, associate professor at ASU, led this new research. He said, “Determining the geology of exoplanets is tough since we can’t use telescopes or send rovers to their surfaces. So we try to simulate the geology in the lab.”

For this experiment, scientists brought their samples to two APS beamlines: GeoSoilEnviroCARS (GSECARS) at beamline 13-ID-D, operated by the University of Chicago, and High-Pressure Collaborative Access Team (HPCAT) at beamline 16-ID-B, operated by Argonne’s X-ray Science Division.

The samples were compressed in diamond anvil cells, essentially two gem-quality diamonds with tiny flat tips. Place a sample between them, and you can squeeze the diamonds together, increasing the pressure.

Yue Meng, a physicist in Argonne’s X-ray Science Division and a co-author on the paper, said, “We can raise the pressure to multiple millions of atmospheres. APS is one of the few places in the world where you can conduct this kind of cutting-edge research. The beamline scientists, technicians, and engineers make this research possible.”

Shim said, “The pressure of exoplanets can be calculated, even though the data we have on these planets is limited. Astronomers can measure the mass and density, and if the size and the mass of the planet are known, the right pressure can be determined.”

Vitali Prakapenka, a beamline scientist at GSECARS, a research professor at the University of Chicago and a co-author on the paper, said, “Once the sample is pressurized, infrared lasers — which can be adjusted to smaller than the width of a human blood cell — are used to heat it. We can bring the sample up to thousands of degrees Fahrenheit. We have two high power lasers that shine on the sample from both sides precisely aligned with an ultra-bright APS X-ray probe and temperature measurements along the optical paths with sub-micron accuracy.”

Once the sample is pressurized and heated up, the APS’ ultra-bright X-ray beams can allow scientists to take snapshots of atomic-scale structure changes during the chemical reactions as they happen. In this case, Shim and his team immersed a small amount of silica in water, increased the pressure and temperature, and monitored how the materials would react.

They found that at high temperature and pressure of about 30 gigapascals (about 300,000 times the standard atmospheric pressure on Earth), the water and rock start to merge.

Shim said, “If you were to build a planet with water and rock, you would assume that the water forms a layer above the rock. What we found is that it is not necessarily true. With enough heat and pressure, the boundary between rock and water becomes fuzzy.”

Prakapenka said, “This is a new idea that will need to be incorporated into models of exoplanets. The main point is that it tells the people modeling the structure of these planets that the composition is more complicated than we thought. Before we believed that there was a separation between rock and water, but based on these studies, there is no sharp boundary.”

Shim said“It’s a starting point to build the way chemistry works on these planets. How water interacts with rock is important for life on Earth, and therefore, it is also important to understand the type of life that might be on some of these worlds.”

Shim acknowledges that this research is not the first thing one might picture when thinking about a light source like the APS. But it’s precisely that diversity that he said is an advantage of large-scale user facilities.

Journal Reference:
  1. Carole Nisr et al. Large H2O solubility in dense silica and its implications for the interiors of water-rich planets, PNAS. DOI: 10.1073/pnas.1917448117
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