The China National Space Administration (CSNA) has released photos of the mysterious gel-like substance encountered by its Chang’e-4 mission on the Moon. According to scientists, the substance resembles the samples taken Apollo 17 astronauts during their lunar expedition.

In July, during the eighth lunar day of the Chang’e-4 mission, its rover Yutu-2 came across a crater containing a strange substance as it was traveling across the surface of the Moon. According to the CSNA, the substance had an unusual color and a gel-like texture.

Due to the discovery, the mission team decided to postpone the rover’s scheduled activities to focus on the substance.

Recently, the space agency released the photos of the substance that were taken by Yutu-2’s Visible and Near-Infrared Spectrometer (VNIS) instrument. The photos were unveiled through WeChat account of the agency.  

In the photos, the strange substance can be seen near the center of the crater. It is highlighted by a rectangular frame and red circle, which could be caused by the VNIS’ field of view.

Although CNSA has not yet provided an explanation regarding the real nature of the substance, there has been speculation that the gel-like material came from melted glass caused by meteorite strikes on the surface of the Moon.

Lunar scientist Clive Neal of the University of Notre Dame said that it closely resembles the sample taken by astronaut Harrison Schmitt during the Apollo 17 mission in 1972. According to Neal, the substance, known as 70019, is composed of dark broken fragments of minerals that were cemented together. Sample 70019 also features traces of black, shiny glass, Space.com reported.

Dan Moriarty, a postdoctoral program fellow at NASA’s Goddard Space Flight Center, noted that it’s a bit hard to determine what Yutu-2’s discovery actually is due to the quality of the photos. But, since the substance appears to be brighter than its surroundings, Moriarty noted that it could indicate that it is made of various materials.

“Chang’e-4 landed in a mare basalt-filled crater, which is typically dark,” he told Space.com. “Highlands crustal materials are typically brighter, so that would be a potential candidate.”

“It will be very interesting to see what the spectrometer sees, and if any higher-resolution images become available,” he added.