The lunar lander for China’s Chang’e 4 mission encountered what appeared to be alien substance on the Moon. According to the scientists working on the mission, the strange substance had an unusual color and had gel-like properties.

The lunar rover, known as Yutu 2, encountered the unknown material during the lunar day 8 of its mission, which began on July 25. The rover spent the day navigating a path across a region on the lunar surface that was filled with impact craters, Space.com reported.

On July 28, the mission’s team at the Beijing Aerospace Control Center was preparing to power down Yutu 2 in order to protect the rover from the radiation and high radiation caused by the Sun. Before powering down the rover, the team checked the images collected by Yutu 2.

As the team members were going through the photos, they came across a photo of a small crater that contained gel-like material. They noted that the substance had a color that’s very different from the surrounding area.

Due to the discovery, the team postponed Yutu 2’s other activities, which mainly involved proceeding west. Instead, the team members decided to focus on the strange substance in an attempt to determine its exact nature.

Yutu 2 then returned to the crater where it first spotted the substance to observe its surroundings. The team used the rover’s Visible and Near-Infrared Spectrometer to study the area.

Despite the follow-up observations, the Chang’e 4 team was not able to come up with a solid explanation as to what the substance might be. Although there are speculations that the substance might be alien in nature, researchers who are not part of the team suggested that it could be the remains of melted glass that formed due to meteor impacts on the lunar surface.

Interestingly, this isn’t the first time that a strange material was encountered on the Moon. During NASA’s Apollo 17 mission in 1972, astronaut Harrison Schmitt came across orange-colored soil during a moonwalk. It was determined that the soil was created following a volcanic eruption on the Moon 3.64 billion years ago.