Earth is constantly being hit by space rocks. Several tons rain down on the planet each day in the form of dust. And larger strikes have created more visible features, including giant craters. But which of our planet’s extraterrestrial scars is the oldest?

Researchers reported on Tuesday in Nature Communications that they have pinpointed it, in Western Australia. It was caused by an impact more than 2.2 billion years ago. Interestingly, that timing roughly coincides with the end of one of our planet’s ice ages. An impact in the ice would have liberated an enormous amount of water vapor, the researchers suggest, perhaps enough to alter Earth’s climate.

The Yarrabubba impact structure, about a day’s drive northeast of Perth, isn’t much to look at today. The original crater, believed to have been roughly 40 miles in diameter, is long gone. That’s because the combined effects of wind, rain, glaciation and plate tectonics have washed several miles off the surface of the planet, effectively erasing the crater. The extent of erosion suggests that the impact structure is very, very old.

Existing clues yield “a pretty giant” age range of about a billion and a half years, said Timmons Erickson, a geochronologist at NASA Johnson Space Center in Houston and the study’s lead author. But Dr. Erickson knew that it was possible to do far better, by reading the tiny geological clocks that hide within rocks.

In 2014, Dr. Erickson collected roughly 200 pounds of granitic rocks from Yarrabubba. Back in the laboratory, based on a series of experiments, Dr. Erickson and his team calculated that the Yarrabubba impact occurred 2.229 billion years ago, with an uncertainty of 5 million years. The next­oldest impact structure, Vredefort Dome in South Africa, is over 200 million years younger.

The age of the Yarrabubba impact structure happens to line up with the end of an ice age, which makes for a compelling coincidence, Dr. Erickson said: “Would an impact event like Yarrabubba be enough to terminate a glacial time in Earth’s history?”

To help answer that question, the scientists modeled the effects of a roughly four­mile­wide impact object striking ice sheets of different thicknesses. They found that more than 100 billion tons of water vapor would have been jetted into the upper atmosphere. Water vapor is an effective greenhouse gas; suddenly having much more of it in the atmosphere could have caused a warming that ended an ice age, the team suggested. That idea still needs to be tested with climate models, the researchers noted.

Christian Koeberl, a geochemist at the University of Vienna and not involved in the research, agreed. Inferring what might have happened to Earth’s ancient climate is “Where things get a lot more speculative,” he said.

A lunar rover(月球车) has spotted a strange cube-shaped object and will change its official course to check it out, needing 2-3 months to arrive. Official observations suggest it could be a rock pushed upward from the impact(撞击) of an outside object that clearly landed next to it, or a technological relic from previous exploration, or hopefully, perhaps something that can’t be explained.

The Chinese Yutu 2 lunar rover spotted it in its cameras while passing a C-shaped enclosure on the moon’s far side. “Under the dark and deep sky, a circle of winding mountains stood on the extension line of the sky and the moon. On the side, people can’t help but admire the extraordinary work of the universe.” wrote Our Space, a Chinese -language blog connected with the national space agency. The drivers zoomed in(镜头拉近) on the pictures, slowly admiring them one by one. Suddenly, a cube on the northern skyline attracted their attention. The object looked like a “mysterious hut(小屋)”.

Right angles are unusual in nature, usually marking out something artificial if we see them through the faint light of a forest. On the moon, however, there’s not a lot of room for possibilities for a cube. After some days, the camera produced an image to share with the world of their “mysterious hut(小屋)”. The blog post suggested it could be evidence of previous moon missions, but noted the presence of a small impact crater(陨石坑) next to the cube, which could have led to the rising of rocks.

Popular Science magazine said the image could simply be pixelated, as it was taken from far away, Yutu2 landed with Chang’e 4, the first two spacecraft ever to land on the moon’s far side. They’ve in fact had the place to themselves since they arrived in 2019.

Do you ever get the feeling that you are being watched? Well, you might be right.

According to a study published in Nature on June 23, astronomers have found that 1,715 stars have had a direct view of Earth since humans have been here.

In order to do this, scientists used a previous method that looked for life on other planets. But instead, they changed the method so it could try to determine what places could see us.

The team looked at 331,312 stars within 326 light-years of Earth, with each light-year equaling 9.4 trillion kilometers. Out of all those stars, only 1,715 of them could see Earth within the last 5,000 years, with an extra 319 stars that will be able to see us in the next 5,000 years.

“When I look up at the sky, it looks a little bit friendlier because it’s like, maybe somebody is waving,” said Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University, US, and the study’s lead writer.

If a planet circling around one of those 1,715 stars is home to advanced life, they could easily see that there is life here because of the oxygen on Earth. If that didn’t give it away, then the radio waves we have sent out into space would also be an indicator. In fact, human-made radio waves have already traveled through 75 of the closest stars on Kaltenegger’s list.

Why haven’t we heard from anyone yet, then?

It takes a long time for messages to travel between star systems. By the time a message could be received, that advanced civilization would probably not exist anymore.

Alan Boss, a scientist at the Carnegie Institution for Science in the US who wasn’t part of the study, wrote in an email that this long time would limit the chances for different life to exchange “emails and TIKTOK videos”.

“So we should not expect aliens to show up anytime soon,” Boss said.