A possible ancient shoreline has been found in the region of Mars explored by the Chinese rover Zhurong, providing further evidence that an ocean may once have covered a vast area of the lowlands in the planet’s northern hemisphere.

The rover landed in southern Utopia Planitia in May 2021 and remained active for almost a year. Researchers studying data from the rover have found hints of an ancient ocean or liquid water as recently as 400,000 years ago.

Now, Bo Wu at Hong Kong Polytechnic University and his colleagues have conducted a comprehensive analysis of the topographic features in the landing area, combining remote sensing data from satellites with observations from the rover.

They say they have found features consistent with the existence of a shoreline in southern Utopia, including troughs and sediment channels. They also dated and identified the composition of surface deposits in the area. Based on this, the team thinks the ocean existed 3.68 billion years ago, but froze and then disappeared around 260 million years later.

“The findings not only provide further evidence to support the theory of a Martian ocean but also present, for the first time, a discussion on its probable evolutionary scenario,” says Wu.

The area can be divided into a shallow section to the south and a deep section in the north. Even the shallow part of the ocean may have been up to 600 metres deep, says Wu, but there isn’t enough data to estimate the maximum depth of the ocean.

“Water is a key ingredient for life, and the past presence of an ocean on Mars raises the prospect that Mars once might have been capable of harbouring microbial life at its early stage,” he says.

Mathieu Lapôtre at Stanford University in California says whether oceans existed on early Mars is a highly contentious question with critical implications for the planet’s past habitability. He says future missions will need to test the findings of this new research.

“Utopia Planitia may constitute an invaluable record of early Mars’s near-shore and coastal environments,” says Lapôtre.

Flights taken on private jets should be subject to a carbon tax to curb the runaway growth in carbon emissions from the sector, researchers have said.

Emissions from private aviation jumped 46 per cent between 2019 and 2023, according to analysis of 18.7 million flights by almost 26,000 aircraft.

Flights were mainly for leisure reasons, with 1846 private flights to the 2022 FIFA World Cup in Qatar alone. Other popular destinations were the Cannes Film Festival, the Super Bowl, the COP28 climate conference in Dubai, and the World Economic Forum in Davos. Trips to the south of France, Ibiza and other destinations in Spain peaked during the summer months as travellers jetted in for long weekends of sun.

“A rather small group of very wealthy individuals, because of their lifestyles and investments, is pushing emissions quite quickly up,” says Stefan Gössling at Linnaeus University, Sweden.

Alongside colleagues, Gössling used flight tracker data for millions of flights to build a picture of private aviation use around the world.

Flying by private jet is the most polluting way to travel, with a single flight emitting 3.6 tonnes of CO2 on average, equivalent to the annual carbon impact of someone living in Sweden.

Most flights on private jets are short, the analysis found, with almost half of all flights covering a distance less than 500 kilometres. Most were within the US and Europe.

Total emissions from private jets in 2023 were 15.6 megatonnes of carbon dioxide, equivalent to the annual emissions of Tanzania. That is up from 10.7 megatonnes in 2019.

Growth rates were distorted by the covid-19 pandemic. Unlike commercial aviation, which was heavily restricted in 2020 and 2021, private aviation only showed a small dip in flight numbers and emissions in 2020 before rebounding to growth the following year.

Many of the most extensively used private jets are owned by very rich celebrities, including Tesla CEO Elon Musk, former Google CEO Eric Schmidt, pop star Jay-Z and entertainment personality Kim Kardashian, according to data compiled by the website Celebrity Jet.

“This is about the inequality in the production of greenhouse gases,” says Mark Maslin at University College London. “It’s not even the 1 per cent – it’s the 0.1 per cent richest people in the world who click their fingers and use a private jet.”

The high personal emissions of the super-rich risks eroding public appetite for cutting personal emissions, says Gössling. “If the very wealthy don’t don’t have to reduce their emissions… then we don’t have any reason for anybody else to reduce their emissions, because everybody else is emitting less,” he says.

Gössling would like to see a carbon tax applied to private jet use. “We can put a price tag on every tonne [of carbon] that is emitted, and I think everybody will agree that it’s fair that the affluent should pay the cost of the damage that they are causing,” he says.

Others would like governments to go even further. Sean Currie at the campaign group Stay Grounded wants to see a total ban on the use of private jets. “Around half of these flights are short-haul flights,” he says. “They could easily be replaced by trains if we were to ban private jets and then invest in real infrastructure.”