New Tongan island offers clues to water on Mars

The Earth’s newest island is a geological treasure trove that may, among other things, hold clues to questions about water on Mars in ancient times, scientists say.

The island, known as Hunga Tonga Hunga Ha’apai, rose from the South Pacific in a month-long volcanic eruption through December 2014 and January 2015. It lies north of New Zealand in the Kingdom of Tonga. 

Initially, scientists expected it to last only a few months before the pounding of the South Pacific surf eroded it away. But it persists, standing 120 metres tall at its highest point and measuring about two kilometres across, scientists said this week at a meeting of the American Geophysical Union, in New Orleans, Louisiana, US.

Unlike another island that recently formed in the Red Sea, this one was created not from erosion-resistant lava but from an explosive process that produced a large cone of loosely consolidated volcanic ash. 

In 2009, another island had formed close to the present site of Hunga Tonga Hunga Ha’apai in 2009, but it eroded away within a matter of months, says Jim Garvin of NASA’s Goddard Space Flight Centre in Greenbelt, Maryland.

Hunga Tonga Hunga Ha’apai, however, has persisted, although erosion has changed its shape from circular to something more elongated. It is also now connected to two neighbouring islands via low-lying peninsulas formed from eroded material that piled up in the shallow waters between them. 

The island continues to erode, but is washing away slowly enough that today it is predicted to last for between 26 and 30 years, says Gavin’s NASA Goddard colleague Dan Slayback. 

Garvin compares the rapid changes to watching your kids grow up, though in this case, the island isn’t actually growing. “It’s losing mass but creating a large land bridge that’s allowing the system to partially survive,” he says.

The heart of the island is also protected by a sandbar that keeps ocean waves from penetrating into the sea-level lagoon that fills its central crater. Were that to break down, waves would probably erode the crater’s steep, unstable cliffs very rapidly, Slayback says.

The island’s continued survival draws comparisons to Surtsey, an island off the southern coast of Iceland birthed by volcanic eruption in 1963 and which has likewise defied predictions of a rapid demise. 

In the case of Surtsey, survival is attributed to a cap of hard lava that is not present on Hunga Tonga Hunga Ha’apai. Surtsey is also preserved by chemical reactions in the rock due to interactions with hydrothermal water. This geothermally heated water, circulating through the ash, created concrete-like materials known as pelagonite.

 “Those things fortify volcanoes in this marine environment,” says Gavin. “Weak ash you can dig with a shovel turns into hard stuff you can’t break.”

Pelagonite might also be forming on Hunga Tonga Hunga Ha’apai, he adds. “We think we’ve found evidence in some samples,” he says.

Vicki Ferrini of the Lamont-Doherty Earth Observatory at Columbia University, New York, adds that the island may also be surviving due to its location. Like the two neighbouring islands (both of which appear to be remnants of much larger eruptions thousands of years ago), it sits on the inner slopes of the caldera of a seamount that rises 1.4 kilometres from the seabed, she says. 

She and colleagues recently mapped that caldera, using multibeam sonar from an oceanographic research vessel. They found it to be five kilometres wide, with a flat floor only 155 metres below the surface, which may help stabilise the island’s base.

But what’s really exciting about the new island is what studying its evolution it might tell us about Mars.

That’s because Mars has many volcanic features that look much like it. “There are thousands of them in multiple regions,” says Gavin. 

Many scientists think these features may have been formed as a result of volcanic eruptions beneath shallow seas, about one to two billion years ago. But instead of being eroded away, these one-time islands were frozen in time when the ocean dried up and left them high and dry, in what Gavin calls “an arrested state of development.”

Knowledge gained by studying the evolution of Hunga Tonga Hunga Ha’apai, he says, may make it possible to determine the depth of the water in which these Martian islands sat and how long that water may have persisted. “This will give us windows onto the times on Mars when we think there were standing bodies of water,” he says. “Do we see cliff lines with erosion, and the patterns we see here?”

Not that the parallel is perfect. Hunga Tonga Hunga Ha’apai is in the Pacific Ocean, and whatever oceans existed on Mars weren’t anywhere close to that large. So, he says, while Hunga Tonga Hunga Ha’apai provides scientists an opportunity to observe the evolution of such islands, the processes occurring on it will not be identical to ones that would have occurred on Mars. “It’s accelerated because it’s in this big marine environment,” Gavin says.

Still, he says, watching Hunga Tonga Hunga Ha’apai evolve can help scientists start to put time constraints on the length of time when those Martian oceans might have been around. And when it comes to studying ancient Martian history, he says, trying to determine how long water persisted “is one of those Holy Grails.”

Related reading: Mars: New water cycle discovered