by It is a tiny world, just 310 miles in diameter, and until recently was considered one of the least interesting moons in the solar system. But Enceladus, one of the 146 moons that orbit Saturn, is a hot topic of astronomical interest – scientists have found that it offers one of the best prospects for life on another planet in our solar system.
The European Space Agency (ESA) has announced that it has begun planning a mission to take a robotic probe across a billion miles of space to investigate.
It will be an extremely taxing project. Besides the huge distance the probe will have to travel, it will need huge reserves of fuel to move itself into orbit around Enceladus and then land on the ice-covered surface.
However, the prospect of studying the small moon is attractive to astronomers who have discovered that Enceladus – first observed by William Herschel in 1789 – is home to geysers that regularly erupt from its surface and spew. water into space. Even more surprising, these plumes contain complex organic compounds, including propane and ethane.
“Enceladus has three main ingredients that are thought to be essential for the appearance of life,” said astronomer Professor Michele Dougherty of Imperial College London. “It found liquid water, organic matter and a source of heat. That combination makes it my favorite moon in the entire solar system.”
This view is shared by Esa, who has recently booked a mission to travel to the moons of Jupiter or Saturn as her next target for a major scientific endeavour. Such a project would be expected to provide a “transformative scientific result”, according to a panel of expert scientists who studied three main targets: Europa, Jupiter’s ice-covered moon; Titan, Saturn’s hydrocarbon-rich moon; and Enceladus. They all have underground oceans that hold the promise of holding alien life forms and would make prime targets for scientific scrutiny.
After months of deliberation, the panel reported a few weeks ago that the last on this interplanetary list, Enceladus, should be prioritized over the other targets. A mission should be launched by 2040 with the aim of landing on the moon or flying through the geysers spraying water and carbon chemicals from its surface into space. It would be best if both goals were attempted, the panel added.
“The mission would provide a great scientific result and would be fundamental to the successful detection of biomarkers on icy moons,” said Dr. Zita Martins, an astronomer at Instituto Superior Técnico, who chaired the panel.
Achieving goals like these won’t be easy, however, as Dougherty pointed out last week. “It’s one thing to put a probe into orbit around a moon or a large planet that has a strong gravitational field that could slow down an incoming spacecraft. But Enceladus is small with weak gravity, which means you’ll need a lot of fuel to slow it down so it doesn’t overshoot its target in deep space. That is a difficult question for those designing the mission.”
Dougherty’s particular interest in Enceladus stems from his role as principal investigator for the magnetometer flown on the Cassini mission that studied Saturn and its moons between 2004 and 2017.
“At one point, Cassini passed by Enceladus and our instrument indicated that Saturn’s magnetic field was pulling the moon around in a way that suggested the small moon had an atmosphere,” Dougherty said.
Cassini’s managers agreed to instruct the probe to take a closer look and, in July 2005, the spacecraft swept over the surface of the moon at an altitude of 173km – and saw significant amounts of water vapour. “It was great,” says Dougherty.
Even more surprises came from brushing after that. The picture showed large geysers of water erupting from geological fault lines at the south pole. The only other body in the solar system, apart from Earth, was revealed to have liquid water on its surface. Finally came the discovery of the organisms in those plumes and Enceladus went from being rated as a small, unimportant moon to a world that is now the subject of billions of euros and years of effort by European astronomers and space engineers.
Life on other moons?
Titan
Another moon of Saturn and one of the largest in the solar system, Titan has hydrocarbon lakes and seas; river channels; large stretches of dunes; and signs that it has an underground ocean that could provide a home for primitive life. It is also very cold.
Mars
The red planet was a hot, watery world 4bn years ago and had suitable conditions for life to appear. However, Mars later lost its magnetic field, as well as its water and atmosphere, and was hit by intense ultraviolet radiation. It would be difficult for life to survive on the surface but it could still exist, as a microbe, underground.
Europe
One of Jupiter’s main moons, Europa is completely covered in ice and has the smoothest surface of any known solid object in the Solar System. Beneath its surface, there is an ocean of water that scientists believe could contain extraterrestrial life composed of primitive entities such as bacteria.
