It is a tiny world, a mere 310 miles in diameter, and was considered until recently to be one of the least interesting moons in the solar system. But Enceladus, one of 146 moons that orbit Saturn, has become a hot astronomical attraction – scientists have discovered that it offers one of the best prospects of finding life on another world in our solar system.
The European Space Agency (Esa) has announced it has begun planning a mission to take a robot probe across a billion miles of space to investigate.
It will be an extraordinarily taxing project. Apart from the colossal distance the probe will have to travel, it will need huge reserves of fuel to manoeuvre itself into orbit around Enceladus and then land on the ice-coated surface.
Nevertheless, the prospect of studying the little moon is enticing for astronomers who have discovered that Enceladus – first observed by William Herschel in 1789 – possesses geysers that regularly erupt from its surface and spray water into space. Even more astonishing, these plumes contain complex organic compounds, including propane and ethane.
“Enceladus has three key ingredients that are considered to be essential for the appearance of life,” said astronomer Prof Michele Dougherty of Imperial College London. “It has got liquid water, organic material and a source of heat. That combination makes it my favourite moon in the whole solar system.”
This view is shared by Esa, which recently earmarked a mission to travel to a moon of Jupiter or Saturn as its next target for a major scientific endeavour. Such a project would be expected to deliver “a transformational scientific return”, according to a panel of expert scientists who studied three prime targets: Europa, the ice-covered moon of Jupiter; Titan, the hydrocarbon-rich moon of Saturn; and Enceladus. All possess subsurface oceans that hold the promise of sustaining alien lifeforms and would make first-class targets for scientific scrutiny.
After months of consideration, the panel reported a few weeks ago that it was the last on this interplanetary list, Enceladus, that should be given priority over the other targets. A mission should be launched by 2040 with the aim of either landing on the moon or flying through the geysers spraying water and carbon chemicals from its surface into space. Preferably, both goals would be attempted, the panel added.
“The mission would provide tremendous scientific return and would be fundamental for the successful detection of biosignatures on icy moons,” said Dr Zita Martins, an astrobiologist at Instituto Superior Técnico, who chaired the panel.
However, such goals will not be easy to achieve, as Dougherty pointed out last week. “It is one thing to put a probe into orbit round a large moon or planet that has a strong gravitational field that can slow down an incoming spacecraft. But Enceladus is small with weak gravity, which means you will need a lot of fuel to slow it down so that it does not whiz past its target into deep space. That is going to be a tricky issue for those designing the mission.”
Dougherty’s special interest in Enceladus stems from her 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 close to Enceladus and our instrument indicated Saturn’s magnetic field was being dragged round the moon in a way that suggested the little moon had an atmosphere,” said Dougherty.
Cassini’s managers agreed to direct the probe to take a closer look and, in July 2005, the spaceship swept over the moon’s surface at a height of 173km – and detected significant amounts of water vapour. “It was wonderful,” recalls Dougherty.
Subsequent sweeps produced even greater wonders. Huge geysers of water were pictured erupting from geological fault lines at the south pole. The only other body in the solar system, apart from Earth, possessing liquid water on its surface had been revealed. Finally came the discovery of organics in those plumes and Enceladus went from being rated a minor, unimportant moon to a world that is now set to trigger the expenditure of billions of euros and decades 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 lakes and seas of hydrocarbons; river channels; great stretches of dunes; and signs that it has a subsurface ocean that could provide a home for primitive life. It is also extraordinarily cold.
Mars
The red planet was a warm, watery world 4bn years ago and possessed ideal conditions for the appearance of life. However, Mars later lost its magnetic field, as well as its water and atmosphere, and was battered by intense ultraviolet radiation. Life would have found it difficult to survive on the surface but may have persisted, as microbes, underground.
Europa
One of the main moons of Jupiter, Europa is entirely covered with ice and has the smoothest surface of any known solid object in the Solar System. Beneath its surface, it has an ocean of water that scientists believe could harbour extraterrestrial life that would most likely consist of primitive bacteria-like entities.
