Iit is a small world, a mere 310 miles across, and until recently was considered one of the least interesting moons in the solar system. But Enceladus, one of 146 moons orbiting Saturn, has become a hot astronomical attraction – scientists have discovered that it offers one of the best prospects for finding life on another world 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.
This will be an extraordinarily taxing project. Apart from the colossal distance the probe will have to travel, it will need large reserves of fuel to move itself into orbit around Enceladus and then land on the ice-covered surface.
Nevertheless, the prospect of studying the moon is attractive to astronomers who have discovered that Enceladus – first observed by William Herschel in 1789 – has geysers that regularly erupt from its surface, spewing water into space. Even more amazing, these plumes contain complex organic compounds, including propane and ethane.
“Enceladus has three key ingredients that are thought to be essential for the appearance of life,” said astronomer Prof Michele Dougherty of Imperial College London. “It has 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 recently earmarked a mission to travel to a moon of Jupiter or Saturn as his next target for a major scientific endeavor. Such a project is expected to produce “a transformational scientific yield,” according to a panel of expert scientists who studied three main targets: Europa, the ice-covered moon of Jupiter; Titan, the hydrocarbon-rich moon of Saturn; and Enceladus. All possess subterranean oceans that hold the promise of sustaining alien life forms and would make prime targets for scientific investigation.
After months of consideration, the panel reported a few weeks ago that it is the last on this interplanetary list, Enceladus, that should take precedence over the other targets. A mission is to be launched by 2040 with the goal of either landing on the moon or flying through the geysers to spray water and carbon chemicals from its surface into space. Both goals will preferably be used, the panel added.
“The mission will yield tremendous scientific returns and will be fundamental to 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 easily achieved, as Dougherty pointed out last week. “It’s one thing to put a probe into orbit around 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’ll burn a lot of fuel need to slow it down so it doesn’t float past its target in deep space. That’s going to be a difficult 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 that Saturn’s magnetic field was being dragged around the moon in a way that suggested the moon had an atmosphere,” Dougherty said.
Cassini’s managers agreed to direct the probe to take a closer look, and in July 2005 the spacecraft swept across the moon’s surface at an altitude of 173 km – and detected significant amounts of water vapor. “It was great,” Dougherty recalled.
Subsequent sweeps produced even greater wonders. Huge geysers of water have been pictured erupting from geological fault lines at the South Pole. The only other body in the solar system, apart from Earth, that has liquid water on its surface has been revealed. Finally, the discovery of organic substances in those plumes came and Enceladus went from a minor, insignificant moon to a world that will now cause the expenditure of billions of euros and decades of effort by European astronomers and space engineers.
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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; large stretches of dunes; and signs that it has an underground ocean that could provide a home for primitive life. It is also unusually cold.
Mars
The red planet was a hot, watery world 4 billion years ago and had ideal conditions for the appearance of life. However, Mars later lost its magnetic field, as well as its water and atmosphere, and was hit by intense ultraviolet radiation. Life would have found it difficult to survive on the surface, but may have persisted, as microbes, underground.
Europe
One of the main moons of Jupiter, Europa is completely covered in 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 harbor extraterrestrial life that would most likely consist of primitive bacteria-like entities.
