The solar system's icy moons, May 30th 2015 P75 (惑星の月)
For decades, researchers looking for life elsewhere in the solar system have stuck to a simple rule：“follow the water”. The stuff is essential to life on Earth, and its peculiar properties mean those hunting aliens think the same will be true elsewhere. This is one reason that Mars has received so much of their attention. Mars has water vapour in its atmosphere, and water ice in the shinning cap at its north pole. Liquid water - which is the form life needs - may well exist underground, and may still sometimes be seen at the surface, where dried-up rivers and lakes suggest it was once commonplace. Orbiting spacecraft have seen mysterious dark channels appear on crater walls；recent results from Curiosity, the newer of the two American rovers now exploring Mars, suggest these could be trickles of brine, kept liquid in the bitter cold by their high salt content.
But for all that, Mars is at best a desert with a few wet patches. Farther from the sun there are a number of icy bodies that seem to boast whole oceans of liquid water beneath their solid surfaces. Even by the standards of space exploration, this water - billions of kilometers away and hidden from direct inspection - is a challenging subject for study. But there plans afoot to meet that challenge. On May 26th NASA announced the suite of scientific instruments it has selected for Europa Clipper, a $2 billion mission which could be launched in the early 2020s. Despite its nicely nautical name, it will not actually sail the seas of Europa, an icy moon of Jupiter. But it might be able to find out something of their chemistry - and, just possibly, their biochemistry.
It is biochemistry - evidence of molecules made by life - that Mars has failed to provide. For decades ago two Viking probes landed and subject samples of its surface to various chemical tests as a way of looking for life. The results were frustrating；not quite what was expected from lifelessness, but not seemingly compatible with life. For all the satellites and rovers that have studied Mars since, there has been no sign of so much as bacterium.
It is thus hardly surprising that interest in water elsewhere has risen. And it has turned out that Enceladus, a small moon of Saturn and Europa have it in abundance：Enceladus may have as much as Lake Superior. Europa may have more than the whole Earth.
That may sound odd. Jupiter and Saturn are much father from the sun than Mars is. The average temperature on Europa is a chilly -171°C. But sunlight is not the only way to heat an astronomical body. Gravity can do it, too. To most Earthlings, the tides produced by the gravity of the moon and sun matter only to see levels. But the tides that a huge planet like Jupiter or Saturn can produce in an orbiting moon are powerful enough to knead the rocks of the moon's core, warming them up via friction. Such heating from below allows both Enceladus and Europa to harbour oceans under their cold, icy surfaces.
In the case of Enceladus bits of this ocean have been observed escaping into space. A probe called Cassini, which has been orbiting Saturn for more than a decade, has spotted jets of water shooting up from near the moon's south pole. Europa, too, may spew out water on occasion. In 2013 the Hubble space telescope spotted what seemed to be plumes erupting from the moon's surface.
Life cannot live by water alone. Other elements are needed - notably carbon and nitrogen. And that is what makes Enceladus and Europa so tantalising. There is evidence that the oceans on both are in contact with a rocky seafloor that can provide the elements life needs. For Europa, that evidence comes indirectly, from computer modelling of its interior and analysis of the brown streaks that cross its surface, which are thought to be salt deposits discoloured by solar radiation.
For Enceladus, by contrast, the evidence is solid. Soon after Cassini spotted the plumes coming from the moon's surface, its controllers directed it to plunge through one of them. Although it had not been designed for such a mission, the craft was able to capture some of the spray and analyse it. It turned out to be full of exactly the sorts of complicated carbon-based molecules that might serve as precursors to life.
Whether they have actually given rise to life is, of course, another question. But the more researchers look, the more intriguing the Saturnian moon becomes. On March 12th, Sean Hsu of the University of Colorado and his colleagues published a paper in Nature analysing tiny silicate grains spewed out by the plumes. They concluded these were almost certainly produced at the bottom of a salty, alkaline ocean in hydrothermal vents - structures that form when mineral-laden water circulates through hot rocks on an ocean's floor. Such hydrothermal vents are found in Earth's oceans too, and are a strong candidate for being the site of the origin of life. Laboratory experiments have shown that tiny pockets inside them can act like primitive cells, concentrating organic chemicals inside and maintaining a voltage difference across the “cell” wall that could drive chemical reactions. These things are both, literally, vital features of living cells.
But for all that, Mars is at best a desert with a few wet patches. Farther from the sun there are a number of icy bodies that seem to boast whole oceans of liquid water beneath their solid surfaces. Even by the standards of space exploration, this water - billions of kilometers away and hidden from direct inspection - is a challenging subject for study. But there plans afoot to meet that challenge. On May 26th NASA announced the suite of scientific instruments it has selected for Europa Clipper, a $2 billion mission which could be launched in the early 2020s. Despite its nicely nautical name, it will not actually sail the seas of Europa, an icy moon of Jupiter. But it might be able to find out something of their chemistry - and, just possibly, their biochemistry.
It is biochemistry - evidence of molecules made by life - that Mars has failed to provide. For decades ago two Viking probes landed and subject samples of its surface to various chemical tests as a way of looking for life. The results were frustrating；not quite what was expected from lifelessness, but not seemingly compatible with life. For all the satellites and rovers that have studied Mars since, there has been no sign of so much as bacterium.
It is thus hardly surprising that interest in water elsewhere has risen. And it has turned out that Enceladus, a small moon of Saturn and Europa have it in abundance：Enceladus may have as much as Lake Superior. Europa may have more than the whole Earth.
That may sound odd. Jupiter and Saturn are much father from the sun than Mars is. The average temperature on Europa is a chilly -171°C. But sunlight is not the only way to heat an astronomical body. Gravity can do it, too. To most Earthlings, the tides produced by the gravity of the moon and sun matter only to see levels. But the tides that a huge planet like Jupiter or Saturn can produce in an orbiting moon are powerful enough to knead the rocks of the moon's core, warming them up via friction. Such heating from below allows both Enceladus and Europa to harbour oceans under their cold, icy surfaces.
In the case of Enceladus bits of this ocean have been observed escaping into space. A probe called Cassini, which has been orbiting Saturn for more than a decade, has spotted jets of water shooting up from near the moon's south pole. Europa, too, may spew out water on occasion. In 2013 the Hubble space telescope spotted what seemed to be plumes erupting from the moon's surface.
Life cannot live by water alone. Other elements are needed - notably carbon and nitrogen. And that is what makes Enceladus and Europa so tantalising. There is evidence that the oceans on both are in contact with a rocky seafloor that can provide the elements life needs. For Europa, that evidence comes indirectly, from computer modelling of its interior and analysis of the brown streaks that cross its surface, which are thought to be salt deposits discoloured by solar radiation.
For Enceladus, by contrast, the evidence is solid. Soon after Cassini spotted the plumes coming from the moon's surface, its controllers directed it to plunge through one of them. Although it had not been designed for such a mission, the craft was able to capture some of the spray and analyse it. It turned out to be full of exactly the sorts of complicated carbon-based molecules that might serve as precursors to life.
Whether they have actually given rise to life is, of course, another question. But the more researchers look, the more intriguing the Saturnian moon becomes. On March 12th, Sean Hsu of the University of Colorado and his colleagues published a paper in Nature analysing tiny silicate grains spewed out by the plumes. They concluded these were almost certainly produced at the bottom of a salty, alkaline ocean in hydrothermal vents - structures that form when mineral-laden water circulates through hot rocks on an ocean's floor. Such hydrothermal vents are found in Earth's oceans too, and are a strong candidate for being the site of the origin of life. Laboratory experiments have shown that tiny pockets inside them can act like primitive cells, concentrating organic chemicals inside and maintaining a voltage difference across the “cell” wall that could drive chemical reactions. These things are both, literally, vital features of living cells.
