January 27, 2014 | 5
Reading the scientific headlines recently one would be forgiven for thinking that we’re experiencing a bout of interplanetary gastrointestinal distress.
First, Saturn’s diminutive moon Enceladus continues to spew what we think are giant sprays of salty water from gnarled creases in its southern icy surface – captured in glorious imagery by the Cassini spacecraft over much of the past decade. Second, just a few weeks back the news hit that the Hubble Space Telescope had caught the Jovian moon Europa seemingly doing the same. Plumes of gaseous water were registered sprouting from this icy satellite’s southerly regions (what is it about the south poles on these objects?) in synch with the apojove, or farpoint, of its 85 hour orbit. And, most recently, the now-defunct Herschel space telescope has left us data indicating that the dwarf planet Ceres (or big asteroid, or big comet-nucleus-like-body, take your pick, whatever you want to call it, really) routinely blows water from patches of its surface.
So what’s going on?
Much is to do with the fact that our ability to sense and measure phenomena across the solar system is getting better and better. But it’s also a plain truth that there is water all over the place, doing all sorts of things; we simply haven’t spotted it squirting out of these bodies before.
In the case of the 1,900 mile diameter Europa we have long suspected that a vast ocean of liquid water might exist dozens of miles beneath its outer icy crust. The evidence has been indirect and diverse, but taken all together it’s extremely compelling.
For example, Europa’s surface features are a very good match for what we’d expect for a world whose innards are kept warm by the combination of radioactivity in a rocky core and tidal flexure due to Jupiter’s fearsome gravity field – maintaining a subsurface layer of liquid water that could be twice the volume of all Earth’s oceans.
There are great regions of ‘chaotic’ terrain, akin to re-frozen fields of tumbling icebergs. Smoother zones show little cratering, appearing geologically young and filled in from below. And strangely raised, welt-like cracks or lineae, criss-cross the surface – as if icy plates have opened and closed to extrude some of the interior. Furthermore, in the 1990′s the Galileo spacecraft sensed an induced magnetic field emanating from Europa, consistent with a dielectric substance beneath the surface. A salty ocean would handily fit the bill, and could also leave the curious sulfate deposits now detected as a blush atop Europa’s frigid exterior.
The water plumes that Hubble has spotted (admittedly at the hairy edge of detection) come and go in synch with Europa’s orbit and its tidal flexing – fitting beautifully with the picture of a deeper ocean that can seep, or even erupt, upwards to refurbish the moon’s exterior. It seems that Europa has indeed reported, offering a glimpse of that mysterious abyss.
Little Enceladus, at 314 miles across, is half the size of dwarf planet Ceres, and its icy geysers erupt vigorously to feed Saturn’s E-ring with salty particles. Here too the culprit must be subsurface water laced with dissolved compounds, but we don’t yet know the precise configuration of the interior. It could be pocket-like lakes fed by upwelling water that has had contact with a rocky core, or a true hidden ocean. Nor do we understand how this tiny moon can keep generating the giga-watts of power needed for either scenario. Considerable radioactive decay and tidal flexing seem necessary, but are hard to explain.
For both Europa and Enceladus the possible existence of volumes of liquid, mineral enriched water raise hopes for finding life beyond the Earth. Analogs to terrestrial chemoautotrophic microbes (rock eaters if you prefer) could be very good at living in the inky depths of such places. Whether or not they could originate there is another question.
So now we come to Ceres, a 600-mile wide body orbiting the Sun in a gentle ellipse that takes it between about 2.55 astronomical units from the Sun to 2.98 in the course of its 1,680 day year. Compared to distant Europa or Enceladus it is in a much more precarious position when it comes to water. This is a zone where there can be sufficient solar heating to cause the ‘boil-off’, or sublimation, of solid ice into the vacuum of space. The current data seem to suggest that this boil-off, like that of a cometary nucleus, is the more likely culprit for Ceres’s watery outbursts – coinciding with its closest approaches to the Sun. However it’s still possible that there is an interior layer where liquid water could exist, warmed by a rocky core and insulated by a frozen crust and prone to cryovolcanic eruption.
The great news is that in 2015 NASA’s Dawn spacecraft will be arriving at Ceres and parking itself in orbit. This has got to count as one of the greatest cases of lucky timing for space exploration in recent years. Dawn will confirm the surface composition of Ceres and could bear direct witness to the active water loss spied by the Herschel instrument.
Whether Ceres is merely acting comet-like or is geophysically active the bigger puzzle may actually be why Ceres got to have this much indigenous water, while Dawn’s previous host, the 320 mile-wide minor-planet/asteroid Vesta, didn’t. Vesta has a similar orbit but very different composition, showing the signs of extensive past heating and volcanism that has flooded its surface with igneous rock.
The most probable explanation is that Ceres formed further from the Sun in our nascent solar system, acquiring a larger amount of frozen water that in turn altered its geophysical history. But for it to be where it is now there must have been considerable positional mixing of objects 4.6 billion years ago. The suspicion is that the outer planets underwent some orbital rejiggering and that their gravitational pulls messed with what we now call the asteroids, but we don’t quite know how this played out. Seeing what Ceres is really made of will help fill in a few more blanks.
The eruption of water across the solar system is therefore not just a case of planetary indigestion, but another clue to the deeper origins and evolution of all that is around us.