Euorpa’s icy, mineral-stained surface as imaged by NASA’s Galileo mission – see below (credit: NASA / JPL)

NASA have been teasing the press and pundits with news that they have a “surprising” announcement to make about Europa, one of Jupiter’s four Galilean moons (so-called as they were first recorded by Galileo Galilei).

Slightly smaller than our own Moon, Europa is covered by shell of water ice, much of it discoloured by mineral deposits and by deep cracks. This icy surface might only be relative thin, on the order of a handful of kilometres in extent, or it might be tens of kilometres thick, and sits over an ocean which is mostly likely liquid water, although some argue it might actually be an icy slush, perhaps extending to 100 km (62.5 miles) in depth.

The ocean is made possible by tidal flexing enacted by the massive gravity of Jupiter as well as from the other large Galilean moons. This generates heat within Europa, and this heat stops the water from freezing solid.

An artist’s impression of how a huge plume of water, over 200km (125 mi) high, which erupted from Europa in 2012 and was “seen” by the Hubble Space Telescope, might have looked like if witnessed from the vicinity of Europa. Credit: NASA / ESA / M. Kornmesser.

Exactly how much heat is generated as a result of this flexing isn’t known, but it has been suggested that the ocean floor could be home to volcanic activity with hydrothermal vents and fumeroles responsible for pumping huge amounts of minerals into the water, as well as supplying energy, potentially marking Europa’s ocean as a place where basic microbial life might arise.

The discovery of life on Europa isn’t going to be the subject of the NASA press conference. It will instead reveal the findings of a Europa observation campaign using the Hubble Space Telescope linked to the potential for a liquid water ocean being present under the moon’s ice. I’ll likely have more next week.

Nor is Europa likely to be alone in harbouring a subsurface ocean among the Galilean moons of Jupiter. In 2015 data from the Hubble Space Telescope confirmed that Jupiter’s largest moon, Ganymede, has an underground ocean that contains more water than all of Earth’s combined. Hubble was used to carry out a spectrographic observation of Ganymede’s aurorae, displays of light in the atmosphere. Because aurorae are controlled by a moon or planet’s magnetic field, observing changes in how they behave offers insights into what is happening beneath the surface of the planet or moon. In Ganymede’s case, the aurorae allowed scientists to confirm a long-suspected subsurface salt water there.

Pluto’s Liquid Heart

A global mosaic of Pluto captured by New Horizons from a distance of  450,000 km (280,00 mi) from Pluto byt New Horizons on July 14th, 2015, coloured from data received by the RALPH instrument on the spacecraft, reveals the planet’s heart-shaped mark, the left “lobe” of which is formed by the massive depression dubbed “Sputnik Planum”. Credit: NASA/JPL / JHU/APL / SwRI

In June, I wrote about a paper proposing Pluto harbouring a liquid water ocean beneath its surface. The paper, by Planetary Science Institute Senior Scientist Amy C. Barr and Noah P. Hammond of Brown University, reached its conclusion after a prolonged study of Pluto’s geological features, including “Sputnik Planum”, a massive depression on the planetoid which forms one “lobe” of Pluto’s distinctive “heart”.

Barr and Hammond’s work focused on the lack of ice II on Pluto – a place where ice II should be expected to form. Had it done so, it would have caused volume contraction, resulting in the formation of compressional tectonic features on the surface of the planet. However, Barr and Hammond found no evidence for such features on Pluto in all of the images returned by the New Horizons spacecraft which flew past Pluto and its twin, Charon, in July 2015. This led them to conclude that Pluto’s interior is warmer than might be expected, which would both prevent ice II from forming and potentially give rise to a liquid ocean beneath Pluto’s frozen crust.

Now, a second paper has been published in Geophysical Research Letters, offering a suggestion as to how deep that ocean is, and its potential composition. Another research team at Brown University have been investigating the dynamics between Pluto and Charon, and the likely formation and development of the “Sputnik Planum” depression, which is thought to have been initially created by the impact of an object some 200 km (125 mi) across at some point in Pluto’s formative years.

Pluto and Charon are tidally locked with each other, so they always show each other the same face as they rotate. “Sputnik Planum” sits directly on the tidal axis linking the two worlds. This suggests the basin has what’s called a positive mass anomaly — it has more mass than average for Pluto’s icy crust. As Charon’s gravity pulls on Pluto, it would pull proportionally more on areas of higher mass, which would tilt the planet until “Sputnik Planum” became aligned with the tidal axis.

The surface ice on “Sputnik Planum” is constantly being renewed both by atmospheric deposition from above, and convection action from below, suggesting a source of heat beneath the ice, which in turn could be keeping any subsurface ocean liquid. Credit: NASA/JPL / JHU/APL / SwRI

But why would a crater – essentially a hole in the ground – be a positive mass anomaly? Part of the answer probably lies in the huge amount of nitrogen ice which has accumulated in the basin over the aeons, adding mass to the basin.

But the ice isn’t thick enough on its own to create the amount of mass needed to make “Sputnik Planum” have positive mass. Water, however, could have sufficient mass.

An impact creates a dent on a planet’s surface, followed by a rebound. That rebound pulls material upward from deep in the planet’s interior. If that material is denser than what was blasted away by the impact, the crater ends up with the same mass as it had before the impact happened. Any material added to it after the impact and rebound would therefore add mass to it, creating a positive mass anomaly.

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