Space Sunday: From Pluto’s ocean to Mercury’s darkness

Water ice had been identified by the RALPH instrument suite on New Horizons, now it seems Pluto may have a liquid ocean beneath its surface

Water ice had been identified by the Ralph instrument suite on New Horizons (shown on blue), now it seems Pluto may have a liquid ocean beneath its surface (Credit: NASA/JHUAPL/SwRI)

It may be so far away that the Sun appears to be a particularly bright star it is sky, but it now seems that Pluto has a liquid ocean just beneath its icy surface, just as might have once been the case with its companion, Charon, billions of years ago.

Since passing through the Pluto-Charon system in July 2015, NASA’s New Horizons space craft has been returning the data it gathered at a steady rate, focusing initially on the high-resolution images collected during the probes high-speed run by the two tiny worlds (both smaller than the Moon). These images have revealed Pluto and Charon to be remarkably complex little worlds, with glacial flows, rotated ice blocks, volcano-like mounds and other features rivalling the geology found on much larger, warmer planets like Mars.

“What we see really has exceeded all of our collective expectations and imagination,” said William McKinnon, a planetary scientist at Washington University, Missouri, and one of those working on the project. “We think on the insides of these bodies were very cold ammonia rich oceans,” said McKinnon, noting that ammonia is a “fantastic antifreeze” that can lower the freezing point of water by 100 C.

A close-up of the canyons on Charon, Pluto's big moon, taken by New Horizons during its close approach to the Pluto system last July. Multiple views taken by New Horizons as it passed by Charon allow stereo measurements of topography, shown in the color-coded version of the image. The scale bar indicates relative elevation. Credits: NASA/JHUAPL/SwRI

A close-up of the canyons on Charon, taken by New Horizons during its close approach to the Pluto system last July, reveal how the surface was “cracked open” as the subsurface ocean froze-out and expanded (credit: NASA/JPL / John Hopkins University Applied Physics Laboratory  / SwRI)

Data from New Horizons indicate that Charon’s ocean probably froze solid around 2 billion years ago, expanding as it did so, cracking open the outer shell of the world. This freezing-out was likely due to Charon being too small to remain geologically active, its internal processes quickly slowing down as it cooled. Pluto, however, being larger, shows every sign of still being active and with a warm interior, so its subsurface ocean probably still exists, marking it as another in a handful of the solar system’s smaller bodies which are home to sub-surface oceans.

“We now have half a dozen worlds, like Enceladus (a moon orbiting Saturn), Europa and Ganymede (moons of Jupiter), and now Pluto, that seem to have oceans in their interiors,” New Horizons’ lead scientist Alan Stern said when discussing the potential and significance of Pluto’s ocean.

Fumeroles (hydrothermal vents) support exotic life on Earth's seabed

Fumeroles (hydrothermal vents) support exotic life on Earth’s seabed

We know that life is remarkably tenacious and is extraordinary for surviving in unlikely places. All that is required is heat, a source of energy and water. On Earth, for example, volcanic fumeroles on the deep ocean floor can become havens for exotic life in places where sunlight never reaches.

This has led to speculation that places like Europa, which generates a lot of internal heat due to gravitational flexing thanks to the presence of Jupiter and the other large Galilean satellites, may well have similar, mineral-rich fumeroles on its ocean floor which may be havens for life exotic, basic forms of life. Could Pluto have the same?

“All we can say is that we think that Pluto has an ocean and we think that this ocean has survived to the present day. It’s the kind of ocean that is deep inside the interior of Pluto, in total darkness,” McKinnon stated.

“But, it would lie between a floating water ice shell and the rocky interior, so it would be in contact with rock. There would be a modest amount of heat leaking out. You certainly couldn’t rule it out, but anything about life on Pluto is simply speculation.”

Whether or not any basic life has managed to develop deep under Pluto’s icy crust is something we may never discover. However, that a liquid ocean does appear to exist beneath the planet’s icy shell is nevertheless intriguing. That water is present on Pluto has already been confirmed by the Ralph instrument suite aboard New Horizons. However, further evidence of its existence was revealed in February with the publication of images of “floating” hills of water ice on the nitrogen ice “sea” of Sputnik Planum”.

These hills are thought to be fragments which have broken away from the uplands surrounding “Sputnik Planum”. They exist in chains multiple kilometres in length or are grouped together, standing in stark contrast to the relatively flat expanse of the icy plain on which they sit.

Because water ice is less dense than nitrogen-dominated ice, scientists believe these water ice hills are like icebergs in Earth’s Arctic Ocean. In particular, the “chains” of hills have formed along the flow paths of the glaciers, while in the more “cellular” terrain of central “Sputnik Planum”, they become subject to the convective motions of the nitrogen ice, and are pushed to the edges of the cells, where the hills form clusters or groups. One of the largest of these, located towards the north of “Sputnik Planum” and measuring some 60km x 35km (37 mi x 22 mi) has been dubbed “Challenger Colles” in memory of the crew of the lost space shuttle Challenger.

The floating water ice hills of Pluto, slowly drifting over the nitrogen ice of "Sputnik Planum"

The floating water ice hills of Pluto, slowly drifting over the nitrogen ice of “Sputnik Planum” (Credit: NASA/JPL / John Hopkins University Applied Physics Laboratory  / SwRI)

Mercury’s Darkness

From one small planet on the outer reaches of the solar system to another, this one orbiting closest to the Sun: tiny Mercury, the smallest “official” planet in the solar system. At 4,879 km in diameter, Mercury is 1/3 the size of Earth and only slightly larger than our own Moon (at 3,475 km), and is actually smaller than Jupiter’s moon Ganymede and Saturn’s Titan, although it has far greater mass than the Moon or Ganymede or Titan.

A strange world, Mercury is tidally locked with the Sun at a 3:2 resonance, meaning it rotates around its axis three times for every two times it orbits the Sun. This means that if you were able to survive on the surface of Mercury, you would see only one day every two Mercurian years (66 terrestrial days apiece).

One of the major mysteries concerning Mercury is that it has a surprisingly dark surface, reflecting far less sunlight than our own Moon.  The latter’s reflectivity is somewhat controlled by the abundance of iron-rich minerals in the lunar surface material – but Mercury is known to have previous few such minerals in its surface matter, so so in theory should be a lot brighter.

This oblique image of the impact crate Basho on Mercury shows the distinctive dark halo of "Low Reflectance Material" (LRM), originally thought to be carbon contained within the cometary fragment which created the crater (and many like it across Mercury), but which now appears to have come which was excavated from depth when the crater was formed. Basho is also renowned for its bright ray craters, which render the crater easily visible even from very far away. Credit: Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

This oblique image of the impact crate Basho on Mercury shows the distinctive dark halo of “Low Reflectance Material” (LRM), originally thought to be carbon contained within the cometary fragment which created the crater (and many like it across Mercury), but which now appears to have come from inside the planet (credit: NASA/JPL / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington)

It had been theorised that much of Mercury’s lack of reflectivity was due to it having been struck repeatedly by many carbon-rich comets, scattering the carbon they contain over the surface and reducing its reflectivity. Certainly, Mercury does carry much evidence for bombardment from space; it is very heavily cratered, its surface resembling that of the Moon, and many of the craters are surrounded by dark carbon rings.

However, data gathered during NASA’s Messenger (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission, which ended in 2015, suggests that while cometary impacts did lead to carbon deposits across the surface of Mercury,  the majority of the deposited material actually came from within the planet itself, which in turn potentially strange history for the planet.

An enhanced colour image from MESSENGER highlights Mercury’s low-reflectance material (seen in blue) and its association with impact-excavated material around relatively young crater's on the planet's surface (credit: NASA/JPL / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

An enhanced colour image from MESSENGER highlights Mercury’s low-reflectance material (seen in blue) and its association with impact-excavated material around relatively young crater’s on the planet’s surface (credit: NASA/JPL / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington)

The theory is that when Mercury was very young, much of the planet was likely so hot that there was a global “ocean” of molten magma. As this cooled, heavier minerals solidified and sank, eventually leaving the planet with a surface crust of buoyant graphite. Then, over time, bombardment from space and volcanic activity gradually covered this carbon crust with the minerals and material deposited beneath it, with the result that more recent impacts in Mercury’s past have reversed this process: throwing up the now buried carbon of the ancient crust and depositing across the planet’s more recent surface material, darkening it in the process.

Ceres Winks as We Wait

As I’ve been reporting over the past few months, the joint NASA / ESA Dawn mission has been mapping Ceres, one of the solar system’s three “protoplanets” located in the asteroid belt between the orbits of Mars and Jupiter. Like Mercury and the Pluto system, Ceres has proven to be a surprising and mysterious little place, complete with a mountain where none should exist and – in particular – the appearance of odd bright spots within one of its craters.

It is anticipated that more information on the Occator bright spots will be given by members of the Dawn mission team during a NASA-hosted conference taking place between March 21st and 25th. However, surprising news about the bright spots has come from another source here on Earth.

The highest resolution image of the Occator bright spots so far returned to Earth by Dawn, taken in in September 2015, when the vehicle was in its "mid-altitude" mapping survey. Now orbiting much closer to Ceres, the next images of the 90 km wide Occator are expected to reveal the bright spots in considerable detail

The highest resolution image of the Occator bright spots so far returned to Earth by Dawn, taken in September 2015, when the vehicle was in its “mid-altitude” mapping survey. Now orbiting much closer to Ceres, the next images of the 90 km (60 mi) wide Occator are expected to reveal the bright spots in considerable detail

In 2015, a team of researchers used the High Accuracy Radial velocity Planet Searcher (HARPS) instrument on the European South Observatory’s 3.6-metre telescope located at La Silla, on the edge of the Atacama Desert, Chile, to observe the Occator bright spots. In particular, they were intending to measure the Doppler shift exhibited by the spots as Ceres spins on its axis every nine hours, in an attempt to better understand what they might be. Not only did they get the data they were expecting on the Doppler shift, they also found the spots appear to be “winking”  – dimming and then brightening again.

It had been thought the bright spots could be associated with water ice or salt within the crater. The finding from HARPs suggest that whatever the material, it is perhaps volatile, and reacts to solar radiation, evaporating under the sun into bright plumes that reflect light more effectively than the spots themselves. As Ceres rotates, the Occator is carried away from the direct influence of the Sun, so the evaporated materials cool and freeze back out in the crater – but never quite in the same location, helping to explain the apparently changing pattern of the bright spots.

If this theory is correct, it would likely point to the material comprising the bright spots containing water ice, marking Ceres as very different from Vesta, the planetoid previously visited by Dawn. It also might explain why Dawn apparently  saw a very localised “atmosphere” within the region of the spots just after it had commenced its initial mapping survey of Ceres in mid-2015.

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