New Horizons is continuing outbound from the Pluto-Charon system, its primary mission complete. A new phase of the mission has now begun: returning all the data gathered safely to Earth; a process that is going to take an estimated 16 months to complete. Even so, and as indicated in my last report, what has already been received has been enough to turn much of planetary science on its head.
During a mission briefing on July 24th, 2015, Alan Stern, the New Horizons principal investigator and members of the science team provided a further update on the mission, and revealed some of the more stunning images captured by the spacecraft during the close approach phase of the mission. One of the most striking of these was a picture snapped by New Horizons just seven hours after close approach, when it was already 2 million kilometres (1.2 million miles) from Pluto.
The image shows the dark disc of Pluto’s night side (which will not see the light of the Sun for another 20 years), surrounded by a halo of atmosphere, 130 kilometres (80 miles) thick, backlit by the distant Sun. Within the atmosphere sit two bands of thick haze, one around 50 kilometres (30 miles) altitude and the second at around 80 kilometres (50 miles) altitude.
These bands of haze are believed to be the result of ultraviolet sunlight striking the upper reaches of Pluto’s atmosphere, breaking apart the methane gas there, giving rise to more complex hydrocarbon gases such as ethylene and acetylene. These heavier gases then descend into the colder regions of Pluto’s atmosphere, condensing as ice particles, which are seen by New Horizon’s instruments as the bands of haze.
The ice particles are further acted upon by ultraviolet sunlight so that tholins are formed. Tholins are large complex organic aerosols thought to contain some of the chemical precursors of life. These gradually fall out of the atmosphere to mix with hydrocarbons on Pluto’s surface, giving it the distinctive colouring we see in images like those given below.
The July 24th briefing also revealed some of the most detailed images of Pluto’s sunlit side yet published, starting with the true colour image shown above. This shows Pluto in twice the level of detail as the July 13th image published by NASA, revealing surface features as small as two kilometres across (the ultra-high resolution images LORRI has captured will eventually reveal surface features as small as 50 metres across). Featured prominently and unmistakably in the image is Pluto’s light-coloured “heart”, informally named the “Tombaugh Regio” in honour of Pluto’s discoverer, Clyde Tombaugh.
This huge region is divided into two parts, defined by the two “lobes” of the heart. On the left (west side) is the relatively smooth expanse of the “Sputnik Planum”, roughly the size of Texas.The is largely composed of a thick layer of nitrogen, methane and carbon monoxide ice. That it is almost completely without craters suggests it is much younger than the rest of Pluto’s visible surface; but how it formed has yet to be determined.
The right side of the “heart” is also brightly-coloured, indicating the presence of ices similar in nature to those in “Sputnik Planum”, but it also shows a much rougher terrain as well. Further bright, icy material also extends from the “point” of the “heart” into the southern polar regions of Pluto, again mixing with rougher terrain.
While it is not clear what actually gave rise to the icy expanse of “Sputnik Planum”, it is not believed the same mechanism is responsible for the ice in either eastern lobe or which extends southwards from the “heart”. These are believed to be the result of material from “Sputnik Planum” being carried into these areas, where it is gradually “painting over” surface features there.
Quite what the mechanism responsible for this is unclear; it might be that Pluto’s winds are scooping up surface material from “Sputnik Planum” and driving them east and south as snow to be deposited within them as the winds meet their more chaotic terrain. Or it might be the result of sublimation; the chemical compounds in the ice turning directly to gas as the result of some process, and then carried east and south in the prevailing weather, where they condense out once more and fall as snow. It might even, at least in part, be the result of the ice itself slowly flowing into these regions.
The best evidence found so far that the ice in and around “Sputnik Planum” has at some point been moving – and may still be moving, depending on what processes are at work on and within Pluto – has been found in the north-west of the region. This exhibits light and dark swirl-like patterns which, when seen on Earth or Mars, are indicative of flowing ice, such as with a glacier. On Pluto, these patterns seem to flow around surface obstacles and into depressions. In some areas, they appear to have broken through or over crater walls, slowly flowing into them. There is no way to directly tell if the ice is still in motion; however, it is a further pointer to Pluto having once been geologically active, and perhaps still being active today.
Continents on Mars?
A French team of scientist studying data gathered from 22 different rock samples by NASA’s Curiosity rover using its ChemCam laser while it was still operating on the floor of Gale Crater, have revealed surprisingly high levels of silica, a rock-forming compound containing silicon and oxygen, commonly found on Earth as quartz.
Silica is primarily of interest to scientists, because high levels of it within rocks could indicate ideal conditions for preserving ancient organic material, if present. However, to the French team, the Curiosity findings indicate something else: that Mars might have a geological history far more similar to Earth’s that had previously been thought.
Earth is currently the only known planet in the solar system with a surface divided into continents, composed of a thick crust rich in silica, and sea beds, composed of a thin, dense crust rich in silica-poor basaltic rock. Theory has it that silica rich rocks are believed to be the result of complex activity in a planet’s interior, potentially related to the onset of plate tectonics, something considered unique to Earth.
By contrast, Mars appears to have a crust primarily made-up of basaltic rock with just some traces of silica rock, suggesting its crust was formed by processes very different to those found on Earth.However, the data studied by the French team shows that may of the rocks sampled by Curiosity and found to contain silica are similar in composition to some of Earth’s oldest preserved continental crust.
This suggests that the early geological history of Mars may have been far more similar to that of Earth than had previously been thought, and that the silica-rich rocks are the last remnants of an ancient Martian continental crust, analogous to that of ancient Earth. Given that silica-rich rocks can be indicative of plate tectonics, the findings also lend further weight to a 2012 study which suggests that Mars might still be at a primitive stage of plate tectonics.
Kepler Finds Earth’s Big Cousin
NASA’s Kepler Space Telescope, launched in 2009 with the express purpose of locating Earth-size extrasolar planets in or near the habitable zone of their parent stars, has achieved its most remarkable result to date.
This comes in the form of an announcement given on July 23rd, that the science team managing the mission had confirmed Kepler has discovered the 1,030th extra-solar Earth-sized planet. However, what makes this discovery so special is that it is the first time such a planet has been found occupying the habitable zone (also called the “Goldilocks region” because everything is “just right” for an Earth-like planet to exist) of a star belonging to the same spectral class of our own Sun.
The star in question, Kepler-452, is located some 1,400 light years away, and lies within the constellation of Cygnus as seen from Earth. It is G2 class star, putting it in precisely the same class as our own Sun, but is around 1.5 billion years older than the Sun. It is also around 10% larger than the Sun 20% more luminous, although it has nearly the same surface temperature.
The planet, officially called Kepler-452b, is about 60% bigger than Earth, is around 1.5 billion years older, and orbits its star once every 385 days. Initial analysis suggests it has a relatively dense atmosphere, and while there has been some speculation it might be a small gas planet (that is, somewhat similar in nature to Jupiter or Saturn), it would seem more likely to be a rocky planet like the Earth. If so, it would appear to be well placed to support liquid water on its surface, although this cannot be confirmed.
There is much to be determined about Kepler-452b, including its mass (statements that it is around 5 times more massive than Earth with a gravity twice that of our own are at best speculative), so it is currently only ranked 6th on the Earth Similarity Index, used to rank exoplanets in their likeness to Earth; but there are still a lot of blanks in the data. Nevertheless, it has excited the exoplanet science community.
“We can think of Kepler-452b as an older, bigger cousin to Earth, providing an opportunity to understand and reflect upon Earth’s evolving environment,” said Jon Jenkins, Kepler data analysis lead at NASA’s Ames Research Centre in Moffett Field, California, and the man who led the team that discovered Kepler-452b.
“It’s awe-inspiring to consider that this planet has spent 6 billion years in the habitable zone of its star,” he continued, “that’s [a] substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist [there].”
Kepler-452b was actually the first of a new batch of planets being added to the catalogue of of mission findings, bringing the overall total to 4,696 for planets of all sizes located by the telescope, despite it suffering a major failure in its pointing mechanism in 2013, which forced observations to switch to a different mode of operation in 2014.
All images of Pluto and Charon, courtesy of NASA / APL / JHU, unless otherwise indicated. All other images as credited.