Cassini – Close Read

Why Titan is awesome #11

Titaaaaan!

Here we go again. 😄 As has been reported, NASA has been interested in sending a robotic submarine to Saturn’s moon Titan to explore the hydrocarbon lakes near its north pole. Various dates have been mentioned and in all it seems likely the mission will be able to take off around 2040. In the 22 years we have left, we’ve got to build the submarine and make sure it can run autonomously on Titan, where the sea-surface temperature is about 95 K, whose ~~waterbodies~~ liquid-hydrocarbon-bodies are made of methane, ethane and nitrogen, and with density variations of up to 30%.

So researchers at Washington State University (WSU) tried to recreate the conditions of benthic Titan – specifically as they would be inside Kraken and Ligeia Mare – by working with the values of four variables: pressure, temperature, density and composition. Their apparatus consisted of a small, cylindrical cartridge heater submerged inside a cell containing methane, ethane and nitrogen, with controls to measure the values of the variables as well as modify conditions if needed. The scientists took a dozen readings as they varied the concentration of methane, ethane and nitrogen, the pressure, sea temperature, the heater surface temperature and the heat flux at bubble incipience.

The experimental setup used by WSU researchers to recreate the conditions inside one of Titan's liquid-hydrocarbon lakes. Source: WSU/NASA — The experimental setup used by WSU researchers to recreate the conditions inside one of Titan’s liquid-hydrocarbon lakes. Source: WSU/NASA

The data logged by WSU researchers pertaining to the conditions inside one of Titan's liquid-hydrocarbon lakes. Source: WSU/NASA — The data logged by WSU researchers pertaining to the conditions inside one of Titan’s liquid-hydrocarbon lakes. Source: Hartwig and Leachman, 2017/WSU

Based on them, they were able to conclude:

The moon’s lakes don’t freeze over even though their surface temperature is proximate to the freezing temperature of methane and ethane because of the dissolved nitrogen. The gas lowers the mixture’s freezing point (by about 16 K below the triple point), thus preventing the formation of icebergs that the robotic submarine would then have had to be designed to avoid (there’s a Titanic joke in here somewhere).
However, more nitrogen isn’t necessarily a good thing. It dissolves better in its liquid-hydrocarbon surroundings as the pressure increases and the temperature decreases – both of which will happen at lower depths. And the more nitrogen there is, the more the liquids surrounding the submarine are going to effervesce (i.e. release gas).

What issues would this pose to the vehicle? According to a conference paper authored among others by Jason Hartwig, a member of the WSU team, and presented earlier this year,

Effervescence of nitrogen gas may cause issues in two operational scenarios for any submersible on Titan. In the quiescent case, bubbles that form may interfere with sensitive science measurements, such as composition measurements, in acoustic transmission for depth sounding, and sidescan sonar imaging. In the moving case, bubbles that form along the submarine may coalesce at the aft end of the craft and cause cavitation in the propellers, impacting propulsive performance.

The quantity of effervescence and the number of sites on the submarine’s surface along which bubbles formed was observed to increase the warmer the machine’s outer surface got.

If NASA engineers get all these details right, then their submarine will work. But making sure the instruments onboard will be able to make the observations they’ll need to make and the log the data they’ll need to log presents its own challenges. When one of the members of the WSU team decided to look into the experimental cell using a borescope (which is what an endoscope is called outside a hospital) and a video recorder, this is what he got:

(Source)

Oh, Titan.

(Obligatory crib: the university press release‘s headline goes ‘WSU researchers build -300ºF alien ocean to test NASA outer space submarine’. But in the diagram of the apparatus above, note that the cartridge heater standing in for the submarine is 5 cm long. So the researchers haven’t built an alien ocean; they’ve simply reconstructed a few thimblefuls.)

Featured image: A radar image obtained by Cassini during a near-polar flyby on February 22, 2007, showing a big island in the middle of Kraken Mare on Saturn’s moon Titan. Caption and credit: NASA.

Note: This post was republished from late February 15 to the morning of February 16 because it was published too late in the night and received little traffic.

A problem worth its weight in salt

Pictures of Jupiter’s moon Europa taken by the Galileo space probe between 1995 and 2003 support the possibility that Europa’s surface has plate tectonics. In fact, scientists think it could be one of only two bodies in the Solar System – the other being Earth – to display this feature. But it must be noted that Europa’s tectonics is nothing like Earth’s if only because the materials undergoing this process are very different – compare the composition of Earth’s crust and Europa’s ice shell. There are also no arc volcanoes or continents on Europa.¹ But this doesn’t mean there aren’t any similarities either. For example, scientists have acknowledged that shifting ice plates on the moon’s surface, with some diving over others and pushing them down, could be a way for minerals on the top to plunge further interior. Because Europa has been suspected of harbouring a subsurface ocean of liquid water, a mineral cycle could be boosting the chances of finding life there. Plate tectonics played a similar role in making Earth habitable.

The biggest giveaway is that the moon’s surface is not littered with craters the way other Jupiter moons are. This meant that cratered patches of the ice shell were disappearing into somewhere and replaced with ‘cleaner’ patches. There are also kilometre-long ridges on the shell suggesting that something had moved along that distance, and they ended abruptly in some places. In 2014, a pair of geologists from Johns Hopkins and the University of Idaho used software like Photoshop to cut up Galileo’s maps of Europa and stitch them back together such that the ridges lined up. They found that there were some areas with a “big gap”. One way to explain it was that the patch there had dived beneath a neighbouring one – a simple version of plate tectonics. But tantalising as the possibility is, more evidence is needed before we can be sure.

If we’re hoping to find the first alien life inside a Jovian moon, we’ll need good models that can help us predict how life might’ve evolved there. A new paper from researchers at Brown University tries to help by trying to figure out why the plates might be shifting (To say something could be happening, it helps to have a simple way it could be happening and with the available resources). On Earth, interactions between the crust and the mantle are motivated among other factors by differences in temperature. The crust is cooler than the magma it ‘slides’ over, which means it’s denser, which assists its subduction when it happens. Such differences aren’t mirrored on Europa, where scientists think there’s a thin, cold ice shell on top and a relatively warmer one below. When a patch of ice from the top slides down, it becomes warmer because the upper layer provides insulation, which prevents the sliding layer from sliding further down because the density has been evened out.

Instead, the Brown University fellows think the density differences could arise thanks to salt content (which, by the way, could also be useful when reading their press release. It says, “A Brown University study provides new evidence that the icy shell of Jupiter’s moon Europa may have plate tectonics similar to those on Earth.” You know it’s not similar, especially if left unqualified like that.) Salt is denser than water, so ice that has more salt is more dense. A 2003 study also suggested that warmer ice will have lesser salt because eutectic mixtures could be dissolving and draining it out. So using a computer model and making supposedly reasonable assumptions about the shell’s temperature, porosity and salinity ranges, the Brown team calculated that ice slabs made up of 5% salt and saltier than their surroundings by 2.5% would be able to subduct. However, if the distribution of salt was uniform on Europa’s surface (varying by less than 1% from slab to slab, e.g.), then a subducting slab would have to have at least 22% salt → very high.

I said “supposedly reasonable assumptions” because we don’t exactly know how salinity and porosity vary around and through Europa. In their simulations, the researchers assumed that the ice has a porosity of 10% (i.e. 10% of the material is filled with pores), which is considered to be on the higher side of things. But the study remains interesting because it’s able to establish the big role salts can play in how the ice moves around. This is also significant because Galileo found the Europan magnetic field to be stronger than it ought to, suggesting the subsurface ocean had a lot of salt. So it’s plausible that the cryomagma² on which Europa’s upper shell moves could be derived from the waters below.

The researchers also claim that if the subducting slab doesn’t lose all its salt in about one million years, it will remain dense enough to go all the way down to the ocean, where it could be received as a courier carrying materials from the surface that help life take root.³ But of you think this might be too out there, look at it in terms of the planned ESA Jupiter Icy Moons Explorer (JUICE) and NASA Clipper missions for the mid-2020s. Both Cassini and Galileo data have shown that there’s a lot going on with the icy moons of the gas giants Jupiter and Saturn, with observations of phenomena like vapour plumes pointing to heightened chances for the formation and sustenance of alien life. If JUICE and Clipper have to teach us something useful about these moons, then they’ll have to go in prepared to study the right things, the things that matter. The Brown University paper has shown that salt is definitely one of them. It was accepted for publication in the Journal of Geophysical Research: Planets on December 4, 2017. Full text here.

Featured image: An artist’s impression of water vapour plumes erupting from Europa’s south pole, with Jupiter in the background. Credit: NASA-ESA.

¹Venus has two continent-like areas , Ishtar and Aphrodite terra, and also displays tectonic activity in the form of mountains and volcanoes, e.g. But it does not have plate tectonics because its crust heals faster than it is damaged during tectonic activity.

²One of the more well known cryovolcanoes in the Solar System is Doom Mons on where else but Titan.

³ On Earth, tectonic plates that are pushed downward also take a bunch of carbon along, keeping the surface from accumulating the element in amounts that could be deleterious to life.

Why Titan is awesome #10

Titaaaaan!

How much I’ve missed writing these posts since Cassini passed away. Unsurprisingly, it’s after the probe’s demise that we’ve really begun to realise how much of Cassini’s images and data we were consuming on a daily basis, all of which is gone. There’s no more the steady stream of visuals of Saturn’s rings, bands, storms and panoply of moons – in fact all of which have been replaced by Jupiter’s rings, bands, storms and panoply of moons thanks to Juno. Nonetheless, one entire area of the Solar System has been darkened in my imagination. Until the next full mission to the Saturnian system (although nothing of the kind is in the works), we’ll have to make do with what Cassini data trickles down through NASA’s and ESA’s data-processing sieves.

One such is a new study about the temperature of the air high above Titan’s poles. Before Cassini’s death-dive into Saturn, the probe spent some time studying the moon’s polar atmosphere. Researchers from the University of Bristol who obtained this data noticed something odd: the part of the atmosphere over Titan’s poles began to develop a warm spot over late 2009 but that by 2012, it had become a ‘cold spot’. By 2015, the temperature at about 550 km above had dropped to 120 K (that’s a little below the temperature at which supercooled water turns into a glass).

On Earth, a warm spot forms over the poles because of two principle reasons: how Earth’s wind circulates around the planet and because of the presence of carbon dioxide. During winter, air over the corresponding hemispheric pole sinks down, becomes compressed and heats up. Moreover, the carbon dioxide present in the air also emits the heat it has trapped in its chemical bonds.

In 2012, astronomers using Cassini data had found that Titan also exhibits a wind circulation process that is moon-wide. It can be understood as Titan having two atmospheres, or layers, one on top of the other. In the lower atmosphere, there are three Hadley cells; each cell represents a distinct air circulation system wherein air rises up for 10 km or so from near the equator, moves up/down towards subtropical regions, sinks back down and returns to the equator along the surface. In the second, upper atmosphere, air moves between the two poles directly in a unified, global Hadley cell.

Titan_south polar vortex

Now, remember that Titan’s distance from the Sun means that one Titan-year is 29.5 Earth-years, that each Titanic season lasts over seven Earth-years and that seasonal shifts are much slower on the moon as a result. However, in 2012, scientists studying Cassini data found that the rate at which the air over one of Titan’s poles was sinking into the pole – like the air does on Earth – was happening really quickly: according to Nick Teanby, a researcher at the University of Bristol and also the lead author of the latest study, the rate of subsidence increased from 0.5 mm/s in January 2010 to 1.5 mm/s in June 2010. In other words, it was a shift that, unlike the moon’s seasons, happened rapidly (in just 12 Titanic days).

The same study concluded that Titan’s atmosphere was thicker than previously thought because trace gases like ethane, hydrogen cyanide, acetylene and cyanoacetylene were found to be produced at an altitude of over 500 km over the poles thanks to photochemical reactions induced by ultraviolet radiation and high-energy electrons streaming in from the Sun. These gases would then subside into the lower atmosphere over the polar region – which brings us to the latest study. It says that, unlike what carbon dioxide warming Earth’s atmosphere, the (once) trace gases actually cool the atmosphere, resulting in the dreadfully cold spot over Titan’s poles. They also participate in the upper Hadley cell circulation.

This is similar to a unique phenomenon observed over Saturn’s south pole in 2005.

Changes in trace gas abundances over Titan's south pole. Credit: ESA — Changes in trace gas abundances over Titan’s south pole. Credit: ESA

What a beauty you are, Titan. And I miss you, Cassini, more than I miss many other things in life.

~~I couldn’t find a link to the paper of the latest study; here’s the press release.~~ Update: link to paper.

Links to previous editions:

Featured image: Cassini’s last shot of Titan, taken with the probe’s narrow-angle camera on September 13, 2017. Credit: NASA.

The significance of Cassini’s end

Many generations of physicists, astronomers and astrobiologists are going to be fascinated by Saturn because of Cassini.

I wrote this on The Wire on September 15. I lied. Truth is, I don’t care about Saturn. In fact, I’m fascinated with Cassini because of Saturn. We all are. Without Cassini, Saturn wouldn’t have been what it is in our shared imagination of the planet as well as the part of the Solar System it inhabits. At the same time, without Saturn, Cassini wouldn’t have been what it is in our shared imagination of what a space probe is and how much they mean to us. This is significant.

The aspects of Cassini’s end that are relevant in this context are:

The abruptness
The spectacle

Both together have made Cassini unforgettable (at least for a year or so) and its end a notable part of our thoughts on Saturn. We usually don’t remember probes, their instruments and interplanetary manoeuvres during ongoing missions because we are appreciably more captivated by the images and other data the probe is beaming back to Earth. In other words, the human experience of space is mediated by machines, but when a mission is underway, we don’t engage with information about the machine and/or what it’s doing as much as we do with what it has discovered/rediscovered, together with the terms of humankind’s engagement with that information.

This is particularly true of the Hubble Space Telescope, whose images constantly expand our vision of the cosmos while very few of us know how the telescope actually achieves what it does.

From a piece I wrote on The Wire in July 2015:

[Hubble’s] impressive suite of five instruments, highly polished mirrors and advanced housing all enable it to see the universe in visible-to-ultraviolet light in exquisite detail. Its opaque engineering is inaccessible to most but this gap in public knowledge has been compensated many times over by the richness of its observations. In a sense, we no longer concern ourselves with how the telescope works because we have drunk our fill with what it has seen of the universe for us…

Cassini broke this mould by – in its finish – reminding us that it exists. And the abruptness of the mission’s end contributed to this. In contrast, consider the story of the Mars Phoenix lander. NASA launched Phoenix (August 2007 to May 2010) in August 2007. It helped us understand Mars’s north polar region and the distribution of water ice on the planet. Its landing manoeuvre also helped NASA scientists validate the landing gear and techniques for future missions. However, the mission’s last date has a bit of uncertainty. Phoenix’s last proper signal was sent in November 2, 2008. It was declared not on the same day but a week later, when attempts reestablish contact with Phoenix failed. But the official declaration of ‘mission end’ came only in May 2010, when a NASA satellite’s attempts to reestablish contact failed.

Is it easier to deal with the death of someone because their death came suddenly? Does it matter if their body was found or not? For Phoenix, we have a ‘body’ (a hunk of metal lying dormant near the Martian north pole); for Cassini, we don’t have a ‘body’. On the other hand, we don’t have a fixed date of ‘mission end’ for Phoenix but we do for Cassini, down to the last centisecond and which will be memorialised at NASA one way or another.

Spectacle exacerbates this tendency to memorialise by providing a vivid representation of ‘mission end’ that has been shared by millions of people. Axiomatically, a memorial for Cassini – wherever one emerges – will likely evoke the same memories and emotions in a larger number of people, and all of those people will be living existences made congruent by the shared cognisance and interpretation of the ‘Cassini event’.

However, Phoenix’s ‘mission end’ wasn’t spectacular. The lander – sitting in one place, immobile – slowly faded to nothing. Cassini burnt up over Saturn. Interestingly, both probes experienced similar ‘deaths’ (though I am loth to use that word) in one sense: neither probe knew the way an I/AI could that they were going to their deaths but both their instrument suites fought against failing systems all guns blazing. Cassini only got the memorial upper hand because it could actively reorient itself in space (akin to the arms on our bodies) and because it was in an environment it was not designed for at all.

The ultimate effect is for humans to remember Cassini more vividly than they would Phoenix, as well as associate a temporality with that remembrance. Phoenix was a sensor, the nicotine patch for a chain-smoking planet (‘smoking’ being the semantic variable here). Cassini moved around – 2 billion km’s worth – and also completed a complicated sequence of orbits around Saturn in three dimensions in 13 years. Cassini represents more agency, more risk, more of a life – and what better way to realise this anthropomorphisation than as a time-wise progression of events with a common purpose?

We remember Cassini by recalling not one moment in space or time but a sequence of them. That’s what establishes the perfect context for the probe’s identity as a quasi-person. That’s also what shatters the glaze of ignorance crenellated around the object, bringing it unto fixation from transience, unto visibility from the same invisibility that Hubble is currently languishing in.

Featured image credit: nasahqphoto/Flickr, CC BY-NC-ND 2.0.

Titan’s lakes might be fizzing with nitrogen bubbles

Featured image: A shot by Cassini of the lakes Kraken Mare and Ligeia Mare near Titan’s north pole. Credit: NASA.

TITAAAAAAAAAAN!

One more study reporting cool things about my favourite moon this week. Researchers from Mexico and France have found that the conditions exist in which the lakes of nitrogen, ethane and methane around Titan’s poles could be fizzy with nitrogen bubbles. In technical terms, that’s nitrogen exsolution: when one component of a solution of multiple substances separates out. In this case, the nitrogen forms bubbles and floats to the surface of the lakes, becoming spottable by the Cassini probe. The results were published in the journal Nature Astronomy on April 18.

The Cassini probe has been studying Saturn and its moons since 2004. In 2013, its RADAR instrument – which makes observations using radio-waves – found small, bright features on some of Titan’s lakes that winked out over time. These features have been whimsically called ‘magic islands’ and there has been speculation that they could be bubbles. The Mexican-French study provides one scientific form for this speculation.

The researchers used a numerical model to determine how and why the nitrogen could be degassing out of the lakes. Specifically, they extracted estimates of the temperature and pressure on the surface and interiors of the Ligeia Mare lake from past studies and then plugged them into simulations used to predict the properties of Earth’s oil and gas fields. They found that the bubbles could form if the solution of methane, ethane and nitrogen was forced to split up at certain temperatures and pressures. So, the researchers had to figure out the simplest way in which this could happen and then the likelihood of finding it happening in a Titanic lake.

When the lake’s innards are not forced to split up, they’re thought to exist in a liquid-liquid-vapour equilibrium (LLVE). In an LLVE, two liquids and a vapour can coexist without shifting phases (i.e. from liquid to vapour, vapour to liquid, etc.). The researchers write in their paper, “In the laboratory, LLVEs have been observed under cryogenic conditions for systems comparable to Titan’s liquid phases: nitrogen + methane + (ethane, propane or n-butane).” While cryogenic conditions may be hard to create on Earth’s surface, they’re the natural state of affairs on Titan because the latter is so far from the Sun. The surfaces of its lakes are thought to be at 80-90 K (-190º to -180º C), with the lower reaches being a few degrees colder.

For an LLVE-like condition to be disrupted, the researchers figured the lake itself couldn’t be homogenous. The reasons: “A sea with a homogeneous composition that matches that required for the occurrence of an LLVE at a specific depth is an improbable scenario. In addition, such a case would imply nitrogen degassing through the whole extent of the system.” So in a simple workaround, they suggested that the lake’s upper layers could be rich in methane and the lower layers, in ethane. This way, there’s more nitrogen available near the surface because the gas dissolves better in methane – and also because it could be dissolving into the top more from the moon’s nitrogen-rich atmosphere.

Over time, the lake’s top layers could be forced to move downward by weather conditions prevailing above the lake, and push the material at the bottom to the top. But during the downward journey, the rising pressure breaks the LLVE and forces the nitrogen to split off as bubbles. Given the size and depth of Ligeia Mare, the researchers have estimated that nitrogen exsolution can occur at depths of 100-200 m. The bubbles that rise to the top can be a few centimetres wide – not too small for Cassini’s RADAR instrument to spot them, as well as in keeping with what previous studies have recorded.

Of course, this isn’t the only way nitrogen bubbles could be forming on Ligeia Mare. According to another study published in March, when an ultra-cold slush of ethane settling at the bottom of the lake freezes, its crystals release the nitrogen trapped between their atoms. Michael Malaska, of NASA’s Jet Propulsion Lab, California, had said at the time:

In effect, it’s as though the lakes of Titan breathe nitrogen. As they cool, they can absorb more of the gas, ‘inhaling’. And as they warm, the liquid’s capacity is reduced, so they ‘exhale’.

The Mexican-French researchers are careful to note that their analysis can’t say anything about the quantities of nitrogen involved or how exactly it might be moving around Ligeia Mare – but only that it pinpoints the conditions in which the bubbles might be able to form. NASA has been tentative about sending a submarine to plumb the depths of another Titanic lake, Kraken Mare, in the 2040s. If it does undertake the mission, it could speak the final word on the ‘magic islands’. Ironically, however, NASA scientists will have to design the sub keeping in mind the formation of LLVEs and nitrogen exsolution.

But won’t the issue be settled by then? Maybe, maybe not. Come April 22, Cassini will fly by Titan’s surface at a distance of 980 km, at 21,000 km/hr. It will be the probe’s last close encounter with the moon, as mission scientists have planned to take a look at some of the smaller lakes. After this, the probe will fly a path that will take it successively through Saturn’s inner rings. Finally, on September 15, NASA will perform the probe’s ‘Grand Finale’ manoeuvre, sending it plunging into Saturn’s gassy atmosphere and unto its death, bringing the curtains down on a glorious 13-year mission that has changed the way we think about the ringed planet and its neighbourhood.

Published in The Wire on April 20, 2017.

What you need to know about the Pluto flyby

The Wire
July 14, 2015

In under seven hours, the NASA New Horizons space probe will flyby Pluto at 49,900 km per hour, from a distance of 12,500 km. It’s what the probe set out to do when it was launched in January 2006. The flyby will allow it to capture high-resolution images of the dwarf planet’s surface and atmosphere as well as take a look at its biggest moon, Charon. For much of the rest of the day, it will not be communicating with mission control as it conducts observation. The probe’s Long-Range Reconnaissance Imager (LORRI) has already been sending better and better pictures of Pluto as it gets closer. During closest approach, Pluto will occupy the entire field of view of LORRI to reveal the surface in glorious detail.

Fourteen minutes into the Pluto flyby, New Horizons will make its closest approach to Charon, which is about 24,000 km away. Next: 47 minutes and 28 seconds after the Charon flyby, the probe will find itself in Pluto’s shadow where its high-gain antennae will make observations of how the dwarf planet’s atmosphere affects sunlight and radio signals from Earth as they pass through it. Then, 1 minute and 2 seconds after that, New Horizons will again be in sunlight. Finally, 1 hour and 25 minutes later, it will be in Charon’s shadow to look for its atmosphere.

That New Horizons survived the flyby will be known when, on early Wednesday morning (IST), it starts to send communication signals Earthward again. The timings of various events announced by NASA will have to be adjusted against the fact that New Horizons is 4.5 light-hours away from Earth. NASA has called for a press conference to release the first close-up images at 0030 hrs on July 16 (IST). The entire data snapped by the probe during the flyby will be downloaded over a longer period of time. According to Emily Lakdawalla,

Following closest approach, on Wednesday and Thursday, July 15 and 16, there will be a series of “First Look” downlinks containing a sampling of key science data. Another batch of data will arrive in the “Early High Priority” downlinks over the subsequent weekend, July 17-20. Then there will be a hiatus of 8 weeks before New Horizons turns to systematically downlinking all its data. Almost all image data returned during the week around closest approach will be lossily compressed — they will show JPEG compression artifacts. Only the optical navigation images are losslessly compressed. [All dates/times in EDT]

Downloading the entire science dataset including losslessly compressed observations will take until around November 2016 to complete. Until then, the best will always be yet to come. As always, all communications will be via the Deep Space Network – whose Goldstone base is currently all ears for the probe.

Incidentally, the ashes of the astronomer Clyde Tombaugh, who discovered Pluto in 1930, are onboard New Horizons.

What do we know about Pluto?

Among the last images taken by LORRI before the flyby revealed a strange geology on Pluto. Scientists noted dark and bright polygonal patches (in the shape of a whale and a <3, respectively) as well as what appeared to be ridges, cliffs and several impact craters. However, these features on the side of Pluto facing New Horizons as it flies in. During the flyby, it will image the other side of Pluto, where these features may not be present. The probe can’t hang around to wait to see the other side either because Pluto rotates once every 6.4 Earth-days.

An annotated image of Pluto snapped by the New Horizons probe. Credit: Applied Physics Lab/NASA

During the flyby, images of Charon will also be taken. Already, the probe has revealed that, like Pluto, the moon also has several intriguing features – while until recently both bodies were thought to be frozen and featureless balls of ice and rock – like giant craters and chasms. In fact, NASA noted one crater near Charon’s south pole, almost 100 km wide and another on Pluto, some 97 km wide, both appearing to have been the result of recent impacts (in the last billion years). The particularly dark appearance of the Charon crater has two theories to explain it. Either the ice at its bottom is of a different kind than the usual and is less reflective or the ice melted during impact and then refroze into larger, less bright grains.

An annotated image of Charon snapped by the New Horizons probe. Credit: Applied Physics Lab/NASA

All these details will be thrown up in detail during New Horizons’ flyby. They will reveal how the two bodies evolved in the past, the structure and composition of their interiors, and if – for some astronomers – Charon might’ve harboured a subsurface ocean in its past. Complementarily, NASA will also be training the eyes of its Cassini, Spitzer and Keplerspace-borne instruments on Pluto. Cassini, from its orbit around Saturn, will take a picture of New Horizons just around the time of its flyby. From July 23 to July 30, the Spitzer Space Telescope will study Pluto in the infrared, mapping its surface ice. Then, in October, the exoplanet-hunting Kepler telescope, in its second avatar as K2, will start focusing on the changes in brightness off of and around Pluto to deduce the body’s orbital characteristics.

Then, there are also post-flyby missions whose results, when pieced together with the July 14 flyby and other observations, will expand our knowledge of Pluto in its larger environment: among the Kuiper Belt, at whose inner edge it resides.

Finally, as Dennis Overbye of The New York Times argued in a poignant essay, the Pluto flyby marks the last of the Solar System’s classical planets to explored, the last of the planets the people of our generation will get to see up close. The next frontiers in planetary exploration will be the exoplanets – the closest of which is 4.3 light-years away (orbiting Alpha Centauri B). But until then, be willing to consider the Solar System’s moons, missions to which are less than a decade away. Leaving you with Overbye’s words:

Beyond the hills are always more hills, and beyond the worlds are more worlds. So New Horizons will go on, if all goes well, to pass by one or more of the cosmic icebergs of the Kuiper belt, where leftovers from the dawn of the solar system have been preserved in a deep freeze extending five billion miles from the sun…

But the inventory of major planets — whether you count Pluto as one of those or not — is about to be done. None of us alive today will see a new planet up close for the first time again. In some sense, this is, as Alan Stern, the leader of the New Horizons mission, says, “the last picture show.”