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Scicomm Science

A new map of Titan

It’s been a long time since I’ve obsessed over Titan, primarily because after the Cassini mission ended, the pace of updates about Titan died down, and because other moons of the Solar System (Europa, Io, Enceladus, Ganymede and our own) became more important. There have been three or four notable updates since my last post about Titan but this post that you’re reading has been warranted by the fact that scientists recently released the first global map of the Saturnian moon.

(This Nature article offers a better view but it’s copyrighted. The image above is a preview offered by Nature Astronomythe paper itself is behind a paywall and I couldn’t find a corresponding copy on Sci-Hub or arXiv nor have I written to the corresponding author – yet.)

It’s fitting that Titan be accorded this privilege – of a map of all locations on the planetary body – because it is by far the most interesting of the Solar System’s natural satellites (although Europa and Triton come very close) and were it not orbiting the ringed giant, it could well be a planet of its own accord. I can think of a lot of people who’d agree with this assessment but most of them tend to focus on Titan’s potential for harbouring life, especially since NASA’s going to launch the Dragonfly mission to the moon in 2026. I think they’ve got it backwards: there are a lot of factors that need to come together just right for any astronomical body to host life, and fixating on habitability combines these factors and flattens them to a single consideration. But Titan is amazing because it’s got all these things going on, together with many other features that habitability may not be directly concerned with.

While this is the first such map of Titan, and has received substantial coverage in the popular press, it isn’t the first global assessment of its kind. Most recently, in December 2017, scientists (including many authors of the new paper) published two papers of the moon’s topographical outlay (this and this), based on which they were able to note – among other things – that Titan’s three seas have a common sea level; many lakes have surfaces hundreds of meters above this level (suggesting they’re elevated and land-locked); many lakes are connected under the surface and drain into each other; polar lakes (the majority) are bordered by “sharp-edged depressions”; and Titan’s crust has uneven thickness as evidenced by its oblateness.

According to the paper’s abstract, the new map brings two new kinds of information to the table. First, the December 2017 papers were based on hi- and low-res images of about 40% of Titan’s surface whereas, for the new map, the authors write: “Correlations between datasets enabled us to produce a global map even where datasets were incomplete.” More specifically, areas for which authors didn’t have data from Cassini’s Synthetic Aperture Radar instrument for were mapped at 1:2,000,000 scale whereas areas with data enabled a map at 1:8,000,000 scale. Second is the following inferences of the moon’s geomorphology (from the abstract the authors presented to a meeting of the American Astronomical Society in October 2018):

We have used all available datasets to extend the mapping initially done by Lopes et al. We now have a global map of Titan at 1:800,000 scale in all areas covered by Synthetic Aperture Radar (SAR). We have defined six broad classes of terrains following Malaska et al., largely based on prior mapping. These broad classes are: craters, hummocky/mountainous, labyrinth, plains, lakes, and dunes [see image below]. We have found that the hummocky/mountainous terrains are the oldest units on the surface and appear radiometrically cold, indicating icy materials. Dunes are the youngest units and appear radiometrically warm, indicating organic sediments.

SAR images of the six morphological classes (in the order specified in the abstract)

More notes once I’ve gone through the paper more thoroughly. And if you’d like to read more about Titan, here’s a good place to begin.

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Science

Could there be life on Europa? NASA okays mission to find out

The Wire
June 19, 2015

Artist concept of NASA’s Europa mission spacecraft approaching its target for one of many flybys. Credit: NASA/JPL-Caltech
Artist concept of NASA’s Europa mission spacecraft approaching its target for one of many flybys. Credit: NASA/JPL-Caltech

On Thursday, NASA okayed the development of a probe to Jupiter’s moon Europa, currently planned for the mid-2020s, to investigate if it has conditions suitable for life. The milestone parallels the European Space Agency’s JUICE (Jupiter Icy Moons Explorer) mission, also planned for the mid-2020s, which will study the icy moons of the Solar System’s largest planet.

The NASA mission has tentatively been called Clipper, and its proposal comes on the back of tantalizing evidence from the Galileo mission that Europa could have the conditions to harbour life. Galileo conducted multiple flybys of the moon in the 1990s and revealed signs that it could be harbouring a massive subsurface ocean – with more than twice as much water as on Earth – under an ice shell a few kilometres thick. It also found that the ocean-floor could be rocky, there were tidal forces acting on the water-body, and that the thick ice shell could be host to plate tectonics like on Earth.

These characteristics make a strong case for the existence of habitable conditions on Europa because they mimic similar conditions on Earth. For example, plate tectonics on Earth moves a jigsaw of landmasses on the surface around. Their resulting interactions are responsible for moving minerals on the surface into the ground and dredging new deposits upward, creating an important replenishment cycle that feeds many lifeforms. A rocky seafloor also conducts heat efficiently toward and away from the water, and tidal forces provide warmth through friction.

With NASA’s okay, the Europa mission moves to the “formulation stage”, when mission scientists and engineers will start technology development. The agency’s fiscal year 2016 budget includes $30 million for just this, according to a May 26 statement, out of a total of $18.29 billion that Congress has awarded it. NASA has already also asked for $285 million through 2020 for the Europa mission, with the overall mission expected to cost $2 billion notwithstanding delays at the time of a launch planned for 2022.

The same statement also announced the scientific payload that would accomplish the mission. Out of 33 proposals submitted, NASA selected nine – all geared toward exploring the ice- and water-related properties of the moon. They could also be pressed into observing other moons in the Jovian neighbourhood – many of which are icy and have curious surface and atmospheric characteristics resembling Europa’s. These include another of Jupiter’s moons, Ganymede, and Saturn’s Dione, Enceladus, Hyperion, Iapetus, Phoebe and Tethys.

ESA’s JUICE mission – part of its broader Cosmic Vision strategy for a class of long-term missions in the 2020s – is planned to launch in 2022 and reach Jupiter by 2030. At one point, it will enter into orbit around Ganymede. If NASA’s Clipper is at Europa by then, what the two probes find could be complementary, and be compared to infer finer details.

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Science

A close encounter with the mid-sized, icy kind

Cassini looks over the heavily cratered surface of Rhea during the spacecraft's flyby of the moon on March 10, 2012, from a distance of about 43,000 km.
Cassini looks over the heavily cratered surface of Rhea during the spacecraft’s flyby of the moon on March 10, 2012, from a distance of about 43,000 km. Image: NASA/JPL-Caltech/Space Science Institute

In three days, NASA’s Cassini mission will fly by Saturn’s second-largest moon Rhea. While interest in the Saturnian moons has been hogged by the largest – Titan – Cassini‘s images of Rhea could provide important new information about a class of natural satellites that it exemplifies: the so-called ‘mid-sized’ moons. While Titan is big enough to be a planet, it is also an exception. Only 13 of Saturn’s 62 confirmed moons are bigger than 50 km in diameter. Among them, Rhea is the largest, and its diameter is still less than a third of Titan’s.

As the artist Michael Carroll wrote,

With the exception of planet-sized Titan, these moons make up a mid-sized family of bodies that are more poorly understood than the larger Galilean satellites.

Cassini will take two sets of images of Rhea during its fly-by scheduled for February 10. The first set, an 11-frame mosaic of its northern hemisphere facing away from Saturn, will be taken from a distance of 46,943 km. The second set will be a 16-frame mosaic of the southern hemisphere shot from a distance of 53,700 km. The images will have a peak resolution of 305 m/pixel. On both occasions, the probe will use the Imaging Science Subsystem (ISS), whose cameras are specifically designed to be able to study the moons’ surfaces, as well as the planet’s glorious rings.

Including Rhea, Saturn has seven known sizeable moons with icy surfaces; the other six are Dione, Enceladus, Hyperion, Iapetus, Phoebe and Tethys). Of them, Dione and Enceladus are thought to be significant contributors to the halo of charged particles that also orbits Saturn, around its giant magnetosphere. Enceladus is especially known to eject fountains of water vapor from near its south pole, whose constituent droplets become ionized by the magnetosphere. On the other hand, Rhea does the opposite of contribute to the halo: it absorbs charged particles, as a result of which its surface is charged. In fact, its surface has the strongest negative potential among the mid-sized icy moons.

Extensive studies of Earth’s moon have revealed that such an electrostatic potential (caused due to charged particles streaming in from the Sun) has accelerated dust particles into space. Astronomers have reason to believe a similar mechanism at play on some of Saturn’s moons – but with more intensity – could be moving dust particles between the moons. And Rhea could be at the center of this dusty relay.

Moreover, its tendency to absorb charged particles promotes a feeble radiolysis of its surface ice, feeding a thin atmosphere of ozone, hydrogen peroxide and molecular oxygen around it. Similar mechanisms have been used to explain molecular oxygen in the atmospheres of Dione and Jupiter’s moons Europa and Ganymede, and possibly on other Saturnian and Jovians moons, and on exoplanets, as well.

These finds could be further augmented by continued observations of Rhea. The last of them happened in 2013, during Cassini‘s last targeted mission for the moon when it came within 1,000 km. Then, it had found a surface studded with more craters than were seen on the other icy moons. Astronomers think this implies Rhea was battered with comets that could have been the source of a whiff of carbon dioxide that has been detected emanating from it. At the same time, Rhea alone might not have been battered. It is farther outward than Dione, Enceladus and Tethys, whose innards are warmed by tidal forces generated by Saturn’s gravitational pull. The warmth melts some of the surface ice and could cover up possibly hidden craters.

These features – mid-range mass, a highly charged surface with an unusual number of craters, oxygen in its atmosphere, and a depravity of tidal warming – all together keep Rhea in the spotlight, if only a fluctuating one. Even now, as Cassini embarks on the first of its 20 flybys of Saturn’s icy satellites in 2015, the ISS’s cameras will be turned toward Rhea only on February 10. On February 12, it will image Titan during the second of seven flybys, all slated for this year. In fact, at this moment, Europa remains the most talked-about icy moon in the Solar System.

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Science

What life on Earth tells us about life ‘elsewhere’

Plumes of water seen erupting form the surface of Saturn's moon Enceladus. NASA/JPL-Caltech and Space Science Institute
Plumes of water seen erupting form the surface of Saturn’s moon Enceladus. NASA/JPL-Caltech and Space Science Institute

In 1950, the physicist Enrico Fermi asked a question not many could forget for a long time: “Where is everybody?” He was referring to the notion that, given the age and size of the universe, advanced civilizations ought to have arisen in many parts of it. But if they had, then where are their space probes and radio signals? In the 60 years since, we haven’t come any closer to answering Fermi, although many interesting explanations have cropped up. In this time, the the search for “Where” has encouraged with it a search for “What” as well.

What is life?

Humankind’s search for extra-terrestrial life is centered on the assumption – rather hope – that life can exist in a variety of conditions, and displays a justified humility in acknowledging we really have no idea what those conditions could be or where. Based on what we’ve found on Earth, water seems pretty important. As @UrbanAstroNYC tweeted,

And apart from water, pretty much everything else can vary. Temperatures could drop below the freezing point or cross to beyond the boiling point of water, the environment can be doused in ionizing radiation, the amount of light could dip to quasi-absolute darkness levels, acids and bases can run amok, and the concentration of gases may vary. We have reason to afford such existential glibness: consider this Wikipedia list of extremophiles, the living things that have adapted to extreme environments.

Nonetheless, we can’t help but wonder if the qualities of life on Earth can tell us something about what life anywhere else needs to take root- even if that means extrapolating based on the assumption that we’re looking for something carbon-based, and dependent on liquid water, some light, and oxygen and nitrogen in the atmosphere. Interestingly, even such a leashed approach can throw open a variety of possibilities.

“If liquid water and biologically available nitrogen are present, then phosphorus, potassium, sodium, sulfur and calcium might come next on a requirements list, as these are the next most abundant elements in bacteria,” writes Christopher McKay of the NASA Ames Research Center, California, in his new paper ‘Requirements and limits for life in the context of exoplanets’. It was published in Proceedings of the National Academy of Sciences on June 9.

Stuff of stars

McKay, an astro-geophysicist, takes a stepped approach to understanding the conditions life needs to exist. He bases his argument on one inescapable fact: that we know little to nothing about how life originated, but a lot about how, once it exists, it can or can’t thrive on Earth. Starting from that, the first step he devotes to understanding the requirements for life. In the second step, he analyzes the various extreme conditions life can then adapt to. Finally, he extrapolates his findings to arrive at some guidelines.

It’s undeniable that these guidelines will be insular or play a limited role in our search for extraterrestrial life. But such criticism can be partly ablated if you consider Carl Sagan’s famous words from his 1980 book Cosmos: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”

In 1991, RH Koch and RE Davies published a paper (titled ‘All the observed universe has contributed to life’) presenting evidence that “a standard 70 kg human  is always making about 7 3He, 600 40Ca, and 3,000 14N nuclei every second by radioactive decay of 3H, 40K, and 14C, respectively”. In other words, we’re not just made of starstuff, we’re also releasing starstuff! So it’s entirely plausible other forms of life out there – if they exist – could boast some if not many similarities to life on Earth.

To this end, McKay postulates a ‘checklist for habitability’on an exoplanet based on what we’ve found back home.

  • Temperature and state of water – Between -15° C and 122° C (at pressure greater than 0.01 atm)
  • Water availability – Few days per year of rain, fog or snow, or relative humidity more than 80%
  • Light and chemical energy sources
  • Ionizing radiation – As much as the bacterium Deinococcus radiodurans can withstand (this microbe is the world’s toughest extremophile according to the Guinness Book of World Records)
  • Nitrogen – Enough for fixation
  • Oxygen (as the molecule O2) – Over 0.01 atm needed to support complex life

McKay calls this list “a reasonable starting point in the search for life”. Its items show that together they make possible environmental conditions that sustain some forms of chemical bonding – and such a conclusion could inform our search for ‘exo-life’. Because we’re pretty clueless about the origins of life, it doesn’t mean we’ve to look for just these items on exoplanets but the sort of environment that these items’ counterparts could make possible. For example, despite the abundance of life-friendly ecosystems on Earth today, one way life could have originated in the first place is by meteorites having seeded the crust with the first microbes. And once seeded, the items on the checklist could have taken care of the rest.

Are you sure water is life?

Such otherworldly influences present yet more possibilities; all you need is another interstellar smuggler of life to crash into a conducive laboratory. Consider the saturnine moon Titan. While hydrocarbons – the principal constituents of terran life – on Earth are thought to have gassed up and out from the mantle since its formative years, Titan already boasts entire lakes of methane (CH4), a simple hydrocarbon. A 2004 paper by Steven Benner et al discusses the implications of this in detail, arguing that liquid methane could actually be a better medium than water for certain simple chemical reactions that are the precursors of life to occur in.

Another Solar System candidate that shows signs of habitability is Titan’s peer Enceladus. In April this year, teams of scientists studying data from the Cassini space probe said there was evidence that Enceladus hosts a giant reservoir of liquid water 10 km deep under an extensive ice shell some 30-40 km thick. Moreover, Cassini flybys since 2005 had shown that the moon had an atmosphere of 91% water vapor, 3-4% each of nitrogen and carbon dioxide, and the rest of methane.

These examples in our Solar System reveal how the conditions necessary for life are possible not just in the Goldilocks zone because life can occur in a variety of environments as long some simpler conditions are met. The abstract of the paper by Benner et al sums this up nicely:

A review of organic chemistry suggests that life, a chemical system capable of Darwinian evolution, may exist in a wide range of environments. These include non-aqueous solvent systems at low temperatures, or even supercritical dihydrogen– helium mixtures. The only absolute requirements may be a thermodynamic disequilibrium and temperatures consistent with chemical bonding.

As humans, we enjoy the benefits of some or many of these conditions – although we know what we do only on the basis of what we’ve observed in nature, not because some theory or formula tells us what’s possible or not. Such is the amount of diversity of life on Earth, and that should tell us something about how far from clued-in we are to understanding what other forms of life could be out there. In the meantime, as the search for extra-terrestrial life and intelligence goes on, let’s not fixate on the pessimism of Fermi’s words and instead remember the hope in Sagan’s (and keep an eye on McKay’s checklist).