Cassini’s Legacy: Enigmatic Enceladus Will Inspire Us for Generations to Come

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NASA’s Cassini mission captured this view of icy moon Enceladus on March 29, 2017. The crescent is lit by the sun, but the near-side green hue is reflected sunlight bouncing off Saturn’s atmosphere — a.k.a. “Saturn glow” (NASA/JPL-Caltech/Space Science Institute)

The day before Cassini plunged into Saturn’s atmosphere, dramatically ending 13 years of Saturn exploration (and nearly two decades in space), I was sitting on a bench outside the Von Karman Visitor Center on the NASA Jet Propulsion Laboratory campus in La Cañada Flintridge with Linda Spilker, who served as the mission’s project scientist since before Cassini was launched.

What was supposed to be a quick 5-minute chat before lunch, turned into a wonderful 20-minute discussion about Cassini’s discoveries. But it was also about what the spacecraft meant to Spilker and how other space missions have shaped her life.

“I feel very fortunate to be involved with Cassini since the very beginning … and just to be there, to be one of the first to see SOI [Saturn Orbital Insertion] with those first incredible ring pictures,” she told me. “I love being an explorer. I worked on the Voyager mission during the flybys of Jupiter, Saturn, Uranus and Neptune; that sort of whet my appetite and made me want more, to become an explorer to go to the Saturn system.”

Spilker especially loved studying Saturn’s rings, not only from a scientific perspective, but also because they are so beautiful, she continued. “It’s been a heartwarming experience,” she said.

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Before Cassini crashed into Saturn’s atmosphere, it took a series of observations that created this mosaic of Saturn and its rings. Cassini plunged into the Saturnian atmosphere on Sept. 15 (NASA/JPL-Caltech/Space Science Institute/Mindaugas Macijauskas)

But Cassini’s “legacy discovery,” said Spilker, was the revelation that the tiny icy moon of Enceladus is active, venting water vapor into space from powerful geysers emerging from the moon’s “tiger stripes” — four long fissures in the moon’s south pole. After multiple observations of these geysers and direct sampling of the water particles during flybys, Cassini deduced that the icy space marble hides a warm, salty ocean.

“What Cassini will be remembered for — its legacy discovery — will be the geysers coming from Enceladus with the ocean with the potential for life. It’s a paradigm shift.” — Linda J. Spilker, Cassini project scientist, NASA Jet Propulsion Laboratory (JPL), Sept. 14, 2017.

Alongside Jupiter’s moon Europa, Enceladus has become a prime destination for future explorations of life beyond Earth. Its subsurface ocean contains all the ingredients for life as we know it and Cassini was the mission that inadvertently discovered its biological potential. So now we know about this potential, Spilker is keen to see a dedicated life-hunting mission that could go to Enceladus, perhaps even landing on the surface to return samples to Earth.

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Artist impression of Cassini flying through Enceladus’ water plumes venting from the moon’s south pole (NASA/JPL-Caltech)

As Enceladus is much smaller and less massive than Europa, its gravity is lower, meaning that landing on the surface is an easier task. Also, the radiation surrounding Saturn is much less aggressive than Jupiter’s radiation belts, meaning less radiation shielding is needed for spacecraft going to Saturn’s moons.

But if we ever send a surface mission to Enceladus (or any of the icy moons in the outer solar system), the planetary protection requirements will be extreme.

“If any life were found on these moons, it would be microbial,” said Larry Soderblom, an interdisciplinary scientist on the Cassini mission. “Some [terrestrial] bacteria are very resilient and can survive in hot acid-reducing environments. They can be tenacious. We have to make sure we don’t leave any of these kinds of Earthly bacteria on these promising moons.”

Soderblom has a unique perspective on solar system exploration. His career spans a huge number of NASA missions since the 1960’s, including Mariner 6, 7, 9, Viking, Voyager, Galileo, Magellan, Mars Pathfinder, the Mars Exploration Rovers, Deep Space 1, to name a few. While chatting to me under the shade of a tree on the JPL campus, he pointed out that the outer solar system was seen as a very different place over half a century ago.

“When I started to explore the solar system as a young guy just out of graduate school, our minds-eye view of the outer solar system was pretty bleak,” he remembered. “We expected lifeless, dead, battered moons with no geologic activity.”

After being involved with many outer solar system missions, this view has radically changed. Not only have we discovered entire oceans on Enceladus and Europa, there’s active volcanoes on Jupiter’s tortured moon Io, an atmosphere on Titan sporting its own methane cycle and surface lakes of methane and ethane. Other moons show hints of extensive subsurface oceans too, including distant Triton, a moon of Neptune. When NASA’s New Horizons flew past Pluto in 2015, the robotic spacecraft didn’t see a barren, dull rock as all the artistic impressions that came before seemed to suggest. The dwarf planet is a surprisingly dynamic place with a rich geologic history.

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With a diameter of only 313 miles, tiny Enceladus is a surprising powerhouse of internal activity. Subsurface oceans are heated through tidal interactions with Saturn (and, possibly, radioactivity in its rocky core), forcing water through its south pole fissures (NASA/JPL-Caltech)

Sending our robotic emissaries to these distant and unforgiving places has revolutionized our understanding of the solar system and our place in it. Rather than the gas and ice giant moons being dull, barren and static, our exploration has revealed a rich bounty of geologic variety. Not only that, we’re almost spoilt for choices for our next giant leap of scientific discovery.

Missions like Cassini are essential for science. Before that spacecraft entered Saturn orbit 13 years ago, we had a very limited understanding of what the Saturnian system was all about. Now we can confidently say that there’s a tiny moon there with incredible biological potential — Enceladus truly is Cassini’s legacy discovery that will keep our imaginations alive until we land on the ice to explore its alien ocean.

For more on my trip to JPL, read my two HowStuffWorks articles:

Why Cassini Crashed: Protecting Icy Moon Enceladus at All Costs

What Epic Space Missions Like Cassini Teach Us About Ourselves

Sorry, Proxima Centauri Is Probably a Hellhole, Too

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The surface of Proxima b as imagined in this artist’s impression. Sadly, the reality probably doesn’t include an atmosphere (ESO/M. Kornmesser)

The funny thing about habitable zones is that they’re not necessarily habitable. In fact, depending on the star, some of them are likely downright horrible.

Take, for example, the “habitable zone exoplanet” orbiting our neighboring star Proxima Centauri. When the discovery of Proxima b was announced last year, the world erupted with excitement. After all, astronomers had detected an Earth-sized world right on our galactic doorstep, a mere four light-years away.

Immediately there was discussion about Proxima b’s habitable potential (could there be aliens?) and the possibility of the world becoming an interstellar target (might we one day go there on vacation?).

Alas, for the moment, these exo-dreams are pure fantasy as the only things we know about this world are its mass and its orbital period around the star. We have no clue about the composition of this exoplanet’s atmosphere — or even if it has an atmosphere at all. And, according to new research published in The Astrophysical Journal Letters, Proxima b would probably be a very unlikely place to find extraterrestrial life and you’d be ill advised to invest in a vacation home there.

Like TRAPPIST-1 — that other star system that contains “habitable, but probably not so habitable” exoplanets — Proxima Centauri is a red dwarf star. By their nature, red dwarfs are small and cooler than our sun. Their habitable zones are therefore very compact; to receive enough heating energy to keep water in a liquid state on their surfaces, any “habitable” red dwarf exoplanets would need to snuggle up really close to their star. Liquid water (as we all know) is essential for life. So, if you want to find life as we know it (not that weird Titan life), studying habitable zone planets would be a good place to start. And as red dwarfs are abundant in our galaxy, seeking out habitable zone planets in red dwarf star systems would, at first, seem like an even better place to start.

Except, probably not.

Red dwarfs are angry. They erupt with powerful flares, have powerful stellar winds and their habitable zones are awash with intense ultraviolet radiation. And, like TRAPPIST-1, Proxima Centauri probably wouldn’t be a great place to live.

But the researchers decided to test this hypothesis by throwing Earth in at the deep end.

“We decided to take the only habitable planet we know of so far — Earth — and put it where Proxima b is,” said Katherine Garcia-Sage, a space scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and lead author of the study.

The big advantage for Earth is that it possesses a powerful global magnetic field that can deflect our sun’s solar wind and coronal mass ejections with a minimum of effort. But put Earth in a habitable zone orbit around Proxima Centauri and bad stuff starts to happen, fast.

At this location, the intensity of extreme ultraviolet radiation becomes a problem. Using data from NASA’s Chandra X-ray Observatory, the researchers could gauge the star’s activity and how much radiation would hit Proxima b. According to their calculations, the exoplanet receives hundreds of times more extreme ultraviolet radiation than Earth receives from our sun and, even if we assume Proxima b has an “Earth-like” magnetosphere, it will lose its atmosphere very quickly.

As ultraviolet radiation will ionize the exoplanet’s atmosphere, electrons (that are negatively charged) will be readily stripped from light atoms (hydrogen) and eventually the heavier atoms too (like oxygen and nitrogen). As the electrons are lost to space, a powerful “charge separation” is created and the positively charged ions that are left behind in the atmosphere will be dragged with the electrons, causing them to also be lost to space. Granted, the global magnetic field will have an effect on the rate of atmosphere loss, but the researchers estimate that this process will drain an atmosphere from Proxima b 10,000 times faster than what happens on Earth.

“This was a simple calculation based on average activity from the host star,” added Garcia-Sage. “It doesn’t consider variations like extreme heating in the star’s atmosphere or violent stellar disturbances to the exoplanet’s magnetic field — things we’d expect provide even more ionizing radiation and atmospheric escape.”

In the worst-case scenario, where the outer atmospheric temperatures are highest and the planet exhibits an “open” field line configuration, Proxima b would lose the equivalent of the whole of Earth’s atmosphere in just 100 million years. If the atmospheric temperatures are cool and a “closed” magnetic field line configuration is assumed, it will take 2 billion years for the atmosphere to be completely lost to space. Either way you look at it, unless the atmosphere is being continuously replaced (perhaps by very active volcanism), Proxima b will have very little chance to see life evolve.

“Things can get interesting if an exoplanet holds on to its atmosphere, but Proxima b’s atmospheric loss rates here are so high that habitability is implausible,” said Jeremy Drake, of the Harvard-Smithsonian Center for Astrophysics and study co-author. “This questions the habitability of planets around such red dwarfs in general.”

Weird Form of Alien Life May Be Possible on Saturn’s Moon Titan

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Artist’s impression of Titan’s surface and atmosphere (credit: Benjamin de Bivort, debivort.org / CC BY-SA 3.0)

Titan is a very strange moon.

Orbiting the ringed gas giant Saturn, Titan is the only moon in the solar system that sports a thick atmosphere. Although the moon is extremely cold, its atmosphere is very dynamic; exhibiting seasons, precipitation and even creating vast seas.

Although this may sound very much like Earth’s atmosphere — where water evaporates from the oceans, condenses as clouds and precipitates as rain, forming rivers that flow back into the oceans — Titan’s atmosphere is dominated by a methane cycle, not a water cycle.

This may sound like the antithesis of Earth’s life-giving chemistry, but astrobiologists have been gradually finding clues to Titan’s habitable potential and today (July 28) scientists have announced the confirmation of a key molecule that could be the proverbial backbone to a weird kind of “alternative” alien life on Titan — based not on liquid water, but on liquid methane.

“The presence of vinyl cyanide in an environment with liquid methane suggests the intriguing possibility of chemical processes that are analogous to those important for life on Earth,” said astrochemistry researcher Maureen Palmer, of NASA’s Goddard Space Flight Center in Greenbelt, Md.

Palmer is lead author of a study published in Science Advances describing the detection of vinyl cyanide (also known as acrylonitrile) at Titan using the awesome power of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

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B. Saxton (NRAO/AUI/NSF); NASA

Previous observations of Titan’s atmosphere by NASA’s Cassini mission and chemical modeling of the moon’s surface have hinted that it is the ideal environment for vinyl cyanide to form. But it was only when analysis of archived data collected by ALMA between February to May 2014 was carried out that its presence was confirmed. And there appears to be a lot of the stuff.

So what is vinyl cyanide and why is it so important?

The molecule (C2H3CN) has the ability to form membranes and, if found in liquid pools of hydrocarbons on Titan’s surface, it could form a kind of lipid-based cell membrane analog of living organisms on Earth. In other words, this molecule could stew in primordial pools of hydrocarbons and arrange itself in such a way to create a “protocell” that is “stable and flexible in liquid methane,” said Jonathan Lunine (Cornell University) who, in 2015, was a member of the team who modeled vinyl cyanide and found that it might form cell membranes.

“This is a step forward in understanding whether Titan’s methane seas might host an exotic form of life,” Lunine, who wasn’t a member part of the team that announced today’s results, said in a statement.

Life As We Don’t Know It

When studying Titan’s nitrogen-rich atmosphere, ALMA detected three unambiguous millimeter-wavelength signals produced by vinyl cyanide that originated from 200 kilometers above Titan’s surface. It is well known that the moon’s atmosphere is a vast chemical factory; the energy of the sun and particles from space convert simple organic molecules into more complex chemistry. These chemicals then cycle down to Titans rich hydrocarbon surface.

But speculating about life on Titan is a hard task. The moon’s atmosphere is often compared with that of early Earth’s, but there are some huge differences. Titan is crazy-cold, averaging around 95 Kelvin (that’s an incredible -178 degrees Celsius or -288 degrees Fahrenheit); at no time in history has Earth’s atmosphere been that cold. Also, it’s thought that early Earth had large quantities of carbon dioxide in its atmosphere, Titan does not. As for water? Frozen. Oxygen? Forget about it.

So this research underpins our quest to find the chemistry of life as we DON’T know it, using the building blocks that follow the pattern of life that we do know, but swapping out key components (like water) to see if an analog of life’s chemistry can under very alien conditions.

“Saturn’s moon, Enceladus is the place to search for life like us, life that depends on — and exists in — liquid water,” said Lunine. “Titan, on the other hand, is the place to go to seek the outer limits of life — can some exotic type of life begin and evolve in a truly alien environment, that of liquid methane?”

Perhaps it’s time for a return mission to Titan’s extreme surface.

TRAPPIST-1: The ‘Habitable’ Star System That’s Probably a Hellhole

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Red dwarfs can be angry little stars (NASA/GSFC/S. Wiessinger)

There are few places that elicit such vivid thoughts of exotic habitable exoplanets than TRAPPIST-1 — a star system located less than 40 light-years from Earth. Alas, according to two recent studies, the planetary system surrounding the tiny red dwarf star may actually be horrible.

For anyone who knows a thing or two about red dwarfs, this may not come as a surprise. Although they are much smaller than our sun, red dwarfs can pack a powerful space weather punch for any world that orbits too close. And, by their nature, any habitable zone surrounding a red dwarf would have to be really compact, a small detail that would bury any “habitable” exoplanet in a terrible onslaught of ultraviolet radiation and a blowtorch of stellar winds. These factors would make the space weather environment around TRAPPIST-1 extreme to say the least.

“The concept of a habitable zone is based on planets being in orbits where liquid water could exist,” said Manasvi Lingam, a Harvard University researcher who led a Center for Astrophysics (CfA) study, published in the International Journal of Astrobiology. “This is only one factor, however, in determining whether a planet is hospitable for life.”

The habitable zone around any star is the distance at which a small rocky world can orbit and receive just the right amount of heating to maintain liquid water on its hypothetical surface. Orbit too close and the water vaporizes; too far and it freezes. As life needs liquid water to evolve, seeking out exoplanets in their star’s habitable zone is a good place to start.

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The peaceful surface of a TRAPPIST-1 habitable zone exoplanet as imagined in this artist’s rendering (NASA/JPL-Caltech)

For the sun-Earth system, we live in the middle of the habitable zone, at a distance of one astronomical unit (1 AU). For a world orbiting a red dwarf like TRAPPIST-1, its orbital distance would be a fraction of that — i.e. three worlds orbit TRAPPIST-1 in the star’s habitable zone at between 2.8% and 4.5% the distance the Earth orbits the sun. This is because red dwarfs are very dim and produce meager heating — for a world to receive the same degree of heating that our planet enjoys, a red dwarf world would need to snuggle up really close to its star.

But just because TRAPPIST-1 is dim, it doesn’t mean it holds back on ultraviolet radiation. And, according to this study, the three “habitable” exoplanets in the TRAPPIST-1 system are likely anything but — they would receive disproportionate quantities of damaging ultraviolet radiation.

“Because of the onslaught by the star’s radiation, our results suggest the atmosphere on planets in the TRAPPIST-1 system would largely be destroyed,” said co-author Avi Loeb, who also works at Harvard. “This would hurt the chances of life forming or persisting.”

Life as we know it needs an atmosphere, so the erosion by UV radiation seems like a significant downer for the possible evolution of complex life.

That’s not the only bad news for our extraterrestrial life dreams around TRAPPIST-1, however. Another study carried out by the CfA and the University of Massachusetts in Lowell (and published in The Astrophysical Journal Letters) found more problems. Like the sun, TRAPPIST-1 generates stellar winds that blast energetic particles into space. As these worlds orbit the star so close, they would be sitting right next to the proverbial nozzle of a stellar blowtorch — models suggest they experience 1,000 to 100,000 times stellar wind pressure than the solar wind exerts on Earth.

And, again, that’s not good news if a planet wants to hold onto its atmosphere.

“The Earth’s magnetic field acts like a shield against the potentially damaging effects of the solar wind,” said Cecilia Garraffo of the CfA and study lead. “If Earth were much closer to the sun and subjected to the onslaught of particles like the TRAPPIST-1 star delivers, our planetary shield would fail pretty quickly.”

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The TRAPPIST-1 exoplanet family. TRAPPIST-1 e, f and g are located in the system’s habitable zone (NASA/JPL-Caltech)

So it looks like TRAPPIST-1 e, f and g really take a pounding from their angry little star, but the researchers point out that it doesn’t mean we should forget red dwarfs as potential life-giving places. It’s just that life would have many more challenges to endure than we do on our comparatively peaceful place in the galaxy.

“We’re definitely not saying people should give up searching for life around red dwarf stars,” said co-author Jeremy Drake, also from CfA. “But our work and the work of our colleagues shows we should also target as many stars as possible that are more like the sun.”

Great Balls of ‘Space Mud’ May Have Built Earth and Delivered Life’s Ingredients

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Artist’s impression of the molten surface of early Earth (NASA)

When imagining how our planet formed 4.6 billion years ago from the protoplanetary disk surrounding our sun, images of large pieces of marauding space rock slamming into the molten surface of our proto-Earth likely come to mind.

But this conventional model of planetary creation may be missing a small, yet significant, detail. Those massive space rocks may not have been the conventional solid asteroids — they might have been massive balls of space mud.

This strange detail of planetary evolution is described in a new study published in the American Association for the Advancement of Science (AAAS) journal Science Advances and it kinda makes logical sense.

Using the wonderfully-named Mars and Asteroids Global Hydrology Numerical Model (or “MAGHNUM”), planetary scientists Phil Bland (Cornell University) and Bryan Travis (Planetary Science Institute) simulated the movement of material inside primordial carbonaceous chondrite asteroids — i.e. the earliest asteroids that formed from the sun’s protoplanetary disk that eventually went on to become the building blocks for Earth.

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A simulated cross section of a 200-meter wide asteroid showing its internal temperature profile and convection currents (temperatures in Celsius). Credit: PSI

It turns out that these first asteroids weren’t cold and solid lumps of rock at all. By simulating the distribution of rock grains inside these asteroids, the researchers realized that the internal heat of the objects would have melted the icy volatiles inside, which then mixed with the fine dust particles. Convection would have then dominated a large portion of these asteroids, causing continuous mixing of water and dust. Like a child squishing a puddle of dirt to create sloppy “mud pies,” this convection would have formed a ball of, you guessed it, space mud.

Travis points out that “these bodies would have accreted as a high-porosity aggregate of igneous clasts and fine-grained primordial dust, with ice filling much of the pore space. Mud would have formed when the ice melted from heat released from decay of radioactive isotopes, and the resulting water mixed with fine-grained dust.”

In other words: balls of mud held together by mutual gravity, gently convected by the heat produced by the natural decay of radioactive materials.

Should this model hold up to further scrutiny, it has obvious implications for the genesis of life on Earth and could impact the study of exoplanets and their habitable potential. The ingredients for life on Earth originated in the primordial protoplanetary soup, but until now the assumption has been that the space rocks carrying water and other chemicals were solid and frozen. If they were in fact churning away in space as dynamic mud asteroids, they could have been the “pressure cookers” that delivered those ingredients to Earth’s surface.

So the next question would be: how did these exotic asteroids shape life on Earth?

This Is NASA’s Future Mars 2020 Rover Looking for Biosignatures on the Red Planet

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NASA/JPL-Caltech

While Opportunity and Curiosity continue to explore the surface of Mars, the launch date of NASA’s next big rover mission is on the horizon. And here’s a stunning artist’s impression of the mission that NASA released on Tuesday.

Wait. Isn’t that Curiosity?

No. While the Mars 2020 rover will certainly look like Curiosity, as many of the current rover’s design features will be worked into NASA’s next six-wheeled robot, there will be some key differences in the next rover’s science.

Rather than seeking out past and present habitable environments (as Curiosity is currently doing on the slopes of Mount Sharp), one of Mars 2020’s stated science goals is to directly search for biological signatures of past and present microbial life on Mars. This next-generation rover will also feature a drill that can bore deep into rocks, pull samples and store them on the Martian surface for a possible future sample return mission.

For more on Mars 2020, check out NASA’s mission site.

This Super-Hot, Super-Weird Space Doughnut Could Be a New “Planetary Object”

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The structure of a planet, a planet with a disk and a synestia, all of the same mass (Simon Lock and Sarah Stewart)

Pluto is going to be pissed.

After studying computer simulations of planetary collisions, scientists have discovered a possible phase of planetary formation that has, so far, been overlooked by astronomy. And they think this phase is so significant that it deserves its own planetary definition.

After two planetary objects collide, researchers from the University of California Davis and Harvard University in Cambridge, Mass., realized that a bloated, spinning mass of molten rock can form. It looks a bit like a ring doughnut with the hole filled in. What’s more, it is thought that Earth (and other planets in the solar system) probably went through this violent period before becoming the solid spinning globes we know and love today.

They call this partly vaporized rock “synestia” — “syn-” for “together” and “Estia” after the Greek goddess of architecture and structures.

Over a range of masses and collision speeds, planetary scientist Sarah Stewart (Davis) and graduate student Simon Lock (Harvard) simulated planetary collisions and focused on how the angular momentum of colliding bodies might influence the system. Their study has been published in the Journal of Geophysical Research: Planets. Basically, when two bodies — with their own angular momentum — collide and merge, the sum of their momenta must be conserved and this can have a dramatic effect on the size and structure of the merged mass.

“We looked at the statistics of giant impacts, and we found that they can form a completely new structure,” said Stewart.

After colliding, the energetic event causes both planets to melt and partially vaporize, expanding with a connected ring-like structure. And this structure — a synestia — would eventually cool, contract and solidify. It could also form moons; post-collision molten debris in the synestia doughnut ring may emerge in a stable orbit around the planet.

The synestia phase would be a fleeting event in any planet’s life, however. For an Earth-sized mass, the post-collision synestia would likely last only 100 years or so. But the larger the mass, the longer the phase, the researchers theorize.

So, giving this theoretical “planetary object” a classification might be a little generous — a move that would raise recently “demoted” Pluto’s eyebrow — but as telescopes become more advanced, we might one day be lucky enough to spy a synestia in a young star system where dynamic instabilities are causing planets to careen into one another.

Curiosity Is Getting Diggy With It in Mars’ Ripply Dunes

NASA/JPL-Caltech/MSSS

There are few sights on Mars more satisfying than its oddly familiar — yet weirdly alien — dunes.

On the one hand, the Martian dunes look much like the dunes we have on Earth — aeolian (“wind-driven”) formations undulating across the landscape have similarities regardless of which planet they were blown on.

But there’s something uncanny about Martian dunes. Maybe it’s the “extra” tiny ripples that NASA’s Curiosity itself discovered — a phenomenon that is exclusive to the Martian atmosphere. Or maybe it’s just that I know these dunes are being seen through synthetic eyes on a world millions of miles across the interplanetary void.

Who knows.

But right now, the six-wheeled robot is sampling grains of wind-blown regolith from a linear dunes on the slopes of Mount Sharp to help planetary scientists on Earth build a picture of how this ancient landscape was shaped.

Curiosity scooped samples of linear dune material into the rover’s Sample Analysis at Mars (SAM) so they could be compared with material from other dunes it had visited in 2015 and 2016. Samples are also planned to be delivered to the mission’s Chemistry and Mineralogy (CheMin) instrument. As NASA points out, this is the first ever study of extraterrestrial dunes. (Dune fields also exist on Saturn’s moon Titan, but as recent research indicates, those are very different beasts and haven’t been directly sampled.)

“At these linear dunes, the wind regime is more complicated than at the crescent dunes we studied earlier,” said Mathieu Lapotre, of the California Institute of Technology (Caltech), in Pasadena, Calif., who led the Curiosity dune campaign. “There seems to be more contribution from the wind coming down the slope of the mountain here compared with the crescent dunes farther north.”

All of the dunes Curiosity has sampled are a part of the Bagnold Dunes, a dune field that stretches up the northwestern flank of Mount Sharp. Within the field, depending on the wind conditions, different types of dunes have been found.

“There was another key difference between the first and second phases of our dune campaign, besides the shape of the dunes,” said Lapotre in a NASA statement. “We were at the crescent dunes during the low-wind season of the Martian year and at the linear dunes during the high-wind season. We got to see a lot more movement of grains and ripples at the linear dunes.”

 

Cassini Finds ‘Nothing’ in Saturn’s Ring Gap

NASA/JPL-Caltech

It’s official, there’s a whole lot of nothing in Saturn’s innermost ring gap.

This blunt — and slightly mysterious — conclusion was reached when scientists studied Cassini data after the spacecraft’s first dive through the gas giant’s ring plane. At first blush, this might not sound so surprising; the 1,200-mile-wide gap between Saturn’s upper atmosphere and the innermost edge of its rings does appear like an empty place. But as the NASA spacecraft barreled through the gap on April 26, mission scientists expected Cassini to hit a few stray particles on its way through.

Instead, it hit nothing. Or, at least, far fewer particles than they predicted.

“The region between the rings and Saturn is ‘the big empty,’ apparently,” said Earl Maize, Cassini’s project manager at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.”

Using Cassini’s Radio and Plasma Wave Science (RPWS), the scientists expected to detect multiple “cracks and pops” as the spacecraft shot through the gap. Instead, it picked up mainly signals from energetic charged particles buzzing in the planet’s magnetic field. When converted into an audio file, these signals make a whistling noise and this background whistle was expected to be drowned out by the ruckus of dust particles bouncing off the spacecraft’s body. But, as the following audio recording proves, very few pops and cracks of colliding debris were detected — it sounds more like an off-signal radio tuner:

Compare that with the commotion Cassini heard as it passed through the ring plane outside of Saturn’s rings on Dec. 18, 2016:

Now that is what it sounds like to get smacked by a blizzard of tiny particles at high speed.

“It was a bit disorienting — we weren’t hearing what we expected to hear,” said William Kurth, RPWS team lead at the University of Iowa, Iowa City. “I’ve listened to our data from the first dive several times and I can probably count on my hands the number of dust particle impacts I hear.”

From this first ring gap dive, NASA says Cassini likely only hit a handful of minute, 1 micron particles — particles no larger than those found in smoke. And that’s a bit weird.

As weird as it may be, the fact that the region of Cassini’s first ring dive is emptier than expected now allows mission scientists to carry out optimized science operations with the spacecraft’s instruments. On the first pass, Cassini’s dish-shaped high-gain antenna was used as a shield to protect the spacecraft as it made the dive. On its next ring dive, which is scheduled for Tuesday at 12:38 p.m. PT (3:38 p.m. ET), this precaution is evidently not needed and the spacecraft will be oriented to better view the rings as it flies through.

So there we have it, the first mysterious result of Cassini’s awesome Grand Finale! 21 ring dives to go…

KCRW ‘To The Point’ Interview: Cassini’s Grand Finale

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Artist’s impression of Cassini passing through the gap between Saturn and the planet’s rings (NASA/JPL-Caltech)

After all the excitement of last night’s Cassini mission checking in and transmitting data to NASA’s Deep Space Network, I joined Warren Olney on his NPR-syndicated show “To The Point” this morning to chat about the mission and why the “Grand Finale” is an awesome, yet bittersweet, part of Saturn exploration. Listen to the 10 minute segment here. It was great as always to chat with Warren, thanks for having me on the show!