What do asteroids 44 Nysa, 64 Angelina and the Galilean satellites Io, Europa and Ganymede have in common? They are all Solar System objects without an atmosphere that are highly reflective. They are also the subject of study in a new paper from Robert Nelson (Planetary Science Institute) that investigates a feature common to all: At small phase angles (the angle from the Sun to the target being observed), they show negatively polarized light.
Light reflected from objects in the Solar System is usually polarized, meaning that the electric and magnetic vibrations of the electromagnetic wave occur in a single plane. The amount of polarization depends upon the reflective material, but also on the geometry, as a good astronomy textbook makes clear (I’m checking against Karttunen et al.’s Fundamental Astronomy, 6th ed., 2016).
Image: The phases of Rhea. Emily Lakdawalla used these Cassini images to explain phase angle in a useful 2009 backgrounder. Her caption: The angle from the Sun, to a moon, to the observer is called “phase angle.” This montage shows Saturn’s moon Rhea as seen by Cassini through a clear filter at a variety of phase angles. The images have been resized to a constant pixel scale and rotated so that the terminator is up-and-down; the images sample a variety of latitudes and longitudes. Credit: NASA / JPL / SSI / montage by Emily Lakdawalla.
Polarization at phase angles of greater than about 20° from a body without an atmosphere are positive, while closer to opposition, the polarization is negative. Looking at the polarization behavior of the Jovian moons and asteroids he studies, Nelson and team deduce that their long-observed negative polarization at low phase angles can be explained by extremely fine-grained particles with void space above 95 percent. The surface material, in other words, would feature grain sizes on the order of the wavelength of light of the observations, or a fraction of a micron.
The inference is that these objects are covered with material that is less dense than newly fallen snow. This is a significant result, given that in the long term, we have hopes of placing a lander on Europa that can explore some of the intriguing surface material that may represent upwellings from the ocean below. The question becomes: Would a lander simply sink into the surface before it could do its work?
“Of course, before the landing of the Luna 2 robotic spacecraft in 1959, there was concern that the Moon might be covered in low density dust into which any future astronauts might sink,” Nelson said. “However, we must keep in mind that remote visible-wavelength observations of objects like Europa are only probing the outermost microns of the surface.”
Image: This enhanced-color view from NASA’s Galileo spacecraft shows an intricate pattern of linear fractures on the icy surface of Jupiter’s moon Europa. Credits: NASA/JPL-Caltech/ SETI Institute.
Notice that three Galilean satellites are implicated in this work, meaning that we may have comparable issues at Io and Ganymede. Nelson’s reminder of the Luna 2 landing has in turn reminded me of a 1963 novel by Jeff Sutton called Apollo At Go, an extrapolation based on where the manned space program was at that time of what a lunar landing might involve. Here, too, we had dust as a major feature, this time of a science fiction plot. Indeed, the cover blurb for the book includes this look at a projected first lunar landing:
They were hurtling over it, hurtling and dropping. He fought his harnessing to learn closer to the port… There a small crater, here a hill, a crevice, a milky rectangle that looked absolutely smooth and rock hard — this was an inferno without fire, a hell, the architecture of a maddened nature. The House of Lucifer…
“Zero percent fuel,” Kovac rapped out edgily.
“May says zero on the fuel. Still going down. Trying to hold steady, but can’t. Ash looks deep, deep… Rocks right next to us jagged and — Oh, there she goes… main engines off. Small jets can’t… We’re falling…”
I would think ‘zero percent fuel’ would indeed make Kovacs’ voice a bit edgy.
Sutton, a newspaperman, public relations professional and former Marine, may have been wrong about the dangers posed by the lunar surface, but he was spot on in a different regard. He sets up his Moon landing on July 8, 1969, just twelve days before the actual event. And as to the dust, he was actually writing before we had enough data from the Soviet Luna program and the Surveyor landings to ease scientists’ worries. Let’s hope we’re as well informed about Europa’s surface against the day when our first robotic lander sets down there.
The paper is Nelson et al., “Laboratory simulations of planetary surfaces: Understanding regolith physical properties from remote photopolarimetric observations,” Icarus Vol. 302 (1 March 2018), 483-488 (abstract).
No expert here, but my first reaction to the choice of al2o3 as regolith analogue was that it seemed untenable. Europan regolith for example has a high volatile content. 95% porosity seems under the circumstances unlikely. I would expect radiolytic processes to create a settling that reduced any porosity with time.
“[Polarization at p]hase angles of greater than about 20° from a body…]
I think that’s what you meant to say.
“Would a lander simply sink into the surface before it could do its work?”
I would expect the top layer to be mostly void since the grains won’t easily compact under the low gravity. Density will increase with depth due to the overburden. This is true right here whether with snow, sand or soil.
The first question I have is at what depth the “soil” can bear the lander’s weight. The second is what will be the effect of the thrusters on the soil since there would be melting, and the lander could tip and even be held firm by the re-freezing water. I would imagine they have thought about both these questions among many others.
Ron, thanks for that — my original was clumsy indeed! I’ve adjusted the text.
It is funny how at first Europa’s ice crust was considered too thick to drill through to that global ocean below, then some argued that it was thin enough to dig through:
Now we are worried about our landers staying atop the ice at all.
The Soviet Moon probe Luna 2 only “landed” on Earth’s natural satellite in the sense that it was deliberately crashed on the surface in order to make another space first for the USSR.
Luna 2 was probably vaporized by this “landing”, which probably even destroyed the metal spheres composed of pennants with the Soviet Coat of Arms which were supposed to burst open upon impact and spread themselves all over the alien landscape to ensure anyone who ever found the crater knew what happened, and by whom.
The honor of proving that the lunar dust was too compacted for any vessel to sink into would have to go the Soviet Luna 9 and USA Surveyor 1 landers, both of which set down safely upon the Moon in 1966. The later and much heavier manned Apollo Lunar Landers merely confirmed just how solid the lunar crust really is.
So is there a possibility of someone writing a homage to Clarke’s “A Fall of Moondust” as “A Fall of Europan Ice”?
If we ever fully explore the Moon, it would be interesting if we find pockets of deep dust as Clarke speculated for his novel. As Clarke shifted his attention to Europa in “2010: Odyssey 2” and “2061: Odyssey 3”, it seems fitting that this moon might also have parts of its surface as low resistance snow suitable for potentially floating on with a suitable vehicle.
In a scene that only took place in the novel version of 2010, the Chinese were the first to land a manned mission on Europa, only to have it sink into the ice when the ship’s floodlights attract a native creature below.
Other SF that comes to mind are Arthur C Clarkes “A Fall of Moondust” in which 21st century ‘boats’ cruise dust lakes on the moon, and some of Larry Nivens early Mars stories where an entire ecosystem and intelligent species live under the Martian dust seas.
There is also the 1964 episode of The Outer Limits titled “The Invisible Enemy”, where the first two manned missions to Mars sent in the early 2020s discover the hard way that something big and hungry lives in the sands of the Red Planet, moving through them like sharks in a terrestrial ocean:
The U.S. also slammed probes (Rangers) into the Moon. IIRC they would transmit pictures right up until the moment of impact. Maybe the first probe(s) to Europa should be similar? I guess it depends on the cost breakdown? If a lander is a substantial part of the mission cost then it might be worth doing some preliminary missions to make sure we can actually land. On the other hand if most of the cost is just getting there than taking a chance that a lander will work might be more economical…
Only if such an impact missions would actually reveal anything useful regarding the surface of Europa. The Ranger impactors did show small craters right up to the very last images which did further information about the strength of the lunar surface.
Still, if we are going all 400 million miles to Europa, a soft lander would be preferable.
We just might be able to “have our snow cone and eat it too” if, say, a CubeSat spacecraft (perhaps a “daughter” sub-probe) equipped with a solar sail was used as both an orbiter *and* a lander for Europa. On the moon’s surface, the sail would function like a hestetruger; these are circular–and sometimes square–equine snowshoes (see: http://www.snowshoemag.com/2015/10/03/snowshoes-for-more-weighty-creatures/ ) which, despite their small size, support horses (even draft horses)–who are quite heavy–on top of snow, and:
A square, hexagonal, octagonal, decagonal, or circular sail, which could be spin-rigidized (and spin-stabilized), *or* could have lightweight composite “fishing pole” sail support booms that could be either spin-stiffened or self-supporting, need not function as a propulsive sail (the sunlight out at Jupiter is only about 1/27th as intense as at Earth’s distance from the Sun). In circum-Europan orbit, the sail would work as a propulsive device, just that much more slowly. It could also serve as a large direct-to-Earth communications antenna while the probe was orbiting Europa, and breaking out of orbit to land there wouldn’t be difficult:
The Block II Ranger spacecraft (numbers 3, 4, and 5 in the spacecraft series, see: http://en.wikipedia.org/wiki/Ranger_program ) carried hard-landing “seismometer ball” payloads which–had any of those spacecraft succeeded–would have been braked sufficiently to survive impact. These payloads were larger and heavier than CubeSats, and had to be slowed from direct impact trajectory velocities that were somewhat in excess of our Moon’s escape velocity (about 6,000 mph), yet the solid propellant retro-rocket fitted to each one was quite small. The *orbital* velocity around Europa, which is significantly less massive than the Moon, is considerably lower, so that the de-orbit and soft-landing velocity total should be considerably less than what the Ranger hard-landing seismometer payloads required (two quite small solid rocket motors, or a low-thrust de-orbit system and one solid motor for the landing, might be sufficient). Also:
The sail need not prevent surface examination or interfere with imaging (by “dazzling” the optical sensors) after landing. Many spin-rigidized and boom-supported solar sail designs, like IKAROS and L’Garde’s built-but-cancelled Sunjammer, have a sizable center opening in which the spacecraft payload rides, and a Europa orbiter/lander sail vehicle could also be arranged in this way. Also, since the sail would only operate far from the Sun, its anti-sunward side–which would face upward after landing–need not have high thermal emissivity (as sails for inner solar system operation require, especially if they’re made of metallized plastic film), so that side of the sail could be fairly dark in color. In addition:
A second antenna boom, which would extend above the surface after landing (the sunward-side one might double as a surface penetrator probe, or be made frangible so as to crumple and absorb the landing shock), could be fitted with a camera (perhaps one of two, with the other one being affixed to the payload itself) to provide higher-elevation and horizon views. After landing, the vehicle could relay data and images to Earth at a high bit rate through the “mother” spacecraft, and it could also communicate directly, at a lower bit rate, through a secondary medium-gain or low-gain antenna. Thin-film solar cells “embedded” in the sail as with IKAROS’s sail (but with two “sets,” facing both directions) could, along with rechargeable batteries, power the vehicle.
I’ve always supported any Europa mission and it has been painful for more than 15 years seeing cancellation after cancellation while more and more Mars missions were approved and launcher one after the other.
However, I’m not a great fan of a lander for the first mission. The reason being that better landing location and more targeted instruments can be had after an hi-res survey from space.
Of course given NASA’s recalcitrance in launching anything but Mars missions, one should perhaps get as much as possible in that single, isolated opportunity that might be given.
BTW, on the Youtube channel of Lunar and Planetary society there are two quite recent talks on Europa and Enceladus and they have mentioned another two on Titan and Trappist-1.
The talk is from Pappalardo, a proponent of the thick shell (~20Km) which I sincerely hope to be wrong (for habitability and accessibility).
Enceladus is the new star : now that the ocean appears to be global and more stable in time than originally thought. And, of course, it is habitable with water and organics that Mars people can dream of :-)
Russian billionaire Yuri Milner of Breakthrough Initiatives fame might get a life-seeking probe to Enceladus sooner than anyone else, including NASA:
Perhaps he can do the same for Europa. And let us not forget the smooth, egg-shaped Saturnian moon Methone, which may be all fluffy dust on its surface:
How can we not send a probe to that world?!
Enceladus is another reason I don’t favor a first time lander on Europa : if it turns out that the ice is thick ~20 km or so with sporadic/absent interaction with the surface, then Enceladus’ priority goes up a lot. And so, the money saved by not doing the Europa lander could go to Enceladus instead.
If I were being asked where to place a lander with life-detection capabilities on Europa, I would aim it for one of the many long, dark linear lines that cross the icy face of that moon.
No doubt those cracks are upwellings from the liquid water ocean below and the dark material could very well be organic material if not life itself. No drilling or melting involved if we want ocean samples that way.
Has anyone scanned those regions for their makeup?
As for having to choose Europa or Enceladus, this should not be a competition. We can do both and for far less than any typical military project budget. We need to explore both.
Brad Dalton of Nasa Aimes Research, years ago speculated that the dark lines on Europa could be a result of bacterial life.
“Streaks of reddish-brown color highlight cracks in Europa’s outer layer of ice. Some scientists have speculated that microorganisms suspended in Europa’s ice may be the cause of these colorations. To test this theory, planetary geologist Brad Dalton of the NASA Ames Research Center compared the infrared (IR) signature of Europa’s ice with the IR signature of microorganisms living at hot water vents in Yellowstone National Park. Dalton discovered that the IR signatures are very similar.”
Thank you. That should further clinch the plan on where to place a Europa lander: Where the stuff that looks like organic goop is, and they have a lot of places to choose from.
Might those reddish-brown streaks be tholins instead? But since we can’t be sure with distant observations from Earth (or even from spacecraft passing near Europa while in circum-Jovian orbit), we must either set a lander down there, or snag samples of that material using a “surface-walking” bolo tether spacecraft system. (Designs that have been studied could also set instrument packages on the surface, and surface sampler ones could “dwell” on the surface for a minute or so before “retracting” back into the sky–plenty of time for acquiring and even photo-documenting a sample and its surroundings.)
Recent experiments have determined that under simulated Europan conditions salt(NaCl)turns red. This lead to the conclusion that the streaks stem from the ocean.
Astronauts on the surface of Europa will have to also deal with high energy elections and ions, since Europa is inside Jupiter’s electromagnetic field and radiation belt.
The crew in Europa Report had lots of trouble with sinking into the ice of that alien moon:
Not a word on what material there might be on 64 Angelina and 44 Nysa except I found they are considered to be of the same group – and here about the polarized light.
Perhaps I should not ask what nobody else know, but I am intrigued – would ice be stable or sublimate at that distance? Or is it small bits of silicone that reflect light so well?
Very nice picture of Europa surface, I did not know so good images exist.
Despite having a stuck antenna which reduced the amount of data and images it could return to Earth, the Galileo orbiter probe did take some nice photographs of Europa:
I wonder too–although this is more speculative (but sufficiently hardy terrestrial micro-organisms exist, so it isn’t impossible)–if Io has been considered by exobiologists? That moon has a temperature range (between about our Moon’s night-time temperature and the molten, liquid sulfur temperatures of its volcanoes) where hospitable conditions should occur, possibly underground within some distance of the sulfur magma reservoirs. Such places might also provide adequate to excellent shielding from Jupiter’s intense trapped radiation. While Io is said to be–in terms of water–the driest world in the Solar System, there should be some water (possibly including chemically bound water) underground, but:
Designing a probe that could survive and function in those thermal (in Io’s volcanic precincts) and surface radiation conditions would be challenging, but there is a way to, in a sense, explore Io without landing there, which would also explore another moon while providing an opportunity to “work up” to exploring Io:
An Amalthea orbiter *and* lander, which could easily explore the whole tiny–about 75 miles long–moon because its gravity is so feeble (at its sub-Jovian and anti-sub-Jovian points, Amalthea’s escape velocity is only about 1 meter per second due to Jupiter’s gravity), could sample its orange surface covering, whose particles spiral inward from Io. A spring- or solenoid-operated “hopper” mechanism (like on the ill-fated Phobos spacecraft’s hopper lander) would enable the lander to sample surface and subsurface materials at many points. Using very little propellant, it could orbit Amalthea just a few tens of meters to a few kilometers from its surface, in order to gather “global” geochemical spectra and topographical images of that irregular world.
Assuming life needs water to evolve in the first place, Io is a low probability object. Constant tidal flexing and attendant vulcanism have desiccated it. If life might have a chance today in lava tubes as some suggest, by what means did life get inside them? There is no known subsurface water layer and surface migration is blocked due to radiation. The arguments aren’t convincing imo.
Here are links to some articles on the subject of Ioian life:
The conclusion doesn’t hold out much hope for anything living on the surface of that volcanic moon, but deep underground in caves could be another matter.
The Galilean moons have been surprising us since 1979, so who knows what awaits us once we finally decide to get really serious exploring them.
Thank you. I was surprised to read that Ionian life might even be somewhat more likely than I’d thought; I didn’t know that Io formed in a water ice-rich zone around Jupiter, and I hadn’t considered the alternative organic solvents that could have gradually replaced water as Io’s surface water leaked away (hydrogen sulfide, sulfur dioxide, and sulfuric acid [even some terrestrial microbes have partly replaced their carbon with sulfur, and can live happily in boiling sulfuric acid]), and:
Io certainly has abundant heat energy to support subsurface life, and underground micro-environments could harbor water ice and liquid water. I’m not suggesting that Io is a more likely abode for life than Europa (or even possibly Jupiter itself), but it seems too relatively likely to overlook.