The Sounds of Europa

by Paul Gilster on May 11, 2017

Although there are no plans at present to send a lander to Europa, we continue to work on the prospects, asking what kind of operations would be possible there. NASA is, for example, now funding a miniature seismometer no more than 10 centimeters to the side, working with the University of Arizona on a project called Seismometers for Exploring the Subsurface of Europa (SESE). Is it possible our first task on Europa’s surface will just be to listen?

The prospect is exciting because what we’d like to do is find a way to penetrate the surface ice to reach the deep saltwater ocean beneath or, barring that, any lakes that may occur within the upper regions of the ice shell. The ASU seismometer would give us considerable insights by using the movements of the ice crust to tell us how thick it is, and whether and where ocean water that rises to the surface can be sampled by future landers.

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Image: Close-up views of the ice shell taken by the Galileo spacecraft show uncountable numbers of fractures cutting across each other. Reddish colors (enhanced in this view) come from minerals in ocean water leaking through the shell and being bombarded by Jupiter’s radiation. The ASU-designed seismometer would land on the shell and detect its movements. Credit: NASA/JPL-Caltech.

Europa’s story is all about tides. The moon (a bit smaller than our own Moon) is constantly being tugged by the large Galilean moons Io and Ganymede, preventing its orbit from circularizing completely. In turn, that small orbital eccentricity allows Jupiter to stress the ice shell. Alyssa Rhoden is an ASU geophysicist working on the SESE project. She points out in this ASU news release that seismometers can tell us how active the ice shell is.

Acknowledging that we’re dealing with a geologically young surface — probably between 50 to 100 million years old, based on crater counts and resurfacing — Rhoden adds: “It may have undergone an epoch of activity early in that period and then shut down.” Equally plausible is the idea that even today the shell is undergoing uplifts and fracturing from below, with the opportunity for ocean water to reach the surface. Recent observations of plumes on Europa, based on Hubble data from 2012 and 2016, support the idea.

Seismometers would help us detect ongoing activity in the shell. ASU envisions a seismometer mounted on each leg of a lander — four to six seismometers in all, depending on lander design. These would be driven deep into the ground, avoiding the kind of loose surface materials that would isolate the instruments from seismic waves passing through the shell. And that calls for the kind of rugged instrument ASU is building. Able to operate at any angle, the prototype can survive landings hard enough to ensure deep penetration for each seismometer.

Edward Garnero is an ASU seismologist who points out that the instrument package will need to sample a wide range of potential vibrations, combining observations from each seismometer to pinpoint the source of seismic activity:

“We can also sort out high frequency signals from longer wavelength ones. For example, small meteorites hitting the surface not too far away would produce high frequency waves, and tides of gravitational tugs from Jupiter and Europa’s neighbor moons would make long, slow waves.”

The sound of Europa? Garnero adds:

“I think we’ll hear things that we won’t know what they are. Ice being deformed on a local scale would be high in frequency — we’d hear sharp pops and cracks. From ice shell movements on a more planetary scale, I would expect creaks and groans.”

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Image: Four sensors arranged in a box measuring about 10 centimeters on a side make up the test module for the SESE project seismometer. The various sensor orientations allow the instrument to work no matter how it lands on the surface. Credit: Hongyu Yu/ASU.

The Seismometers for Exploring the Subsurface of Europa project avoids the mass-and-spring sensor concept used in conventional instruments because that design is delicate enough that it needs to be put in place without any serious jolts and must be installed in an upright position. The SESE seismometer avoids those problems and uses a micro-electromechanical system with a liquid electrolyte as its sensor, offering high sensitivity to a wide range of vibrations.

Finding pockets of water within the upper ice would offer further areas of astrobiological interest and add to the likelihood of nutrients being transported from the ocean to the surface. Thus the findings of a seismometer like this could be crucial for future lander missions. Galileo imagery has shown us long linear cracks and ridges broken by areas of disrupted terrain where surface ice has refrozen. If Europa remains active today, we can use what SESE hears on the surface to predict the best areas for future lander operations.

“We want to hear what Europa has to tell us,” adds Hongyu Yu (ASU School of Earth and Space Exploration), who heads up the project. “And that means putting a sensitive ‘ear’ on Europa’s surface.”

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{ 23 comments… read them below or add one }

Alexander Tolley May 11, 2017 at 11:20

I’m guessing testing on Earth will provide ground truth samples to compare with the Europa data to classify the signals.

On later missions that drill deep into the ice, such sensors could be deposited at various depths should the surface data prove very useful.

While less sensitive, we know that the accelerometers in cell phones can be used to detect earthquake tremors, usually by detecting the same signal in a number of phones and using that data to detect the approximate epicenter. This indicates to me that it may be possible to use abundant, extremely small sensors, to make useful spatial observations of seismic data on celestial bodies.

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J. Jason Wentworth May 12, 2017 at 0:39

Yes–and the SESE seismometers, because they must be so rugged, will also help make “stand-alone” penetrator probes (see: http://www.google.com/#q=planetary+penetrator+probes ) practical. This would be of great benefit for exploring Europa (in concert with “traditional”-type landers) and any other worlds with solid surfaces (and for collecting “three-dimensional” [well-triangulated, that is] seismic data via arrays of penetrators, even on small bodies such as asteroids and comets). Also:

Penetrators that have been tried so far–such as the pair of Deep Space 2 Mars penetrators–haven’t worked (or didn’t make it to their destination impact sites, as in the Mars 96 mission). Such mini-probes can also be equipped with surface analysis instruments and imagers, as were the two Mars 96 penetrators.

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Andrew Palfreyman May 12, 2017 at 2:06

Ballistic penetrator or slow melt drill?

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J. Jason Wentworth May 12, 2017 at 9:09

On an icy (or ice-covered) world, either option–a ballistic penetrator or a slow melt drill–could be used. The choice would depend on cost and the exploration objectives, although a mixture of both types could be used, too.

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Michael May 12, 2017 at 3:04

These small sensors or lab on a chip could be attached to the surface of the seismometer to look at organic material. I would also like to see a number of very reflective plates, very thin, dropped all over the surface so the orbiting spacecraft can map the up and down and side ways motions of the surface as the other moons and Jupiter squeeze it. May be it would be a good idea to see how much the cracks open and close which could be used to narrow the error on the thickness of the ice sheet.

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J. Jason Wentworth May 12, 2017 at 9:34

That would work (and would be very useful), but corner reflectors could be smaller and would enable very precise measurements to be made by means of lasers aboard Europa orbiters (or elsewhere, as is described below). This was/is done with Earth-based lasers, using the corner reflectors (retroreflectors) that were placed on the Moon by the Apollo and Lunokhod (Soviet robotic rover) missions. Also:

This tidal measurement tool could also be used on Io (until the retroreflectors got coated with ejecta from the volcanoes there; that might take a while, depending on where they were emplaced). The laser(s) could be located in orbit around Jupiter beyond its radiation belts, or it/they could be landed (or set up) on Callisto, the outermost of the Galilean moons. (If Callisto isn’t far enough from the worst radiation, Themisto or Leda could be used.) Except for the spacecraft that dropped or set up the retroreflectors on Io, this would be perhaps the only Io-based instrument that wouldn’t get fried by the intense radiation there.

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Michael May 12, 2017 at 16:16

It may be as simple as using the radio communication system to reflect of these reflectors which would more likely be mesh like, perhaps graphene membranes coated with reflective material would work, and survive high g impacts.

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J. Jason Wentworth May 13, 2017 at 20:36

Yes–compared with the relatively low radio reflectivity of the natural ice, frozen mud, or dirt surface, a fine metal mesh or a metallized plastic film dropped onto the surface would produce a very bright echo (a metallized plastic film would be very optically reflective as well). Dropped onto the surfaces of airless worlds, these “markers” would enable seismic and tidal land motions to be measured by reflecting radio and/or laser beams off them. Also:

A small, spherical (because it permits using thinner-walled, lighter cases) solid propellant retro-rocket could slow down such a mesh or film package to ensure a gentle landing on the object’s surface. For deployment from penetrator probes (where surviving the impact deceleration loads and compact “packaging” are important design factors), some sort of metallic crystal powder, which the penetrators could disperse upon impact, might serve as radio- and/or laser-
reflective “marker patches.”

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Michael May 15, 2017 at 3:11

The metallic patch idea has some merit, perhaps these membranes and powders could be embedded in the solid fuel used to slow a penetrator down. I am also thinking of the idea of using a collapsible solid fuel motor as the impactor, should take a lot of energy of impact and reduce the high ‘g’ loading phase.

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J. Jason Wentworth May 15, 2017 at 22:47

I seem to vaguely recall a solid rocket motor or a flare with a powdered metal (not just the commonly-used powdered aluminum solid rocket fuel, or the powdered magnesium flare fuel) added to the un-cured propellant, which left tiny molten droplets or solid particles in the exhaust that gave a bright radar echo. Something like that could deposit a reflecting patch on the surface of a celestial body. Also:

By “collapsible solid fuel motor,” do you mean a rocket motor case that would deform on impact to absorb much of the energy (like the “crush zones” in car bodies that crumple on impact, to absorb the energy)? That should work very well, using a retro-rocket motor case made of a suitable alloy (or alloys) that would crumple on impact. The retro-rocket-equipped spherical “hard-landing” seismometer capsules that were carried aboard the Ranger 3 – 5 lunar impact probes (none of which delivered their hard-landing capsules to the lunar surface at a survivable impact velocity) used a rather similar approach. The exterior of each capsule was sheathed in a thick layer of crush-able balsa wood (its outside surface was painted black and white to moderate the capsule’s temperature), which would have absorbed the impact energy.

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Michael May 16, 2017 at 9:37

‘By “collapsible solid fuel motor,” do you mean a rocket motor case that would deform on impact to absorb much of the energy (like the “crush zones” in car bodies that crumple on impact, to absorb the energy)?’

Yes- If we say have the solid rocket motor in a concertina design which is very strong under pressure when the motor is firing and collapses under impact. The electronic unit can be at the back so takes a lot less damage as the motor is compressed or it could have a sharper edge to fly through the middle of the motor absorbing energy.

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J. Jason Wentworth May 16, 2017 at 21:10

Thank you for confirming that. Hmmm…your suggestion (about providing the payload with a sharper edge, to fly through the middle of the [collapsing] motor case to absorb impact energy) could enable penetrator probes to survive landings at higher velocities, and enable them to survive landing on harder surfaces (such as granite, or solid-metal asteroids’ surfaces). A sensor-equipped spike could cut down through the “re-accordionizing,” crumpling retro-rocket motor case (and then down into the body’s surface); also, the retro-rocket would relieve the carrier spacecraft of much of the task (which would require much more propellant) of slowing down to ensure a survivable penetrator impact velocity.

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Michael May 17, 2017 at 10:27

I like your idea of a probe drilling down through the broken material field underneath the impact site, it may not need to drill much as the material will be well and truly broken.

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J. Jason Wentworth May 18, 2017 at 0:36

Thank you. The two Deep Space 2 penetrator probes (see: http://www.google.com/#q=Deep+Space+2 ) were designed sort of like that.
Each one consisted of a rather cup-shaped, “surface retention” after-body containing the transmitter; this was fitted with a hemispherical-tipped, instrumented cylindrical “spike” that was to penetrate some distance (up to 2 feet) into the ground, trailing a cable back up to the after-body.
This arrangement, which utilized the penetrator’s impact velocity, made a drill unnecessary (although the “spike” forebody contained a small, horizontal, sample-collecting drill).

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Melissa May 17, 2017 at 13:20

In his book ” 2010″ , Arthur C Clarke described the global ocean of Europa as being noisier than Earth’s ocean due to the stronger global tidal forces. Could Clarke be right in that there would be more noisy sounds due to things like submarine earthquakes and hissing escaping gases due to the strong tidal influence of Jupiter?

I would think that whatever multicellular life is in Europa’s ocean, if such life was as evolved as fish or squid….that such alien life would have developed hearing and the alien equivalent of ears/auditory organs. Europan life would possibly be able to hear the noises of the Europan ocean.

If we were to drill into the ice of Europa and send a submarine probe down there, it could possibly disturb and scare off the Europan life forms by making a lot of noise when it tunnels to the moon’s ocean. That is, if there are Europan equivalents of fish and squid.

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Melissa May 17, 2017 at 13:29

I would be very worried about disturbing the normal behavior of any multicellular animals such as fish we might find in Europa’s ocean by putting a submarine probe in said ocean. This would be an equivalent of sending robots to study an undersea vent, because such machines would not be a normal event in the everyday life of aquatic/marine animals. The animals could react to the robotic probe in an unusual way, which would complicate observation of their normal behavior.

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J. Jason Wentworth May 18, 2017 at 0:48

Europa’s ice surface might also be noisy, also due to the tidal “tug-of-war” between Jupiter and the other Galilean satellites. On Earth, large masses of ice that are subjected to stress (such as “calving” icebergs, before they break free) or press against each other produce almost painfully loud “singing” sounds. Sections of Europa’s ice crust may also do this. Also:

One of the subsurface exploration proposals involves a robotic submarine that melts its way through the ice crust, which should be quieter than drilling.

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Michael May 20, 2017 at 17:50

It is this noise that may be very useful in giving us information about the crust thickness and ocean depth.

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J. Jason Wentworth May 21, 2017 at 0:49

Indeed–such natural oceanic and/or ice crust noises could be utilized as the signal sources for a “bi-static SONAR” system. Also, being natural sounds, these noises wouldn’t affect any Europan oceanic lifeforms differently, as artificially-produced SONAR pulses might, as Melissa mentioned above. (Any creatures might still retreat from sources of natural ocean sounds–either to avoid danger [or what they perceive as being threatening; maybe predators “mask” their presence via such noise?], or to avoid the sheer loudness of the sound, but they would still be reacting normally to natural sounds of their environment.)

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Michael May 22, 2017 at 2:39

I love this idea, really simple, leave the power source on the surface and use fibre optic lasers to melt your way through the ice and a great communication path and camera system.

https://www.wired.com/2012/04/bill-stone-laser-powered-europa-rover/

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Michael May 22, 2017 at 14:28

Thinking a bit more about this technique perhaps we could have a smaller lander version. The fibre optic laser could be built very thin and allow us to melt a few meters down into the ice, the sides of part of the fibre can be used to look at the material to the sides and sample it as well.

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J. Jason Wentworth May 23, 2017 at 5:28

We already have high-velocity versions of these devices: anti-tank missiles. While the early ones (such as TOW–Tube-launched, Optically-tracked, Wire-guided) used trailing wires that enabled the shooter to keep the missile on target by keeping the optical sight of the launcher trained on the target, newer anti-tank missiles use trailing fiber optic cables (some of these missile systems use a television camera in the missile itself). They now have ranges of at least 9.3 miles (15 km), see: http://www.designation-systems.net/dusrm/m-157.html ! This is one technology that–if used in “sub-ice” probes (with cameras) deployed from Europa landers–appears to be ready to use for this application.

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Michael May 23, 2017 at 13:01

We could perhaps drill with a UV laser fibre, the ice would photo decompose and not refreeze in hole, the last bit we could use an infrared laser to burrow a little deeper without damaging organics.

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