APOD: Galileo's Europa (2022 Jan 15)

[APOD Robot]

Galileo's Europa

Explanation: Looping through the Jovian system in the late 1990s, the Galileo spacecraft recorded stunning views of Europa and uncovered evidence that the moon's icy surface likely hides a deep, global ocean. Galileo's Europa image data has been remastered here, with improved calibrations to produce a color image approximating what the human eye might see. Europa's long curving fractures hint at the subsurface liquid water. The tidal flexing the large moon experiences in its elliptical orbit around Jupiter supplies the energy to keep the ocean liquid. But more tantalizing is the possibility that even in the absence of sunlight that process could also supply the energy to support life, making Europa one of the best places to look for life beyond Earth. What kind of life could thrive in a deep, dark, subsurface ocean? Consider planet Earth's own extreme shrimp.

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[orin stepanek]

Hmmm! I wonder what that Europan shrimp would taste like?
Can you imagine it having more water than all of Earth?

[neufer]

Hmmm! I wonder what that Europan shrimp would taste like?

As decapods, Europan shrimp can probably count to ten.

However, as prey to cephalopods (and Forest Gump) they will never be the most intelligent of species.

Click to play embedded YouTube video.

<<The Decapoda or decapods (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, shrimp and prawns. Most decapods are scavengers. The order is estimated to contain nearly 15,000 species in around 2,700 genera, with around 3,300 fossil species. Nearly half of these species are crabs, with the shrimp (about 3,000 species) and Anomura including hermit crabs, porcelain crabs, squat lobsters (about 2500 species) making up the bulk of the remainder. Decapods originated in the Late Ordovician around 455 million years ago, with the Dendrobranchiata (prawns) being the first group to diverge. The remaining group, called Pleocyemata, then diverged between the swimming shrimp groupings and the crawling/walking group called Reptantia, consisting of lobsters and crabs. High species diversification can be traced to the Jurassic and Cretacious periods, which coincides with the rise and spread of modern coral reefs, a key habitat for the decapods.>>

• Prawn vs. Prains: After Dorothy and her friends have completed their mission, the Wizard gives the Scarecrow brains (made out of BRAN, PINS and needles, as he has been the most intelligent of the group all along).

<<Dendrobranchiata is a suborder of decapods, commonly known as prawns. There are 540 extant species in seven families, and a fossil record extending back to the Devonian. They differ from related animals, such as Caridea and Stenopodidea, by the branching form of the gills and by the fact that they do not brood their eggs, but release them directly into the water. They may reach a length of over 330 millimetres (13 in) and a mass of 450 grams (1.0 lb), and are widely fished and farmed for human consumption. While Dendrobranchiata and Caridea belong to different suborders of Decapoda, they are very similar in appearance, and in many contexts such as commercial farming and fisheries, they are both often referred to as "shrimp" and "prawn" interchangeably. In the United Kingdom, the word "prawn" is more common on menus than "shrimp", while the opposite is the case in North America. The term "prawn" is also loosely used to describe any large shrimp, especially those that come 15 (or fewer) to the pound (such as "king prawns", yet sometimes known as "jumbo shrimp"). Australia and some other Commonwealth nations follow this British usage to an even greater extent, using the word "prawn" almost exclusively. When Australian comedian Paul Hogan used the phrase, "I'll slip an extra shrimp on the barbie for you" in an American television advertisement, it was intended to make what he was saying easier for his American audience to understand, and was thus a deliberate distortion of what an Australian would typically say.>>

[johnnydeep]

So many questions to try to answer by reading more...that is, should the others here not clue me in first

- When we say "approximate what the human eye would see", what do we mean? When the object - in this case, Europa - is receiving the same amount of sun as we do on earth (or perhaps as the moon does, with it's lack of atmosphere to interfere)?

- Those "extreme shrimp" live in an incredibly hot environment - 400 degrees C from the link!. Just how hot can it get at the lower ocean surface boundary on Europa?

- And about that tidal flexing caused heating: I always find it amazing that it can provide so much energy, and I would guess for quite a long time too. Are the tidal forces due to Europa not being tidally locked with one side always facing Jupiter? Just what's causing the flexing anyway? I know it's due to the difference in the strength of gravity of Jupiter depending on the distance of the two sides of Europa from it. Is that all that's going on? Being tidally locked would put a stop to that changing across the surface of Europa over time, right?

Oh well. Just some Idyll thoughts for a lazy Saturday afternoon in sub-freezing - and I assume not currently tidally heated - NY.

[Chris Peterson]

So many questions to try to answer by reading more...that is, should the others here not clue me in first :)

- When we say "approximate what the human eye would see", what do we mean? When the object - in this case, Europa - is receiving the same amount of sun as we do on earth (or perhaps as the moon does, with it's lack of atmosphere to interfere)?

There's less sunlight. But that doesn't much matter for how our eyes see things, because we compensate. The lighting in an office is orders of magnitude dimmer than outdoor lighting, but we don't really notice a difference. I think it just means that the color and contrast are similar to how it would look to our eyes.

- Those "extreme shrimp" live in an incredibly hot environment - 400 degrees C from the link!. Just how hot can it get at the lower ocean surface boundary on Europa?

The shrimp will die if they get into 400° water. They live in water that ranges from about freezing to well under boiling.

- And about that tidal flexing caused heating: I always find it amazing that it can provide so much energy, and I would guess for quite a long time too. Are the tidal forces due to Europa not being tidally locked with one side always facing Jupiter? Just what's causing the flexing anyway? I know it's due to the difference in the strength of gravity of Jupiter depending on the distance of the two sides of Europa from it. Is that all that's going on? Being tidally locked would put a stop to that changing across the surface of Europa over time, right?

It is tidally locked. But its inclination isn't quite zero, and its eccentricity isn't quite zero. So there are stresses as it orbits and distances change.

[MoonRockMan]

Does anyone know what the scars/missing chunks at the bottom left of the image are?

[rj rl]

To follow up johnnydeep's question: I imagine the amount of energy needed to heat an entire planet's worth of ice to melt should be quite substantial. Does it noticeably alter Europa's orbit? Shouldn't it be decaying?

[Ann]

Hmmm! I wonder what that Europan shrimp would taste like?
Can you imagine it having more water than all of Earth?

All the Earth's water (large drop), Earth's total amount of liquid fresh water, and the water in rivers and lakes (smallest drop). Credit: Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution.

---

It's not hard imagining a planet with more water than the Earth. What is unique about the Earth in the Solar system is that most of the Earth's water is liquid and located on the surface of our planet. Moreover, much of Earth's water is quite shallow, so that water, land and relatively bright sunlight meet, and so many life forms love that.

Ann

[neufer]

To follow up johnnydeep's question: I imagine the amount of energy needed to heat an entire planet's worth of ice to melt should be quite substantial. Does it noticeably alter Europa's orbit? Shouldn't it be decaying?

The standard rule of thumb is that a moon (like our own) that orbits a planet slower than the planet rotates will kick up a tidal bulge on the planet that precedes the moon and thereby also pulls that moon along raising it's orbit. Since this effect is strongest at periapsis it presumably also kicks the moon into a higher apoapsis thus also increasing the ellipticity of the orbit.

Europa currently has the highest ellipticity of Jupiter's Galilean moons resulting in an 85 hour tidal flexing that is about hundred times our own Earth's 12 hour tidal flexing.

<<Tidal heating occurs through the tidal friction and tidal flexing processes caused by tidal acceleration: orbital and rotational energy are dissipated as heat in the core of the moon, the internal ocean, and the ice crust. Ocean tides are converted to heat by frictional losses in the oceans and their interaction with the solid bottom and with the top ice crust. In late 2008, it was suggested Jupiter may keep Europa's oceans warm by generating large planetary tidal waves on Europa because of its small but non-zero obliquity. This generates so-called Rossby waves that travel quite slowly, at just a few kilometers per day, but can generate significant kinetic energy. For the current axial tilt estimate of 0.1 degree, the resonance from Rossby waves would contain 7.3×10¹⁸ J of kinetic energy, which is two thousand times larger than that of the flow excited by the dominant tidal forces. Dissipation of this energy could be the principal heat source of Europa's ocean.

Tidal flexing kneads Europa's interior and ice shell, which becomes a source of heat. Depending on the amount of tilt, the heat generated by the ocean flow could be 100 to thousands of times greater than the heat generated by the flexing of Europa's rocky core in response to gravitational pull from Jupiter and the other moons circling that planet. Europa's seafloor could be heated by the moon's constant flexing, driving hydrothermal activity similar to undersea volcanoes in Earth's oceans.

Experiments and ice modeling published in 2016, indicate that tidal flexing dissipation can generate one order of magnitude more heat in Europa's ice than scientists had previously assumed. Their results indicate that most of the heat generated by the ice actually comes from the ice's crystalline structure (lattice) as a result of deformation, and not friction between the ice grains. The greater the deformation of the ice sheet, the more heat is generated. In addition to tidal heating, the interior of Europa could also be heated by the decay of radioactive material (radiogenic heating) within the rocky mantle. But the models and values observed are one hundred times higher than those that could be produced by radiogenic heating alone, thus implying that tidal heating has a leading role in Europa.

The Hubble Space Telescope acquired an image of Europa in 2012 that was interpreted to be a plume of water vapour erupting from near its south pole. The image suggests the plume may be 200 km high. It has been suggested that if they exist, they are episodic and likely to appear when Europa is at its farthest point from Jupiter, in agreement with tidal force modeling predictions. In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated critical analysis of data obtained from the Galileo space probe, which orbited Jupiter between 1995 and 2003. Galileo flew by Europa in 1997 within 206 km of the moon's surface and the researchers suggest it may have flown through a water plume. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon. The only other moon in the Solar System exhibiting water vapor plumes is Enceladus. The estimated eruption rate at Europa is about 7000 kg/s compared to about 200 kg/s for the plumes of Enceladus. If confirmed, it would open the possibility of a flyby through the plume and obtain a sample to analyze in situ without having to use a lander and drill through kilometres of ice.

In November 2020, a study was published in the peer-reviewed scientific journal Geophysical Research Letters suggesting that the plumes may originate from water within the crust of Europa as opposed to its subsurface ocean. The study's model, using images from the Galileo space probe, proposed that a combination of freezing and pressurization may result in at least some of the cryovolcanism activity. The pressure generated by migrating briny water pockets would thus, eventually, burst through the crust thereby creating these plumes. In a press release from NASA's Jet Propulsion Laboratory referencing the study, these suggested sources for Europa's plumes would potentially be less hospitable to life. This is due to a lack of substantial energy for organisms to thrive off of, unlike proposed hydrothermal vents on the subsurface ocean floor.>>

[rj rl]

To follow up johnnydeep's question: I imagine the amount of energy needed to heat an entire planet's worth of ice to melt should be quite substantial. Does it noticeably alter Europa's orbit? Shouldn't it be decaying?

The standard rule of thumb is that a moon (like our own) that orbits a planet slower than the planet rotates will kick up a tidal bulge on the planet that precedes the moon and thereby also pulls that moon along raising it's orbit. Since this effect is strongest at periapsis it presumably also kicks the moon into a higher apoapsis thus also increasing the ellipticity of the orbit.

hmm, but here it says the orbit approaches circular as time goes by

[johnnydeep]

Does anyone know what the scars/missing chunks at the bottom left of the image are?

I presume you are referring to these:

Depressions on Europa

Not sure exactly, but they seem to be uncommon. Localized "sink holes" in the ice of some sort I guess.

[neufer]

To follow up johnnydeep's question: I imagine the amount of energy needed to heat an entire planet's worth of ice to melt should be quite substantial. Does it noticeably alter Europa's orbit? Shouldn't it be decaying?

The standard rule of thumb is that a moon (like our own) that orbits a planet slower than the planet rotates will kick up a tidal bulge on the planet that precedes the moon and thereby also pulls that moon along raising it's orbit. Since this effect is strongest at periapsis it presumably also kicks the moon into a higher apoapsis thus also increasing the ellipticity of the orbit.

hmm, but here it says the orbit approaches circular as time goes by

It does indeed say that... and it makes no sense to me.

[johnnydeep]

The standard rule of thumb is that a moon (like our own) that orbits a planet slower than the planet rotates will kick up a tidal bulge on the planet that precedes the moon and thereby also pulls that moon along raising it's orbit. Since this effect is strongest at periapsis it presumably also kicks the moon into a higher apoapsis thus also increasing the ellipticity of the orbit.

hmm, but here it says the orbit approaches circular as time goes by

It does indeed say that... and it makes no sense to me.

This link has a huge bunch of math which is too deep for me, but maybe not for you - http://astro.pas.rochester.edu/~aquille ... al.key.pdf

Starting on slide 25 of the 57 slide powerpoint. Also found an all words explanation here:

https://yarchive.net/space/orbits/tidal ... arize.html

[neufer]

hmm, but here it says the orbit approaches circular as time goes by

It does indeed say that... and it makes no sense to me.

This link has a huge bunch of math which is too deep for me, but maybe not for you - http://astro.pas.rochester.edu/~aquille ... al.key.pdf

Starting on slide 25 of the 57 slide powerpoint. Also found an all words explanation here:

https://yarchive.net/space/orbits/tidal ... arize.html

Ah, very good!

So it is the tidal bulge of the moon which is driving the orbital changes here.

[rj rl]

hmm, but here it says the orbit approaches circular as time goes by

It does indeed say that... and it makes no sense to me.

This link has a huge bunch of math which is too deep for me, but maybe not for you - http://astro.pas.rochester.edu/~aquille ... al.key.pdf

Starting on slide 25 of the 57 slide powerpoint. Also found an all words explanation here:

https://yarchive.net/space/orbits/tidal ... arize.html

great explanation in the second link, very clear. Thank you guys, I learned something new! One interesting question that remains is what are the timescales for this kinda circularization. Life could arise on a Europa-like world only to be frozen solid a few hundred million years later.

[neufer]

great explanation in the second link, very clear. Thank you guys, I learned something new! One interesting question that remains is what are the timescales for this kinda circularization. Life could arise on a Europa-like world only to be frozen solid a few hundred million years later.

Like it says:

<<When the moon is also raising tidal bulges in the planet,
the situation gets distinctly more complicated, of course...>>

The 4 Galilean moons are also interacting with each other
(as well as with Jupiter's strong magnetic field).

That being said (and taking into account Chaos Theory)
I doubt that anything specific can said for sure about
what evolves qualitatively ... much less quantitatively.

[MoonRockMan]

Does anyone know what the scars/missing chunks at the bottom left of the image are?

I presume you are referring to these:


Depressions on Europa.JPG


Not sure exactly, but they seem to be uncommon. Localized "sink holes" in the ice of some sort I guess.

Exactly. They're intriguing; thanks for your thoughts on them