Day 17: Europa's Ocean
by Youry Aglyamov
As we all know, Earth has an ocean. The question arises: what other places in the Solar System have oceans? Venus and the giant planets are likely to have at least part of their atmosphere in a supercritical fluid state, but it’s not clear whether that counts. Titan has its lakes, but they’re far from being planetary in scale. The most indisputable ocean in the Solar System outside Earth, then, is an ocean on one of the icy bodies. And of those, the most interesting, and (as I understand it) the most proven, is the one on Europa.
Today we talked about proving that ocean’s existence. We started with questions; there was a discussion about why the lecture said Europa had an “iron or iron-sulfur” core, that is, why any light elements in Europa’s core must be sulfur, when we don’t even know the light elements in our own core. The consensus was that sulfur was a best guess. Then, we were given a table of four questions and individually solved the first two.
The first question asked, quite simply, how we know that Europa has an ocean; the second asked why Ganymede’s ocean, sandwiched between two layers of ice (like those of most other icy moons), is less interesting in terms of astrobiology. After working on them individually, we discussed them, first in pairs, and then as a whole class.
The conclusion? That there are no less than five major lines of evidence for Europa’s ocean:
Secondly (because who starts with #1?), water ice dominates the surface.
Thirdly, there are tectonic-ish features on Europa’s surface, suggesting a dynamic planet.
Fourthly—well, the original fourth line was that clay minerals were found on Europa’s surface in impact ejecta, but Professor Brown explained that this was invalid evidence, and that moreover the evidence for clay minerals on Europa’s surface is probably not real. Thus, the fourth line of evidence became the presence of a time-varying magnetic field in a fashion that suggested a conductor near the surface.
Fifthly, density and moment of inertia measurements imply a thick water or ice layer covering Europa to a depth of over 100 km.
And sixthly and finally, tidal forces due to Europa’s orbital eccentricity (which comes from the Galilean satellites other than Callisto being in a resonance and thus preventing Jupiter from circularizing their orbits) should in theory cause heating.
As for Ganymede’s ocean, it’s less interesting simply because it lacks a liquid water to rock interface. Such an interface is great for reactions in ways that a water to ice or ice to rock interface isn’t, because there is both a liquid component and chemical variety. “But wait,” you might say, “why is the water sandwiched anyway? Doesn’t ice float on water?” The answer is that it does, but at high pressure ice adopts a new crystalline structure that sinks. The result is that the low-pressure ice on the surface floats, but the high-pressure ice on the bottom sinks, and boom—sandwich.
And that brings us to the question of how, exactly, time-varying magnetic fields are affected by conductors. We looked at the problem in groups, until eventually Professor Brown reminded us that
* A perfect electric conductor cannot have an electric field inside itself, and therefore by Maxwell’s equations cannot have a time-varying magnetic field;
* Therefore, magnetic field lines will bend around an electric conductor;
* Therefore, an entirely conducting moon will bend the field lines around it;
* A moon with a conducting core will bend the field lines around the core, but because an insulator has no effect on magnetic field lines, will do so to a lesser extent than an entirely conducting moon;
* And a conducting planet with a nonconducting core, although able to have some sort of loop magnetic field in the core, from the exterior will appear magnetically the same as an entirely conducting moon;
* And thus, class time is up.
The habitable zone is dying.
But not actually—the idea of the habitable zone, a comfy Goldilocks annulus precisely the right distance away from the sun to maintain liquid water, is what’s being quietly set aside. Habitability, as it is so defined by the presence of liquid water, is no longer solely a function of a planet’s orbital radius, because we now know of several worlds which are hosts to liquid water—worlds where, based on their distance from the sun, there should only be ice.
These worlds are not actually even planets—they’re moons. Specifically, we’ll be discussing Europa and Ganymede, two Galilean moons of Jupiter. Both Europa and Ganymede are homes to an enormous ocean of liquid water underneath a shell of ice. There are several pieces of evidence for these oceans: density measurements, time-varying magnetic fields, and tidal forcings from Jupiter.
Europa and Ganymede are a lot like a fat and slightly senile person—they’re bulging and eccentric. By having eccentric orbits around Jupiter, they do a kind of dance from Jupiter’s gravity. The pull of the planet squeezes and releases them periodically, causing them to bulge out all over the place. The main effect of this strange dance is that it inputs energy into the moons, heating them up internally such that ice melts and you’ve got a liquid ocean.
On Jupiter’s closest moon, Io, the gravitational tugging and squeezing is even more severe. Thus, Io is the most volcanic place in the solar system, as its interior is so active and hot.
Three of the Galilean moons of Jupiter (Io, Europa, and Ganymede) are in Laplacian resonances with each other. This is why they have such eccentric orbits.
Europa’s ocean is much more interesting than Ganymede’s. On Ganymede, there are layers of ice with oceans sandwiched in between. With no water-rock interface, the organic chemistry needed to create prebiotic conditions can’t happen. Europa, on the other hand, has an ocean on top of a silicate mantle, allowing cool chemical reactions to occur.
Day 18: Final Review
by Alec Brenner
The course has quickly come to an end—today we discussed the logistics of the final exam, as well as the material it would cover. This began with a question from Mike Brown: What unit did you like the best?
A quick survey of the class found a clear favorite: 5 people preferred the giant planets unit (myself included), while 2 each liked the Mars and small bodies units, and only 1 liked life in the universe. Using these as the basis of discussion groups, we did some “rearranging,” until we had groups of a few people each.
The discussion topic for the day was to come up with 2-3 open-ended questions pertaining to each unit that could theoretically be placed on the final exam. These questions would require ~paragraph-length answers.
After ten twenty thirty minutes of deliberation, each group had 2-3 questions. The next order of business was answering them, so we selected a question from each group to pose to the class. They were (approximately):
• What were the geological ages of Mars, when were they, what happened during them, and how do we know?
• Imagine you're conducting a survey for earth-mass habitable exoplanets, and imagine you're using the radial velocity detection method. What conditions/parameters might make a star system most amenable to detection of such an exoplanet?
• What is the composition of a comet, and what does a comet’s composition tell us?
• What kind of evidence is there for life on Mars? What about against life on Mars?
And the class-wide answers, respectively:
• During the Noachian (4.5-3.7 Ga), Mars experienced heavy bombardment, and had a water-covered surface. Magnetic lineations appeared in Noachian terrains (plate tectonics, perhaps?), and hydrothermal processes in deep rocks produced phyllosilicate minerals.
During the Hesperian (3.7-2.5 Ga), Mars began to dry and cool, and experienced fewer impacts. The formation of the Tharsis bulge and associated volcanics drove out groundwater in catastrophic flooding events, producing huge canyon/floodplain features such as Valles Marineris. Clay minerals were deposited at the surface during this interval.
During the Amazonian (2.5 Ga-recent), Mars experienced very little other than minor cratering. It is currently cold, dry, and mostly static.
• A low-luminosity star, such as a red dwarf, would be ideal, as the planet could execute a very close orbit while remaining habitable (i.e., in the liquid water zone). Additionally, a low-mass star would allow for a shorter orbital period, giving astronomers the opportunity to track the RV curve through multiple orbital cycles, thereby greatly improving the signal/noise and data quality. (Also, a close orbit would allow for greater potential for modification/broadening of the habitable zone by tidal heating, but this is a minor effect).
Systems with planets at low inclinations relative to one another, as well as low eccentricities, would be the most stable over secular timescales, and would therefore also be ideal.
A system with a low inclination relative the observer (us) would be preferable, so as to generate the best possible signal (msini).
Finally, a star would give a better signal if it is closer to us (say, within a few tens of light years).
• The canned answer is that comets are “dirty snowballs.” Comets are composed mostly of ices (specifically water – this is known from comparing the point at which comets begin to develop comas to the point at which various ices reach a high vapor pressure – but also ammonia). Also present are significant amounts of carbonaceous and organic dusts and volatiles, which ablate from the comet near perihelion and interact with UV photons to give radicals, which are detectable in a comet’s coma/tail by spectroscopic methods. The icy, volatile compositions of comets, as well as their highly eccentric or unbound orbits, points to a place of formation beyond the frost line, so that they could condense from solidified volatiles in the early solar nebula.
• The labeled release experiment on the Viking Lander suggested the presence of microbes in Martian soil which could metabolize nutrients, although results remain inconclusive (as per the findings of the Phoenix Lander, which discovered perchlorates in the soil which could have destroyed organics when heating samples). Additionally, two findings of the Curiosity Rover suggest the possibility of ancient life: the neutral pH of the soil formed in the ancient lake at Yellowknife Bay, and the presence of a strong hydrogen sulfide signal in a GCMS experiment. Both indicate good growing conditions for life during the Martian Hesperian period. Evidence against the presence of life on Mars includes, among other considerations, the absence of methane in the atmosphere, discrediting claims of methanogenic microbes.
We eventually got to all the questions, and (running out of time) briefly posed the rest of the questions that each group had formulated, before filling out TQFRs and heading off to lunch:
• What evidence is there for a subsurface ocean Europa? Compare it to any subsurface ocean that could(n’t) exist on Ganymede?
• What are the differences between a stable resonant orbit and an unstable one?
• What are all the steps, in order, of gas giant formation?
• Draw a schematic flowchart (equations are not needed) that could be used to calculate heat flux into the Martian subsurface?
• What evidence is there for water on Mars?
by Valerie Pietrasz
For our last day in class, we started reviewing for the dreaded final exam by splitting into groups based on our favorite unit from the class. From there, we looked back at the lectures from our unit and formed questions we thought would make good final questions. The questions we discussed are below:
Small Bodies (My personal favorite):
Q: Briefly describe the composition of a comet and explain what it tells us about its origin.
A: Water & carbon (di)oxide ices, dust & other stuff, radicals in coma. Ice implies that the comets needed to form in an orbit outside the frost line, then become perturbed into the orbits we find them in. Moreover, since they contain these volatiles, they could not have spent a lot of time in their current orbits, or they would have melted as they passed the sun—so they've spent a lot of time in long orbits located far outside the inner solar system (Oort Cloud?)
– How do we know small bodies with unusual orbits didn't start there?
– What is wrong with Mars and how can we explain it?
– Explain the difference between a stable resonant orbit and an unstable one and give examples.
Water on Mars:
Q: Describe the three time periods of Mars, their features, ages, and what we know about them.
A: Noachian [4.1 – 3.7Bya]. We know the Noachian was wet because we see dendritic channels that imply rain. This was the team during which we see the most meteorite impacts.
Hesperian [3.7 – 3.0Bya]. In the Hesperian, we see outflow channels that imply periodic floods of liquid water, as well as the rise of Tharsis and the creation of the volcanic rock we see on much of Mars' crust.
Amazonian [3.0 Bya – now]. The Amazonian is the dry, uninteresting time period we are currently in.
– Draw a schematic of heat flux on Mars that could be used to solve for surface temperature.
– List and describe evidence for water on Mars.
Inside Giant Planets (the class's favorite):
Q: Using the radial velocity method of detection of earth-sized (habitable) planets, what system conditions would be the best for finding the planet?
A: We want a system that is close to us, so we receive more light and can make more precise measurements. We want a small, dim star, so that the variation in light caused by a smaller, earth-sized planet can be detected. Moreover, a planet with a smaller orbit (as opposed to a larger orbit) will create a greater wiggle for us to detect. Lastly, we want to be in the orbital plane of the planet so that the sin(i) term in radial velocity that we get from inclination is maximized.
– Describe all the steps in planet formation.
Life in the Universe:
Q: List three pieces of evidence that Mars could've support life and evidence that it did not.
A: Evidence for life: Liquid water necessary for life implied by drainage/dendritic channels, an atmosphere in the past that could protect life from radiation and regulate surface temperature, and minerals on Mars such as sulfates, chlorates, phosphites/phates that would allow organic chemistry.
Evidence against life: We haven't yet found any organics that suggest past life.
– Give four pieces of evidence for a subsurface ocean on Europa, and compare to the evidence we have for a subsurface ocean on Ganymede. Which is more likely?
And that's that! While I don't look forward to studying for the final, I had tons of fun in this class (planetary science is my major, so no surprise there!) and definitely learned a lot. :)