PRE2019 1 Group2

- Group -

• Kasper Dols - 0953689
• Marco Luijten - 1008931
• Wouter Meekes - 1011988

Main tutor: Tijn Borghuis

• Week 1
• Week 2
• Week 3
• Week 4
• Week 5
• Week 6

Mission to Europa

Introduction

Europa is a very interesting, mysterious moon of Jupiter, discovered by Galileo Galilei in the year 1610. The moon raised a lot of interest in the past couple of decades, because there are some indications of liquid water on the moon. Since water is at the top of the list of ingredients that make life possible, the speculations for extraterrestrial life on Europa began to rise. Water dissolves nutrients for organisms to eat, transports important chemicals within living cells and allow those cells to get rid of waste. [1] But due to the circumstances at Europa, the water is believed to be hidden underneath a thick coat of ice. This coat is estimated to be 10 kilometers around the whole moon, with a deviation of 160. [2] Calculations will be performed using this 10 km; the 160 m deviation will be considered negligible with respect to 10 km.

Presence of liquid water on the surface

But can there be water present in liquid form somewhere at the surface of Europa? Probably not. One of the reasons to assume this, is based on the phase diagram of water, shown to the right. As can be seen in the image, the lowest pressure at which water can still exist in liquid form is its triple point at 611.73 Pa (0.0061 atm), at the usual temperature of 273.15 K (0 °C). Below that pressure, water has no liquid form. Since the pressure at Europa’s surface is about 10^-10 atm, this means that liquid water can not stably exist on the surface of Europa. Some water may come to surface for a brief moment, but will almost instantaneously either freeze or boil, leaving no water remaining. It should be noted that indeed this diagram does not extend below 10^-5 atm, and that based on this image it is thus technically not possible to say that water does not have a liquid form at such ultra-low pressures. However, it is first of all unlikely that such an out-of-place phase change exists based on this and other phase diagrams. Secondly, this ‘liquid’ may not be liquid as we know it and still be unable to support life. Much like solid water has different crystalline structures at different temperatures and pressures, so can this liquid water have very different properties based on the environment it is in. Hence based purely on physical grounds it is unlikely that liquid water in a familiar form exists on the surface of Europa. [3]

Presence of a sub-surface ocean

Why do researchers believe there is an sub-surface ocean? The first theories that the planet has a sub-surface ocean came after the fly-by mission of Voyager 1. This spacecraft was, in march 1979, the first spacecraft that made images in significant detail of Europa’s surface, with a resolution of about 2 kilometers per pixel. These images revealed a surprisingly smooth surface, brighter than that of earth’s moon, crisscrossed with numerous bands and ridges. Researchers noted that some of the dark bands had opposite sides that matched extremely well, comparable to pieces of a jigsaw puzzle. These cracks had separated, and dark, icy material appeared to have flowed into the opened gaps, suggesting that the surface had been active at some time in the past. The images also showed only a handful of big craters, which are expected to build up over billions of years as the planetary surface is bombarded by meteorites, until the surface is covered in craters. Thus, a lack of much craters suggested that Europa’s surface was relatively young and implied that something erased the craters, such as icy, volcanic flows. Next to that, scientists found patterns of some of the longest linear features in the images that did not match the predicted patterns of the features, created by tides as Europa orbits Jupiter. They determined that the found patterns would fit very well if Europa’s surface could move independently and was not locked to the rest of the interior. These interesting findings led to the next mission to Europa, Galileo. This spacecraft was launched in 1989 and entered orbit around Jupiter in 1995. Galileo eventually made 12 close flybys of the icy moon, including images of Europa at a range of scales, revealing new details about the surface and providing context for how those details were related to the moon as a whole. One important measurement made by the Galileo mission showed how Jupiter’s magnetic field was disrupted in the space around Europa, implying that a special type of magnetic field is being created within Europa by a deep layer of some electrically conductive fluid beneath the surface. Scientists believe, based on Europa’s icy composition, that the most likely material to create this magnetic signature is a global ocean of salty water. Above described are four strong indications of a sub-surface ocean on Europa, which is why the common belief under scientists is that the ocean really exists. [4]

Indications for life

The three basic requirements for life to be present are liquid water, chemical building blocks and a source of energy. The first requirement is explained in previous paragraph. The second requirement, the chemical building blocks, are also believed to be partly present. The ice and other materials on Europa’s surface are bombarded with radiation from Jupiter, that could alter them into some of the chemical building blocks of life, like oxygen (O₂), hydrogen peroxide (H₂O₂), carbon dioxide (CO₂) and sulfur dioxide (SO₂). If these compounds reach the sub-surface ocean, they can be valuable nutrients to start and sustain life. Besides, the ocean water can react with the rocks and minerals of the subsurface ocean’s floor to liberate other nutrients to support life. The third requirement is a source of energy. Europa’s position in space is within the powerful gravitational field of Jupiter, causing the moon into an orbit with one hemisphere constantly facing Jupiter. This elliptical orbit takes Europa alternatingly closer to and further away from Jupiter. This constant increase and decrease of gravitational force on Europa results in elongating and relaxing of the moon with each trip around the planet. This internal movement, combined with gravitational forces caused by neighboring moons, produces internal friction and heat within Europa. This internal heat could be the energy source that keeps the subsurface ocean from freezing and sustains any life that exists there. Next to that, there could be hot water vents on the floor of the subsurface ocean that deliver energy and nutrients from the planet’s interior. On earth, organisms have been discovered in the subglacial lakes of Antarctica and in hot ion-rich waters of hydrothermal vents. Life in Europa’s sub-surface ocean could be supported in a similar way. [5] These indications for life in Europa’s ocean have led to a future mission of NASA to the moon. They planned to launch the Europa Clipper mission in 2025. The spacecraft will conduct an in-depth exploration of Europa, investigating whether the moon could harbor conditions suitable for life. [6]

The goal

The above described mission of NASA is of course very interesting, but with the strong indications for life as described above, the interest rises to search for life on the spot. Since the presence of liquid water at Europa’s surface is unlikely as explained, the goal of this project is as follows:

“Investigate whether it is possible to land on Europa, dig through the icy layer and send a submarine into the sub-surface ocean, to search for life, signs of life, or conditions that may support life in or on Europa.”

Users

The question who is helped by going to Europa starts by asking why anyone would want to go to Europa in the first place. Ultimately, the humanity wants to learn stuff. In particular, the search for life outside earth. This can teach the humanity about the origin of life, and help to answer the age-old question: “Is there other life in this universe?” The reason to go to Europa and not just any other satellite in the solar system (possibly much closer) is that Europa is very likely to contain liquid water, which is one of the prerequisites for biological life, like explained in the introduction. The first most obvious question to ask then: “Is there (the possibility of) life on Europa?” This is what the mission first and foremost should answer. Furthermore, knowledge about Europa can help to learn about other exoplanets. By comparing our long-distance observations of Europa to the on-site observations, long-distance observations of exoplanets can be translated to planetary conditions. This may allow a more accurate prediction whether an exoplanet may be habitable. Lastly, a mission to put a lander on a planetoid like Europa has never been undertaken, and hence going to Europa will be a proof of concept showing that it is possible to go such a hostile environment. This is convenient information for a possible similar mission to, for instance, Pluto or an exoplanet. In the end, it is unknown what will be found on Europa. Maybe it contains about as much life as the centre of the sun, maybe it will show that life would be possible but never sprung up, or perhaps it turns out that it is home of the Atlanteans, who sunk their city on earth when they found earth with its dense atmosphere and high temperatures would not make for a habitable colony. Either way, it is also important to take into account the lives that might be encountered on Europa. It would be a pity to find all new types of bacteria on Europa, only to kill them with a stowaway extremophile hidden on the lander. Sterilised equipment

The users can largely be divided into 3 categories:

• Those executing this and other missions (SA’s)
• Those processing and using the results (scientists)
• Those potentially found during the missions (life)

The vehicle must be brought to Europa in the first place. SA’s will want a solution for that. Since direct communication over this distance is not possible, SA’s will want to be able to send commands to the vehicle such as ‘Go there’ or ‘Investigate this’, which the vehicle will carry out autonomously. For ‘investigate this’-commands, the vehicle should be able to recognise ‘this’ (‘this’ being whatever object it was instructed to investigate) and know how to investigate ‘this’. In ‘Go there’-commands, the vehicle should be able to know where it is on Europa and where its destination is. Furthermore, it should travel the distance and avoid or clear any obstacles it may come across. Furthermore, SA’s will want to be kept up-to-date on how the vehicle is doing. It should be capable of sending status updates to mission control about its own state. Furthermore, if something is found to be wrong, an ability to repair the vehicle could possibly save the mission. This updating will also give information to people planning a similar mission, about the feasibility and problems that are yet to be overcome. To avoid having to restart the mission on a monthly basis to accomplish the mission goals, some longevity on the vehicle is required. Both the energy and durability should last for a minimum t.b.d. period of time.

Scientists will want information on Europa itself; the chemical makeup of the crust, the atmosphere and the subsurface ocean, and the terrain of the crust. They will also want information on whether life exists there and/or could exist. This information will also help in the search for other habitable planets. For instance, measurements of Europa’s atmospheric density are done in terms of the column density (which counts the number of particles in a column with a particular ground surface area reaching all the way up into space), rather than the density of the atmosphere at surface level. Now, with a lander, the density of the atmosphere at surface level can be determined. This will yield a comparison between column density and surface density, which can be used for estimating the surface density of exoplanets based purely on column density. This may in the long run allow to find new planets to colonise, to redistribute the human load on the earth.

In case there is sentient life on Europa, they will most likely want to not be massacred. (This is deduced from the simple fact that if they are a civilization that would - for whatever reason - like to be massacred, they would’ve massacred themselves already.) Some form of communication is required. Furthermore, mission command will want them not to destroy the vehicle. For that, there is hope that they will not.

Space agencies:

• People responsible for the journey to Europa
  • Vehicle operators
  • Executives for other missions
• Scientists
  • Astronomers
  • Biologists
  • Biohistorians (that’s a profession now)
  • Humanity/ sociologists
• Life
  • Civilizations

User Requirements

These are the requirements based on what the different users want.

External

• 1 Get to Europa
• 2 Build vehicle
  • 2.1 Assuming that the mission is paid for, the builders will build it, so long as it is legal

SA’s

• 3 Command vehicle
  • 3.1 Autonomous execution of following category of commands
    • 3.1.1 Go there
    • 3.1.2 Investigate this
• 4 Longevity
  • 4.1 Sufficient energy
  • 4.2 Sufficient durability

Scientists

• 5 Info on Europa
  • 5.1 Atmosphere
    • 5.1.1 Ionosphere; plasma density, magnetic field, current
    • 5.1.2 Density and pressure
  • 5.2 Subsurface ocean
    • 5.2.1 Density
    • 5.2.2 Viscosity
    • 5.2.3 Salinity
  • 5.3 Crust
    • 5.3.1 Terrain
  • 5.4 All environments
    • 5.4.1 Chemical makeup
• 6 Info on life
  • 6.1 Possibility
    • 6.1.1 Required chemicals
    • 6.1.2 Required environment
  • 6.2 Itself
    • 6.2.1 Chemical makeup
    • 6.2.2 Habitat (Link data of life to data of the habitat)
    • 6.2.3 Enzymes

Life

• 7 Do not go all genocide on it
  • 7.1 Preserve habitat (as little perturbing as possible)
  • 7.2 Non-lethal research methods
  • 7.3 Sterilised equipment

User Preferences

These are the preferences based on what the different users would like, given unlimited resources.

• Longevity
  • 4.3 Keep up-to-date on vehicle status
    • 4.3.1 Recognise and report on faulty equipment
    • 4.3.2 Possibly repair faulty equipment

• 6 Info on life
  • 6.2 Itself
    • 6.2.4 DNA
    • 6.2.5 Complexity

Constraints

These are the constraints resulting from the implications of the user requirements and preferences. For instance, 'surviving Europa' implies being able to operate at temperatures between 86 and 132 K.

External

• 1 Get maximum capacity with falcon heavy
  • 1.1 Must fit inside cylindrical capsule: (L=13.1 m, r=2.6 m)
  • 1.2 Must be under 3500 kg (possibly a bit more, but if at all it is negligible)
• 2 Must be legal

SA’s

• 3 Command
  • 3.1 Autonomous execution
    • 3.1.1 'Go there'
      • 3.1.1.1 Know current and destiny locations
      • 3.1.1.2 Move
      • 3.1.1.3 Recognise obstacles on the way
    • 3.1.2 Investigate this
      • 3.1.2.1 Recognise ‘this’
      • 3.1.2.2 Know how to and be able to investigate ‘this’ (possibly in the command)
• 4 Longevity

Things prone to wear and tear or able to run out should run for at least 5 years.

• • 4.1 operate for preferably several years, Either:
    • 4.1.1 carry enough energy
    • 4.1.2 Produce energy there
  • 4.2 Durability
    • 4.2.1 Iono- & Magnetosphere

[7] Magnetic fields estimated at 5.0*10^-7 T Electron densities of up to 10¹⁰ m^-3 with energies up to 250 eV Ionospheric currents up to .42 A/m

• • • 4.2.2 Low gravity

Calculations of non-uniform gravity Suggestions for zero-G car

• • • 4.2.3 Low atmospheric pressure

Oxygen densities of around 10^-10 that of earth (~1.801*10^23 cm^-2)(see also calculation: Barometric formula) 3D-Plasma source-sink model Spectrometry model Monte Carlo model

• • • 4.2.4 Low temperatures

86-132 K Europan temperature

• • • 4.2.5 Possibly rough or slippery surface

Bases should be able to maintain their position on the surface

• • • 4.2.6 Withstand tectonic activity

Life

• 7 Do not kill
  • 7.1 Radioactive sources amply shielded
  • 7.2 Research methods that do not kill the subject
  • 7.3 No biological earthly life brought along on the mission

Measurability Requirements and Constraints

These are the conditions to be fulfilled in order for the main requirements to be met.

1. The requirement to actually get to Europa has been laid into the hands of SpaceX. Requirement 1 has been fulfilled if the total lander system fits inside a capsule with a length of 13.1 m and a radius of 2.6 m, and is no heavier than 3500 kg.
2. Requirement 2, which asks for the lander system to be built, is fulfilled if the lander system contains only technology which is legal right off the bat, or for which it is possible to get a permit.
3. The requirement of autonomous execution of commands will be fulfilled if the vehicle has systems in place that allow it to know where it is, where it should end up, and how to get there without colliding with obstacles. Furthermore, it should be able to receive commands to research the things specified under requirements 5 and 6, it should be able to find these things, and know how to research them.
4. Requirement of survival is met if the lander system carries enough energy to sustain itself for at least 2 years or can produce on the spot enough energy for that period of time. Furthermore, it should hold on for these five years under the following conditions:
   1. External magnetic field of up to 1 mT, 2000 times stronger than what is estimated for Europa’s magnetosphere. Furthermore, the lander system should hold up at plasma densities of up to 10¹⁰ m^-3.
   2. The lander system should still function at gravities down to 10% of earth’s and should be able to account for a non-uniform gravity not always perpendicular to the surface.
   3. The lander system can still operate under pressures down to 10^-10 atmospheres. At the same time, the digger must be able to withstand pressures of at least 105 atm, and preferably up to 3000 atm.
   4. Temperatures between 86 and 132 K do not damage the lander system and its components, nor make it thusly prone to damage that normal operation will result in damage to the system.
   5. System bases (components of the system that do not move around on Europa) should be able to maintain a fixed position on or in Europa.
   6. The lander system should not be destroyed by the tectonic activity of Europa.
   7. Preference for survival is met if the lander system can recognise faulty equipment and report on it, and can repair or replace said equipment.
5. Requirement of planet research will be met if the lander system can gather data on the aforementioned aspects of Europa.
6. Point a. can be deduced from requirement 5, but to that end point 5 should be expanded to include the criteria for life. Point b. will be met if the lander system can gather data on the aforementioned aspects of any life found on Europa.
7. The requirement of ethical treatment of life and its environment will be met if research can be conducted in such a way as to avoid unethical harm done to relevant moral agents.

The Plan

9 overview of the entire Europa mission

8 interesting points on Europa

Before outlining the research and design in detail, a quick overview of the general mission will be presented here. All this will be expanded on in the following chapters. First of all, a surface base will land on Europa’s crust. This base can conduct research at surface level, upholds contact with the earth. Furthermore, it holds a digger, that will begin to go through the crust right after the surface base has landed. Once the digger has breached through to the ocean, it will anchor itself in the ice and release a submarine which will do the bulk of the research. The digger maintains direct contact with the surface base. The submarine will sail through the ocean to find life, signs of life, or the possibility of life. It will do research in as large a range of circumstances as possible. The lower picture on the right shows 8 interesting points on Europa. The broadest range of circumstances is considered based on 2 parameters: Depth in the ocean (point A vs B or S vs T in the picture) and proximity to Jupiter (A vs J). The latter difference is interesting because of the large impact that Jupiter has on Europa, in particular through tectonic heating. Thus, an optimal trajectory through Europa’s ocean would be ABOKJ or ABTKJ as seen in the picture.

Performing Research

Experiments

Seismic experiments: A seismometer can be used by the station to determine the thickness of the ice of Europa’s crust, and possible determine a suitable entry point for the ice digger to start digging at. Furthermore, the seismic experiments may also result in observations about the size of the underwater ocean(s), giving scientists new data to correlate already made observations to. Since the lander will not be going under the ice, the seismic experiments will be performed solely on the surface. Similarly, since the station will not be changing its location on the surface, the seismic experiments will only be performed on the location that the surface lander lands on.

Spectrometry experiments: The submarine will carry a spectrometer to perform spectrometric experiments on the water below the ice. These experiments will show scientists the composition of the water, and maybe even the presence of life-sustaining molecules. The life-sustaining molecules that the submarine looks for belong to one of four categories:

• Carbohydrates, which supply an organism with energy and structure
• Lipids, which are organic molecules that consist of a hydrophobic and a hydrophilic side
• Proteins, which may serve a very wide array of functions within an organism
• Nucleic acids, which are molecules in an organism that carry information about said organism[8]

As for the location of performing spectrometric experiments, they will be performed at several locations. Since on Earth, different depths in the ocean harbor different forms of life, the mission probably has the highest chance of success if the search for life is performed at different depths, ranging from just below the ice to as deep as the submarine can go without being crushed.

Photos: Taking photos from the surface of Europa is not only cool, if the lander is equipped with a microscope or a likewise instrument, evidence or traces of life might be found, similar to how fossils and microscopic organisms on Earth provide evidence of past and present life. Of course, the surface is not the only thing that should be photographed. Having a camera on board of the submarine will enable photos of the ocean to also be taken. These photos could reveal the internal structure of Europa, or, in the very best case scenario, living organisms themselves in the ocean. Similar to the spectrometric experiments, the submarine will take photos at different depths, to maximize the chances of taking pictures of actual life. The submarine will start taking pictures just below the ice, and intermittently take new pictures as it reaches new depths.

Gas emission experiments: Similar to how the Viking landers performed gas release experiments on Mars, the same experiments could be performed on Europa. If a small sample of soil, or, in this case, water, is injected with radioactively labeled nutrients, the release of metabolised molecules may indicate that life is present. These gas emission experiments can not only be performed to find organisms that metabolise to CO2, it can also used to find organisms that metabolise CO2 itself, for example photosynthetic organisms. Once again, similar to the spectrometric experiments and the photos, the gas emission experiments will be performed at different depths, in hopes of obtaining evidence of the presence of life in Europa’s ocean. At these different depths, different organisms may be found, maximizing the chances of successful execution of the mission.

How to let water in and out of the submarine

Sketch of the system

To perform the spectrometric experiments and the gas emission experiments, a sample of water will have to be let into the submarine, so the internal spectrometer can evaluate this sample. Just below the ice, this will not be a problem, but the deeper the submarine goes, the more pressure the water will put on it. These pressures will become very high, up to .3 GPa. The main problem when letting water in and out is to overcome the .3 GPa of pressure without having to do too much work, as this work requires energy that is probably not available. To achieve this, a rotating disk with a small slot for sampling water is proposed, as shown to the right. This disk will be encapsulated in the hull of the submarine, with sufficient strengthening around it to compensate for the puncturing of the hull. As the disk rotates, it will take with it a volume of water equal to the volume missing from the full disk. After half a rotation, it will deposit the water on the inside where it can flow to where it needs to be sampled via a simple tube. A disadvantage is that the submarine needs a particular orientation for the water to be able to fall in or out of the slot. However, being a submarine this should not be a problem. The volume of water sampled is equal to t*a*(R^2-(R-d)^2), where t is the disk thickness, a the angle that the slot spans in radians, R the outer radius of the disk and d the depth of the slot. For instance, at t=.3 cm, R=14 cm and d=2 cm, a needs only be 1.28 radians (73.4 degrees) for the slot to contain 10 mL. This sample size is based on one of the volumes that a cuvette of a spectrometer normally has, 1 mL. Given that 2 different experiments are performed, this gives a sample size of 2 * 1 mL = 2 mL. If, then, a margin of safety is added to this sample size, to account for any loss of sample size during intake, an intake volume of 10 mL would be a safe sample size to perform the experiments on. The biggest advantage of this system is that because the water is allowed to push on both sides of the slot in the disk, it attempts to push the disk both forward and backward with equal force, resulting in a net force due to water pressure of 0 N. By comparison, the total torque applied on either side of the slot at 150 km depth with the above parameters is 4680 Nm. By comparison, the SSC Tuatara‘Hypercar’ caps out at a meagre 1735 Nm of torque, and it has a motor weighing nearly 200 kg.[9] This goes to show the significant advantage of using the rotating slot. Of course it will still require some force, as the disk needs to rotate without gaps for the water to seep through. Hence both the disk and the casing around it need to be extremely smooth to decrease the coefficient of friction as much as possible. Hwang and Zum Gahr found the friction coefficient between 2 plates of polished steel to be about 0.116.[10] With a slot depth of 6 cm and volume of 10 ml, this means that as the water pushes down on the disk through the opening - 7 cm wide - with a total force of .3*10^9*.003*.07=62962 N, the friction force to be overcome is 7304 N, resulting in a torque of 1023 Nm. This is still a lot, but the benefit is that the disk does not need to turn extremely fast, like would be desirable for the Tuatara. With gears or a worm drive, torques significantly below this 1023 Nm can be transformed into much higher torques. As an order-of-magnitude example, the following motor will be looked at.[11] The relevant parameters are the:

• Box volume (the volume of the smallest rectangular box one can place it in): 4.5 L
• Mass: 4.5 kg
• Power: 120 W
• Torque: 1.3 Nm
• Maximum speed: 850 RPM
• Input voltage: 230 V AC

The required torque factor is 1023/1.3=787. This is a lot of tooth for one gear, and it is intended to keep it compact. To that end, 2 the same worm drives might be used, each having a ratio of at least 28:1 (30 is a customary number of teeth, which is convenient as it thus goes about 15% over the required torque). These are available at industrial levels at diameters below 5 cm. If custom-made, they can probably be made to fit inside a volume of about 1 L. The total transmission then becomes 30*30=900. At this ratio, the disk would be made to turn at .944 RPM. That means that the disk will turn halfway in about 28 seconds. The total energy requirement for this turn will be 120*28=3400 Joules. A battery on board the submarine should be able to take in a sample and deposit it outside again at least three times in a row + some to spare to allow for repeated and thus more reliable sampling. Thus, the battery should be able to store at least 23800 (7 half turns) but ideally 34000 Joules of electric energy. 34000 Joules is not a lot. A typical AA rechargeable battery can easily store such energies. The problem is in the 230 V AC requirement, with which you are automatically looking at heftier battery packs, such as the following one.[12] In a custom design this can probably be made a lot smaller, especially if it needs only 1 or 2 output ports. The volume of this pack is 5.6 L and it has a mass of 5.4 kg. This number will be divided by 3 for our battery.