In our last article, we took a look at NASA’s JEO and the main object of its observations, the intriguing moon Europa.  In this article, we will look at the ESA’s JGO, which will orbit Jupiter’s largest moon, Ganymede.

As we said above, the opening of the mission, upon arrival at Jupiter, is a collaborative overview of the entire Jovian system.  Because of its enormous gravity pull and magnetic field, the great planet has a big influence over its satellites, and the system must be viewed as a whole.  In particular, the magnetic interaction between all of these bodies is very complex, and will be studied in great detail.

In connection with this, there will be a contribution from the Japan Aerospace Exploration Agency.  JAXA is designing and building a probe called the Jupiter Magnetospheric Orbiter (JMO), which will perform some stand-alone observations of the Jovian magnetic environment, as well as collaborating with the JGO and JEO on multi-point measurements of Jupiter’s magnetic field and its interaction with its satellites.

So far, we haven’t mentioned the JMO because it will travel onboard the JGO until insertion into Jupiter orbit.  At that time, it will be deployed from the JGO and enter a separate orbit.

The Jupiter Magnetospheric Orbiter will be a spinning probe weighing about 400 kg., and it will be carrying a scientific payload of 25 kg.  When it is deployed, the spin axis will be tilted toward Earth.  This craft will employ technology borrowed from three earlier missions: the Nozomi Mars probe, the BepiColombo Mercury probe and the Solar Sail project.  (The BepiColombo and Solar Sail missions were mentioned in earlier articles at this website.  Nozomi was a Japanese unmanned Mars probe which was unfortunately lost due to technical problems, but its failure had nothing to do with the technology that will be recycled for use on the JMO.)  For its post-deployment maneuvers, it will be carrying 60 kg. of propellant.  For all of its other functions, it will be powered by electricity from two “paddle-wheel” solar panels.

(This info comes from the original mission proposal, and may change before completion.  In particular, the proposal expressed the wish to increase the payload weight.  Hopefully they will be successful in this.)

The JMO has an ambitious schedule of observations to make after being deployed from the JGO.  It will perform the first complete survey of the Jovian magnetosphere, mapping lines of magnetic force which run from Jupiter to its moons.  It will also address various questions relating to the interaction of the solar wind with the Jovian magnetosphere, as well as the effects of these interactions on the atmosphere of Jupiter.  As mentioned above, there will be some studies which will be performed in coordination with the JGO and JEO.  The presence of a third point of observation will allow the construction of three-dimensional images of the magnetosphere.

After separating from the JMO, the JGO will first take part in an elaborate series of maneuvers in coordination with the JEO, in which they will conduct a large-scale survey of Jupiter and its moon system.  The JGO’s primary goal is to study the moon Ganymede, but there will also be observations of Callisto, Io and Europa, performed either alone or in cooperation with the JEO.  After that, the JGO will go into orbit around Ganymede for prolonged observations.  (This is a greatly simplified description.  The full itinerary will include an ambitious- and confusing- array of maneuvers, a full discussion of which is far beyond the scope of this humble article.  For those who want more details, the websites of all three space agencies offer fascinating reading.)

Ganymede is Jupiter’s largest moon.  In fact, if this body orbited the sun instead of Jupiter, it would be classified as a planet.  It is larger than either Mercury or Pluto, and is three-quarters of the size of Mars.  It actually has an oxygen atmosphere, though the pressure is too low to support any kind of life known on Earth.

One interesting point about Ganymede, and the subject of some of the study on this mission, is that it is the only moon in the solar system with an active magnetic field.  Like so many other things in the Jupiter system, Ganymede’s magnetism is a result of Jupiter’s gravity.  In our last article, we saw that the big planet’s gravity causes tidal heating in the interiors of its moons, especially the closer ones.  This is the reason why Europa almost certainly has a liquid ocean beneath its surface, and it is also the reason for Io’s excessive volcanic activity.  Here we see another example, for without Jupiter’s gravity, Ganymede would not have its magnetic field.

As we saw in our article on the BepiColombo Mercury probe a couple of weeks ago, a body can only have an active magnetic field if it has a solid inner core with a molten mantle above it.  The molten mantle contains iron, and the heat which keeps it molten causes it to move in convection currents.  The solid core contains iron, too, and the movement of the molten iron mantle around the solid iron core is what makes the electrical current that forms the magnetic field.  Without this movement of iron around iron, electricity will not be generated, and the body will not have a magnetic field.  In the case of Ganymede, it is the tidal heating caused by Jupiter’s gravity that keeps the moon’s mantle liquid, allowing the convection movement that makes the whole process work.

During the time when the JGO is orbiting Ganymede, there will be times when the JEO is also passing close enough to allow coordinated observations involving both probes.  This will allow detailed study, with the JGO performing close study while the JEO observes from farther away.  In this way, it is hoped that they will be able to do precise mapping of the Ganymedan magnetic field.

We already know that Ganymede’s history has been long and complicated.  About forty percent of its surface is highly cratered, darker areas, while the other sixty percent is lighter in color, and is covered by an elaborate pattern of grooves.  The surface of Ganymede is mostly water ice, and these grooves are thought to be caused by tensional faulting of this ice, or by liquid water flowing up from below the surface.  Here, as on Europa, we have the near certainty of large amounts of liquid water beneath the surface.   The presence of liquid water always raises the possibility of life, and here again we get back to the main goal of the entire EJSM/LaPlace mission,  to search for habitable worlds.  This actually encompasses two separate questions: do these worlds have life now, and could they support human beings in the future?

The fourth moon to be examined will be Callisto, and these observations will be conducted primarily by the JGO, though the JEO will take part in some of them.  Callisto is the third largest moon in the solar system, and is almost as big as Mercury.  The unique and interesting thing about Callisto is that it seems to be a geologically dead world, with no visible seismic activity, vulcanism, or anything else to alter its surface.   Unlike the three other moons we have looked at, this one orbits much farther from Jupiter, so the kind of volcanic activity that we saw on Io, for example, is absent here.  Because of this, it is thought that Callisto has the oldest landscape in the solar system, preserving impact craters from the very early history of planetary formation.  All of the other bodies in the solar system have changed since then, but Callisto remains as a snapshot of the system’s childhood.  As you can imagine, this moon will be the subject of much study in the future, and the work that will be done by this mission will pave the way for that research by giving us our first close-up view of the body.

The EJSM/LaPlace mission is a huge undertaking which promises to yield a staggering amount of data.  Even if some of the science doesn’t come off as planned, this project will undoubtedly be one of the most productive and thorough space projects ever.  For years to come, we will be poring over the information that this project will give us.

We have always had our eyes on the moons of Jupiter.  Ever since Galileo looked through his homemade telescope and drew his first crude pictures of them, we have wanted to visit them.  The more we learn about them, the more interesting they get.  These are real worlds, with atmospheres, heat and water.  They may have life, and they certainly will have it in the future, for we will visit these bodies in person someday.  While it is unlikely that hellish Io will ever be settled by humans, it certainly makes an interesting place to study, and the other three Galilean moons will almost certainly feel the tread of human feet someday.  When that happens, it will be a direct result of the knowledge we gain from this mission.

Sources:

Solar System Exploration: Moons and Planets of the Solar System at NASA website:  solarsystem.jpl.nasa.gov/planets/

OPFM: Outer Planet Flagship Mission at website of Jet Propulsion Laboratory, California Institute of Technology:  opfm.jpl.nasa.gov/europajupitersystemmissionejsm/

OPFM: Outer Planet Flagship Mission- Jupiter Ganymede Orbiter (JGO) Concept at website of Jet Propulsion Laboratory, California Institute of Technology:  opfm.jpl.nasa.gov/europajupitersystemmissionejsm/jupiterganymedeorbiterjgoconcept/

Synergy Between JMO, JGO, and JMO at ISAS/JAXA website:  sprg.isas.jaxa.jp/jupiter/pukiwiki/index.php?Synergy%20between%20JMO%2C%20JGO%2C%20and%20JEO

Scope and Purpose: The Europa Jupiter System Mission (EJSM) at ISAS/JAXA website:  sprg.isas.jaxa.jp/jupiter/pukiwiki/index.php?I.%20SCOPE%20and%20PURPOSE

III “The Spacecraft” and III.1: “Current Plan” at ISAS/JAXA website:  sprg.isas.jaxa.jp/jupiter/pukiwiki/index.php?III.%20Spacecraft

II.2.1 “Main Objectives” at ISAS/JAXA website:  sprg.isas.jaxa.jp/jupiter/pukiwiki/index.php?II.2%20Magnetospheric%20and%20Space%20Sciences

IV.1 “The Current JAXA Plan” at ISAS/JAXA website:  sprg.isas.jaxa.jp/jupiter/pukiwiki/index.php?IV.%20Orbit%20and%20Operation

As often happens in the field of space exploration, the idea was way ahead of the reality.  The mission’s unusual name is a salute to Italian astrophysicist Guiseppe “Bepi” Colombo, who originally figured out the course that such a flight should take.  In 1975, Colombo proposed a plan to NASA by which a spacecraft could swing close to Mercury three times by making gravity-assist maneuvers around Venus.  The plan would have worked, but was too ambitious for NASA at the time; it got shelved and forgotten.  Now the years have passed, and things that were once unattainable seem within reach.

Unfortunately, Bepi Colombo died in 1984.  Another sad fact about space exploration is that those who think up the great ideas often don’t live long enough to see them come of age.  The mission that he proposed years ago is going to happen at last, and it will bear his name.

BepiColombo will be launched aboard a Soyuz-Fregat rocket from Kourou, French Guiana.   The date seems to be uncertain; the JAXA website says it’s 2013, but the ESA site says 2014.  (It’s possible that the mission plans were changed in the early stages, as often happens, and that these two postings reflect different versions of the plan.)  BepiColombo’s objectives are to make precise observations of the planet’s magnetic field, magnetosphere, interior and surface.

One of the main questions that it will try to answer is why, of all the planets in the solar system, only Earth and Mercury have active magnetic fields.   Actually, we know why Earth has one, but knowing that just makes Mercury more perplexing.   Earth has an active magnetic field because it has a molten layer surrounding its solid inner core.  The innermost core is compressed into a solid by gravity, but above that, there is a viscous, molten layer called the outer mantle.  The fluid contains iron, and as it moves around the core, the entire planet becomes a giant electrical generator.  The principal is exactly the same as generators here on Earth, in which a rotor moves around a stator, creating an electrical field.  In this case, we see the basic concept expanded to a planetary scale, and the field that results is so strong, it forms lines of magnetic force that stretch around the whole planet.  The important point here is that without the molten outer layer and the solid inner layer moving past each other, you wouldn’t get the magnetic field.  It’s being constantly generated and replenished by that spinning motion inside the planet.

If Earth’s molten outer core were to cool down and become solid- which will happen eventually, in our planet’s old age- the magnetic field would no longer be generated.  Of course, this does not mean that the earth’s magnetic field would suddenly disappear.  We all know that when you place a piece of iron in a magnetic field for a while, it becomes magnetized, too.  If you cut off the magnetism, the iron will still retain some of that energy, but it will be weaker than the original source, and it will fade with time.  If the iron is all mixed up with a bunch of rocks and other stuff, and the mixture is of a very uneven consistency, then the magnetism will fade at an uneven rate.  If there’s more iron in one spot than another, then of course, that spot will hold the magnetism longer.  Eventually you will end up with a glob of iron and rock and stuff which has a very weak, uneven magnetic field around it.  If you had the instruments to measure it, it would be immediately obvious that this was an old, fading field rather than an active one that is being constantly replenished from within.

Let’s leave Mercury for a moment and look at Mars.  This planet has a weak, spotty magnetic field, which tells us that it once had a molten outer core like Earth’s, but that core has now become solid and is no longer generating magnetism.  This is not surprising, because Mars is smaller than Earth, and must have lost the initial heat of its birth faster than Earth has.  Mars is said to have a “fossil” magnetic field.

So here we have the two extremes: Earth with its active field, and poor old Mars with its weak, faded one.  In the old days, when we were looking at Mercury from afar, the popular thought was that it would probably have no magnetic field at all, or if it did, the field would only be a fossil field.   The reasoning was that since Mercury is considerably smaller than either Earth or Mars, it should have long since lost all of the original heat from its formation.  And despite the fact that Mercury is the closest planet to the sun, it still isn’t hot enough to melt rock.  After all, the planet’s surface is still quite solid.  Since the inner region of the planet should have lost all of its original heat, and is insulated by all the rock that lies above it, it must be cooler than the sun-baked surface.  So, if the surface is solid, the core should be, too.  Looking at it logically, you get a picture of a small planet with a relatively cool and very solid interior.

Or so we thought, back in the olden days.  When Mariner 10 made its flyby of Mercury in March of 1974, it revealed what nobody had expected to find: an active magnetic field.  Amazingly, this little planet seems to have a molten portion in its interior, just as Earth does.  Either that, or the current theory about how planetary magnetic fields are generated is completely wrong.  If that’s the case, it means we’ll have to throw away all the textbooks and start over at square one- and since this would be a scandalous waste of paper, we can only hope that it won’t be necessary.  A less revolutionary explanation is that Mercury may have large amounts of radioactive material in its interior, which generates enough heat to melt iron and form a molten mantle- but at the moment, this is pure speculation.

In any case, the planetary scientists certainly want to have a closer look.  The part of BepiColombo’s instrumentation which was contributed by JAXA is the Mercury Magnetospheric Orbiter (MMO) which will observe the planet’s magnetic field with unprecedented accuracy in an effort to better understand its configuration and how it is being created.  It is hoped that the probe can also reveal how Mercury’s magnetosphere differs from Earth’s.  Since these are the only bodies known to have magnetospheres, it is possible that comparing the two will help us understand  something about both of them.

Another job of the MMO is to observe the tenuous exosphere of Mercury.  Rather than a gaseous atmosphere like Earth’s, Mercury is surrounded by a very thin layer of atoms, mainly sodium, that have been blasted off the surface by the fierce solar radiation and the impacts of micrometeorites.  These atoms quickly heat up and escape into space, so Mercury never builds up as much pressure as Earth’s atmosphere has.  This exosphere has never been studied in detail before, and the MMO will observe its structure and the process of formation and escape into space.

The other component of the BepiColombo mission is the Mercury Planetary Orbiter (MPO), contributed by the European Space Agency.  The function of this probe is to study Mercury as a planet: its form, interior, structure, geology, composition and craters.  This will result in a more accurate and detailed map of Mercury than we have ever had before, as well as a better understanding of just what the planet is made of.

One specific question that will be addressed by the MPO is whether there is frozen water at Mercury’s poles.  In the flyby of Mariner 10 back in 1974, the satellite received radar reflections from the polar regions, so we know that there’s something highly reflective there.  Nature doesn’t produce many things that are that reflective; frozen water is the most common.  And it makes sense, because there are areas at Mercury’s poles that are permanently shaded, and probably have been been since the planet was formed.  As strange as it seems, the closest planet to the sun has places that have never felt sunlight, and may be the coldest piece of ground in the solar system.  The MPO will subject these areas to closer scrutiny, looking for ice.

Like some other places in the solar system, Mercury is interesting because it can tell us something about the system’s early times.  On Earth, any substances that we find have been crushed, melted, reformed, eroded and then reformed again, possibly many times over.  There really isn’t much pristine matter from the solar system’s birth for us to study here.   On some of the other bodies in the system, there may be more unchanged matter that can give us a better picture of the original accretion disc which gave birth to all of this.  In particular, if there is ice at the Mercurian poles, it may be a sample of the water vapor that was present in that disc around the sun, which froze in those early times and has remained frozen ever since.  As you can imagine, the planetary scientists are itching to get a peek at it.

The questions are there, just waiting to be answered- and BepiColombo will bring us some of those answers.  By the time it arrives in 2020, we should already have a lot of new information on Mercury from NASA’s MESSENGER probe, which will have been in orbit around the planet for nine years.  BepiColombo will fill in more of the gaps in our knowledge, providing us with our most complete picture yet of the sun’s first planet.    The projected mission is only supposed to last for one Earth year, but there is the possibility of an extension beyond that.

BepiColombo is a big, ambitious mission with huge potential.  It is a remarkable technical achievement, and in the coming years it will bring us some fascinating science about one of the last unexplored bodies in our system.

The fun is just beginning!  As the news comes in, you get always get it here.

Sources:
Mercury Exploration Mission “BepiColombo” at website of Japan Aerospace Exploration Agency:  jaxa.jp/projects/sat/bepi/index_e.html

BepiColombo page at website of the European Space Agency:  sci.esa.int/science-e/www/area/index.cfm?fareaid=30

BepiColombo Overview on the Science and Technology page at website of the European Space Agency:  esa.int/esaSC/120391_index_0_m.html

World Book at NASA- Mercury:  nasa.gov/worldbook/mercury_worldbook.html

Mercury: Closest Planet to the Sun at website of National Geographic:  science.nationalgeographic.com/science/space/solar-system/mercury-article.html

BepiColombo: Mercury Interior at website of the European Space Agency:  sci.esa.int/science-e/www/object/index.cfm?fobjectid=31272