David Rothery

David Rothery

Prof of Planetary Geosciences, Open Univ
Moons Educator
Moons (2015) OUP
Planet Mercury (2014) Springer
Planets (2010) OUP

Location Milton Keynes


  • 1) Yes, the gravity assist is still very much used. BepiColombo, on its way to get into orbit about Mercury had one Earth flyby, 2 at Venus and next month has its 3rd (of 6) at Mercury itself.
    2) I would have to Google to find that answer. I suggest you search for yourself, and report back here :-)
    3) No optical telescope at Earth has any hope of seeing an...

  • Yes, absolutely.

  • It’s about moons, not planets. If one moon’s orbital period is a simple ratio of another moon’s (orbiting the same planet), then they are resonant. Divide the longer period by the shorter. If it is a small whole number (or very nearly) such as 2, 3 or 4 or a simple fraction such as 3/2 then that qualifies.

  • @NiraRamachandran yes, that’s right.

  • No. it is onboard New Horizons, curretnly heading out through the Kuiper belt http://pluto.jhuapl.edu/Mission/2019-Onward.php

  • I've done research work on volcanoes, using gravitymeters (which measure local acceleration due to gravity - the 'strength of gravity' if you like) as a way of tracking the movement of magma below ground. To do that you have to remove the effect of tides on the local gravity. When the ground is flexed up, local gravity is less (because you are further from...

  • A cosmic ray hit, or a bad detector in the image array, can give you one faulty (in this case bright) pixel. Also artefacts can be introduced if the image has been compressed, for example into a jpeg file.
    It's not impnrtant here. Look at the moons' surfaces, not the real (or fake) stars in the background.

  • Feldspar probably began to crystallise before pyroxene, because we can see feldspar crystals enclosed inside large pyroxene crystals. The ilmentite looks late to me. More than one kind of crystal can have been growing at once.

  • Neither of those. Anything rich in carbon, as in carbonaceous meteorites. it could just be organic molecules made by linking a few methane molecules together.

  • I've changed the text - however I am not certain that any of the three are real stars. They could be bad data (noise).

  • Yes they are cooler because of the geometry of illumination from the Sun. If the Moon were a smooth sphere, there would be only grazing incidence sunlight at the poles. However, the situation is even more extreme in reality because the Moon has craters and the parts of the floors for craters near the poles are NEVER illuminated directly by the Sun. That's what...

  • @JanStallard That one is not on FutureLearn anymore but the same material is available (free, but without mentors) on OpenLearn at https://www.open.edu/openlearn/science-maths-technology/an-introduction-geology/content-section-overview?active-tab=description-tab

  • @RobertMawson @NiraRamachandran There is almost certainly too little internal heat leaking out of the Moon for the temperature at the base of the polar ice to be warm enough for ice to melt. Remember, the ice in polar craters is not going to be 10s of km thick, like the ice on Europa. It might be 10 m thick, so its base wont be much warmer than its top.

  • Exposure to the vacuum of space (especially on the night side of the globe) would do it.

  • No. We use an optical microscope (white light), with polarizing filters, as described in the 3rd paragraph.

  • Why would you expect them to be different?

  • You are supposed to choose the characteristic of the moon on your card that is most likely to beat any other moon. It is a gamble, but drawing on your general knowledge of moons.

  • "no gravity on the moon, means less energy required to launch" The Moon does have gravity. At the Moon's surface, the strength of gravity is about 1/6th of wat we experience at the Earth's surface.

  • I dont think a magentic field has much importance when considering life in an ocean below surface ice. The surface of an airless body without a magnetic field is exposed to the solar wind (charged particles from the Sun), and the flux of charged particles is much more severe if your orbit takes you through radiation belts associated with (say) Jupiter's...

  • Gravity and magnetism have no relation to each other.

  • I don’t know why, but the LROC Quickmap doesn’t seem to have Mare Orientale in its nomenclature database. Instead it has Montes Cordillera, which is the name given to the raised basin rim. Easiest to find if you chose the cylindrical map projection.

  • I don’t think you’ve understood about light transmission through minerals. You put one polarising filter on the light before it passes through and another after. To do what you suggest would require a filter across the whole Sun to polarise the light before it hit the lunar surface, and even then you be dealing only with light reflexes from, rather than...

  • This is not really like a “shooting star”.
    That’s a trail of light across the sky representing an incoming object (usually sand grain size) burning up in the atmosphere. The impact flashes on the Moon are at the point of impact on the ground (and the ones we can see from Earth are rather bigger than most meteors in Earth!s sky.

  • Because Mars has a radius of nearly 3400 km. Orbital radius is measured from the centre of the planet.

  • 'mare' (singular) is pronounced 'MAH-rey'.
    'maria' (plural) is pronounced 'MAH-ri-a' (not like the girl's name Maria)

  • Amalgam is not a geological term. Conglomerate and breccia both describe sedimentary rocks made of pebble to boulder size lumps. In a conglomerate the lumps are rounded. In a breccia the lumps are angular in shape.

  • It probably was when Pluto was named, and that original Greco-Roman underworld theme has been applied to all Pluto's moons. Mythologies (and other cultural concepts) from other traditions are used for other solar system bodies, and in the names of surface features.

  • Well, as there is currently no FutureLearn basic geology course, how about this? https://www.waterstones.com/book/geology-a-complete-introduction-teach-yourself/david-rothery/9781473601550
    However, the mineral names and optical mineralogy in this week are here just to expose you to the nature of lunar samples. We are not trying to train you to be a geologist.

  • Amorphous solids are glass. If they are crysalline, they are not glass. Some rocks consist of crystals embedded in glass.

  • Water mainly.

  • Khatera - the history of life on Earth is beyond the scope of this course. However, the oldest fossils (just simple cells) have been dated by scientific methods at about 3.5 billion years, the oldest animals on land (millipedes, which ate moss) 428 million years, dinosaurs were alive 250 to 65 million years ago, the oldest human-like apes evolved 6-7 million...

  • Yes, "impact speed" means the speed at which the projectile hits the surface, not the speed of the projectile when it is at some distance away in space. However, impactors arrive so fast that there is little time for the target body's gravity to accelerate it by much. The biggest effect of surface gravity is thus in restricting the size of the crater and the...

  • The recording of this year's webcast is available at the same link as the live event.

  • @AllanHaines People tend to forget that being on the surface of the Moon places an astronaut at no more risk from meteorites than the journey to and from the Moon. Both settings lack the protection of Earth's atmosphere which stops/destroys everything less than about a metre is size.

  • @InekeFioole There are many craters on large icy moons, with the exception of Europa which has been largely resurfaced. You should already have seen several examples. At Jupiter and beyond surface temperatures are so cold that ice behaves like rock, and craters hardly degrade at all over time.

  • Mass of a small moon is hard to measure remotely (so is size), and there will always be considerable uncertainties unless you get a spacecraft close. I think any definition ought to depend primarily on something that can be reliably determined.

  • Moons do not disintegrate, unless their orbit evolves to bring them too close to their planet. (That is a possible ring-forming process)

  • I've just watched it again myself, and I agree!

  • You will see some silicate minerals from the Moon in Week 5. Beware though: Earth's mantle is very unlikely to contain quartz, felspar or mica. Those are characteristic of the crust only. Olivine (and pyroxene) can occur both in the matnle and the crust.

  • It's tricky to get your head round. That might be true if the Earth were fixed in space - but both the Earth and the Moon are moving freely through space and both experience a gravitational pull from the Sun. That's why they orbit the Sun (yes, I know the Moon orbits the earth, bit both are orbittig the Sun too)..
    You are using the a similar false argument...

  • @AllanHaines @InekeFioole that BBC article is wrong. "Centrifugal force" has nothing to do with it. There would be equal and opposite tidal bulges in the ocean even if the Earth was not rotating. The correct reason is that just as the ocean on the Moon-ward side of the Earth is pulled towards the Moon a little more strongly than the Earth itself, likewise the...

  • The Earth's mantle is not molten. Are you confusing 'mantle' with 'magma'? Plate tectonics is possible on Earth because a layer near the top of the mantle is weak. This allows the lithosphere (the crust and the very uppermost, rigid, part of the mantle) to slide across the interior. Separate to this, partial melting occurs in the upwelling parts of convection...

  • Yes, 'it' refers to Io.

  • @AllanHaines Ther Solar System formed 4.5 billion years ago. The Universe is nearly 3 times older than that. The Big Bang has no direct relevance to the material from which the Solar System formed. Several generations of high mass stars had lived and died before that happened.

  • @RobertMawson you are right, that's the criterion on which Pluto fails the 'planet test', except that it is not 'debris' in its orbital space that is the issue. Pluto shares its orbital space with objects whose mass is not greatly less than its own (indeed, it also crosses Neptune's orbit, and Neptune has order of magnitude more mass)

  • No there is not.

  • You will learn a little about black smokers later in the course.

  • You are very welcome here Khatera. You have started later than most learners, and most of them are studying week 3 now - but that's OK :-)

  • Gravitational pulls have no tendency to "make things unite" if there is any sideways component in their relative motion. It is gravity that makes things orbit each other.

  • You don't have to do any calculations. How could you? We haven't given you any equations. The calculations have already been done. This is just an exercise in revealing the results, after you have had a quick think about what to expect.

  • @AllanHaines the further you go from the Sun, the colder the environment. Ices made of methane and even nitrogen are encountered - but there is plenty of water-ice too. You will learn about this later in the course.

  • Those terms refer to where something came from, rather than what it is like. In this diagram exogenous substances (i.e., chemicals) originated outside Europa, and endogenous substances originated inside Europa. (The term is not applied here to 'life forms', though it could be)

  • Everything does, unless in an EXACTLY circular orbit. In an elliptical orbit, orbital speed decreases with distance. This is a consequence of the strength of gravitational attraction between two bodies decreasing with the square of the distance between them.

  • From Earth the flash of light when an impactor hits, for example, a moon of Jupiter would be much too faint to see if the impactors were only the size of the example discussed here. The only natural impacts that I am aware of being seen were when fragments of comet Shoemaker-Levy 9 struck Jupiter in July 1994. The brief flash of light was seen, as well as...

  • That's a shame - but of course the recording will be available

  • @TanjaEllenSleeuwenhoek the time difference is only 1 hour! I hope you can join us :-)

  • Aren’t you going to ask us a question for tomorrow’s webcast?

  • Replenished with what? If you mean water, the likely rate of water loss to space is too slow to worry about. Nutrients for life is more of a problem; reactions between water and ocean floor rock and/or meteorites could be the answer here.

  • The Sun!

  • @NiraRamachandran it was worked out by tracking the planets’ motion against the background stars over decades. The ancient Babylonians gathered good data.

  • You are misunderstanding the origin of these meteors. From our perspective they approach the Earth from the direction in which the star Gamma Norma lies in March. However they originate much closer to home , WITHIN the Solar System. They have not travelled interstellar distances.

  • Actually 'friction' has little or nothing to do with the heating during atmospheric entry. This is a lazy myth that you will find repeated in many places. The source of heat is compression of the air in front of the incoming object.

  • The inner Galilean moon is called Io (not Lo). It begins with a letter-i.
    If you divide Callisto's period by Io's period you don't get a (nearly) exact whole number.

  • Alternatively Raymond: why do you assume that orbits should be circles? That assumption hamstrung western science for 1500 years until Kepler came along!

  • Fascinating article!

  • @JohnLateano thee was enough natual light

  • @RobertMawson if water is in contact with warm rock (warmed by tidal heating or by radioactivity) there will inevitably be some chemical reactions going on, so that the water will end up with salts dissolved in it - not necessarily dominated by sodium chloride (as in Earth's oceans) but salt none the less.

  • Yes

  • You are experiencing a well-known optical illusion. Remind yourself that the illumination in the caldera image is coming from the lower right, and in the lunar image it is coming from the bottom (you can tell this by interpreting the shadows).
    If your brain still insists that the relief is inverted, try turning your screen upside down.

  • @NiraRamachandran no. Naked eye observation of planets’ motion in the sky even before telescopes were invented revealed the orbital planes of each visible planet.

  • David Rothery replied to [Learner left FutureLearn]

    @AdrienneCullen-Morgan Please consider whether what you have said (including "Woke nonsense") might be upsetting to some of your fellow learners who have chosen 'they' as their preferred pronoun. It is legitimate to ask whether moons are masculine or feminine, but the rest of what you wrote doesnt really belong here.
    To answer your question, moons are named...

  • 'Geologically active' does not have to mean volcansim. Fault movements and other crustal deformation; surface erosion, transport and deposition of sediment - all these are geological activity.

  • @GilbertDavis 'Geologically active' does not have to mean volcansim. Fault movements and other crustal deformation; surface erosion, transport and deposition of sediment - all these are geological activity.

  • The size of Deimos was measured by spaceprobes orbitting Mars and taking pictures of Deimos from a known distance (at closest about 1000 km).

  • The planets dont have 'perfectly aligned orbits' anyway. Mercury's orbit is inclined by 7.0 degrees (relative to Earth), Venus 3.4 degrees, Mars 1.9 degrees. Pluto's orbit is more steeply inclined (and more eccentric), that's all.

  • David Rothery replied to [Learner left FutureLearn]

    @AdrienneCullen-Morgan Louise was referring to the ages of the different areas of surface. Of course Ganymede as a body has a single age (probably 4.5 billion years), but various areas of surface were formed or sculpted at different times, because it has been geologically active. It's the same on Earth.

  • @MarkJackson @MoragLanzendorf the pebbles mad eof water-ice have been eroded (and rounded) during transport by flowing liquid methane. The process is 'fluvial', analogous to fluvial processes involving liqud water on Earth. (Fluvial comes from the the latin for 'river' - that's the important bit, not what the river is made of)

  • @AdrienneCullen-Morgan Look at the surface details. The two views have opposite edges (opposite limbs) of the Moon illuminated.

  • Not quite. Huygens landed on Titan (Saturn's largest moon) in 2005. You will learn about this later.

  • @NiraRamachandran No. 1) cirlces are 2 dimensional, whereas a body is 3 dimensional. If you mean 'spehrical' (the 3-dimensional equivlaent of a circle, that's not what happens. It depends on the itnenral strength of the body. If spinnig fast and weak internally, it can become a flattened ellipsoid. Some small rubble-pile asteroids take on the shape of...

  • If there is no transcript option visible, then there is no sound (at least no spoken word).

  • @TanjaEllenSleeuwenhoek nicely done Tanja :-)

  • Imagine Pluto and Charon being joined together by a rod (that had no mass of its own!), and further imagine you were trying to find the point on this rod where you could balance the two. That's their centre of mass.

  • Try it Morag, and report back to us :-)

  • You mean planets of other stars? We'd expect each to orbit in the same direction that their star rotates, but stars' rotstion axes are pretty much randonly distributed.

  • Yes, orbits are all elliptical, but some have such low eccentricty that they are almost indistiguishable from circles. A circle is an ellipse with an eccentricity of exactly zero.

  • @JohnLateano no sound on your device? Then use the 'view transcript' option that appears at the lower left of each video picture :-)

  • David Rothery replied to [Learner left FutureLearn]

    @AdrienneCullen-Morgan you dont have to remember them! You can look them up any time you like, thanks to the internet :-)

  • Yes: "periapsis" is a general term, whereas "perigee, perihelion, periherm, perijove ..." are particular to each orbited body (Earth, Sin, Mercury, Jupiter ...). If you don't apprvoe of this, would you approve of calling all dogs "dogs" rarther than distringuishing them as "spaniel, poodle, collie, terrier ...."? The distinctions can be useful...

  • Whether moons can have moons was addressed in the previous step.

  • @NiraRamachandran nobody is planniing to do this. In fact, there are protocols in place tro try to prevent accidental contamination. You will learn about these later in the course.

  • You don't yet know how long each step will take Ineke.

  • The visible shape of Moon is actually slighly flattened by refraction in the atmosphere when it is close to the horizon. You see the same with the Sun at sunset. This is just flattening, not sideways stretching, so if you were to measure its apparent area the Moon in fact is smaller when close the the horizon than when it is high in the sky.

  • @NiraRamachandran probably. If you took a carefully selected sort of microbe from Earth and put it at the right palce on the floor of Europa's internal ocean it could survive, feed and multiply.

  • Welcome back. There is a link to that story coming up soon. They are small moons and don’t add significantly to our understanding of the Jupiter system.

  • David Rothery replied to [Learner left FutureLearn]

    @johnadams Aren't you here to learn John? There are sound reasons based on increased understanding of the bodies that orbit the Sun behind the International Astronomical Union's decision to exclude Pluto from its 2006 definition of the term 'planet' .

  • Not so. Asteroids are part of the Solar System and formed at the same time as the rest.