David Rothery

David Rothery

Prof of Planetary Geosciences, Open Univ
Moons Educator
Books:
Moons (2015) OUP
https://www.youtube.com/watch?v=l7FkCOwpCdk
Planet Mercury (2014) Springer
Planets (2010) OUP
http://www.open.ac.uk/people/dar4

Location Milton Keynes

Activity

  • I can't speak for 'most geologists', but as usual in journalism the impact of the study it reports is exaggerated. It remains very widely accepted that very large volume eruptions on the Moon ended before 3 billion years ago. It is accepted that some substantial areas received big lava flows until about 2 billion years ago (proved when a Chinese sample...

  • @BenC you can see the replay already at the original link. The YouTube version will take a few more weeks because captions have to be added and some image rights need to be cleared.

  • Close, but the Cretaceous ended about 66 million years ago.

  • This may be a language issue. Does “split apart” make more sense to you?

  • Radioactivity never 'ceases'. It decreases by 50% for every half-life that has elapsed. Radioactive heating inside a planet, or moon, therefore decreases over time so that volcanic activity becomes rarer and may eventually cease entirely (because the small amount of generated heat can escape in other ways).

  • Not for Apollo. The Space Shuttle had ceramic tiles as a heat shield on its underside, and some of those came loose.

  • What Julian says is true of Ariel, Umbriel, Titania and Oberon - but for all the more recently discovered moons of Uranus from Miranda (a human character in The Tempest) onwards the names are of Shakespearean characters. Ariel, Titania and Oberon happen to be in Shakespeare, whereas Umbriel is a character in Alexander Pope's "The Rape of the Lock", where Ariel...

  • When you click it reveals a larger image with annotation, to help you understand the text in our written answer.

  • That's the quality of the data we have, so we have to work with it if we want to do science.

  • Some for hours, some for months. Most of the plume material falls back onto Io, but some ends up dispersed along Io's orbit.

  • Not at all. Almost all the erupted material ends up on the surface, and is buried by later eruptions whose weight causes subsidence. The rocky (silicate) component of Io has probably been recycled several times in this manner.

  • @LindaGilbert @InekeFioole sorry about that. I've fixed the link now.

  • @IsaacNumoah the moons of other planets all have names. For example, one of Jupiter's moons is named Europa, and other is named Ganymede. Our moon has a name too, which is "the Moon", with a capital-M.

  • I have just tried it, and it works for me. It is an ESA website and should be accessible for everyone, irrespective of location. Possibly the website was offline temporarily. If you still can't see it, try a different browser.

  • @YvonneWilliams be in no doubt that this is a scam. Paid-for names will not be accepted by the IAU, and the website that you linked to is trying to part gullible people from their money.

  • What do you mean by "understand"? Can you understand the rebound effect that causes a single peak in the smaller complex craters? If so, it is just a small step to accepting the evidence or ring-like rebound within larger craters reading to peak rings instead of a central peak.

  • Io

  • @MaryR This is the effect that exaggerates the eccentricity of their orbits, which is why tidal heating is effective.

  • Why would you expect the difference in size between Ganymede and Titan to be visually apparent? Titan's radius is only slightly less than Ganymede's.

  • @MaryR There are a lot of misconceptions to unpick in this thread. Tidal forces on Earth have not changed (well, they are becoming very slightly less over time as the Moon's orbital radius increases). Volcanism has little to do with heat in the Earth's core (it is driven almost entirely by processes in the mantle). There is no current/recent upwards global...

  • @MichaelBath Caroline Herschel was Wm Herschel's sister, and an astronomer in her own right.

  • Short answer: gravity. Long answer: https://www.youtube.com/watch?v=jZUaJZ4FeUI&t=17s

  • @SueC probably, but we will have to wait patiently to see ;-)

  • I hope by now you have reached 2.8 and learned that Galileo did not give names to 'his' 4 moons.

  • @InekeFioole No, you've misunderstood. You need a third body to carry away the excess kinetic energy if you want a free moving body to end up orbiting a planet that is comes close to.
    What we are saying in the final sentence is that present-day Triton could have been one member of a double object (like present-day Pluto-Charon) that was captured by Neptune,...

  • Orbital speed depends on the strength of gravity and distance. A moon in a larger orbit will travel more slowly than a moon in a smaller orbit about a given body.
    Callisto is not small. It is left off because it orbits outside Callisto's orbit and is not relevant because its orbital period is not in resonance with the other three.

  • Yes that's correct. You will learn about this later. The cause is tidal drag.

  • A planet with the enormous mass of Saturn will scarcely feel any effect if a 100 km icy moon becomes smeared out as a ring rather than being in a single ball.

  • There is no formal rule. At least, the International Astronomical Union has not made a rule to cover this.

  • Everything has gravity. The more mass. the more gravity, so you can't base a definition of a moon on whether or not it 'has gravity'. For moons that have never been studied during a close flyby by spacecraft we can't tell if they have a core or not - so possession of a core would be an unsatisfactory definition.
    As you will see later in this course, a 10...

  • In detail, yes - but there are characteristics that many of them share too, as you will see as the course progresses.

  • He doesn't?! Well, at least he has a nose, which means he doesn't smell terrible ;-)

  • @RoyHolton that was one person's unscripted opinion. Make of it what you will.

  • @YvonneWilliams buzz when you think you spot one ;-)

  • @YvonneWilliams I'm sorry if the video is not playing for you. Try again, and then try with a different browser

  • @JulianLivsey In fact the tidal effect is identical with the Sun and Moon on the exact same side of the Earth to what it is when they are on exact opposite sides of the Earth. You forgot that the Earth is not attached to some kind of cosmic fixed point. If is free in space, so the centre of the Earth gets pulled towards Sun and Moon, so it is the DIFFERNCE...

  • The missing images issue has now been fixed

  • Well spotted. David Mitchell is the narrator in the "60 second adventures" series, including the Rotating Moon example at the top of this step. We have another one later in the course.

  • @RyanWatson can you now see the Two Views of the Solar System image near the top of the page? We are hoping for a proper fix tomorrow.

  • @SueC Hi Sue - can you now see the Two Views of the Solar System image near the top of the page? We are hoping for a proper fix tomorrow.

  • @GeoffBurt yes, but that Moon infographic is a 3rd party item, and is a different issue that I don't think we will be able to solve.
    I have deleted the link to that infographic, but added a new link at the bottom of the list.

  • The Europa oxygen story is misguided. How much oxygen is released by radiation-damaged ice at the surface has no relevance to the conditions experienced by life in the internal ocean, which would not even need to use oxygen for respiration anyway.
    The press have latched on to a single remark in the original paper, and made a mountain out of a molehill.

  • Apologies about the missing images. So far as I can see, someone has (very recently) removed a whole directory where many (fortunately not all) of the images in this course were stored. I have asked for this to be reinstated as a matter of urgency.

  • Hi Ineke The Chang'e 5 video has just run fine for me in Google Chrome. I suggest you try a different browser
    best wishes, Dave

  • 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