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


  • Nice to have you back. Thnak you for your comments :-)

  • Most English people would say Karon (or Kharon) for the mythological ferryman, but for Pluto’s moon Americans (especially) say Sharon, because James Christy, who discovered it, wanted to name it after his wife Charlene (which is pronounced Sharlene).

  • Yes, but even on the Moon those make up only a small fraction. The common minerals on the Moon are also common on the Earth.

  • @TraceyH watch the recording of last week’s Planets and moon chat, and you will see me reporting on a brand new paper claiming an exomoon discovery Kepler-1708 b i.

  • Yes for the Sun. The magnetic poles reverse each solar cycle (about11 years).

  • Hi Paula - sorry we didnt have time to tackle your question. I don't know what we are likely to discover in the future, but I think the wisdom of having held back a lot of samples is shown by the improved and more sensitive analytical techniques that have demonstrated originally unsuspected traces of water inside some lunar minrals (such as apatite).

  • I don't think the solubility of uranium in water is relevant to the internal differentiation of the Earth. Water is only a very minor constituent of the interior. Uranium is enriched in the crust because when partial melting occurs, uranium tends to go into the melt rather than stay in the residual minerals. This melt (magma) rises because it is less dense...

  • No. Most of the near-side mare are in large near-side impact craters (basins). They are unrelated to structures on the far side.

  • Sadly Brian's explanation is misleading. He implies that Io's orbit is very eccentric (strongly elliptical). In fact it is almost exactly circular, but resonance with Europa keeps the orbit just eccentric enough for tidal heating of the magnitude observed.

  • I’m glad you think so. To learn petrography takes a long time. Here we just try to give you a general impression.

  • @BruceArden at 11.5, 6.9 you have a hole in the glass with, near its centre, a bubble in the glue that holds the thin section of rock to the glass slide. I'm not sure about the other thing.

  • @StephenNewton thank you. I have updated the JUICE dates now. The Europa Clipper dates remain current.

  • “Crewed” rather than “manned” these days please :-)

  • Goodwill samples were Apollo 17 samples presented to other countries, many of which were ineptly 'curated' by the recipients. See https://en.wikipedia.org/wiki/Stolen_and_missing_Moon_rocks

  • That’s what we aim to do :-) @fredsherratt

  • Hi Fred, I don't know what you are basing this on, but Pluto is not too small to be a planet. Pluto was denied planetary status by the IAU when it framed its definition of 'planet' in 2006, because Pluto has not 'cleared its orbit'. Specifically its orbit crosses that of Neptune (much bigger and more massive) and of several other Kuiper belt objects whose size...

  • @LeeScott Please stop trying to bait the Mentors (and me). We are here to help you learn. We are not FutureLearn "powers that be". Your registration situation is a matter for FutureLearn, and they are dealing with it.
    best wishes, Dave

  • Sorry about that Paula. The link was to an external site, which appears to have been taken down. I've edited the link to go to that Apollo 16 video that you found instead.

  • None that I can recall.

  • Rock is much denser than ice. When a body forms from a collection of rocky and icy components, the rock naturally sinks inwards. This is called differentiation. The same process is responsible for Earth’s dense iron having sunk inwards to form the core.

  • The can't always reach full moon phase at the same time, because their orbital perods about Mars are different. Phobos looks considerably smaller from Mars than the Moon does from Earth, so 'moonlight' on Mars must the fainter than on Earth. An additional effect is that Mars is more than twice as far from the Sun as the Earth is, so the sunlight shining on...

  • Even worse, Enceladus is a moon of Saturn, not Jupiter ;-)

  • Yes, but it's not clear that the liquid water inside Ganymede would be in contact with rock, rather than sandwiched between layers of high-pressure ice. In the latter case it is hard to conceive of a metabolic pathway for life. Also, and liquid wate rin Ganymede would be deeper i nside than Europa or Enceladus, so harder to access for study.

  • It makes a more invting jump when it's that way up ;-)

  • Future missions to Europa will carry ice-penetrating radar to try to find thin regions of ice.

  • We are not trying to teach you how to do it. We just want to expose you to the concept.

  • Interesting. Have you ever thought why you tend to use a C? Are you writing for southern hemisphere children? Are you intentionally showing a (northern hemisphere) pre-dawn scene? If neither, then have you unwittingly followed a convention?

  • Consider their relative sizes. The Earth is bigger than the Moon - thus the Earth will have protected the Moon more than the Moon will have protected the Earth.

  • It's no more risky on the surface of the Moon than in space in general.

  • The molten part of the Earth's core (its outercore) is molten iron mixed with sulfur. Jupiter's magentic field is generated by dynamo motion in a zone of hydrogen under extreme high pressure ('metallic hydrogen'). Inside Uranus and Neptune their magnetic fields are probbaly mad eby dynamo motion in electrically conducting high pressure 'ices'. 2 of 2

  • You have the wrong idea about planetary (or moon) magnetic fields if by 'magnetic element' you mean an element that in solid from can retain a magentic field and be a so-called 'permanent magnet' (for eample iron, nickel, cobalt). The core of Earth, Io and even the Moon (if it has a core) will be at a temperature exceeding the 'Curie temperature' (770 deg C...

  • Apologies for the broken link to the Io image. I have asked for it to be repaired ASAP. In the meantime, you can find it at the following link, where it is the right-hand image of the pair https://photojournal.jpl.nasa.gov/catalog/PIA02584

  • Its not just density that matters, but also material strength. Dense and weak would become spherical at a smaller size, low density and strong would become spherical at a bigger size.

  • Two reasons why not: 1) the mantle is made of rock, so it s not an electrical conductor, 2) even if party molten it would be too voscous for rapid convection. For dynamo-generation of a magnetic fiel you need a vigorously convectiong electrical conductor (such as molten iron).

  • This is a solid suface, with water-ice pebbles sitting on it. The methane (with some ethane mixed in) lakes are mostl at high northern latitudes. You will see these later. The Huygens lander that took this pictre from the ground was designed to float in case it did land in a methane lake. It also had a penetrometer, built at the Open University, so measure how...

  • Not sure what you mean Susan. The tidal bulges caused by the Sun and Moon on the Earth (or Earth's oceans) dont need to be 'made'. They already exist. Each propagates around the globe as we rotate, trying to stay lined up with the Sun and Moon. We dont experence individual Sun and Moon bulges, we just see the combined effect of both.

  • Jupiter has rings too (so to Uranus and Neptune), but they are much less spectacular than Saturn's. No one seriously suggests that you should try to count all the particles in a ring and call each one a moon.

  • It's not that simple. It depends on what stage you regard as a body's birth. You culd say that the Earth didnt fininish forming until the impact that (probably) created the Moon. In that case they are pretty much the same age. You will learn more later.

  • This is a 10m long object beyond the Moon. No telescope can show any detail. These things are hard even to track, and we cant be sure of its previous trajectory - hence the uncertainty in its identity and origin.

  • Thanks John. I've reported this.
    The link is to a graphic hosted on OpenLearn, which is the OU's own platform. I think it’s a mofre widespread general OpenLearn problem, that with luck should be fixed soon.

  • What fraction of Earth’s sky is hidden by the Moon? The Earth hides somewhat more of the Moon’s sky but that is still only a tiny fraction of incoming projectiles intercepted.

  • Sorry, the 2 March live webcast has been cancelled. I'm not going to offer this while the Universities and College Union is on strike over a fraudulent valuaton of our pension scheme. I'm hoping we can offer the webcast instead in Week 7.
    I have edited step 1 of Week 3 to reflect this.

  • Yes, but it is physically meaningful too.

  • Ken, you are out of date, and also rude to our American friends. The 'f' spelling was agreed by international scientific convention in 1990 and endorsed by the Royal Society of Chemisty (is that good enough for you?) in 1992. The former 'ph' spelling was an historical affectation to make the word look Greek in origin, which it isn't.

  • Waves are caused by wind, not tides.
    There is almost no tide even in the Mediterranean Sea, which is a lot bigger than Lake Michigan (especially east-west, which is the more important dimension).

  • The ecliptic is the plane of the Earth's orbit about the Sun. The equatorial plane is the plane of the Earth's equator, which is tilted at 23 degrees relative to its orbital plane.

  • The Moon is responsible for about 2/3 of the tide in the ocean (1/3 is from the Sun) but is nothing to do with storm winds.

  • You've got it :-)

  • The 'black tendril' in the right-hand image is a surface lava flow, of basalt o something similar.

  • We aren’t going to see why more of Peggy without a spacecraft at Saturn. The Cassini mission has ended.

  • Volcanic dust just scatters more light and would make it even harder to see the Moon close to the Sun.

  • @Jean-JacquesWintraecken I suspect this would not last long, because neither Pluto nor Charon is perfectly spherical and the probe would probably drift, but I'm no expert in this field. Currently we can't get into any kind of orbit about Pluto, because a probe from Earth that took only a few years to get there (like New Horizons) would arrive travelling way...

  • You can have a vollcanc eruption only if there is magma (molten rock) waiting to be erupted. Fracturing the crust may open a pathway, but it wont usually cause melting in the mantle .

  • I suppose if you managed to get a space probe staionary at the Pluto-Charon barycentre that might be a stable location. It wouldnt be pulled apart, but it might drift off-station.

  • It's the air molecules that scatter the sunlight, so the sky is always bright when the Su is above the horizon, no matter what the local conditions.

  • @NickCharlton Kepler's laws concern the orbit of a single body round a primary. They have nothing to say about multiple systems. It is not coincidence, it is repeated gravitational tugs each time an inner moon overtakes an outer moon that can eventually bring orbits into resonance.

  • The barycentre of the Earth-Moon system is inside the Earth, as that Wikipedia article correctly states. Pluto-Charon is the most massive example I know of where the barycentre is not inside one of the bodies.

  • It is anti-clockwise as seen looking down from way above the Earth’s north pole. The direction in which a clock’s hands move mimics the direction of movement of the shadow in a sundial in the northern hemisphere. How we will define N and S in an exoplanet system has not, so far as I know, been decided.

  • Go back to 1.5 and watch the Mythbusting moons video again :-)

  • I'm glad you've enjoyed it so far.

  • Hmm. All known comets come much closer to the Sun than Pluto ever does.

  • You are unlikely to be able to see a waxing crescent as it rises, because the Sun will already have risen and be quite close to the Moon in the sky. It s much easer to see a waxing crescent as it sets, because the Sun will already have set and be out of the way.
    For similar reasons, you can see a waning crescent as it rises (the Sun will rise afterwards), but...