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


  • Wait until the Moon myths video and you will learn that #2 and #3 are wrong.

  • This is not strictly true. It needs the presence of other moons in resonant orbits to maintain a moon’s orbit in a sufficiently eccentric orbit for tidal heating to be effective. You will learn about tidal heating later in the course.

  • Name one lunar mineral resource that would be economic to extract from the Moon and bring to Earth.

  • That’ll be an instrument the size of a tiny moon then ;-)

  • When did you order it? You’ll need to take that up with OUP though.

  • According to this https://planetarynames.wr.usgs.gov/SearchResults?target=TITAN&featureType=Mare,%20maria Punga Mare is named after the father of sharks and lizards, son of the sea god Tangaroa

  • Thanks Paul :-)

  • ALTERED glass would be no use in telling us about the original water content. If we could be sure that the alteration occurred AT THE TIME of eruption (rather than later) then water in the altered glass would tell us that water was present during eruption, but not its concentration, which is what the question asks. Only the tiny melt inclusions, trapped within...

  • Why are you complaining about having to remember stuff? You are free to go back and find out the info to help you answer correctly.

  • Week 1.19. The Moon takes 29 Earth-days to go through one cycle of phases, so if you think about it that has be be its day-night cycle too.

  • Cold traps. No where other than the permanently shadowed regions is cold enough for water molecule to stick indefinitely.

  • To what?

  • It is not. All Uranus’s moons orbit close to Uranus’s orbital plane, and their spin axes are at 90 deg to it. They share the tilt of their planet.

  • It is neither really. It is conversion of gravitational potential energy into kinetic energy (which goes up as the square of the speed, and then conversion of that to heat upon impact.

  • You will be welcome back. I expect the course will run again at the same time next year.

  • There are 3 coronae on the part of Miranda that has been seen. By IAU convention, each is named after a location in a Shakespeare play: Arden, Elsinore and Inverness.

  • Apparently yes, from the phylogenetic evidence. So far no genomes have been found showing evidence of an independent genesis.

  • Probably the same time next year.

  • It’s meant to be a generic “seaweed”. Surely that’s more improbable than the “jellyfish” or the “shrimp”?

  • Nice thought, but there is no way that Enceladus or any of Saturn’s moons has been warm enough at the surface for water-ice to melt. Moons near Enceladus have very ancient heavily cratered regions, and there are plenty of craters on Enceladus too. Globally, surfaces have “always” been frozen

  • This is a mosaic constructed from a series of images. It cannot be otherwise, because at any one instant half of this area must be in darkness. I will amend the caption to make that clear.

  • Now fixed. Sorry about that.

  • Thanks for pointing that out Beate. Sadly there is no 'non-Flash' version of the Tree of Life graphic available, so I have removed the link.

  • Colours in transmitted light using polarising filters bear no relationship to what you see in reflected light. You just have to get used to them. All we attempt here is to give you a flavour.

  • Earth’s rocks are younger than the age of the Earth because there has been a lot of recent geological activity - that’s what makes rocks.

  • I think you mean what mineral will contain potassium. The will be traces of it in feldspars, which are common in basalt and anorthosite @DilysHarlow

  • It is based on measuring infrared radiation at several wavelengths.

  • The (unlabelled) wrinkle ridge is drawn with a couple of steep faults beneath it, not a dyke. I wouldn’t have drawn them so steep - but it’s all a matter of interpretation anyway.

  • I didn't notice an earlier reference, sorry. The 2nd edition of Satellites of the Outer Planets was 1999. I have no plans for a 3rd edition. However I did write this https://global.oup.com/academic/product/moons-a-very-short-introduction-9780198735274 a bargain at only £8.99

  • Fair point, but have you also considered what a VERY long time a million years is?

  • The comparison would be valid if the Earth was airless. However, because we have an atmosphere, air gets drawn in to eruption columns where it is heated, and most of the rise height is because of convection.

  • When you go out at night, how often do you see a 'shooting star' in the sky? Those are mostly sand-dust sized particles. From that you can judge how often you might be hit by one if the Earth had not atmosphere to protect you. The risk would be the same on the Moon.

  • Hmm. Try hitting a golf ball while wearing a spacesuit and using a makeshift club @SallyChurch

  • All correct except at the end. Basalt is the rapidly cooled, hence fine grained, equivalent of gabbro (which has cooled slowly and so has larger crystals).

  • Can you see the picture at the top? S/2004 N1 is labelled and ringed in yellow.

  • Read the caption. It is 214 million years old. There has been a lot of erosion since then.

  • Where have you seen pictures of whales in Lake Manucuoagan? It is fresh water.

  • What would you leave out?

  • What device are you on? I have just tried on my iPhone and it’s fine. If you have a slow internet connection you will need to be patient.

  • It depends what you mean. There are multi-ring basins on the Moon and the Moon survived, despite being small. If the target was a life-bearing world the environmental impact would be very severe though.

  • There is no air where moons orbit.

  • @KarenMartin for the live event this time see step 4.1. There is a link there to the unedited recording.

  • There is no reason for near and far sides to ‘age’ at different rates. The Earth is not a significant heat source, and it’s not heat that ‘ages’ a surface anyway. It is bombardment by meteorites, and exposure to solar and cosmic radiation.

  • 'crewed' mission is the accepted term these days please.

  • Tidal heating can't change one substance into another. However it can cause a moon to lose its more volatile components. For example, Io could have begun like Europa with 100-400 km of ice above its rock, but lost all its ice (to space) because of tidal heating.

  • @LouiseT TheChicxulub crater is 150 km in diameter. If you look on Wikipedia you will see impactor size estimated at 11-80 km. The upper end of that range seems very high to me, and it would have to be a low density body. It depends where you define outer space as beginning, because the Earth’s atmosphere has no sharp edge. The legal definition, the Karman...

  • That’s right.

  • I have re-read it carefully and I can find nothing out of date. What specifically do you think is out of date?

  • .

  • Gravity can’t be significantly less at any point on an essentially spherical world @RogerThomas

  • There will be a new live Q and A on 8 Mar. It was announced in at least on weekly email and in step 4.1.

  • Only the tiny ones. Why are there so few decent sized craters?

  • There is a live Q and A session next week, 19:30 GMT Tuesday. It was in weekly email and is noted in Step 4.1.

  • Try turning the picture upside down - that often helps.

  • Temperature does not affect gravity. In a vacuum objects would fall at the same rate irrespective of temperature.

  • Impactors can 'skim' along a curved surface Catriona. Alfie has the right explanation.

  • Catriona is right. 'Tectonics' does not have to imply plates. 'Plate tectonics' refers to the way the Earth's outer shell (its lithosphere) moves as rigid plates over the weaker interior. Europa's outer shell moves differently, and the process isn't the same as plate tectonics on Earth. Deformation of the crust of the Moon or Mars or Venus is just tectonics...

  • There is no need for an external visitor in the Nice Model.

  • That’s not a figure that I recognise, and it’s too high for the cumulative number since this course was first presented. What do you mean by “home page” @JanCantle ? No such number at https://www.futurelearn.com/

  • What ice are you referring too? It is all fairly close to the density of water.