Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION

Transcription

1 Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION CHAPTER 8 Moons, Rings, and Plutoids Lecture Presentation

2 Lower right: Europa and the shadow of Io, have now been joined by 12 recently discovered moons! moons of Jupiter and Counting! Top left: shows Galilean moon Io floating above Jupiter

3 8.0 Moons, rings, plutoids oh my! Largest known trans-neptunian objects, aka Plutoids: newly found pieces of the Solar system!

4 8.1 The Galilean Moons of Jupiter All four Jovian planets have extensive moon systems, and more are continually being discovered. The Galilean moons of Jupiter are those observed by the astronomer Galileo (in 1610): Io, Europa, Ganymede, and Callisto the largest, Ganymede is larger than Mercury! Galilean moons have two major similarities to terrestrial planets: orbits have low eccentricity and the moon densities decrease as the distance from Jupiter increases.

5 8.1 The Galilean Moons of Jupiter The proto-sun and proto-jupiter created conditions favorable to rocky planets/moons near them:

6 8.1 The Galilean Moons of Jupiter The Galilean moons and their orbits. Which orbits closest to Jupiter?

7 8.1 The Galilean Moons of Jupiter Their (probable) interiors: Io is rocky Europa and Ganymede may have (liquid) water in oceans below their frozen (solid) surfaces. Callisto may be an ice/rock conglomerate.

8 8.1 The Galilean Moons of Jupiter Their surfaces are also very different: Io is volcanic, Europa and Ganymede may have been resurfaced by (liquid) water, Callisto has an old, heavily-cratered surface.

9 8.1 The Galilean Moons of Jupiter Io is the densest of Jupiter s moons, and the most geologically active object in the solar system. It has many active volcanoes, some quite large! (How interesting! How is that possible? Remember on Earth this is (mostly) the result of plate tectonics.) Io can change surface features in a few weeks. Io has no craters; they fill in too fast. Io has the youngest surface of any solar system object.

10 8.1 The Galilean Moons of Jupiter Io is very close to Jupiter and also experiences gravitational forces from Europa. The tidal stretching is huge, and provides the energy for the volcanoes.

11 8.1 The Galilean Moons of Jupiter Europa has no craters; surface is water ice, possibly with liquid water below. Tidal (or perhaps plate tectonics) forces stress and crack ice: water flows out from the cracks, keeping surface relatively (new) and flat.

12 8.1 The Galilean Moons of Jupiter On Europa tidal (or perhaps plate tectonics) forces stress and crack ice: water flows out from the cracks, keeping surface relatively (new) and flat.

13 8.1 The Galilean Moons of Jupiter Europa s surface appearance: is quite similar to artic pack-ice: (à )

14 8.1 The Galilean Moons of Jupiter Europa also shows evidence of plumes of water/ice, called (cryo-) ice volcanoes see images below!

15 8.1 The Galilean Moons of Jupiter Ganymede is the largest moon in the solar system larger than Pluto and Mercury! It has some similarities to Earth s Moon, but with water ice ( lava flows ) instead of lunar rock.

16 8.1 The Galilean Moons of Jupiter Ganymede has some similarities to Earth s Moon, but with water ice ( lava flows ) instead of lunar rock.

17 8.1 The Galilean Moons of Jupiter Callisto s surface is the oldest and most heavily cratered in the Solar System with no evidence of resurfacing activity.

18 8.2 The Large Moon of Saturn Titan has been known for many years to have an atmosphere thicker and denser than Earth s: 98.4% nitrogen and 1.4% methane. Titan s cloudy atmosphere makes it impossible to see the surface; the picture at right was taken from only 4000 km away.

19 8.2 The Large Moon of Saturn Trace chemicals in Titan s atmosphere make it chemically complex: methane rain think about it methane rivers, methane lakes!

20 8.2 The Large Moon of Saturn Infrared image of Titan, shows some surface detail and possibly an ice volcano. There are few craters, consistent with surface weathering. Complex chemical interactions occur in atmosphere.

21 8.2 The Large Moon of Saturn Based on measurements made by Cassini and Huygens, the image, lower left, is the current best guess as to what the surface of Titan looks like: a fairly flat surface with hydrocarbon (methane/ethane) lakes! Ligeia Mare, lower right, is similar in size to Lake Superior!

22 8.2 The Large Moon of Neptune Triton is in a retrograde orbit and is tidally locked to Neptune; its surface has few craters, indicating active resurfacing. Nitrogen geysers have been observed on Triton, contributing to the surface features. Also, there appear to be ice volcanoes (see frozen lake in top image.)

23 8.3 The Medium-Sized Jovian Moons Densities of these medium-sized moons suggest that they are rock and water ice.

24 8.3 The Medium-Sized Jovian Moons Medium-sized moons of Saturn, in natural color Note the similarities, as well as the large crater on Mimas.

25 8.3 The Medium-Sized Jovian Moons Saturn s medium-sized moons are all tidally locked this means they have leading and trailing surfaces check the text to see how this makes a difference to the appearance of these moons! Mimas, Enceladus, Tethys, Dione, and Rhea are relatively close to Saturn with orbits between 3 and 9 planetary radii from Saturn Iapetus orbits 59 radii away and is also tidally locked

26 8.3 The Medium-Sized Jovian Moons Enceladus appears to be an ice-covered moon with youthful terrain in the south (region with few craters). The long blue tiger stripes are about 1km wide fractures in the ice through which water/ice volcanoes spread icy ash over Enceladus surface.

27 8.3 The Medium-Sized Jovian Moons Are the water/ice volcanoes that spread icy ash over Enceladus surface something like snowmaking machines used in ski resorts to re-surface the slopes with fresh snow??

28 8.3 The Medium-Sized Jovian Moons

29 8.3 The Medium-Sized Jovian Moons Medium-sized moons of Uranus and Neptune

30 8.3 The Medium-Sized Jovian Moons Miranda shows evidence of a violent past, although the origin of the surface features is unknown.

31 8.4 Planetary Rings The ring system of Saturn is large and complex and is easily seen from Earth. The other Jovian planets have ring systems as well.

32 8.4 Planetary Rings (Saturn) Saturn s ring particles range in size from fractions of a millimeter to tens of meters (in diameter). Composition: Water ice similar to snowballs Why rings? For moons with their own gravity holding them together, tidal forces near large planets tear them apart (see Roche limit next)! Details of formation of Saturn s rings are unknown: -- Too active to have lasted since birth of solar system -- Either must be continually replenished, or are the result of a (somewhat recent) catastrophic event

33 8.4 Planetary Rings The Roche limit is where the tidal forces of the planet are too strong for a moon to survive, numerically about 2 ½ planet radii, where rings are formed.

34 8.4 Planetary Rings (Saturn) Saturn s rings are not solid; they are composed of small rocky and icy particles. All but E are within the Roche limit. We learn in a few pages why the E -ring is different!

35 8.4 Planetary Rings The ring systems of all 4 Jovians are consistent with the Roche limit.

36 8.4 Planetary Rings (Saturn) Voyager probes showed Saturn s rings to be much more complex than originally thought: see next few pages! Earth is shown on the same scale as the rings.

37 8.4 Planetary Rings Shepherd moons define the edges of some of the rings. The strangest ring is F -ring; it appears to have braids and kinks F -ring s oddities are probably caused by two shepherd moons, one of which can be seen here:

38 8.4 Planetary Rings (Saturn) This backlit view shows the fainter F, G, and E rings (Cryo-) ice volcanoes on Enceladus are providing the particles in the E -ring!

39 8.4 Planetary Rings Recall that all 4 Jovians have ring systems. Jupiter has the most minimal ring system with one, small, thin ring.

40 8.4 Planetary Rings Uranus has nine thin rings, named after the first 9 Greek letters. The inset (top) shows the Epsilon ring.

41 8.4 Planetary Rings Two shepherd moons keep the Epsilon -ring of Uranus from diffusing away.

42 8.4 Planetary Rings Neptune has five rings, three narrow and two wide.

43 8.5 Beyond Neptune Pluto was discovered in It was thought to be needed to explain irregularities in the orbits of Uranus and Neptune, but it turned out that there were no such irregularities.

44 8.5 Beyond Neptune For 85 years little more was known about Pluto We now know that Pluto is similar in size and density to Triton (probably a captured Kuiper belt object). The amazing images of Pluto were taken by the New Horizons satellite in July 2015 This backlit image ( à ) shows a 40 km deep nitrogen atmosphere containing more than a dozen layers of haze.

45 8.5 Beyond Neptune Pluto s moon, Charon, was discovered in It is about ½ the size of Pluto. Charon and Pluto are mutually tidally locked: thus one side of Pluto faces Charon and one side of Charon faces Pluto! Pluto also has four smaller moons: Nix, Hydra, Styx, and Kerberos.

46 8.5 Beyond Neptune Charon also displays a surprisingly complex geological history with: smooth planes, surface fracturing, tall mountains and heavy cratering.

47 8.5 Beyond Neptune (some history) From 1930 until relatively recently Pluto was a planet so what happen? What happened was that other Pluto-like objects were discovered, past the orbit of Neptune, creating a dilemma. In 2006, the International Astronomical Union (IAU) made the first official definition of planet. A planet must: 1. Orbit the Sun 2. Be massive enough that its gravity keeps it spherical 3. Clear its orbit of other debris Pluto does 1 and 2 but nominally not 3.

48 8.5 Beyond Neptune Pluto s orbit is eccentric and inclined to the plane of the ecliptic; it also crosses the orbit of Neptune

49 8.5 Beyond Neptune The Kuiper belt is outside the orbit of Neptune and has many icy chunks The first Kuiper belt objects were observed in the 1990s, and now more than 2000 are now known. Some of them are comparable in size to Pluto (this was the dilemma ). This image show Eris and its moon Dysnomia.

50 8.5 Beyond Neptune This figure shows several of the largest known Kuiper belt objects. Most recently these are also called trans-neptunian objects with the largest called Plutoids. So the 9 th planet dilemma morphed into a new class of large Moon-size objects called Plutoids!

51 8.5 Beyond Neptune And the LSST, currently under construction, should discover 10s of thousands of new trans-neptunian objects and effectively map the Kyper Belt. LSST Image Gallery The Large Synoptic Survey Telescope ( 2/27/18, 11:45 AM PUBLIC & SCIENTISTS (/) ( LSST CORPORATION ( SEARCH (/) Large Synoptic Survey Telescope () Opening a Window of Discovery on the Dynamic Universe ABOUT (/ABOUT) NEWS (/NEWS) SCIENCE GOALS (/SCIENCE) PARTICIPATE (/PARTICIPATE) GALLERY ( FOR SCIENTISTS (/CONTENT/LSST-INFORMATION-SCIENTISTS) Home (/) / LSST Image Gallery Telescope LSST Image Gallery (/gallery/telescope) (21) Site (/gallery/site) (84) CHECK OUT THE NEW GALLERY! ( Mirror Fabrication (/gallery/mirror-fabrication) (31) Artist Conceptions (/gallery/artist-conceptions) New! Construction Update (/gallery/collection/construction-update-16) ( (13) Community (/gallery/community) (21) Data (/gallery/data) (8) Camera (/gallery/camera) (15) ( ( Logos (/gallery/logos) (3) Videos (/gallery/videos) (41) A three dimensional rendering of the baseline design of the dome with a cutaway to show the telescope within.

52 8.5 Beyond Neptune No objects (other than long period comets) have been observed in the Oort cloud it is simply too far away.

53 Summary of Chapter 8 The outer solar system has 6 large moons, 12 medium ones, and many smaller ones. Titan has a thick atmosphere and may have flowing rivers of methane. Triton has a fractured surface and a retrograde orbit. Medium-sized moons of Saturn and Uranus are mostly rock and water ice. Saturn s rings are complex, and some are defined by shepherd moons.

54 Summary of Chapter 8 (con t) The Roche limit is the closest a moon can survive near a planet; inside this limit, rings form instead. Jupiter, Uranus, and Neptune all have faint ring systems. Pluto has five moons: Charon, Nix, Styx, Kerberos, and Hydra. Dwarf planets beyond Neptune (including Pluto) are now known as Plutoids.