Download Week 5 File

Astronomical telescopes and basic observing techniques There are 3 different types of telescopes that are commonly used 1) Refractor – this uses a system of lenses to collect and focus the light from distant objects. Galileo was the first person to use a refractor (or any telescope) to undertake astronomical observaFons in 1610. 2) Reflector – this uses a system of mirrors to collect and focus the light from distant objects. Newton was the inventor of the reflecFng telescope, and most modern telescopes used by professional astronomers are reflectors 3) Catadioptric telescopes use a combinaFon of lenses and mirrors to focus light from distant objects. These have the advantage that the opFcal tube can be shorter than for Newtonian reflectors. And it is easier to aNach a CCD camera and balance the telescope. Basic concepts Astronomical telescopes have two basic funcFons: 1) CollecFng a large number of photons so that faint objects may be studied 2) FormaFon of a large image of a distant object so that it may be studied in detail Compared to the size of the telescope, all astronomical objects are so far away that they can be considered to be at infinity -­‐> all rays from an object (e.g. a star) may be considered to be parallel (and perpendicular to the wave-­‐fronts) The figure below shows a schemaFc diagram of a basic telescope. Light from two distant sources (i.e. stars, or two points associated with an extended object), separated by an angular distance θ on the sky, is collected through an aperture of diameter D and is focused (by a lens or curved mirror) onto the focal plane. The size of the image, s, is given by s = F x θ (assuming θ is small, so Tanθ = θ) where F is the focal length of the objecFve lens that sits in the aperture. To study a distant object in detail it is clear that s needs to be large, so the focal length F needs to be large. The focal raFo of a telescope is defined as f = F/D. i.e. If a telescope is described as being f/11 then f = F/D = 11. Refractor telescope at CincinnaF Observatory in 1848 The largest refractor telescope at Yerkes Observatory (40-­‐inches) The simple telescope shown in the previous slide works by taking the parallel rays associated with the distant source and focusing them to a point on the focal plane. We see from the diagram on the right that the planar wave-­‐fronts associated with the parallel rays become curved as they pass through the lens in the aperture. A lens focuses the incoming light rays by refracFng them. Obtaining a unique focal point for parallel rays emanaFng from a point-­‐like source is achieved by shaping the lens surface appropriately. Hence the name refractor. A reflecFng telescope uses a curved mirror to focus light rays – a parabolic-­‐shaped mirror is required to provide the best focus. The analysis of how telescopes work provided in the next few slides uses refracFng telescopes as the basis of discussion, but the same analysis also applies to reflectors. SchemaFc view of refracFng telescope with eyepiece for visual observing Astronomers do not normally use telescopes for visual observing because the eye is not a very sensiFve detector, and cannot register a permanent image of an object for later analysis. The objecFve lens forms an image of a distant object of finite size, or of two or more point-­‐like objects (e.g. two stars) in the focal plane. The size of the image formed is s = F x θ1 (where F is the focal length of the objecFve) and θ1 is the angle separaFng the two stars on the night sky) The eyepiece magnifies the image formed in the focal plane by the objecFve lens. It converts the diverging rays formed by the objecFve into parallel rays, and these are then focused into an image on the reFna by the human eye. We see from the diagram that s = Fe x θ2 where Fe is the focal length of the eyepiece, and θ2 is the apparent angular separaFon of the two stars on the sky. The magnificaFon is defined by the raFo of the angular sizes of the image: M = θ2 / θ1 = F / Fe. This shows that the magnificaFon is not an intrinsic property of a telescope, since we can increase the magnificaFon by simply changing the eyepiece to one with a smaller focal length Fe. Remember: M = F / Fe Observing with a CCD camera or similar imaging device Now consider the situaFon where a CCD detector is placed in the focal plane, so that a focused image is formed on the detector. Note that in the diagram on the right the image size is denoted by ‘y’ instead of by ‘s’ as was the case in the previous diagrams. SchemaFcally, this is the set-­‐up used in professional observatories. F The diagram shows that two objects (stars) with angular separaFon θ on the sky will form an image in the focal plane, with the two objects being separated by distance y. Simple trigonometry tells us that y = F x θ where F is the focal length (and we have assumed Tan θ = θ because the angle is small). The plate scale is defined by p = θ / y (the number of radians per mm if we measure the lengths F and y in mm). Thus we have p = 1 / F -­‐> as we increase the focal length the linear separaFon of two points that have angular separaFon θ increases. Astronomers normally measure angles in arcseconds (because the angles they deal with tend to be small), so the plate scale then becomes p = 206265 / F (there are 206265 arcsecs in a radian). p is measured in arcseconds per mm Why is the plate scale important? A parFcular telescope comes with a fixed focal length. A CCD detector comes with a fixed size (normally close to 10mm x 10mm). The plate scale tells us how much of the sky (the observable area in arcsecs2) can be imaged with a parFcular telescope and CCD camera. A telescope with a plate scale p combined with a CCD detector measuring A x B mm2 will give an image that is (p.A x p.B) arcsecs2. A CCD detector will have a specific number of pixels (N x M pixels, where N and M are normally about 1000). We can therefore work out the angular distance on the sky that is imaged by each pixel (the CCD pixel scale), and therefore the maximum resoluFon of the set up (i.e. the minimum separaFon between two stars that can be imaged) A telescope with plate scale p combined with a CCD detector that is of size A x B mm2 and has N x M pixels will give rise to an image that corresponds to (p x A) / N arcsecs in the ‘x-­‐direcFon’ and (p x M) / B arcsecs in the ‘y-­‐direcFon’. i.e. The number of arcsecs per pixel can be calculated as: -­‐> (the number of arcsecs per mm) / (the number of pixels per mm) OR -­‐> (the number of arcsecs imaged on the CCD) / (the number of pixels on the CCD) Both these ways of thinking give rise to the expressions above. (See the worked example in the on-­‐line hand-­‐wriNen notes). ESO’S European-Extremely
Large Telescope (E-ELT)
f/0.93 Primary mirror=39 m 798 segments Thirty Meter Telescope (TMT)
Thirty Meter Telescope (TMT)
James Webb Space
Telescope (JWST)
DiffracFon limited resoluFon and “astronomical seeing” The ability to resolve two objects that have small angular separaFon, θ, on the sky is not just a quesFon of choosing a telescope with an appropriate focal length and a CCD camera with the right number of pixels. A fundamental limit on our ability to resolve two objects arises because of diffracFon of the advancing wave-­‐fronts as they pass through the aperture of the telescope. To illustrate how this phenomenon works we first consider single-­‐slit diffracFon (see next slide). The lek diagram looks down on a narrow slit of width D and shows two light rays incident on the screen at distance L from the slit. The right diagram shows the single-­‐slit diffracFon paNern Consider light passing through a narrow slit of width D, and being incident on the focal plane (or screen) that is a distance L from the slit. Consider the posiFon on the screen that is distance y from the opFcal axis. A ray that emanates from the edge of the slit can be associated with another ray that emanates from the centre of the slit at distance D/2 away. The difference in the path length of the two rays is given by D/2 sinθ, where θ is the angle between the opFcal axis and line joining the point on the screen and the centre of the slit. If this path length difference equals λ/2, where λ is the wavelength of the incident light, then we will have destruc1ve interference and a dark interference fringe. Thus we have a dark fringe when D/2 sinθ = λ/2 or when sinθ = λ/D We can also consider a ray that emanates from D/4 away from the edge of the slit, and we will have a dark fringe if D/4 sinθ = λ/2 or sinθ = 2λ/D We can also consider a ray that emanates from D/6 away from the edge of the slit, and we will have a dark fringe if D/6 sinθ = λ/2 or sinθ = 3λ/D Generalising, we can see that we will have dark fringes when sinθ = mλ/D where m is an integer Remember: θ is the angle subtended by the opFcal axis and the locaFon on the screen being considered A circular aperture gives rise to a circular diffracFon paNern called an Airy disc: Easily resolved Just above resoluFon limit Just below resoluFon limit A central peak containing approximately 80% of the light is surrounded by rings that correspond to minima and maxima of brightness. The first minimum occurs where sinθ = 1.22 λ/D For two point-­‐like objects on the sky, separated by a small angle θ, we say that these can be just resolved by a telescope of aperture D when observing at wavelength λ if: sinθ = 1.22 λ/D Ex.: The diffracFon limited resoluFon of an 8.2-­‐m telescope at 2.5μm is 0.077 arcseconds. Note: 1radian = 206265 arcseconds (i.e. when the angular distance between them just equals the angular distance between the central maximum and the first minimum of the Airy disc of one of the stars). This is called Rayleigh’s criterion. Examples of star images close to the resoluFon limit are shown in the top right diagram. Astronomical seeing or just “seeing” refers to the fact that turbulence in the Earth’s atmosphere causes blurring of astronomical images, as shown in the two animaFons: Seeing is due to the fact that density and temperature changes in the Earth’s atmosphere, associated with turbulent cells, change the local refracFve index of the atmosphere and therefore distort the paths of light rays as they pass through the atmosphere. Two ways around this problem: Go to space AdapFve opFcs Our Solar System – an overview The solar system consists of the Sun around which orbit 8 planets, 5 dwarf planets, and many 100’s of thousands of minor bodies (asteroids, kuiper-­‐belt objects, comets). 6 of the planets have one or more orbiFng satellites. Dwarf planets, asteroids and kuiper belt objects may also have satellites. Except for Mercury the planets have orbits that are nearly circular – their orbits are ellipFcal as described by Kepler’s law, but the eccentriciFes are small (e < 0.1). The orbits also almost lie in the same plane -­‐ inclinaFons relaFve to Earth’s orbit plane are small, typically < 4o, except for Mercury. DefiniFons: A planet The definiFon of a planet set in 2006 by the InternaFonal Astronomical Union (IAU) states that, in the Solar System, a planet is a celesFal body which: 1). is in orbit around the Sun, 2). has sufficient mass to assume hydrostaFc equilibrium (a nearly round shape), and 3). has "cleared the neighbourhood" around its orbit. A dwarf planet A non-­‐satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet". According to the IAU, "planets and dwarf planets are two disFnct classes of objects” Although controversial, these definiFons can be understood more clearly by noFng that the 5 dwarf planets of the solar system are members of either the asteroid or kuiper-­‐belt, so their gravitaFonal influence has clearly been unable to clear their neighbourhood of other bodies. The planets, on the other hand, occupy orbits that are disFnct from other bodies, except for co-­‐orbital trojan asteroids which are trapped in special orbits by Jupiter and Neptune. Orbital characterisFcs The planets can be divided into: -­‐ bodies orbiFng relaFvely close to the Sun with compact orbits – the 4 inner planets -­‐ bodies with more distant orbits that are more widely separated – the 4 outer planets We note that Mercury has the most eccentric orbit with e = 0.206 and the most inclined orbit with i=7o relaFve to the Earth’s orbit plane (the eclipFc). Physical properBes – Terrestrial planets: The planets fall into two categories – four small inner bodies, and four large outer ones. The inner planets are called terrestrial planets because they resemble the Earth (terra means Earth in LaFn). Mercury, Venus, Earth and Mars are largely made of rock and metal with thin or non existent atmospheres. Their mean densiFes are in the range 3934 ≤ ρmean ≤ 5515 kg m-­‐3. Note: The density of rock on the surface of the Earth is approximately 3000 kg m-­‐3. Physical properBes – Jovian planets: The four outer planets are called Jovian planets because they resemble Jupiter (Jove was another name for the Roman god Jupiter). These planets have thick gaseous atmospheres and generally show disFnct cloud features. Their mean densiFes are in the range 687 ≤ ρmean ≤ 1638 kg m-­‐3 -­‐> their bulk composiFons are very different from the terrestrial planets. The sizes and masses of the Jovian planets are much larger than the terrestrials. Jupiter is 318 Fmes more massive than Earth. Uranus is 14.5 Fmes more massive The mean densiBes of the planets are related to their chemical composiBons. Rocks on the surface of the Earth have a density of 3000 kg m-­‐3. The mean density of the Earth is 5515 kg m-­‐3 –> gravitaFonal compression + Iron + Nickel content. Why is the Iron + Nickel in the core? How do we measure the mean densi1es of planets? Jupiter and Saturn are composed largely of hydrogen and helium, with a rock and ice core located at their centres. Hence they are oken referred to as “gas giants”. Uranus and Neptune also have rocky+icy cores, on top of which lie a layer of compressed water, methane and ammonia liquid mixtures. This liquid mantle is surrounded by a thick hydrogen and helium atmosphere. The original source of water and ammonia is believed to have been icy bodies in the cold outer regions of the solar system – similar to comets. These accreted to form the cores of the giant planets, and they also comprise most of the mass of Uranus and Neptune. Hence these planets are oken referred to as “ice giants”. The mean densiBes of the planets are related to their chemical composiBons. Rocks on the surface of the Earth have a density of 3000 kg m-­‐3. The mean density of the Earth is 5515 kg m-­‐3 –> gravitaFonal compression + Iron + Nickel content. Why is the Iron + Nickel in the core? How do we measure the mean densi1es of planets? Using Newton’s law of gravity. Jupiter and Saturn are composed largely of hydrogen and helium, with a rock and ice core located at their centres. Hence they are oken referred to as “gas giants”. Uranus and Neptune also have rocky+icy cores, on top of which lie a layer of compressed water, methane and ammonia liquid mixtures. This liquid mantle is surrounded by a thick hydrogen and helium atmosphere. The original source of water and ammonia is believed to have been icy bodies in the cold outer regions of the solar system – similar to comets. These accreted to form the cores of the giant planets, and they also comprise most of the mass of Uranus and Neptune. Hence these planets are oken referred to as “ice giants”. The mean densiBes of the planets are related to their chemical composiBons. Rocks on the surface of the Earth have a density of 3000 kg m-­‐3. The mean density of the Earth is 5515 kg m-­‐3 –> gravitaFonal compression + Iron + Nickel content. Why is the Iron + Nickel in the core? How do we measure the mean densi1es of planets? Using Newton’s law of gravity. Jupiter and Saturn are composed largely of hydrogen and helium, with a rock and ice core located at their centres. Hence they are oken referred to as “gas giants”. Uranus and Neptune also have rocky+icy cores, on top of which lie a layer of compressed water, methane and ammonia liquid mixtures. This liquid mantle is surrounded by a thick hydrogen and helium atmosphere. The original source of water and ammonia is believed to have been icy bodies in the cold outer regions of the solar system – similar to comets. These accreted to form the cores of the giant planets, and they also comprise most of the mass of Uranus and Neptune. Hence these planets are oken referred to as “ice giants”. Methods to measure planet’s interiors: 1) Gravity, 2) Oblateness (effects of centrifugal force), 3) seismology (track the path of seismic waves) Satellites All the planets except Mercury and Venus have satellites. Earth has one, Mars has two, Jupiter and Saturn more than 60 each, Uranus at least 27 and Neptune at least 13. Seven of these satellites are substanFally more massive than the rest. Some are of similar size and mass to the planet Mercury. Titan is the only satellite to have an atmosphere. Io shows acFve volcanism, and Triton shows acFve “cryovolcanism” -­‐ ice-­‐volcanoes driven by the Fdal heaFng of Triton by Neptune. Satellites All the planets except Mercury and Venus have satellites. Earth has one, Mars has two, Jupiter and Saturn more than 60 each, Uranus at least 27 and Neptune at least 13. Seven of these satellites are substanFally more massive than the rest. Some are of similar size and mass to the planet Mercury. Titan is the only satellite to have an atmosphere. Io shows acFve volcanism, and Triton shows acFve “cryovolcanism” -­‐ ice-­‐volcanoes driven by the Fdal heaFng of Triton by Neptune. Cryovolcano in Titan Asteroids and Kuiper belt objects The solar system contains relaFvely small bodies composed of rock or rock + ice Kuiper belt Asteroid belt Asteroids and Kuiper belt objects The solar system contains relaFvely small bodies composed of rock or rock + ice The asteroid belt lies between the orbits of Mars and Jupiter at semi-­‐major axes 2 – 3.5 AU. The largest asteroid, Ceres, has a radius ~ 500 km. This is the only dwarf planet in the asteroid belt. Pallas and Vesta have radii ~ 250 km. hNp://www.nasa.gov/feature/jpl/new-­‐animaFon-­‐takes-­‐a-­‐colorful-­‐flight-­‐over-­‐ceres The distribuFon of sizes is conFnuous, ranging from objects the size of boulders (i.e. 10’s of metres) all the way up to Ceres. Small objects are more numerous than large ones. Ceres Vesta The Kuiper belt lies beyond the orbit of Neptune -­‐> a region from 39 – 70 AU containing a large number of icy bodies referred to as “Kuiper belt objects” (KBOs). Pluto+Charon are KBOs. They are caught in a 3:2 mean moFon resonance with Neptune –> Neptune orbits the Sun 3 Fmes for every 2 orbits of Pluto. Numerous other bodies are in the 3:2 resonance, and resonances such as the 2:1 are also populated by KBOs. Pluto & Charon The Kuiper belt hosts 4 dwarf planets (masses in Earth masses are shown in brackets): Pluto (2.2 x 10-­‐3), Eris (2.8 x 10-­‐3), Haumea (6.7 x 10-­‐4) and Makemake (6.7 x 10-­‐4). For an icy body to be spherical its radius > 200 km. Many kuiper belt objects are thought to have radii larger than this, but are not confirmed dwarf planets. There are probably ~ 200 dwarf planets in the solar system. Note that Eris is now believed to be smaller than Pluto, but is sFll believed to have a higher mass due to its larger density. Haumea Pre-­‐Rose[a images Shape ellipsoidal; inferred from observaBons of its light curve. Haumea is a dwarf planet because its gravity is thought to be sufficient for it to have relaxed into hydrostaBc equilibrium. Rapid rotaBon, high density and high albedo (from crystalline water ice in its surface) -­‐> formed by a giant collision. It has two satellites. Post-­‐Rose[a images Spectroscopy – determinaBon of chemical composiBon Measurement of mean densiFes provides informaFon about bulk chemical composiFon, but direct measurement of the composiFon is most accurately provided by spectroscopy. When sunlight reflects off a planetary atmosphere some of the sunlight is absorbed at specific wavelengths by chemical compounds within the atmosphere, giving rise to an absorpFon spectrum. Comparing this absorpFon spectrum with spectra measured in the laboratory allows determinaFon of the composiFon of the atmosphere. For example, the atmospheres of Jupiter and Saturn are known to consist largely of hydrogen and helium because of spectroscopic measurements. Spectra obtained for Saturn’s moon Titan show the existence of methane absorpFon features, demonstraFng the presence of methane in its atmosphere. Spectral features in the UV show that N2 is the main consFtuent of Titan’s atmosphere, and spectral lines in the IR show the existence of complex organic molecules. Spectroscopy can also be used to determine the chemical composiFon of the solid surfaces of planets, satellites, and minor bodies such as asteroids and Kuiper belt objects. Spectral features from solid surfaces do not appear as single discrete lines, but rather as bands of absorpFon over a range of wavelengths. Comparison between the observed spectra and those measured in the laboratory allow idenFficaFon of the chemical composiFons of solar system bodies. Jupiter’s moon Europa has an infrared spectrum very similar to that of water ice measured in the laboratory. The young surface of Europa (indicated by the relaFvely few craters) indicates that the icy surface covers a liquid interior that resurfaces locaFons that become fractured by impacts. The mean density of Europa indicates that its primary consFtuents are a mixture of rock and ice. Tidal interacBons A body of finite size si}ng in the gravitaFonal field of an external mass experiences Fdal forces. These result from the variaFon of the gravitaFonal acceleraFon over the surface of the body. This variaFon arises because the gravitaFonal acceleraFon depends on the distance to the gravitaFng mass. The diagrams to the right show the forces acFng on a body due to a gravitaFng mass. SubtracFng the centre of mass acceleraFon (which just corresponds to the body as a whole being accelerated toward the gravitaFng mass), we see the Fdal forces that act to distort the surface. For the Earth-­‐Moon system, the distorFon of the Earth is manifest as the raising and lowering of ocean Fdes. GravitaFng mass DerivaFon of Fdal acceleraFon is given in the on-­‐line supplementary lecture notes, where we also demonstrate that if Fdal forces are large enough an object can be Fdally disrupted – literally pulled apart by Fdal forces. We also demonstrate how to esFmate the height of the Fdal distorFon. All gravitaFng bodies induce Fdal forces in nearby objects. In the case of the Earth-­‐Moon system, Fdal distorFon of the Earth by the Moon, combined with the Earth’s spin and Moon’s orbital moFon lead to an exchange of angular momentum between the two objects. As demonstrated in the figure, the fact that the Earth spins faster than the Moon orbits (the Earth spins once in 24 hours, the Moon orbits the Earth in 27.3 days) causes the Earth’s Fdal bulge to always be slightly ahead of the Moon. This creates a torque that pulls the Moon forward long its orbit, giving it a posiFve torque. The orbital angular momentum of the Moon is thus constantly increasing, causing it to move away from the Earth. ConservaFon of angular momentum implies that the Earth’s spin rate must be constantly decreasing. So the months are ge}ng longer because the Moon’s orbital period increases as it moves away from the Earth. And the days are ge}ng longer because the spin period of the Earth is increasing. This process will stop when the spin period of the Earth equals the orbital period of the Moon. At the present Fme the Moon is moving away from the Earth at a rate of 3.8 cm per year. Tides explain why the Moon shows the same face to Earth. The Moon was originally spinning faster than the current rate of once per 27.3 days, but its Fdal distorFon by the Earth has caused it to slow down to the point where it has achieved spin-­‐orbit synchronism. MagneFc fields Solar system bodies that possess significant magneFc fields include the Sun, Mercury, Earth, Jupiter, Saturn, Uranus and Neptune. Each of these bodies possess a global dipole magneFc field – similar in structure to a bar magnet. The presence of a magneFc field indicates that the interior of the body contains regions where the material is an electrically conducFng fluid. For the Earth and Mercury a molten iron core generates the magneFc field. In Jupiter and Saturn the deep interior contains highly conducFng “metallic” hydrogen. The magneFc fields of these bodies are conFnuously generated by a dynamo. For a dynamo to operate the following are required: i) An electrically conducFng fluid – ALL magneFc fields are generated by electric currents!! ii) RotaFon iii) ConvecFve moFons Being a small planet, Mars has cooled sufficiently for the core to be solid. Mercury’s interior remains molten because of its eccentric orbit and Fdal interacFon with the Sun. Venus lacks plate tectonics, prevenFng the interior from cooling efficiently through the surface, and therefore switching off the necessary convecFve moFons that require the existence of a strong temperature gradient to operate. Atmospheric composiFon and temperature The atmospheres of the Jovian planets are primarily composed of hydrogen and helium, but the atmospheres of the terrestrial planets are almost devoid of these elements and are composed primarily of the molecules N2, O2 and/or CO2. Why is this ? The temperature of a gas is directly related to the speeds of its molecules. At a given temperature lighter molecules move faster than heavier molecules because they each have the same kineFc energy on average. Molecular hydrogen and helium are thus able to move at the escape velocity from warmer terrestrial planets because of their relaFvely weak gravity. See the supplementary on-­‐line notes for a demonstraFon of this. The approximate calculaFon presented in the supplementary notes ignores the fact that atoms/molecules at a given temperature have a range of velociFes given by the Maxwell-­‐Boltzmann distribuFon. Even though the mean velocity of atoms may be below the escape speed from a planet, there will be a fracFon of them that have velocity above the escape velocity. As a rule of thumb, atmospheric escape is unimportant only if the mean velocity is ≤ 1/6 of the escape velocity, so that only a very small fracFon of atoms move at the escape velocity. What does it escape easier, H or CO? Why do Jupiter/Saturn retain H/He? How does the distance from the Sun affect the probability of an atmosphere to escape? Mercury – key facts (NOT EXAMINABLE) Barren, heavily cratered, devoid of an atmosphere and lacks plate tectonics to renew its surface. Impact craters mainly formed during the first ~ 800 Myr aker formaFon of the Solar System. Low-­‐lying plains were formed by lava flows generated by giant impacts puncturing the young crust. High mean density -­‐> Mercury has a large iron core filling ~ 75% of radius. Maintains a magneFc field with strength ~ 1% of Earth’s -­‐> suggests part of iron core remains molten. Mercury is in a 3:2 spin-­‐orbit resonance: planet spins on its axis 3 Fmes for every two orbital periods. Caused by Mercury’s eccentric orbit, for which the orbital angular velocity varies conFnuously round its orbit, and the fact that Fdal forces depend on the separaFon between Mercury and the Sun. Mercury – key facts Barren, heavily cratered, devoid of an atmosphere and lacks plate tectonics to renew its surface. Impact craters mainly formed during the first ~ 800 Myr aker formaFon of the Solar System. Low-­‐lying plains were formed by lava flows generated by giant impacts puncturing the young crust. High mean density -­‐> Mercury has a large iron core filling ~ 75% of radius. Maintains a magneFc field with strength ~ 1% of Earth’s -­‐> suggests part of iron core remains molten. Mercury is in a 3:2 spin-­‐orbit resonance: planet spins on its axis 3 Fmes for every two orbital periods. Caused by Mercury’s eccentric orbit, for which the orbital angular velocity varies conFnuously round its orbit, and the fact that Fdal forces depend on the separaFon between Mercury and the Sun. The top diagram shows Mercury’s internal structure. The lower diagram demonstrates the 3:2 spin-­‐orbit resonance. Mercury’s spin period deduced in 1965 from radar measurements that showed spreading of the frequency of the returning radar signal due to the Doppler shik induced by the rotaFng planet. Eccentric orbit -­‐> Fdal acceleraFon strongest at perihelion – note the strong dependence of Fdal acceleraFon on the distance between two bodies (i.e. the Sun and Mercury). Tides try to force Mercury to spin with an angular velocity that is close to its orbit angular velocity at perihelion (posiFon labeled 1,5,9). Tides also try to ensure that the axis of the Fdal bulge points toward the Sun at perihelion. Combining these requirements leads to the observed 3:2 spin-­‐orbit resonance. Note: the term resonance is used to described a situaFon when there is a simple integer relaFonship between orbital periods, or orbital periods and spin periods. Venus – key facts Venus shrouded in a dense CO2 atmosphere -­‐> strong greenhouse effect leading to the highest surface temperature for any planet in the Solar System. Atmospheric pressure is 92 Fmes larger than on Earth at sea level. Venus spins in a retrograde direcFon -­‐ caused by a combinaFon of solar Fdes, that slow the rotaFon toward spin-­‐orbit synchronism, and strong atmospheric winds caused by solar heaFng that reverse the rotaFon through fricFon between atmosphere and planetary surface. Radar mapping of surface indicates extensive low lying plains, interspersed with highland regions. Venus appears to be volcanically acFve but does not have plate tectonics -­‐ it is a one-­‐plate planet that experiences localised crust deformaFon. No detectable magneFc field generated by an internal dynamo-­‐> possibly because of slow rotaFon or because of a lack of convecFon in the iron core. Venus – key facts Venus shrouded in a dense CO2 atmosphere -­‐> strong greenhouse effect leading to the highest surface temperature for any planet in the Solar System. Atmospheric pressure is 92 Fmes larger than on Earth at sea level. Venus spins in a retrograde direcFon -­‐ caused by a combinaFon of solar Fdes, that slow the rotaFon toward spin-­‐orbit synchronism, and strong atmospheric winds caused by solar heaFng that reverse the rotaFon through fricFon between atmosphere and planetary surface. Radar mapping of surface indicates extensive low lying plains, interspersed with highland regions. Venus appears to be volcanically acFve but does not have plate tectonics -­‐ it is a one-­‐plate planet that experiences localised crust deformaFon. No detectable magneFc field generated by an internal dynamo-­‐> possibly because of slow rotaFon or because of a lack of convecFon in the iron core. Runaway greenhouse effect 1) Venus originally had liquid water on its surface, and a less dense atmosphere. 2) Being nearer the Sun, a high fracFon of this H2O would have vaporised into the atmosphere, providing an effecFve greenhouse gas. Liquid oceans coexisted with humid atmosphere. 3) The temperature may have been above 100oC, but oceans remained below boiling point because of high atmospheric pressure. This will have lasted 100’s of millions of years, but the Sun’s luminosity increased -­‐> raising the surface temperature -­‐> causing more water to evaporate -­‐> increasing the concentraFon of greenhouse gases in the atmosphere. 4) Eventually the temperature would have risen above 647 K (374oC) -­‐> the criFcal point of water. Above this temperature, no maNer how large the atmospheric pressure, Venus’ oceans would have evaporated fully, increasing the greenhouse effect further. This is the runaway greenhouse effect. In absence of liquid water, CO2 was unable to dissolve in the oceans, so its concentraFon in the atmosphere increased due to volcanic acFvity, raising temperatures even higher, unFl high enough to “bake” the CO2 out of carbonate rocks such limestone. This is how Venus’ thick CO2 atmosphere was formed. The H2O in the atmosphere was photodissociated by UV from the Sun into H and O, and H atoms escaped -­‐> explains why Venus has no H2O today. With H2O removed, and CO2 liberated from rocks and in atmosphere, Venus’ temperature would have leveled off to equal today’s value.
Mars – key facts Dry, wind-­‐blown planet. Hosts the largest volcano in the Solar System (Olympus Mons – 24 km high) and two moons (Phobos & Deimos) Thin CO2 atmosphere with pressure at surface ~ 0.6% of Earth’s. Water can only exist in either solid or vapour state at this low pressure. Spin axis Flted 25o -­‐> strong seasonal variaFons. Polar caps consisFng of H2O and frozen CO2. Freezing of the CO2 atmosphere causes large changes in atmospheric pressure, driving seasonal winds and dust storms Surface is highly cratered, including previously volcanically-­‐acFve regions -­‐> ancient surface and long-­‐exFnct volcanism. Images from orbiFng space-­‐crak and in-­‐situ rovers indicate presence of river valleys and sedimentary rocks -­‐> Mars had a warmer, denser atmosphere in the past and running water on its surface. Radar and in situ measurements have detected frozen water at the poles and permafrost. Mars – key facts Dry, wind-­‐blown planet. Hosts the largest volcano in the Solar System (Olympus Mons – 24 km high) and two moons (Phobos & Deimos) Thin CO2 atmosphere with pressure at surface ~ 0.6% of Earth’s. Water can only exist in either solid or vapour state at this low pressure. Spin axis Flted 25o -­‐> strong seasonal variaFons. Polar caps consisFng of H2O and frozen CO2. Freezing of the CO2 atmosphere causes large changes in atmospheric pressure, driving seasonal winds and dust storms Surface is highly cratered, including previously volcanically-­‐acFve regions -­‐> ancient surface and long-­‐exFnct volcanism. Images from orbiFng space-­‐crak and in-­‐situ rovers indicate presence of river valleys and sedimentary rocks -­‐> Mars had a warmer, denser atmosphere in the past and running water on its surface. Radar and in situ measurements have detected frozen water at the poles and permafrost. Mars’ atmospheric evoluFon – runaway ice-­‐house effect Evidence for running water on Mars points to an earlier period when the atmosphere was substanFally warmer and denser. The carbon cycle maintains an equilibrium in atmospheric CO2 abundance. PrecipitaFon (rain) washes CO2 out of the atmosphere, where it forms carbonate rocks on the surface. Volcanic acFvity releases CO2 into the atmosphere. Carbonate rocks subducted into the sub-­‐surface mantle through plate tectonics. Small size of Mars caused it to cool relaFvely quickly, allowing a deep crust to form that switches off volcanic acFvity. ConFnued precipitaFon reduced the concentraFon of CO2 from the atmosphere, reducing the the greenhouse effect. The consequent reducFon in temperature led to increased precipitaFon (cold air precipitates a greater amount of water vapour), increasing the rate of loss of CO2 and H2O from the atmosphere. This cycle reinforces itself leading to a reducFon in the atmospheric density and surface temperature. A reduced atmospheric density allows greater penetraFon of UV photons from the Sun. These dissociate molecules such as N2, CO2, and H2O. DissociaFon combined with the energy input from the penetraFng UV photons leads to atmospheric escape and further reducFon of the atmospheric density. Atmospheric loss of H atoms lek behind oxygen that reacted with iron bearing compounds on the surface leading to the formaFon of haemaFte – giving Mars its red-­‐ish colour.