1445 Introductory Astronomy I

1
1445 Introductory Astronomy I

• Chapter 7
• Mercury, Venus and Mars
• R. S. Rubins
  Fall, 2009

2
The Inner Planets
3
Mercury the Once Forgotten Planet

• In 1974, Mariner 10 photographed 45 of Mercurys
  surface.
• This Mariner 10 photo shows fog-filled canyons at
  sunrise.

4
Mercury Data

• Average distance from Sun 0.39 AU.
• Mass 5.5 of Earth (0.055 ME).
• Radius 38 of Earth (0.38 RE).
• Average density 98 Earth density surprisingly
  high for a small planet.
• Orbital eccentricity 0.21 cf. Earth (0.02) and
  Venus (0.01).
• Siderial revolution period about the Sun (1
  Mercury year) is 88 Earth days.
• Siderial rotation period (1 Mercury day) is 58.7
  Earth days.
• The ratio of the two periods is 58.7/88 2/3.
• The solar day (or synodic rotation period), which
  is the time from dawn to dawn, lasts two Mercury
  years, which is 176 Earth days.

5
Mercurys Locked Periods Year 1

• In one siderial year (88 Earth days), Mercury
  rotates 1½ times i.e. the siderial day is (2/3)
  x 88 58.7 Earth days.

6
Mercurys Locked Periods Year 2

• After 2 years Mercury returns to its initial
  position.
• Thus the dawn-to-dawn period (the synodic day) on
  Mercury is 3 x 58.7 176 Earth days.

7
Summary of Periods for Mercury

• A solar day (from noon to noon) on Mercury takes
  176 Earth days or 2 siderial years (88 Earth
  years each).

8
About Mercury 1

• Keplers 3rd Law P2 a3 indicates that it
  orbits the Sun at almost twice the speed of the
  Earth, taking 88 days for one complete
  revolution.
• There are no seasons on Mercury, since its
  rotational axis is not tilted like that of the
  Earth.
• As a result, craters at the poles receive no
  direct sunlight, and so may contain water ice.
• Mercury lacks an atmosphere because of its small
  mass, which makes the escape velocity very low.
• Because of its lack of an atmosphere and
  proximity to the Sun, Mercury has the greatest
  temperature extremes in the solar system.

9
About Mercury 2

• Of the traditional planets, only Pluto has an
  orbit more elliptical than Mercury.
• Temperatures can vary from just above 700 K
  (800oF) at mid-day, which is hot enough to melt
  metals, to 100 K ( 280oF) at midnight.
• Because Mercurys orbit is so elliptical, the
  diameter of the Sun as seen from its surface
  would be roughly 50 bigger at its perihelion
  than at its greatest separation, varying from
  about three times to twice its diameter as seen
  from the Earth.
• Newtons Laws do not work perfectly for Mercury,
  since its perihelion point shifts with each orbit
  about the Sun.
• In 1845, this lead Leverrier to predict another
  planet, Vulcan, even closer to the Sun, which
  would affect Mercurys orbit.

10
Mercurys Unusual Orbit

• Vulcan was never found, but Einstein showed in
  1915 that the change in location of Mercurys
  perihelion with each orbit, which could not be
  explained by Newtons theories, provided the
  first quantitative test of his General Theory of
  Relativity.

11
About Mercury 3

• Never more than 27o of arc from the Sun, Mercury
  can only be seen from Earth just before sunrise
  or just after sunset.
• Even the Hubble telescope has not been used on
  Mercury, in case its delicate optics are pointed
  too close to the Sun.
• The 2008and 2009 flybys of the MESSENGER probe
  has revealed many volcanoes and smooth plains
  probably formed by volcanic deposits.
• Because of its remarkably high density for a
  small planet, Mercury must be the most iron-rich
  planet in the solar system.
• A hypotheses for the plentiful iron on Mercury is
  that during its formation, it was struck by a
  large object, which ejected much of the mantle,
  vaporizing the lighter, more volatile elements.

12
Mercury and Earths Moon

• The diameters of Mercury and the Moon are roughly
  4900 km and 3500 km respectively, and there is
  some resemblance in their heavily cratered
  surfaces.

13
Mariner 10 View Craters and Plains

• Although heavily cratered like the Moon, there is
  less overlap between craters than on the Moon,
  and the crater walls are less steep because of
  the larger gravity.

14
Scarp Formation on Cooling

• A scarp is a cliff formed by the vertical
  moment of a section of the planets crust.

15
Discovery Scarp
16
Mariner 10 View Edge of Caloris Basin

• The Caloris Basin, the edge of which was
  photographed by Mariner 10, is a crater of about
  800 mi in diameter (or twice the area of Texas),
  which is surrounded by a ring of mountains about
  2 km (6,500 ft) high.

17
Formation of the Caloris Basin

• The Caloris Basin was probably formed by a major
  impact that occurred early in the planets life,
  the shock waves from the impact producing a hilly
  landscape on the opposite side of the planet.

18
The Messenger Mission 2004 to the Present
19
Mercury Messenger 2008 1

• The following observations were made in the first
  Messenger flyby of January 2008
• i. Mercury is filled with very subtle blue and
  red areas
• ii. wrinkle-like ridges are an indication that
  Mercury is shrinking
• iii. Mercury shows evidence of extensive volcanic
  activity.

20
Mercury Messenger 2008 2
The Spider, containing more than 100 narrow tiny
ridges radiating from a 40 km wide plateau, at
the center of the Caloris Basin.

• The Caloris Basin, shown
• with colors intensified.

21
Mercury Messenger 2008 3

• The unknown blue material indicates that Mercury
  might still be volcanically active a surprising
  observation.

22
Mercury Messenger 2008 4

• The 2nd flyby of Mercury took place in October
  2008, and produced 1200 photos, revealing 30 of
  the surface never before seen, extending the
  observations to about 95 of Mercurys surface.
• Messenger crossed the planets equator at a
  height of just 125 miles and a speed of 14,800
  mph.
• Messengers data has shown that about 40 of
  Mercurys surface consists of hardened lava
  flows, compared to about 20 on the Moon.
• A 3rd flyby took place in September, 2009.
• When Messenger returns in 2011, its speed should
  be slow enough for it to begin orbiting the
  planet.

23
Mercury Messenger 2008 5

• Messengers 2nd flyby revealed the Rembrandt
  Crater, which is 430 miles across.
• Rembrandt was probably formed about 3.9 billion
  years ago by an impacting space-rock.
• The original floor was never filled in by later
  lava flows, and parts are still intact.

24
Mercury Messenger 2009 1
25
Mercury Messenger 2009 2

• A mysterious, unexplained bright spot observed on
  Mercury.

26
Mercury Messenger 2009 3

• Lava-filled Crater

27
Earth and Mercury Compared
28
Mercurys Partially Molten Core

• In a recent experiment, radar signals bounced off
  Mercury in 2007 have shown that Mercury wobbles
  as it rotates, indicating that it has molten
  fluid in its core.
• To see the difference fluid makes, compare
  how differently raw eggs and hard-boiled eggs
  spin on a table.
• This result is surprising because Mercurys small
  size means that it loses heat easily, so that its
  iron core would have been expected to have
  solidified long ago.
• A partially fluid core would explain the weak
  magnetic field detected by NASAs Mariner 10
  mission in 1974.

29
The High Density of the Planet Mercury

• Mercury, with a density just 98 that of the
  Earth, is the second densest planet in the solar
  system.
• If Mercury were made of the same material as
  the Earth, the number would be appreciably
  smaller than 98, since the gravitational
  compression would be much less on the muich
  tinier planet Mercury.
• It is likely that the relatively high density of
  Mercury is due to a higher percentage of
  iron-rich compounds, compared to the constituents
  of the Earth.
• The several theories proposed to explain the high
  density of Mercury all involve mechanisms by
  which the lighter, more volatile compounds are
  boiled off by the solar radiation.

30
About Venus 1

• Because of its proximity, only the Moon outshines
  Venus in our sky.
• Venus, when observed, is never far from the Sun.
• As the Morning Star, Venus heralds the rising
  Sun as the Evening Star, it is a postscript to
  the setting Sun.
• Venus spends nine month as either the Morning
  Star or Evening Star, with a gap of about 50 days
  between them, as it passes in front of or behind
  the Sun.
• In daytime, the white shape of Venus may
  sometimes be seen against the blue of the sky.
  Napoleon saw Venus in this way when making a
  speech, interpreting the sight as a sign of
  victory for his campaign in Italy.

31
About Venus 2

• The closer Venus is to the Earth, the more it
  approaches the crescent phase, with only 1/6 of
  her surface visible at its closest position.
• Unlike our other neighbor Mars, Venus has not
  easily revealed its secrets, because it is
  completely covered by a thick cloud cover of
  sulfuric acid, which prevents the study of its
  surface from the Earth.
• Ten Soviet Venera and Vega spacecraft landed on
  Venus between 1970 and 1984, each being destroyed
  within an hour by the extreme pressures and
  temperatures.
• The US spacecraft Magellen, which circled Venus
  for four years, was able to resolve features of
  the planets surface

32
Lava Plains on Venus

• Over a dozen Soviet Venera spacecraft have
  landed on Venus, none surviving for much more
  than an hour.

33
Venus Data

• Average distance from Sun 0.72 AU.
• Mass 81.5 of Earth (0.815 ME).
• Radius 95 of Earth (0.95 RE).
• Average density 95 Earth density.
• Siderial revolution period 224.7 Earth days.
• Siderial rotation period 243 Earth days
  (retrograde).
• Solar day (or synodic rotation period) 116.8
  Earth days.
• Tilt of rotation axis 177o.
• Surface temperature roughly 750 K at all times
  and latitudes.

34
The Day on Venus

• The solar day on Venus, which is the time between
  consecutive noons, takes just under half a
  Venusian year.

35
About Venus 3

• Venus is of similar size and composition to the
  Earth.
• Like Earth, Venus has a complex evolving climate,
  fueled by geological activity.
• At one time, the atmospheres of both planets
  consisted mainly of carbon dioxide (CO2).
• However, Venus became hotter than the Earth
  because it is 30 closer to the Sun, and also
  lacks a magnetosphere, so that the two planets
  developed quite differently.
• On the Earth, the oceans absorbed the CO2,
  leaving a small amount of nitrogen, which became
  the major component of our atmosphere.
• On Venus, the water boiled off, so that the CO2
  remained in the atmosphere.

36
Effect of the Slow Spin Rate

• Venus is the only planet with a retrograde
  rotation.
• If one were to look down from above its north
  pole, it would rotate clockwise (towards the
  west), so that observed from Venus, the Sun would
  rise in the west.
• Because of its very slow spin rate, the magnetic
  field on Venus very much smaller than on Earth.
• Since it has no magnetosphere, there is no
  protection on Venus from the solar wind, which is
  a stream of high-energy charged particles.
• As a result, hydrogen, helium and oxygen are
  blown away by the solar wind much faster on Venus
  than on Earth, which explains how Venus lost its
  original water to space.

37
About Venus 3

• The surface temperature of 750 K (900oF),
  resulted from a runaway greenhouse effect,
  caused by the absorption of the IR radiation from
  the ground by both CO2 and H2O molecules.
• The very high surface temperature is enough to
  make rocks glow.
• Because of the thick cloud cover and the lack of
  wind, the temperature difference between poles
  and equator is small.
• The atmosphere is unbreathable for us, consisting
  of about 96 carbon dioxide (CO2) and 4
  nitrogen.
• The atmospheric pressure is about 100 times
  greater than on the Earths surface, equivalent
  to being ½ mile under water.

38
Venusian Landscape

• Most of Venus consists of lava plains and
  gently rolling hills, although the highest
  mountain is about 2 km higher than Mt. Everest.

39
The Venusian Atmosphere
40
The Greenhouse Effect on Venus
41
Bright Spot on Venus 2009

• First observed by an amateur astronomer in New
  York State in July 2009, it was confirmed by the
  European Space Agencys Venus Express spacecraft.
  
• It may have been caused by a volcanic eruption.

42
Mars from Hubble
43
Mars Data

• Average distance from Sun 1.52 AU.
• Mass 10.7 of Earth (0.107 ME).
• Radius 53 of Earth (0.53 RE).
• Average density 72 Earth density.
• Siderial revolution period (Martian year) 687
  Earth days.
• Siderial and solar rotational periods (Martian
  day) both a little more than 24½ hr (cf. 24 hr
  on Earth ).
• Tilt of rotation axis 25o (cf. 23.5o on Earth).
• A daily temperature range of 105 oF to 14 oF
  (cf. 280 oF to 800 oF on Mercury), was measured
  by the Mars Pathfinder (1997).

44
Martian Invaders

• In 1877, Giovanni Schiaparelli saw canali
  (channels), which were mistranslated in English
  as canals.
• Percival Lowell observed 160 canals in his
  observatory in Flagstaff, Arizona.
• It was later thought that the canals were being
  used for irrigation of a dying planet, whose
  inhabitants were ready to invade the Earth.
• When actor, Orson Welles, made a live radio
  broacast in 1938 about a Martian invation, based
  on H. G. Wells book War of the Worlds, panic
  ensued in the New York area.
• The canali were later found to be an optical
  illusion.

45
About Mars

• Because of its smaller size, the interior of Mars
  cooled much faster than the Earth, so that
  surface activity has ceased.
• The extremely thin atmosphere, consisting mainly
  of CO2, causes liquid water to boil rapidly and
  leave the planet.
• There are indications that liquid water once
  existed on Mars in the dried-up riverbeds and dry
  lakes.
• The sky on Mars often is reddish, due to the fine
  iron oxide (rust) particles in its atmosphere.
• The great canyon system, Valles Marineris,
  extends for about 4000 km (the distance from NYC
  to LA), and is up to 6 km (4 mi) deep.

46
Viking Orbiter Image 1976
47
Same Image from Mars Orbiter 1998
48
Mars The Red Planet

• There are gigantic volcanoes (left), impact
  craters (upper right), windswept planes, an
  enormous canyon system (lower center), but no
  canali.

49
Water on Mars 1

• Water and the possibility of life on Mars have
  been the major topics of Martian studies reported
  in the last few years.
• The most essential item for life appears to be
  liquid water.
• While there is as yet no direct evidence that
  life ever existed on Mars, features that look
  like gullies, river beds and lake beds strongly
  indicate that water once flowed on the surface.
• While the polar caps are known to contain large
  quantities of water ice, underground glaciers
  have been found at mid-latitudes, which appear to
  contain relatively pure water ice.
• Other studies have indicated that water
  containing perchlorate salts, which can keep
  water liquid down to 70 C, may exist just below
  the Martian surface.

50
Water on Mars 2

• The Mars rovers, Spirit and Opportunity, found
  hydrated sulfur compounds just below the surface.
  
• In 2008, snow was observed in the Martian
  atmosphere, while the Phoenix Mars Lander has
  confirmed the existence of surface water ice.
  
• NOW
  THEN (artists impression)

51
Water on Mars 3

• In 2008, the Mars Phoenix Lander found small
  chunks of ice several inches below the surface,
  which slowly vanished when exposed to the
  atmosphere.
• The ancient riverbed shown below is a strong
  indication of once-abundant water.

52
Water on Mars 4
Clay-like deposits in green (left) and opals in
cream (right) show where water probably once
flowed.
53
Probable Mud Volcano on Mars

• Mars

• Earth

54
Martian Landscape and Red Sky

• Photograph from Mars Pathfinder shows
  Sojourner rover.

55
Dry Martian Lake
56
Martian Surface

• Craters are shown in the 2006 photo taken by the
  Mars Reconnaissance Orbiter (MRO).

57
Victoria Crater from the MRO
58
Victoria Crater from Opportunity

• 2006 photo taken from the edge of the Victoria
  Crater by the Mars rover Opportunity.

59
The Olympus Caldera

• The largest volcano, Olympus Mons, rises 26 km
  (16 mi) above the plane three times the height
  of Mt. Everest.
• The summit collapsed to create a caldera 70 km
  across.

60
The Valles Marineris

• This canyon is over 10 times longer and 5
  times deeper than the Grand Canyon.

61
The North Pole of Mars

• The polar regions are capped mainly with dry
  ice (solid CO2), although there is some water ice.

62
The Spotted Dunes of Mars

• The white surface consists of dry ice (CO2)
  and water ice, which have totally gone by
  mid-summer.

63
Fluid-like Sand Dunes (2009)
64
Sand and Dust Geysers on Mars

• Themis, the Thermal Emission Imaging System
  orbiting Mars have shown jets of CO2 erupting at
  100 mph through ice at the planets South Pole,
  and spewing sand and dust hundreds of feet into
  the air.
• They jets are thought to be due to sunlight
  warming the ice, which turns the solid CO2 below
  the surface into high-pressure gas.

65
A History of the Martian Surface 1
66
A History of the Martian Surface 2
67
A History of the Martian Surface 3
68
Martian Moon Phobos 1

• Potato-shaped Phobos, roughly 28 x 23 x 20 km in
  size, is only 6000 km above the surface.
• Its period of 5½ hours means that it orbits
  faster than Mars rotates, so that it moves from
  west to east across the Martian sky.

69
Martian Moon Phobos 2

• 2008 view from the Mars Reconnaissance Orbiter.
• New data suggests that Phobos is more like a pile
  of rubble.

70
Martian Moon Deimos

• Deimos, roughly 16 x 12 x 10 km in size, moves
  from east to west across the sky in about 3 Earth
  days.