Lecture 17 - Mars and Jupiter Lecture Notes

Mars

Martian Atmosphere

  • The Martian atmosphere primarily consists of carbon dioxide (CO_2), making up 95% of its composition.
  • It also contains small amounts of Nitrogen, Argon, and Oxygen.
  • Trace amounts of water vapor are present, with slight seasonal variations.
  • The atmosphere is very thin, about 1/150th of Earth's atmospheric pressure.
  • Due to its thinness, it retains little heat, causing sharp temperature drops at night.
  • The average surface temperature has little day-to-day variation but is about 50 K colder than Earth.
  • There is significant temperature variation from season to season and location to location.

Troposphere

  • Extends up to 30 km.
  • Occasionally contains clouds of water ice during the daytime.
  • Frequently contains dust, especially during planet-wide dust storms that occur annually.

Stratosphere

  • Located above 30 km.
  • Features a roughly constant temperature.
  • Colder than the troposphere, allowing CO_2 to freeze into ice clouds.
  • The absence of a higher temperature zone suggests the absence of an ozone layer.
  • Above the stratosphere, temperature increases due to solar wind particles interacting with Mars' magnetic field.

Fog Formation

  • Fog can form in low-lying areas when sunlight strikes the surface.
  • As the surface heats up, water vapor rises and condenses in contact with the colder air above, forming temporary water-ice fog.

Greenhouse Effect

  • Earth: CO2 is absorbed in oceans and surface rocks. Geological and volcanic activity returns CO2 to the atmosphere, striking a stable balance and resulting in a modest greenhouse effect.

  • Venus: Has large amounts of CO_2 in the atmosphere from volcanic activity, but no oceans to absorb it, leading to a runaway greenhouse effect. Mars may be victim of a runaway greenhouse effect in the opposite sense of Venus.

  • Mars: CO2 in the atmosphere is depleted by absorption in surface rocks. Less volcanism or geological activity means CO2 is not returned to the atmosphere, which results in a lower surface temperature, causing water ice and CO2 to freeze. The reflective surface decreases temperature, locking more CO2. Lower surface gravity also cannot hold a thick atmosphere like Earth, resulting in a thin atmosphere of CO_2 with a minor greenhouse effect.

  • There is little volcanic activity, and the CO_2 returned was insufficient to replenish the atmosphere.

  • The Martian atmosphere thinned and cooled, losing most of its CO_2 in as little as a few hundred million years.

Martian Internal Structure

  • The magnetic field is weak, about 1/800th that of Earth's.
  • The rapid rotation period of 24 hours and 37 minutes, combined with a low density of 3900 kg/m3, suggests a non-metallic and/or non-liquid core, possibly partially molten, likely composed of iron-sulfide.
  • Mars has a smaller size and cooled faster than Earth.
  • Mars is probably less differentiated than Earth; more iron near the surface.
  • The crust is estimated to be 100 km thick.
  • Plate tectonics likely couldn't get started, and no seismic studies had been done until the InSight mission.

InSight Lander

  • Landed on Mars on November 26, 2018.
  • Deployed a seismometer to listen for Mars quakes.
  • Will burrow a heat probe to study Mars' internal structure.
  • Mars quakes have characteristics between Earth and the Moon.
  • Drier crusts like the Moon remain fractured after impacts, scattering sound waves for tens of minutes.
  • Mars, with its cratered surface, is slightly more Moon-like than Earth-like, with seismic waves ringing for a minute or so.

Moons of Mars

  • Mars has two tiny moons, Phobos and Deimos.
  • Both moons are tidally locked to Mars.
  • Phobos' orbit is 9400 km from the center of Mars, with a period of 7 hours and 39 minutes.
  • Deimos' orbit is 23,500 km from the center of Mars, with a period of 30 hours and 18 minutes.
  • The moons have a composition unlike that of Mars and are likely captured asteroids.
  • Phobos' low orbit interacts with the atmosphere, causing it to slowly lose energy and eventually crash into the planet in a few tens of millions of years.

Jupiter

Orbital and Physical Properties

  • Jupiter is the 5th planet from the Sun.
  • It orbits at a distance of 5.2 AU from the Sun, outside the asteroid belt but well inside the Kuiper belt.
  • The semimajor axis of Jupiter's orbit is a = 5.2 AU.
  • Jupiter's orbit has an eccentricity of e = 0.048.
  • The distance from Jupiter to the Sun varies by about 10% during its orbit.
  • Perihelion: 4.95 AU
  • Aphelion: 5.45 AU
  • Jupiter is easiest to observe during a favorable opposition, when the Earth-Jupiter distance is only about 3.95 AU.
  • At this time, Jupiter can be up to 50" across, allowing for detailed telescopic observations.

Orbit Calculation

  • Use Kepler's Third Law to calculate Jupiter's orbital period, P:
    P^2 \text{ in Earth years} = a^3 \text{ (in AU)}
  • Orbital Period: P = 11.9 Earth years

Observation History

  • Pioneer 10, Pioneer 11 (launched 1972 & 1973): flyby of Jupiter
  • Voyager 1: flyby of Jupiter and Saturn
  • Voyager 2: grand tour of the outer Solar System with flybys of Jupiter, Saturn, Uranus, and Neptune
  • Voyager 1 & 2 (launched 1977) are the most distant man-made objects from Earth (147 AU and 120 AU) and are still sending and receiving radio signals.
  • The Voyager Missions greatly improved our understanding of Jupiter and its moons. Voyager 1 also studied Saturn, and Voyager 2 studied Saturn, Uranus, and Neptune.
  • Planets acted as gravitational “slingshots” to change the direction of spacecraft and accelerate them.
  • Galileo Spacecraft: Launched in 1989 and orbited Jupiter from 1995-2003. It studied the atmosphere and moons of Jupiter, finding evidence for a liquid ocean under Europa's icy surface. It also saw a comet collide with Jupiter in 1994 (Comet Shoemaker-Levy 9).
  • Juno Spacecraft: Orbiting Jupiter since 2016 with a planned mission until 2021. It's studying Jupiter's atmosphere in great detail, along with the magnetosphere and the planet's formation and core composition.

Physical Properties

  • Jupiter is the largest planet in our Solar System.
  • Radius: 71,500 km = 11.2 R_⊕
  • Mass: 1.9 x 10^{27} kg = 318 M_⊕
  • Density: 1300 kg/m3 (much lower than Earth's: 5500 kg/m3)
  • Has 79 moons (as of 2021).
  • Has a faint ring.
  • Very different from terrestrial planets!

Jupiter's Spin

  • Jupiter does not have a solid outer surface.
  • The rotation period is determined by tracking the motions of its colorful clouds.
  • The rotation period differs for clouds in different bands.
  • Jupiter does not rotate as a solid body; it exhibits differential rotation.
  • Jupiter's strong magnetic field rotates with a period of about 10 hours, indicating the rotation period of the planet's deep interior.
  • Jupiter's rotation period is 9 hours 55 minutes, making it the fastest rotating planet in the Solar System.
  • The centripetal force due to the rapid spin causes the gas near the equator to bulge outward, resulting in an oblate (flattened) shape. Its oblateness suggests it has a small, dense, rocky core of about 5-10 Earth masses.
  • Equatorial diameter = 143,000 km
  • Polar diameter = 133,800 km

Jupiter's Atmosphere

  • Major visible features include bands of clouds, various colors (white, pale yellow, brown, red, light blue), and the Great Red Spot.
  • The Great Red Spot is a giant storm twice the size of Earth, similar to hurricanes on Earth.
  • Composition of the atmosphere:
    • Hydrogen (H_2): 86.1 %
    • Helium (He): 13.8 %
    • Small amounts of methane (CH4), ammonia (NH3), and water vapor (H_2O)
  • Jupiter is over 99 % Hydrogen and Helium. Its strong gravity prevents these light gases from escaping.
  • Jupiter's composition closely resembles that of the Sun.
  • If Jupiter's mass were about 80 times larger, the pressure and temperature in its core would have been high enough to generate energy by hydrogen fusion, potentially becoming a star.
  • If Jupiter had become a star, the temperature of the inner Solar System would be higher, its higher mass probably would have made stable planetary orbits impossible, and life on Earth probably wouldn't exist.
  • The outer surface of Jupiter is a gas; the top of the troposphere is considered 0 km.
  • Pressure increases toward the core.

Atmospheric Layers

  • Above the troposphere is a faint haze or smog with a temperature of 110 K, which increases with altitude due to UV absorption.
  • Temperature increases moving down through the troposphere as pressure increases.

Cloud Layers

  • The troposphere contains three main cloud layers, each with different colors and chemistry:
    1. White wispy clouds of ammonia ice
      • Altitude = -40 km
      • Temperature = 125-150 K
    2. Yellow, red, and brown clouds of ammonium hydrosulfide ice
      • Altitude = -60 km
      • Temperature = 200 K
    3. Lowest (bluish) cloud layer: water ice
      • Altitude = -80 km
      • Temperature = 250-275 K
      • Cannot be seen by optical telescopes
  • Cloud chemistry is complex, influenced by chemical composition, chemical reactions, pressure, temperature, and viewing depth. The Galileo atmospheric probe detected trace amounts of phosphine (PH_3), which may be a key coloring agent.
  • The temperature at the cloud tops is about 125 K.
  • Jupiter radiates about twice as much energy as it receives from the Sun.
  • The source of heat is believed to be energy leftover from the initial “squeeze” when Jupiter collapsed under the influence of gravity. Gravitational energy is converted to heat. Because Jupiter is very large, this heat has not all leaked out into space yet.
  • The Galileo probe measured high wind speeds (up to 600 km/h) throughout its descent into Jupiter’s atmosphere, not just at the cloud tops. Heat deep within the planet may be driving Jupiter’s weather patterns.

Belts and Zones

  • Cloud bands can be classified as either Belts (dark colors) or Zones (bright colors).
  • Zones are characterized by rising gas and high pressure, and are higher and cooler than belts.
  • Belts are characterized by sinking gas and low pressure, and are lower and warmer than zones.
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Zonal Flow

  • Jupiter's rapid rotation causes high-low pressure systems to wrap around the planet, creating a strong flow of winds called zonal flow that underlies belts and zones. Wind direction alternates between adjacent bands.
  • The graph shows wind speed in Jupiter’s atmosphere, measured relative to the planet’s internal rotation rate. The average flow is approximately 300 km/h, but the wind speed strongly depends on latitude, with alternations in wind direction associated with the atmospheric band structure.
  • The band structure disappears near the poles. Juno has observed very complex cloud motions and storms near the poles.

The Great Red Spot

  • The alternating wind direction between adjacent bands creates eddies between them. The most famous example is the Great Red Spot, first reported by Robert Hooke in the 17th century.
  • The Great Red Spot has a hurricane structure, about twice the size of the Earth.
  • Gas north of (above) the spot is westward (to the left), and gas south of (below) the spot flows eastward (to the right).
  • The spot itself rotates counterclockwise, suggesting that it is being “rolled” between the two oppositely directed flows.
  • The Great Red Spot has existed for at least 300 years, possibly much longer.
  • Jupiter has no continents, so storms last much longer. Juno data indicate that the storm extends 300 km into Jupiter's atmosphere.
  • The Great Red Spot is dynamic and has been gradually shrinking:
    • 1995: 21,000 km long
    • 2009: 18,000 km long
    • 2014: 15,000 km long

Other Features

  • White Ovals: Smaller storms with color coming from high altitude clouds.
  • Red Spot Jr.: Formed from three merging white ovals and changed color. The red color might be from the storm’s size and strength, lifting cloud tops high above the surrounding clouds.
  • Brown Ovals: Large gaps in the upper cloud layer allowing us to see deeper into the atmosphere, where the clouds are brown.
  • Lightning-like flashes have been seen on Jupiter. Juno observed radio waves emitted from lightning storms, which seem to be mostly near the poles.