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Mars: Basic Planetary Characteristics

Explore Mars' and Earth's orbital features like eccentricity and distance from the Sun, as well as rotational attributes such as axial tilt. Learn how these factors influence insolation patterns on Mars.

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Mars: Basic Planetary Characteristics

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  1. Geography 441/541 F/19 Dr. Christine M. Rodrigue Mars: Basic Planetary Characteristics C.M. Rodrigue, 2019 Geography, CSULB

  2. Mars in Space • Orbital Characteristics • Planetary orbits are elliptical • The major focus of Mars' or Earth's orbit is inside the Sun • The plane of that orbit is the ecliptic • The diameter of the orbit along its long axis is the major axis • Half that distance is the semi-major axis (here shown as a) • The diameter of the planet's orbit along its short axis is the minor axis • Half that is the semi-minor axis (b) • C on this graph is the distance from the center of the orbit to one focus • Eccentricity is c/a – 0 for a perfect circle • Or (Da-Dp)/(Da+Dp), where Da is the distance from Sun to planet at aphelion and Dp is the distance at perihelion (probably the easiest way to calculate eccentricity) C.M. Rodrigue, 2019 Geography, CSULB

  3. Mars in Space • Orbital Characteristics • Mars’ and Earth’s eccentricity • Mars has one of the greatest eccentricities in the solar system at 0.0934 • Earth is one of the more circular at 0.0167 C.M. Rodrigue, 2019 Geography, CSULB

  4. Mars in Space • Orbital Characteristics • Mars’ and Earth’s changes in eccentricity • Planets’ orbital shapes alternate between more circular and more elliptical • Earth’s varies from ~0.01 to ~0.05 over a cycle of ~100,000 years • Mars’ varies from close to 0.00 to ~0.14 over a 96,000 (Earth) year cycle C.M. Rodrigue, 2019 Geography, CSULB

  5. Mars in Space • Orbital Characteristics • Mars’ and Earth’s distance from the Sun and insolation • Mars is about 227,936,640 km from the Sun averaged along the semi-major axis • Earth is 149,597,890 km • Solar irradiance at Mars is about 587 W/m2 versus 1,361 W/m2 at Earth (~43%) • So, Mars can be expected to be pretty cold! C.M. Rodrigue, 2019 Geography, CSULB

  6. Mars in Space • Orbital Characteristics • Mars’ and Earth’s distance from the Sun and insolation • Solar irradiance at Mars is about 587 W/m2 versus 1,361 W/m2 at Earth (~43%) • S = I * (R/D)2 • Solar energy receipt • Irradiance • Radius of Sun • Distance from Sun to planet C.M. Rodrigue, 2019 Geography, CSULB

  7. Mars in Space • Orbital Characteristics • Mars’ and Earth’s distance from the Sun and insolation • Solar irradiance at Mars is about 587 W/m2 versus 1,361 W/m2 at Earth (~43%) • Here, it would be like living on Earth at 54 N or S in March or September where the sun would come in at a 36° angle C.M. Rodrigue, 2019 Geography, CSULB

  8. Mars in Space • Orbital Characteristics • Mars’ and Earth’s distance from the Sun and insolation • Mars at perihelion is 206,700,000 km (Southern Hemisphere summer) • Earth is 147,100,000 km (also Southern Hemisphere summer) • Mars at aphelion is 249,200,000 km • Earth is 152,100,000 km • So, Mars perihelion distance is only 82.9% of its aphelion distance • On Earth, perihelion is 96.7% of aphelion distance • On Earth, this difference is a trivial influence, especially since perihelion hits during the more oceanic hemisphere’s summer C.M. Rodrigue, 2019 Geography, CSULB

  9. Mars in Space • Orbital Characteristics • Mars’ and Earth’s distance from the Sun and insolation • On Earth, this difference is a trivial influence • S = I * (R/D)2 • S=inSolation; I=sun's irradiance or 62,900,000 joules/m2/s; R=sun's Radius or 696,000 km; D=Distance from Sun to a planet • Using Mars' perihelion distance, we get 713 j/m2/s, while using its aphelion distance, we get 491 j/m2/s • Mars' aphelion insolation is, therefore, only 69% that at perihelion! • For Earth, we get 1,408 j/m2/s at perihelion and 1,317 j/m2/s • Earth's aphelion insolation is 94% that of perihelion • What is a fairly trivial difference on Earth is a major seasonal driver on Mars • The Southern Hemisphere of Mars has more extreme seasonality than the Northern Hemisphere C.M. Rodrigue, 2019 Geography, CSULB

  10. Mars in Space • Rotational characteristics • Axial tilt or obliquity: • Mars: 2511’ 24” (25.19 ) from the vertical of the ecliptic • Earth: 2326’24” (23.44) from the vertical of the ecliptic • Mars’ axis precesses 360  in 93,000 Martian years or ~125,000 Earth years • Earth’s axis precesses 1 per 71.6 years or 360 in 25,765 years • Changes in the pole stars, and changes in the timing of equinoces and solstices with respect to the annual "signs of the zodiac" • Affects which hemisphere faces sun at perihelion and aphelion C.M. Rodrigue, 2019 Geography, CSULB

  11. Mars in Space • Orbital Characteristics • Mars’ insolation averaged over the year: tilt and eccentricity • Very freaky: South polar regions get the most solar radiation in summer, due to greater axial tilt, greater eccentricity, and the greater length of day • The length of day overcompensates for the lower sun angle • Note the imbalance between the N and S, due to eccentricity, tilting S pole toward sun at much closer perihelion C.M. Rodrigue, 2019 Geography, CSULB

  12. Mars in Space • Orbital Characteristics • Mars’ insolation averaged over the year • Earth's nearly circular orbit makes two polar regions nearly symmetrical in insolation as perihelion and aphelion distances aren't as divergent as Mars' • Earth's slightly lesser obliquity makes for nearly even insolation all year along equator • Mars' greater tilt creates seasonality at the equator in terms of insolation C.M. Rodrigue, 2019 Geography, CSULB

  13. C.M. Rodrigue, 2019 Geography, CSULB Mars in Space • Size of Planet • Mars and Earth compared: • Mars’ equatorial radius: 3,396 km (Earth: 6,378 km) • Equatorial circumference: 21,344 km (Earth: 40,075 km) • Volume: 163,140,000,000 km3 (Earth: 1,083,200,000,000 km3 • Mass: 641.85 x 1018 metric tons (Earth: 5,973.70 x 1018 metric tons) • Mean density: 3.93 g/cm3 (Earth: 5.51 g/cm3), where water = 1.00 • Equatorial surface gravity: 3.71 m/s2 (Earth: 9.80 m/s2) or about 38% of Earth’s • Escape velocity: 5.03 km/sec (Earth: 11.2 km/sec)

  14. Mars in Space • Size of Planet • Mars’ and Earth’s relative sizes compared C.M. Rodrigue, 2019 Geography, CSULB

  15. Mars in Space • Shape of Planet • Mars is markedly egg-shaped: • Equatorial radius: 3,396 km vs. 6,378 km for Earth • N polar radius: 3,376 and S. polar radius: 3,382 vs. 6,376 km for Earth • Mars' ellipticity is 0.11 vs. 0.08 for Earth • Both are oblate ellipsoids, but Mars is on steroids and quite asymmetrical between the hemispheres C.M. Rodrigue, 2019 Geography, CSULB

  16. Mars in Space • Shape of Planet • Mars is markedly egg-shaped: • The odd shape may reflect an ancient impact that obliterated the northern portion of the planet • The distortion in shape and mass distribution means that Mars' center of mass is offset about 2.5 km from its center of figure (Earth's is offset, too, because of the effects of our oceans, as well as internal mass deviations – ~2.1 km) • This displacement is a cartographic headache, as cartography traditionally bases the geographic grid on center of figure ("planetographic"), but our digital elevation model for Mars is based on gravitational perturbations of spacecraft: center of mass ("planetocentric") • We get noticeably different grids on Mars, as seen at http://planetarynames.wr.usgs.gov/images/mola_regional_boundaries.pdf • Red grid is planetographic "westings" grid • Black grid is planetocentric "eastings" grid C.M. Rodrigue, 2019 Geography, CSULB

  17. Mars in Space • Composition of Planet • Mars is an inner solar system terrestrial (earth-like) planet • Composed primarily of silicates (a great diversity of minerals sharing silicon-oxygen groups) and metals • It is differentiated or gravitationally segregated by density, with iron-nickel drawn down into the core and the lighter silicates displaced outward into the mantle and crust • Differentiation is not as advanced as on Earth: smaller size means smaller gravitation and less internal heat • Core has more sulfur than Earth's • Mantle has about twice as much iron left in it as Earth's • Mars' mantle also has more potassium and phosphorus (on Earth, these are more common in the crust) • Mars crust has less silica (purer silicon-oxygen, SiO2, like quartz) C.M. Rodrigue, 2019 Geography, CSULB

  18. Mars in Space • Magnetism • Mars ancient planetary magnetic field no longer exists • It's believed to have collapsed around 4 billion years ago • Remanent magnetism is found in really old, very mafic rocks, so there once had been a planetary magnetic field • There is no remanent magnetism in the great impact craters, which hit during the Late Heavy Bombardment, from ~4.1 to ~3.7 or 3.8 billion years ago, thus constraining the last existence of the field • Why the collapse? • Did Mars' iron core solidify due to Mars smaller size and coolness, shutting off the dynamo? • Did the Late Heavy Bombardment deposit so much heat in Mars' mantle that the convective movement of heat from the core could not continue with the lessened contrast between core and mantle, thus shutting down the dynamo? • The loss of the magnetic field may have enabled the loss of nearly all of the planet's atmosphere C.M. Rodrigue, 2019 Geography, CSULB

  19. Mars in Space • Moons of Mars • Mars has two moons, asteroid-sized "moonlets" • They might be asteroids that were captured into Mars orbit (or not...) • Phobos is the larger (~22 km in diameter) and nearer (~9,377 km) • It revolves around Mars every 7.66 hours • It rises in the west and sets in the east! • Angular momentum exchange, going in that direction, is tending to slow it down enough to degrade its orbit, so it will eventually crash into Mars • Deimos is the smaller (~13 km in diameter) and farther (~23,460 km) • It revolves around Mars every 30.35 hours • It rises in the east and sets in the west • It gets a slight gravitational assist, so it is moving very slowly away from Mars and will one day simply begin to revolve independently around the sun C.M. Rodrigue, 2019 Geography, CSULB

  20. Mars in Space • Moons of Mars • Mars has two moons, asteroid-sized "moonlets" • Here is a video of Phobos passing by and occulting Deimos taken by MSL C.M. Rodrigue, 2019 Geography, CSULB

  21. Mars in Space • Moons of Mars C.M. Rodrigue, 2019 Geography, CSULB

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