General orientation to the Solar System III
The Terrestrial Planets III: The Earth - moon system
Earth from Apollo 17
- Radius: 6357 km (compared with 1738 for the Moon)
- Density: 5.51 x 103 103kg m-3 (compared with 3.34 for the Moon)
- Average distance from sun: 1.00 AU (149.6 x 106 km)
- Axial rotational period (length of day with reference to background stars): 0.997 days
- Orbital period: 365.26 days
- Orbital eccentricity: 0.017
- Orbital inclination: 0.0 deg. (But then, it would be.)
Points of interest: We know much more about Earth than we do about any other planetary body. It will be our constant standard of comparison, and we will have much to say about it in upcoming lectures. For now:
- Earth beats Venus by a nose, being the largest terrestrial planet.
- Earth's atmosphere is 77% nitrogen, 21% oxygen, and has traces of argon, CO2 and water. The last two are greenhouse gasses that trap solar energy as heat. Earth's average surface temperature is 288 K (15 deg. C). Without an atmosphere this temperature would be roughly 14 degrees C. lower.
- It is the only planet that can support large permanent bodies of liquid water. 71% of its surface is covered by oceans. The evaporation and precipitation of water creates a hydrologic cycle as part of which, water flows across land surfaces as streams.
- Because liquid water is a good medium for life, Earth has an extensive biosphere that radically alters its atmospheric and oceanic chemistry. E.G. photosynthesizers consume CO2 and release oxygen.
Plate tectonics schematic from Prof. J. Tarney, University of Leicester
- Earth is the only planet that currently experiences plate tectonics. I.e., its uppermost layers are divided into rigid plates that glide across deeper layers. A consequence of plate tectonics is the segregation of the rocks of the upper layers into:
- Continental crust: Relatively light, rich in silica (SiO2), collects in topographically high-standing continents.
- Oceanic crust: Relatively heavy, rich in iron and magnesium (Fe and Mg), Forms the broad flat plains of the ocean floors.
The Exit Glacier outwash plain - headwaters of the Resurrection River
- The constant movement of the Earth's crust combines with the movement of water and air to make erosion, transport, and deposition of sediments - remains of preexisting rocks an important shaper of its surface. Between the effects of volcanism, plate tectonics, and the movement of sediment, Earth is constantly being resurfaced. Thus, impact craters are rare. (Arizona tourist site Barringer crater is a mere 49,000 years old.) Volcanism, however, is common and ongoing.
The Moon Wikipedia
- Radius: 1738 km.
- Density: 3.34 x 103 103kg m-3
- Average distance from sun: 1.00 AU (149.6 x 106 km)
- Average distance from Earth: 384 x 103 km - 0.0025 AU)
- Axial rotational period (length of day with reference to background stars): 27.3 days
- Orbital period: 27.3 days
- Orbital eccentricity: 0.055
- Orbital inclination: 5.2 deg.
Buzz Aldrin on the Moon from The Telegraph
Points of interest:
- The only extraterrestrial planetary body to have been visited by humans.
- Synchronous rotation: The moon, like many natural satellites, rotates on its axis in the same interval as it orbits its primary. Thus, it presents the same face to Earth. (An observer on the moon would see Earth hanging in the same place in the sky but undergoing phases.)
- The moon has no atmosphere or magnetic field, and orbits outside Earth's magnetic field. Thus, like Mercury, its surface is directly struck by solar radiation and the solar wind.
- Like Mercury, the moon's surface is dominated by impact craters.
Distinctive feature: The surface presents a dichotomy between bright-colored ancient highlands dominated by the rock anorthosite, and slightly less ancient dark maria (sing. mare) - giant basalt lava flows. The maria are concentrated on the Earth-facing side. The far side consists almost entirely of highlands.
The Terrestrial Planets IV: Mars
Mars, Earth, and the Moon to scale
Mars: Celebrated in fiction, but alas, not like this.
- Radius: 3375 km (compared with 6357 for Earth 1738 for the Moon)
- Density: 3.93 x 103 kg m-3 (compared with 5.51 for Earth 3.34 for the Moon)
- Average distance from sun: 1.52 AU (227.9 x 106 km)
- Axial rotational period (length of day with reference to background stars): 1.03 days
- Orbital period: 686.5 days
- Orbital eccentricity: 0.093
- Orbital inclination: 1.9 deg.
Points of interest:
- Mars is dusty, with fine dust being the main source of its reddish color. Mars' winds redistribute this, resulting in changes of surface color. In Mars' thin air, dust is often shaped into dunes. Interestingly, most of Mars' dust originates in the erosion of a single rock unit - the Medusae Fossae Formation (Ojha et al., 2018.)
- Mars' atmosphere is mostly CO2, like that of Venus, but is much thinner, 6.3 x 10-3 bars. Its butterscotch color mostly comes from tiny dust grains suspended in it. The only time optical scattering of sunlight colors the sky is at sunset, when sunlight passes through enough air to produce blue sunsets. Although thin, Mars' atmosphere supports active weather, including:
water-ice cirrus clouds can be seen.
Martian north polar ice cap from Universe Today.
- Ice: Mars' average temperature is 218 K (-55 deg C), however temperatures vary from 140 K (-133 deg. C) in polar winter to 300 K (27 deg. C, 80 deg. F) in equatorial summer. Mars has permanent polar water-ice caps. The temperature contrast between these ice caps and adjacent warmer ground drives much of Mars' weather. Recent explorations have revealed that considerably more ice is buried beneath thin layers of soil across much wider regions of the planet, or occasionally pooled on the surface.
Martian CO2 sublimation features from NASA-JPL. The scene is ~ 1 km across.
- Dry ice: This water ice seems reassuringly familiar, but Mars is a strange place. Consider:
- During winter, the polar water-ice caps are augmented by deposits of CO2 (i.e. "dry") ice. This ice is transparent so that in spring, sunlight shines through it to warm up the surface beneath. Thus the first dry ice to sublimate (turn to CO2 gas) is at the bottom of the dry-ice layer. This eventually erupts explosively from beneath these caps, carrying sand and dust with it.
- Many of Mars' gullies, rather than ending in an apron of sediment are straight lines that abruptly stop. It is thought that these linear gullies are carved by blocks of sublimating dry-ice sliding downhill like air-hockey pucks.
- Piqueux et al., 2016 report that at higher elevations, Martian night time temperatures drop to levels like those of the ice caps, enabling CO2 frost to form, even in the Martian tropics.
Martian channels from Lunar and Planetary Society.
- Liquid water: Considerable physical and chemical evidence indicates that early in its history, liquid water flowed across Mars. How frequently, for how long, and form what source - rainfall or glacial metwater - is open to debate:
- Many argue that Mars was once covered with extensive oceans. Indeed, Rodriguez, et al., 2016 identify giant tsunami deposits at the margins of the ancient ocean.
- Wordsworth et al., 2015, in contrast argue that observed features are a better match with the episodic melting of equatorial highland glaciers at a time when Mars had a greater axial tilt.
- However Davis et al., 2016 describe networks of channel deposits in Arabia Terra consistent with runoff from rainfall roughly 4 Ga.
- Wilson et al., 2016 describe meltwater channel and lake deposits (also in Arabia Terra) as young as 3 Ga.
- In big news, Orosei et al., 2018 report radar evidence for a "lake" of liquid water roughly 20 km wide beneath the south polar ice sheet - analogous to Antarctica's buried Lake Vostok.
- Over the last decade, a growing body of observations of "recurring slope lineae" or "RSLs" suggests that small scale streams may occasionally flow on Mars today. Hooper and Dinwiddie, 2014 identified an Earth analog to these in the form of debris flows of sand and water in below-freezing arctic sand dunes.
- Ojha et al., 2015, reported the presence of hydrated salts in places of RSL activity. This is what we would expect to be left behind as salty brines boiled away on the Martian surface.
- Throwing cold water on this, several researchers have noted that some gullies and their debris aprons seem to grow during winter and early spring, when one might expect the deposition of CO
2 ice to be a major contributor. (We know that dry ice accumulation can be significant from the effect of a single winter on the Phoenix lander.)
- Most recently, Edwards and Piqueux, 2016 report that whatever causes RSL activity does not result in a temperature contrast between them and surrounding soil, consistent with their being dry. Stay tuned.
Maybe both water, blocks of dry ice sliding down-slope, and layers of dust mobilized by daily condensation and sublimation of dry ice are all active in the formation of contemporary Martian gullies.
But, indisputably, Mars' most ancient landscapes are crossed by the remains of river-sized channels that resembling what we see on Earth. Most researchers accept that at the very least, large amounts of liquid water flowed across Mars during its first billion years. Whether that water:
- Fell from the sky as rain
- Gushed from the ground as volcanic eruptions melted permafrost
But two notes of caution:
- 2010 research suggests that some Martian channels may have been cut by fluid lava.
- On a day to day basis, however, sediment transport on Mars today is mostly performed by the wind.
Topography of Tharsis uplift and Vallis Marineris the Wikipedia.
- The 4000 km long rift valley Valles Marineris, the longest canyon in the Solar System.
- The giant volcanoes of the Tharsis Bulge. These include Olympus Mons, the largest volcano in the Solar System.
Phobos from NASA.
- Mars has two small moons, Phobos (27 x 21.6 x 18.8 km) and Deimos (15 x 12.2 x 11 km). Where did they come from? Conventional wisdom is that they chemically resemble a common class of asteroid (E.G. Pajola et al., 2013), and are probably asteroids that were captured in Mars' orbit. In contrast, however, Citron et al., 2015 propose that Phobos and Deimos actually formed in orbit from material blown off of Mars' surface by a giant impact.
Phobos' orbit is low and decreasing. Eventually, is will be torn apart by tidal forces or crash into Mars' surface. In fact, Phobos' orbit is lower than that of a geostationary satellite, so it completes an orbit faster than Mars rotates. To an observer on the surface, it would appear to rise in the west and set in the East!
- What we know about Mars today makes us intensely curious about what it might have been like back when it had a thicker atmosphere and surface water. Determining when and how this thick atmosphere was lost is a major research goal and the object of NASA's MAVEN mission currently orbiting Mars.
Link to entertaining fictional video of visit to Mars.
Martian atmosphere and surface from Wikipedia. Pareidolia
enthusiasts note the "smiley-face on Mars."
Key concepts and vocabulary. Understand these or you're toast:
- General familiarity with all objects described
- The Moon
- Hydrologic cycle
- Plate tectonics
- Continental / oceanic crust
- Erosion, transport, and deposition of sediment
- Synchronous rotation
- Mare (pl. maria)
- Martian sky - butterscotch with blue sunsets
- Martian ice cap composition
- Stupid CO2 tricks on Mars
- Mars weather
- Evidence for Martian water
- Ancient channels and shorelines
- Ancient tsunami deposits
- Active recurring slope lineae (RSLs)
- Valles Marineris
- Tharsis bulge
- Phobos and Deimos
- J.M. Davis, M. Balme, P.M. Grindrod, R.M.E. Williams and S. Gupta. 2016. Extensive Noachian fluvial systems in Arabia Terra: Implications for early Martian climate. Geology, 44(9).
- Lujendra Ojha, Mary Beth Wilhelm, Scott L. Murchie, Alfred S. McEwen, James J. Wray, Jennifer Hanley, Marion Massé, and Matt Chojnacki. 2015. Spectral evidence for hydrated salts in recurring slope lineae on Mars. Nature Geoscience 8, 829-832.
- M. Pajola, M. Lazzarin, C. M. Dalle Ore, D. P. Cruikshank, T. L. Roush, S. Magrin, I. Bertini, F. La Forgia, and C. Barbieri. 2013. Phobos as a D-type captured asteroid, spectral modeling from 0.25 to 4.0 μm. The Astrophysical Journal, 777(2).
- Sylvain Piqueux, Armin Kleinböhl, Paul O. Hayne, Nicholas G. Heavens, David M. Kass, Daniel J. McCleese, John T. Schofield, James H. Shirley. 2016. Discovery of a widespread low-latitude diurnal CO2 frost cycle on Mars. Journal of Geophysical Research, July 2016 preprint.
- Rodriguez, J. Alexis, Alberto G. Fairen, Kenneth L. Tanaka, Mario Zarroca, Rogelio Linares, Thomas Platz, Goro Komatsu, Hideaki Miyamoto, Jeffrey S. Kargel, Jianguo Yan, Virginia Gulick, Kana Higuchi, Victor R. Baker, and Natalie Glines. 2016. Tsunami waves extensively resurfaced the shorelines of an early Martian ocean. Scientific Reports, 6, Article number: 25106.
- Sharon A. Wilson, Alan D. Howard, Jeffrey M. Moore, and John A. Grant. 2016. A cold-wet middle-latitude environment on Mars during the Hesperian-Amazonian transition: Evidence from northern Arabia valleys and paleolakes Journal of Geophysical Research, Planets, 17 September 2016.
- Wordsworth, Robin D., Laura Kerber, Raymond T. Pierrehumbert, Francois Forget, James W. Head. 2015. Comparison of "warm and wet" and "cold and icy" scenarios for early Mars in a 3-D climate model. Journal of Geophysical Research, 120(6): 1201-1219.