Astronomy 217M e r c u r y , V e n u s &

M a r s

Terrestrial PlanetsAt first glance, the terrestrial planets have little in common with the Earth or each other.

These visible consequences result from Mars and Mercury having little/no atmosphere, Venus has too much.

Orbital SimilarityThe similarities of the terrestrial planets originate in their placement in the solar system.

Mercury has the most eccentric and most inclined orbit. Venus’s rotation is retrograde.

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Maximum Elongation

Orbital Facts

Mercury has commensurate rotational and orbital periods, 2 Prot = 3 Porb.

Venus’s rotation is retrograde and extremely slow.

Mercury Venus Earth Mars

Orbital Semi-Major Axis (AU)

0.39 0.72 1 1.52

Orbital eccentricity 0.206 0.007 0.017 0.093

Orbital Period (yr) 0.241 0.615 1 1.881

Rotational period (days) 58.6 -243 0.997 1.026

Solar Day (days) 175.9 116.7 1 1.028

Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be untrue.

Mercury’s day and year are in a 3:2 resonance; Mercury rotates three times while going around the Sun twice.

Resonant Orbit

Prot = 58.6 days Porb = 87.9 days Psol = 175.9 days

2౫౧ > 2౪౧౬ ѝ 2౧౪ > 23౧౪1 Mercury day = 2 Mercury Years

Long DaySlow, retrograde rotation of Venus results in large difference between solar day (117 Earth days) and sidereal day (243 Earth days).

Prot = 243 days Porb = 225 days Psol = 117 days

Note that Venus’ solar day (Psol) is more than half of the year (Porb), and its sidereal day (Prot) is 8% longer than the year.

2౫౧ > 2౪౧౬ , 2౧౪

Thick clouds prevent observation of Venus’s surface at optical wavelengths.

Observing Venus

Even UV and IR can see only a little deeper into the clouds

Venus’s atmosphere is very dense with a surface pressure 93 times the Earth’s. Venus surface is also very hot, 730 K.

The composition is largely carbon dioxide (97%) and nitrogen (3%).

A solid cloud bank, composed of sulfuric acid, lies 50–70 km above surface, obscuring the surface.

Atmosphere of Venus

Venusian WeatherLike the Earth, the atmosphere of Venus changes over the course of hours.

Upper atmosphere of Venus has high winds (up to 360 km/hr) that blow opposite to the rotational direction, most strongly near the poles, but atmosphere near surface is almost calm (5 km/hr).

Too hot for liquid water, carbon dioxide remained in Venus’s atmosphere, provoking a runaway greenhouse effect as infrared radiation was reabsorbed.

Runaway Greenhouse

Martian atmosphere is also mostly carbon dioxide (95%).

Average surface pressure is < 1/100 of the Earth, the equivalent of an elevation of 35 km.

Thin atmosphere does not retain much heat; allowing temperature to drop sharply at night.

With no magnetosphere, the ionosphere interacts directly with the solar wind.

Martian Atmosphere

The presence of water ice in the Martian soil is revealed by fog forming in low-lying areas, as sunlight strikes. This occurs predominantly at the poles, were water ice caps are seen, and in deep features where the atmosphere is thicker.

Martian Fog

Early MArsA number of lines of evidence suggest that Mars once had liquid water on its surface, perhaps a large ocean in the northern hemisphere.

This would require Mars to have had a warmer and denser atmosphere than it does presently.

3.8 Gyr agoToday

The greenhouse effect on Mars may have runaway in the opposite sense of Venus’s.

Capture of CO2 in water and rock would have depleted Mars’s atmosphere.

As water ice froze, Mars became more reflective, cooling the surface.

Runaway cooling froze more water and eventually carbon dioxide, depleting the atmosphere.

Failed Greenhouse

The best images of Mercury come from the Mariner 10 (1974−75) and Messenger (2004−11) spacecraft.

Mercury shows significant cratering, though less heavily than the Moon.

Mercury also exhibits distinctive scarps or rupes, cliffs several hundred kilometers long and up to 3 km high.

These are thought to form when the solidified crust cracked as a result of contraction by the cooling core.

Surface of Mercury

Mercury’s most prominent geological feature is the Caloris Basin, a 1,500 km diameter impact crater ringed by concentric mountain ranges.

The floor of the basin is volcanic plane, similar to the lunar maria.

Antipodal to the basin lies a patch of “Weird” terrain, possibly caused by converging shock waves or ejecta.

The Surface of Mercury

Surface of VenusRadar mapping by orbiting spacecraft has revealed the Venusian surface to be dominated by relatively smooth volcanic planes, with 2 prominent “continents” rising several kilometers above these planes.

Aphrodite Terra

Ishtar Terra

Comparison of surface maps of Venus and Earth are very informative.

Terrestrial Comparison

Earth shows clear evidence of the movement of tectonic plates, mountain ranges and volcanos defining the edges of continents, while Venus does not.

Rather than gradual recycling of crust by subduction, Venus is thought to undergo periodic global resurfacing.

Mountains of VenusLakshmi Planum is a high plateau on Ishtar Terra. On it’s western edge rises the Maxwell Montes, mountains up to 11 km high.

The meteor crater Cleopatra lies on the western slope of the Maxwell range.

Venus exhibits relatively little cratering, with roughly 1000 craters distributed across the surface.

No craters smaller than 3 km are visible, as Venus’s thick atmosphere causes meteoroids up to 50 m to burn up during entry. The largest crater is Mead, 280 km in diameter.

Venus’s craters show little evidence of degradation by erosion, impacts or volcanism.

This pristine-ness and paucity of craters implies that Venus was globally re-surfaced 300−600 Myr ago.

Venusian Impacts

Volcanoes are common on Venus. Most are shield volcanoes like those in Hawaii, up to 4 km in height.

Venus is home to more than 160 volcanos that are > 100 km across. On Earth, only Hawaiʻi Island, home to Mauna Kea, Mauna Loa & Kīlauea is this large, but it is 10 km high.

Venusian Volcanos

Sif Mons Gula Mons

Volcanic FeaturesVenus exhibits a wide variety of volcanic features.

Coronae: round features with a distinct ring of faults or ridges formed when small mantle plumes raise the surface which fractures and drops when plume cools.

Lava Domes: smaller dome-shapes caused by molten rock bulging out of the ground and then retreating, leaving behind a thin solid crust that later cracks and subsides.

Between 1970 and 1982, the Soviet Union landed 6 Venera probes on the surface of Venus. The probes provided data and pictures for ~ 1 hour before succumbing to Venus’s atmosphere.

Landing on Venus

Mars reveals a wide variety of familiar landforms, some on a scale not seen on Earth.

The Tharsis bulge is a continent sized region raised 10 km above the surrounding surface.

The relative lack of cratering indicates that Tharsis is the youngest surface on Mars, formed 3.8 − 3.5 Gyr ago.

Martian Surface

Valles Marineris is a huge canyon covering 4000 km.

The northern and southern hemispheres of Mars exhibit very different surface features.

Nowhere is there evidence for plate tectonics.

Radar Mapping

Northern hemisphere is rolling volcanic terrain.

Southern hemisphere is heavily cratered highlands.

From cratering, inference is that northern surface is younger than southern.

Average difference in altitude is 5 km.

Implies that northern hemisphere must have been lowered in elevation and then flooded with lava.

Hemispheric Differences

Grandest CanyonValles Marineris is the largest canyon in the Solar system; 4000 km long, up to 120 km wide and 7 km deep.

It was likely created by a rift fault, possibly expanded by water erosion.

Valles Marineri’s formation was likely tied to the formation of the Tharsis Bulge.

Grand Canyon on same scale

Formed as part of the Tharsis bulge, Olympus Mons is the largest volcano in the Solar system; 700 km diameter at base, 25 km high. Its caldera is 80 km in diameter.

Three nearby volcanoes are nearly as large; Arsia Mons, Pavonis Mons & Ascraeus Mons.

Absence of tectonic motion allows mantle hotspot to keep building the same volcano to massive proportions.

Largest Volcanos

The evidence used to support the idea that liquid water existed on Mar’s surface in the past comes from surface features that correspond to water-powered Earth analogues.

For example. these runoff channels resemble those on Earth.

Running Water on Mars

Mars Louisiana

Rivers and Lakes?There are many features that can be attributed to liquid water, but all so far have alternate explanations.

River Delta?

Seashore?

Something seems to have degraded the smaller impact craters on Mars (less than 5 km across). Were these eroded away by water?

There are also examples, like the crater on right, that look like an impact on a liquid surface.

Splashing Craters?

Gully Washer?This intriguing pair of images from the Mars Global Surveyor, appear to show that gully formation may be ongoing. Some have interpreted this as clear evidence of liquid water in Mars’s present.

Planetary FactsMercury Venus Earth Mars Moon

Radius (km) 2439 6052 6370 3396 1737

Mass (1024 kg) 0.33 4.868 5.974 0.642 0.073

Density (kg m−3)

5427 5240 5515 3933 3346

Surface Temperature

100-700 735 184-331 186-293 100-390

Escape Velocity (km s−1)

4.3 10.5 11.2 5 2.4

Magnetic Field (μT)

0.3 <0.003 30 ~0.03 0

Mercury’s magnetic field implies a molten iron core.

It’s high density indicate the core is relatively large, ~ 1800 km in diameter, topped by a thin mantle (~600 km) and crust (~200 km)

This is a sharp contrast to the Moon’s small core.

It’s also more extreme than the Earth’s roughly equal sized core and mantle.

This has suggested to some than Mercury also suffered a large impact which stripped off ~1000 km of mantle.

Mercury’s Interior

Without seismic data, our knowledge of Venus’s internal structure and chemistry is limited.

The absence of subduction zones and clustering of volcanos (as occurs on Earth near plate boundaries) implies a lack of tectonic movement.

It has been suggested that this is due to the absence of water or the high surface temperature making the crust less brittle.

The density of 5204 kg m−3 implies an iron core, and its similarity in size to Earth suggests this core should still be molten.

However, no magnetic field is detected, probably because rotation rate is so slow.

Inside Venus

Seismic data is also unavailable for Mars, limiting our knowledge.

From behavior of crust, it is estimated to be 100 km thick, somewhat more than the Earth’s.

Density is relatively low (3933 kg m−3), raising questions about the degree of differentiation and the core composition. Mars’s mantle is about twice as iron-rich as the Earth’s.

This leads to the suggestion that core has a significant concentration of sulfur, perhaps in the form of iron sulfide.

Absence of magnetic field in a rotating planet is hard to explain unless the core is not liquid, or presence of sulfur and other elements affects the process.

Inside MarS

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