11-5 Both Venus and Mars have volcanoes—and Mars has signs of ancient plate tectonics

Venus has no plate tectonics because its crust is too thin, while on Mars the crust is too thick

From a distance, Venus and Mars appear radically different: Venus is nearly the size of Earth and has a thick atmosphere, while Mars is much smaller and has only a thin atmosphere. But spacecraft observations reveal that these two worlds have many surface features in common, including volcanoes and impact craters. Mars even shows some evidence for ancient plate tectonic activity. By comparing the surfaces of these two worlds to Earth we can gain insight into the similarities and differences between the three largest terrestrial planets.

Observing Venus and Mars from Orbit

To make a detailed study of a planet’s surface, a spacecraft that simply flies by the planet will not suffice. Instead, it is necessary to place a spacecraft in orbit around the planet. Since the 1970s, a number of spacecraft have been placed in orbit around Venus and Mars.

In order to map the surface of Venus through the perpetual cloud layer, several of the Venus orbiters carried radar devices. A beam of microwave radiation from the orbiter easily penetrates Venus’s clouds and reflects off the planet’s surface; a receiver on the orbiter then detects the reflected beam. By measuring the time it takes for reflected waves to return to the orbiter, astronomers can determine the height and depth of Venus’s terrain. The most recent spacecraft to orbit Venus, Magellan, produced the topographic map in Figure 11-16. The same radar method can be used on Mars, and the topographic map of Mars in Figure 11-17 was produced by the Mars Global Surveyor spacecraft (which entered Mars’s orbit in 1997).

Figure 11-16: R I V U X G
A Topographic Map of Venus Radar altimeter measurements by Magellan were used to produce this topographic map of Venus. Color indicates elevations above (positive numbers) or below (negative numbers) the planet’s average radius. (The blue areas are not oceans!) Gray areas were not mapped by Magellan. Flat plains of volcanic origin cover most of the planet’s surface, with only a few continentlike highlands.
(Peter Ford, MIT; NASA/JPL)
Figure 11-17: R I V U X G
A Topographic Map of Mars This map was generated from measurements made by the laser altimeter on board the Mars Global Surveyor spacecraft. As in Figure 11-15, color indicates elevations above or below the planet’s average radius. Most of the southern hemisphere lies several kilometers above the northern hemisphere, with the exception of the immense impact feature called Hellas Planitia. The landing sites for Viking Landers 1 and 2 (VL1 and VL2), Mars Pathfinder (MP), and the Mars Exploration Rovers (Spirit and Opportunity) are each marked with an X.
(MOLA Science Team, NASA/GSFC)

The topographies of Mars and Venus differ in important ways from that of our Earth. Our planet has two broad classes of terrain: About 71% of Earth’s surface is oceanic crust and about 27% is continental crust that rises above the ocean floors by about 4 to 6 km on average. On Venus, by contrast, about 60% of the terrain lies within 500 m of the average elevation, with only a few localized highlands (shown in yellow and red in Figure 11-16). Mars is different from both Earth and Venus: Rather than having elevated continents scattered among low-lying ocean floors, all of the high terrain on Mars (shown in red and orange in Figure 11-17) is in the southern hemisphere. Hence, planetary scientists refer to Mars as having northern lowlands and southern highlands. The implication is that the Martian crust is about 5 km thicker in the southern hemisphere than in the northern hemisphere, a situation called the crustal dichotomy. In this sense Mars resembles the Moon, which has a thicker crust on the far side than on the side that faces Earth (see Section 10-1).

CONCEPT CHECK 11-5

In Figure 11-17 the northern hemisphere is in the top half of the map, with the southern hemisphere in the bottom half. Which hemisphere has the youngest surface, and why?

Tectonics on Venus: A Light, Flaky Crust

Before radar maps like Figure 11-16 were available, scientists wondered whether Venus had plate tectonics like those that have remolded the face of Earth. Venus is only slightly smaller than Earth and should have retained enough heat to sustain a molten interior and the convection currents that drive tectonic activity on Earth (see Section 9-3, especially Figure 9-15). If this were the case, then the same tectonic effects might also have shaped the surface of Venus. As we saw in Section 9-3, Earth’s hard outer shell, or lithosphere, is broken into about a dozen large plates that move slowly across the globe.

291

292

Radar images from Magellan show no evidence of Earthlike plate tectonics on Venus. On Earth, long chains of volcanic mountains (like the Cascades in North America or the Andes in South America) form along plate boundaries where subduction is taking place. Mountainous features on Venus, by contrast, do not appear in chains. There are also no structures like Earth’s Mid-Atlantic Ridge (Figure 9-13), which suggests that there is no seafloor spreading on Venus. With no subduction or seafloor spreading, there has been only limited horizontal displacement of Venus’s lithosphere. Thus, like the Moon (see Section 10-1) and Mercury (see Section 11-3), Venus has a one-plate crust.

Figure 11-18: R I V U X G
A Partially Obliterated Crater on Venus This Magellan image shows how the right half of an old impact crater 37 km (20 mi) in diameter was erased by a fault in the crust. This crater lies in Beta Regio (see the left side of Figure 11-16). Most features on Venus are named for women in history and legend; this crater commemorates Emily Greene Balch, an American economist and sociologist who won the 1946 Nobel Peace Prize.
(NASA/JPL)

Unlike the Moon, however, Venus has had local, small-scale deformations and reshaping of the surface. One piece of evidence for this is that roughly a fifth of Venus’s surface is covered by folded and faulted ridges. Further evidence comes from close-up Magellan images that show that Venus has about a thousand craters larger than a few kilometers in diameter, many more than have been found on Earth but only a small fraction of the number on the Moon or Mercury. We saw in Section 7-6 that the number of impact craters is a clue to the age of a planet’s surface. Such impact craters formed at a rapid rate during the early history of the solar system, when considerable interplanetary debris still orbited the Sun, and have formed at a much slower rate since then. Consequently, the more craters a planet has, the older its surface. The number of craters on Venus indicates that the Venusian surface is roughly 500 million years old. This is about twice the age of Earth’s surface but much younger than the surfaces of the Moon or Mercury, each of which is billions of years old. No doubt Venus was more heavily cratered in its youth, but localized activity in its crust has erased the older craters (Figure 11-18).

Surprisingly, Venus’s craters are uniformly scattered across the planet’s surface. We would expect that older regions on the surface—which have been exposed to bombardment for a longer time—would be more heavily cratered, while younger regions would be relatively free of craters. For example, the ancient highlands on the Moon are much more heavily cratered than the younger maria (see Section 10-4). Because such variations are not found on Venus, scientists conclude that the entire surface of the planet has essentially the same age. This is very different from Earth, where geological formations of widely different ages can be found.

One model that can explain these features suggests that the convection currents in Venus’s interior are actually more vigorous than inside Earth, but that the Venusian crust is much thinner than the continental crust on Earth. Rather than sliding around like the plates of Earth’s crust, the thin Venusian crust stays in roughly the same place but undergoes wrinkling and flaking (Figure 11-19). Hence, this model is called flake tectonics. Earth, too, may have displayed flake tectonics billions of years ago when its interior was hotter.

Figure 11-19: Plate Tectonics Versus Flake Tectonics This illustration shows the difference between plate tectonics on Earth and the model of flake tectonics on Venus.
(Courtesy of John Grotzinger)

Although Venus almost certainly has molten material in its interior, it has no planetwide magnetic field. As we discussed in Section 7-7, Venus may have no magnetic field because it rotates too slowly to generate the kind of internal motions that would produce a magnetic field. With no such field, Venus has no magnetosphere. The planet is nonetheless shielded from the solar wind by ions in its upper atmosphere.

CONCEPT CHECK 11-6

How is it determined that Venus’s surface is older than Earth’s surface, yet younger than the surface of Mercury?

Plate Tectonics on Mars

Like Venus, Mars lacks the global network of ridges and subduction zones like those produced by the seven major plates on Earth. However, evidence has been discovered for the existence of two plates on Mars associated with the 4000-km-long canyon named Valles Marineris (Figure 11-20). While this enormous chasm was first imaged in 1971, it was not until 2012 that astronomers realized the two sides of this canyon are actually plates that have slid past each other horizontally by 93 miles. So far, this is the only known plate boundary on Mars, giving Mars a total of two plates.

Figure 11-20: R I V U X G
Valles Marineris (a) This mosaic of Viking Orbiter images shows the huge rift valley of Valles Marineris, which extends from west to east for more than 4000 km (2500 mi), which is approximately the width of the United States, and is 600 km (400 mi) wide at its center. Its reaches depths of 8 km (5 mi) beneath the surrounding plateau, which is over 4 times as deep as the Grand Canyon, Arizona. At its western end is the Tharsis rise. (b) This perspective image from the Mars Express spacecraft shows what you would see from a point high above the central part of Valles Marineris.
(a: USGS/NASA; b: ESA/DLR/FU Berlin, G. Neukum)

Another piece of evidence about ancient Mars comes from the presence of stripes, caused by crustal magnetism, that display alternating directions of the magnetic field. These magnetic stripes are shown as red and blue bands in the lower middle of Figure 7-15. It is thought that these stripes on Mars might be formed in a similar way that magnetic stripes are formed on Earth, as illustrated in Figure 9-22. On Earth, the stripes arise from an alternating global magnetic field that magnetizes lava as it cools at a rift. As the newly formed crust spreads away from the rift, stripes of magnetism with alternating directions spread across the surface. Two mechanisms are inherent in the forming of Earth’s magnetic stripes that would also take place on Mars—plate tectonics and a magnetic dynamo in a molten core with an alternating magnetic field. It is a remarkable achievement of geology that stripes of magnetism observed on Earth today might tell us so much about another planet’s early history. However, if Martian plate tectonics helped to create these magnetic stripes, another mystery arises: Why can’t we see more than the two tectonic plates associated with Valles Marineris?

293

294

We can also use magnetism detectable in craters to estimate when the magnetic dynamo of Mars shut down since craters that formed after the dynamo shut down would not show magnetism. Using age estimates of the craters without magnetism, we can estimate that the dynamo might have shut down around 500 million years after Mars formed.

Why didn’t Mars experience more plate tectonic activity? Recall that smaller objects cool off more quickly than larger objects (Section 7-6). Because Mars is a much smaller world than Earth, the outer layers of the red planet have cooled more quickly than on Earth, which has led to a thicker crust. Thus, Mars lacks plate tectonics because its crust is too thick for one part of the crust to be subducted beneath another. We see that for a terrestrial planet to have extensive plate tectonics, the crust must not be too thin (like Venus) or too thick (like Mars), but just right (like Earth).

The idea that the Martian crust is too thick to allow for much plate tectonic activity has been verified by carefully monitoring the motion of a spacecraft orbiting Mars. If there is a concentration of mass (such as a thicker crust) in one region on the planet, gravitational attraction will make the spacecraft speed up as it approaches the concentration and slow down as it moves away. A team of scientists analyzed the orbit of Mars Global Surveyor in just this way. They found that unlike Earth’s crust, which varies in thickness from 5 to 35 km, the Martian crust is about 40 km thick under the northern lowlands but about 70 km thick under the southern highlands. Both regions of the Martian crust are too thick to undergo subduction, making plate tectonics very difficult.

Volcanoes on Venus and Mars

Radar images of Venus and visible-light images of Mars show that both planets have a number of large volcanoes (Figure 11-21). Magellan observed more than 1600 major volcanoes and volcanic features on Venus, two of which are shown in Figure 11-21a. Both of these volcanoes have gently sloping sides. A volcano with this characteristic is called a shield volcano, because in profile it resembles an ancient Greek warrior’s shield lying on the ground. Martian volcanoes are less numerous than those on Venus, but they are also shield volcanoes; the largest of these, Olympus Mons, is the largest volcano in the solar system (Figure 11-21b). Olympus Mons rises 24 km (15 mi) above the surrounding plains. By comparison, the highest volcano on Earth, Mauna Loa in the Hawaiian Islands, has a summit only 8 km (5 mi) above the ocean floor.

Figure 11-21: Volcanoes on Venus and Mars (a) The false color in this radar image approximates the real color of sunlight that penetrates Venus’s thick clouds. The brighter color of the extensive lava flows indicates that they reflect radio waves more strongly. To emphasize the gently sloping volcanoes, the vertical scale has been exaggerated 10 times. (b) The volcanoes of Mars also have gently sloping sides. In this view looking down from Mars orbit you can see bluish clouds topping the summits of the volcanoes. These clouds, made of water ice crystals, form on most Martian afternoons.
(a: NASA, JPL Multimission Image Processing Laboratory; b: NASA/JPL/Malin Space Science Systems)

Most volcanoes on Earth are found near the boundaries of tectonic plates, where subducted material becomes molten magma and rises upward to erupt from the surface. This cannot explain the volcanoes of Venus and Mars, since there are no subduction zones on those planets. Instead, Venusian and Martian volcanoes probably formed by hot-spot volcanism. In this process, magma wells upward from a hot spot in a planet’s mantle, elevating the overlying surface and producing a shield volcano.

295

On Earth, hot-spot volcanism is the origin of the Hawaiian Islands. These islands are part of a long chain of shield volcanoes that formed in the middle of the Pacific tectonic plate as that plate moved over a long-lived hot spot (Figure 11-22). On Venus and Mars, by contrast, the absence of significant plate tectonics means that the crust remains stationary over a hot spot. On Mars, a single hot spot under Olympus Mons probably pumped magma upward through the same vent for millions of years, producing one giant volcano rather than a long chain of smaller ones. The Tharsis rise and its volcanoes (see Figure 11-17, Figure 11-20a, and Figure 11-21b) may have formed from the same hot spot as gave rise to Olympus Mons; a different hot spot on the opposite side of Mars produced a smaller bulge centered on the volcano Elysium Mons, shown near the right-hand side in Figure 11-17. The same process of hot-spot volcanism presumably gave rise to large shield volcanoes on Venus like those shown in Figure 11-21a.

Figure 11-22: Hot-Spot Volcanoes on Earth A hot spot under the Pacific plate has remained essentially stationary for 70 million years while the plate has moved some 6000 km to the northwest. The upwelling magma has thus produced a long chain of volcanoes. The Hawaiian Islands are the newest of these; the oldest, the Emperor Seamount Chain, have eroded so much that they no longer protrude above the ocean surface.
(World Ocean Floor, based on bathymetric studies by Bruce C. Heezen and Marie Tharp. Painting by Heinrich C. Berann. Copyright Marie Tharp, 1977)

Volcanic Activity on Venus

About 80% of the surface of Venus is composed of flat plains of volcanic origin. In other words, essentially, the entire planet is covered with lava! This observation shows the tremendous importance of volcanic activity in Venusian geology.

To verify the volcanic nature of the Venusian surface, it is necessary to visit the surface and examine rock samples. Figure 11-23 is a panoramic view taken in 1981 by the Soviet spacecraft Venera 13, one of 10 unmanned spacecraft that the Soviet Union landed successfully on the surface of Venus. Russian scientists hypothesize that this region was covered with a thin layer of lava that fractured upon cooling to create the rounded, interlocking shapes seen in the photograph. This hypothesis agrees with information obtained from chemical analyses of surface material made by the spacecraft’s instruments. These analyses indicate that the surface composition is similar to lava rocks called basalt, which are common on Earth (see Figure 9-18a) and in the maria of the Moon (see Figure 10-16). The results from Venera 13 and other landers are consistent with the picture of Venus as a world whose surface and atmosphere have been shaped by volcanic activity.

Figure 11-23: R I V U X G
A Venusian Landscape (a) This wide-angle color photograph from Venera 13 shows the rocky surface of Venus. The thick atmosphere absorbs the blue component of sunlight, giving the image an orange tint: The stripes on the spacecraft’s color calibration bar that appear yellow are actually white in color. (b) This color-corrected view shows that the rocks are actually gray in color. The rocky plates covering the ground may be fractured segments of a thin layer of lava.
(a: NSSDC/NASA; b: GSFC/NASA)

296

Most of the volcanoes on Venus are probably inactive at present, just as is the case with most volcanoes on Earth. But in 2010, evidence was found for very recent volcanic activity on Venus. By analyzing infrared emission, the European Space Agency’s Venus Express orbiter found material on volcanoes showing signs that it had not experienced significant weathering. However, due to weathering by Venus’s hot and thick atmosphere, the surface of fresh volcanic rock should show weathering fairly quickly. This analysis points to very young lava flows, perhaps a few hundred years old, or as much as 2.5 million years old (which is still geologically young). The presence of such young lava flows suggests that Venus, like Earth, has some present-day volcanic activity.

Another piece of evidence for ongoing volcanic activity on Venus comes from the planet’s atmosphere. An erupting volcano on Earth ejects substantial amounts of sulfur dioxide, sulfuric acid, and other sulfur compounds into the air. Many of these substances are highly reactive and short-lived, forming sulfate compounds that become part of the planet’s surface rocks. For these substances to be relatively abundant in a planet’s atmosphere, they must be constantly replenished by new eruptions. Sulfur compounds make up about 0.015% of the Venusian atmosphere, compared to less than 0.0001% of Earth’s atmosphere. This evidence suggests that ongoing volcanic eruptions on Venus are ejecting sulfur compounds into the atmosphere to sustain the high sulfur content.

Volcanic Activity on Mars

Spectroscopic observations from Mars’s orbit confirm that the planet’s rocks and sands are made almost entirely of the three minerals feldspar, pyroxene, and olivine. These are the components of basalt, or solidified lava. Thus, Mars, like Venus, had a volcanic past.

Unlike lava flows on Venus, however, most of the lava flows on Mars have impact craters on them. These craters suggest that most Martian lava flows are very old and that most of the volcanoes on the red planet are no longer active. This is what we would expect from a small planet whose crust has cooled and solidified to a greater depth than on Earth, making it difficult for magma to travel from the Martian mantle to the surface.

However, a few Martian lava flows are crater-free, which suggests that they are only a few million years old. If volcanoes erupted on Mars within the past few million years, are they erupting now? Scientists have used infrared telescopes to search for telltale hot spots on the Martian surface, but have yet to discover any. Perhaps volcanism on Mars is rare but not yet wholly extinct.

Spacecraft that have landed on Mars have taught us a great deal about the geology and history of the red planet. Before we explore the Martian surface in detail, however, it is useful to examine the unique atmospheres of both Mars and Venus.