Olympus Mons

• 1. Introduction

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  Olympus Mons is a volcano on Mars, and a person standing near its summit would not realize they were on a mountain at all. The horizon there lies only 3 kilometers away, while the slope carries on far beyond it. Even from a great distance on the surface, the whole shape cannot be seen at once, because the curve of the planet and the bulk of the volcano itself hide the full profile. This is nonetheless the tallest volcano on Mars. It is also the tallest mountain on any planet. How does a world with weaker gravity than Earth's end up with a peak like this? Why do two sides of the same mountain show opposite kinds of damage? And how did astronomers come to suspect a mountain was there long before any spacecraft arrived?

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• 2. Olympic Snow

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  Astronomers in the late 19th century knew this place by the name Nix Olympica, which is Latin for Olympic Snow. To them it was an albedo feature, a marking defined by how bright it looked against the rest of the planet. Some already suspected it was a mountain, well before space probes confirmed it.

  Schiaparelli, who lived from 1835 to 1910, watched a Martian dust storm blot out nearly everything except two markings, Nodus Gordis and Olympic Snow. According to the astronomer Patrick Moore, Schiaparelli "guessed correctly that they must be high". Telescopic observers had been recording such storms since the 19th century, and only a few of the Tharsis volcanoes stood tall enough to rise above them.

  In 1971 the Mariner 9 spacecraft arrived in orbit around Mars in the middle of a global dust storm. As the haze began to settle, the first things to emerge were the tops of the Tharsis volcanoes. Their altitude far exceeded that of any mountain on Earth, just as astronomers had expected. Mariner 9's observations confirmed that Nix Olympica was a volcano, and astronomers eventually adopted the name Olympus Mons for the feature. Pinning down its height would prove harder, because the edges of the mountain are complicated.

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• 3. Measuring a Giant

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  The Mars Orbiter Laser Altimeter puts the summit of Olympus Mons at 21.287 km, about 2.5 times the elevation of Mount Everest above sea level. That makes it roughly tied with Rheasilvia on Vesta as the tallest mountain yet found in the Solar System. Still, giving the mountain a single height is difficult, because of the complex structure around its margins.

  Measured from the foot of the cliffs along its northwest edge, the local relief is over 21 km. That is a little more than twice the height of Mauna Kea, measured from its base on the ocean floor. Start instead from the plains of Amazonis Planitia, more than 1000 km to the northwest, and the total change in elevation approaches 26 km.

  Across its base, Olympus Mons spans about 600 km. It covers roughly 300000 square kilometers, an area comparable to Italy or the Philippines.

  An average slope of only 5% marks Olympus Mons as a shield volcano, the same broad shape seen in the Hawaiian Islands. The flanks are steepest around their middle and flatten toward the base, giving a profile that curves upward. On the northwest the slopes are gentler and run farther from the summit than they do on the southeast. People have compared the whole mountain to a circus tent held up by a single pole that sits off center.

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• 4. Built Without Moving

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  A lithosphere 70 km thick supports the weight of Olympus Mons. The mountain owes its size largely to the fact that Mars has no mobile tectonic plates. Unlike on Earth, the Martian crust stays fixed over a stationary hotspot, so a volcano there can keep discharging lava until it reaches an enormous height.

  Weaker gravity on Mars also played a part. It reduces the buoyant forces acting on magma as it rises through the crust. The magma chambers beneath such volcanoes are thought to be much larger and deeper than the ones on Earth. Milder weather systems added to the effect, because they wore the mountain down far less than erosion does on Earth. Together with prolonged volcanic activity, these conditions let the peak climb to nearly 22 km above the Martian surface.

  Olympus Mons is the youngest of the large volcanoes on Mars. It began to form during the Hesperian Period, and its eruptions continued well into the Amazonian Period. Crater counts on its flanks would later show just how recent some of that lava is.

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• 5. Lava and Collapse

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  Many thousands of highly fluid basaltic lava flows poured from vents over a long stretch of time to build the mountain. Chemical measurements give about 44% silicates and 17.5% iron oxides, which are what give the planet its red color. Aluminium makes up 7%, magnesium 6%, calcium 6%, and sulfur dioxide 7%. These figures point to basalts and other mafic rocks, which erupt as thin, runny flows and therefore build gentle slopes.

  Chains of pit craters mark the places where lava tubes have partly collapsed. Many individual flows have levees along their edges, formed when the cooler outer margins harden and leave a central trough of molten lava. Broad lava fans mark the spots where lava emerged from tubes that remain intact underground. Around the base, hardened flows spill onto the surrounding plains in wide aprons and bury the basal escarpment.

  Two impact craters on the volcano have provisional names from the International Astronomical Union. Karzok is 15.6 km across and Pangboche is 10.4 km. Both are among several suspected source areas for shergottites, the most abundant class of Martian meteorites.

  In 2004 the Mars Express orbiter took high-resolution images that allowed crater counts on the northwest flank. They dated the lava flows there from 115 million years old down to only 2 million years. Those ages are very recent in geological terms, which suggests the mountain could still be active in a quiet, episodic way. Other accounts date its last eruption to 25 million years ago, so the timing remains unsettled.

  At the summit, a depression measuring 60 km by 80 km and up to 3.2 km deep holds six nested calderas. A caldera forms when a roof collapses after an eruption drains the magma chamber beneath it, so each one records a separate pulse of activity. The largest and oldest segment appears to have formed as a single large lava lake. Laboratory models of caldera geometry led scientists to place the chamber under that largest caldera about 32 km below its floor. Crater counts date the caldera floors from 350 million years ago to about 150 million years ago, with all of them probably forming within 100 million years of one another. Fresh magma from the mantle may have reached the chambers afterward, inflating them and lifting parts of the summit.

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• 6. Why It Leans

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  The long, shallow northwest flank of Olympus Mons shows signs of stretching, including large slumps and normal faults. The steeper southeast side shows signs of squeezing instead. Step-like terraces across its mid-flank are interpreted as thrust faults, and wrinkle ridges run along the basal escarpment.

  An escarpment up to 8 km tall runs along the outer edge of the mountain, though lava flows hide it in places. No other Martian shield volcano has such a cliff, and enormous flank landslides may have created it. Large shield volcanoes grow by spreading sideways at the base as well as by piling lava on their flanks. As a volcano grows, the stress beneath it shifts from compressional to extensional, and a rift can open in the underlying crust. If the volcano sits on sediments with weak layers, such as beds of water-saturated clay, detachment zones called décollements can form. Stretching in those zones can trigger giant landslides and normal faults on the flanks.

  The mountain sits at the edge of the Tharsis bulge, an ancient volcanic plateau most likely finished by the end of the Noachian Period. When Olympus Mons began forming in the Hesperian, the ground sloped gently from the high of Tharsis down into the northern lowland basins. Those basins collected sediment eroded from Tharsis and the southern highlands. The sediment likely held abundant Noachian clays, made when surface water was plentiful on early Mars, and it was thickest in the northwest where the basins were deepest.

  Friction explains the lean. In the thick northwestern sediments, low-friction detachment zones formed, producing the basal escarpment and broad lobes of debris. Toward the southeast, the Tharsis rise offered more resistance. The sediments there were thinner and probably coarser, so they resisted sliding, and rugged basement rocks of Tharsis added further friction. Numerical models that vary friction along the base reproduce the present shape and asymmetry fairly well. The origin of those debris lobes, however, is still argued over.

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• 7. Moat and Aureole

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  Olympus Mons lies between the northwestern edge of the Tharsis region and the eastern edge of Amazonis Planitia. It stands about 1200 km from three other huge Martian shield volcanoes, Arsia Mons, Pavonis Mons and Ascraeus Mons. Known together as the Tharsis Montes, those three are slightly smaller than Olympus Mons.

  A moat about 2 km deep encircles the base of the mountain. It is thought to result from the volcano's immense weight pressing down on the Martian crust. The depression runs deeper on the northwest side than on the southeast.

  A region of grooved, corrugated ground called the Olympus Mons aureole partly surrounds the volcano and is made of several large lobes. To the northwest it stretches up to 750 km and is known as Lycus Sulci. On the east, lava flows cover part of it, and where it is exposed it carries other names, such as Gigas Sulci.

  Huge landslides or gravity-driven thrust sheets that sloughed off the edges of the shield are the likely explanation for the aureole, though its origin remains debated. Whatever the answer, the same weak, water-rich layers beneath the mountain raise a different question, one about water.

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• 8. Thin Air, Warm Water

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  A typical pressure of 72 pascals greets anyone at the summit of Olympus Mons, about 12% of the Martian surface average of 600 pascals. Both numbers are tiny by Earth's standards. The summit of Mount Everest has 32,000 pascals, about 32% of sea-level pressure on Earth. Even so, high-altitude clouds frequently drift over the Martian summit, and airborne dust is still present there.

  Mars's atmosphere is less thin than its low surface pressure suggests. Average surface pressure there is less than one percent of Earth's. But the planet's weaker gravity raises the atmosphere's scale height, so the air thins out with altitude more gradually than it does on Earth.

  Some researchers have speculated that high-pressure water in the pore spaces of the sediments helped the detachment along those weak layers. That idea carries astrobiological implications. If water-saturated zones still exist under the volcano, a high geothermal gradient and leftover heat from the magma chamber would likely have kept them warm. Springs or seeps around the base of Olympus Mons would then offer many chances to look for microbial life.

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