PURDUE UNIVERSITY

EAS 105-THE PLANETS

Prof. Robert L. Nowack

Lecture 13

Mars

After our investigations on Venus, Mars may be the most likely inner planet for human exploration.

What would it feel like to be on Mars? It is daybreak of an early summer day in the northern hemisphere of Mars. The temperature is a cold 190 K. The wind is a light 2 m/sec (2 mph). The pressure is a low 6 millibars (on Earth the pressure is 1 bar = 1,000 millibars). The Sun, just rising in the east, is met by a tiny disk of light, the moon Phobos, which races from west to east. Five hours later, Phobos will be rising again in the west to begin again. The Sun seems only ½ as big as on Earth. (The moon, Phobos, is only ¼ the size of the Sun's disk.) The air is very thin, although it supports a fine red dust, which coats all the rocks.

As the Sun rises, the air takes on a reddish hue. The field where we are sitting is surrounded by boulders covered by fine red dust.

This enhanced color photo of the Pathfinder landing site faces toward the eastern horizon. The elongated, reddish region in the distance is Roadrunner Flats.

As the day progresses, the rocks will warm. Eventually it will reach a temperature of 240 K. But the air is so thin, it rarely gets above 220 K. Finally, 24 hours and 37 minutes after the first sunrise, the sun will rise again.

As the seasons pass - each season takes 170 days, or about twice as long as on Earth, changes are seen. In the fall:

Great dust storms can arise.

The wind increases to 20 m/s.

Dust reduces the sunlight by more than one half.

The temperatures are cooler, during the day at about 200 K.

Storms can take months to clear.

In the winter:

A thin layer of water frost may appear on the ground at night.

The temperature stays cooler.

Finally, after 686 earth days, it is early summer again.

This is a general description of the weather as seen by Viking Lander 1, one of two spacecrafts that landed on Mars in 1976, and sent back data for the next 6 years.

Mars has an average distance of 1.524 Au from the Sun and an eccentric orbit. Mars' sidereal year is 686.98 Earth days. Mars comes into "opposition" with the Earth every 780 days. This opposition distance varies from 55,700,000 kilometers to 101,200,000 kilometers.

Oppositions of Mars from the Earth; 1939 to 1990. The relative distances are shown by the lines joining the orbits. The seasonal dates on the Earth are indicated.

Mars' period of rotation is 24 hours, 37.4 minutes, very similar to Earth. Also, like Earth, Mars' equator is tipped by 24 degrees from its orbit plane – giving rise to seasons. The radius of Mars is 3397 kilometers which is about ½ of Earth's. Mars has only 11% of the mass compared to the mass of Earth. But, Mars is bigger than either the Moon or Mercury and as a result retains a thin atmosphere. Mars has a bulk density of 3900 kg/m³, less than either Earth or Venus. This implies that Mars has a smaller core, out to 40% of the planet's radius, and possibly an iron sulfide core.

The Canal Controversy

About 100 years ago, an Italian astronomer announced the discovery of faint straight lines on Mars he called "canale". An American amateur astronomer, Percival Lowell, devoted much effort observing the possible “canals” on Mars. He was firmly convinced that intelligent life existed on Mars.

H.G. Well's 1898 novel War of the Worlds about aliens from Mars invading Earth intensified interest in the planet. Many observers remained skeptical because these “canals” were always difficult to see.

Finally, the large telescopes at Mount Wilson failed to see canals. Now it is generally accepted that the canals were an optical illusion caused by poor resolution at that time. Nonetheless, Mars stimulated public interest in planetary science.

Interesting parts of Mars can still be seen from Earth. These pictures show the seasonal changes of the southern (pointed up here) polar ice cap. The polar ice cap is made up principally of frozen carbon dioxide (CO₂), also known as dry ice.

Seasons on Mars

The dates given here are Martian seasonal date taken to correspond to seasonal dates on Earth.

Exploration of Mars

In 1965, the U.S. Mariner 4 spacecraft flew past Mars and sent back the first close-up photographs of Mars. Pictures showed a bleak planet with many impact craters. In 1969, Mariner 6 and 7 relayed more photographs. In 1971, Mariner 9 orbited Mars and mapped the entire surface to a resolution of 1 kilometer. It found giant volcanoes, huge canyons, polar caps and a carbon dioxide atmosphere. These spacecrafts made way for the Viking missions in 1976.

Two orbiter spacecrafts and two lander spacecrafts comprised the Viking missions. They sent back information on weather, surface material, and searched for life on Mars until 1982.

Photograph of a Viking Lander similar to those that reached the surface

In 1981, NASA transferred ownership of the Viking Landers to the National Air and Space Museum – the first museum on Mars!

Sojourner Rover deployed against rock ”Moe”.

In July 1997, the robotic rover Sojourner moved on the surface of Mars as part of Mars Pathfinder Mission.

Mars Exploration Rovers, Spirit and Opportunity landed on Mars in 2004.

Spirit’s landing site was Gusev Crater because a sinuous channel runs into it and it was thought this might have once contained water. Opportunity’s landing site at Meridiani Planum was chosen since orbital scans indicated the mineral hematite known to form in the presence of water.

Opportunity found layered sedimentary rocks and small spherical concretions of hematite that were termed “blueberries”; both indicative of water in the geologic past on Mars. Opportunity also found evidence of sulfates plus bromides and chlorides, which on Earth are compounds left behind when bodies of water dry up. Spirit found the mineral goethite; also suggestive of water in the distant past.

from Seeds (2007)

The Mars Global Surveyor (MGS) orbitor found evidence in 2005 of gullies on the surface that could have resulted form a slurry of water and soild coming from cliff faces. However, given the low atmospheric pressure, any water would not last long.

from Astronomy Magazine, March 2007

The recent Phoenix Mars Lander arrived on Mars in July 2008 and landed on Mars’ northern Arctic plains. This was an area where Mars’ Odyssey orbiter shows large amounts of subsurface water-ice. The robotic arm of Phoenix is designed to dig through the top soil to the layers below.

The Moons of Mars

(a) Phobos "fear" and (b) Deimos "Panic"

Phobos orbits at a distance of 9380 kilometers from the center of Mars with a period of 7 hours, 39 minutes and a radius of 11 kilometers. Deimos orbits at a distance of 23,500 kilometer and has a period of 30 hours, 18 minutes and a radius of 6 kilometers. These Moons are highly irregular "potato moons" with numerous surface craters.

Controversial debate persists whether Phobos and Diemos were captured from the asteroid belt or formed the same time as Mars. Both theories have difficulties.

(a) PHOBOS (Radius of 11 kilometers)

(b) DIEMOS (Radius of 6 kilometers)

Mars’ satellites, photographed by the Viking Orbiters. (a) Phobos; the large 10 kilometer crater at the upper right (partly in shadow) is Stickney. (b) Deimos, from a range of 3,300 kilometers. The satellite is seen in the gibbous phase; the illuminated portion facing the camera has an area of about 12 x 8 kilometers.

Large Scale Surface Topography of Mars

Mars has a huge range of elevations. Four mountains rise to a height of 27 km. This may result since Mars has only 2/5 the surface gravity of Earth, allowing higher structures before they sag under their own weight.

A Comparison of Elevations on Venus, Earth and Mars

The surface of Mars divides into two major terrains. Approximately half the planet, lying primarily in the southern hemisphere, consists of ancient cratered terrain. The other half, primarily to the north, consists of younger volcanic plains which are on average several kilometers lower in elevation than the older southern uplands.

The Major Geological Features of Mars

This map shows the prominent Martian landforms. Heavily battered terrain from numerous impact craters shaped the surface during the heavy bombardment period ending about 3.8 billion years ago. Younger white surface areas have fewer craters and, hence, are smoother. Flood channels can be found on 3 main areas: around the Chryse basin northwest of volcanic region Elysium, and on the northeast rim of Hellas impact basin. Valley channels are confined mostly by the oldest terrain.

Geological features of Mars based on Mariner 9 pictures. Permanent ice caps are mostly water and CO₂ ice. The oldest and most cratered terrain occur in the southern hemisphere. Large volcanoes and sparsely cratered volcanic plains predominate in the northern hemisphere. The absence of mountain chains, transform faults, linear troughs, and ridges indicates a stable crust without plate tectonics.

Mars has impact basins primarily in the southern uplands. Hellas is the largest which is 1800 kilometers wide. The next largest impact basin is Argyre which is 700 km wide.

In addition to a north-south division of the planet, there is also an east-west division. One side of the planet has an immense bulge the size of North America, which rises 10 kilometers above the surroundings, called the Tharsis Bulge. The Tharsis Bulge is crowned with four great volcanoes which rise another 15 kilometers.

Zonal Topography Map of Mars

This map shows topography from the north to the south pole of Mars. It highlights two of the most prominent surface features, the Tharsis Bulge and the Hellas impact crater.

View From the Surface

Viking 1 landed on Mars at 22° N on a 3 billion-year-old wind swept plain near the lowest point of a broad basin. There were numerous angular rocks with dune-like deposits of fine soil.

Viking 1 Site

Viking 2 landed in another lowland region called "Utopia". This site resembles the site of Viking 1 but has substantially more boulders. This could be ejecta from a crater about 200 kilometers away.

Viking 2 view of its landing site in “Utopia” on ejecta from the crater Mie.

Each probe analyzed soil and found clays and iron oxides consistent with the red surface color. Each lander carried a seismometer to detect Mars-Quakes. The one for Viking 1 failed to work. The seismometer for Viking 2 only picked up wind noise. Each spacecraft carried a weather station.

Craters on Mars

Mars has thousands of craters, mostly in the southern hemisphere. Presumably these impact craters formed in the same way as the craters on Mercury and the Moon. However, on Mars, there are no extended rays or streaks, but rather "fluidized' "splosh" craters. Most common is a multi-lobed flower form for the fluidized ejecta.

This photograph of Yuty impact crater on Mars represents a good example of a multi-lobed flower crater type. This distinctive pattern probably formed from the airborne debris saturated with groundwater and therefore tended to flow across the ground as a fluid-like mass after being ejected outward.

These occur mostly in equatorial regions. In the upper latitudes, more uniform type of ejecta craters occur. Evidently, Martian ejecta blankets formed by debris that flow along the ground - the Martian soil was fluidized by the impact. Presumably, there is a permafrost layer in the soil.

Mars has large impact craters, but fewer than the Moon. Erosion on Mars may degrade the oldest features. Also, Mars has a fewer number of very small impact craters than the Moon. This again is an erosional feature.

Mariner 6 photographed this Martian surface on July 30, 1969 at range of 2150 miles (3,459 kilometers). It shows numerous craters. The area is 560 miles (901 kilometers) by 430 miles (692 kilometers) and southeast of Meridiani Sinus about 15° below the Equator. Crater on the right has a diameter of about 160 miles (257 kilometers).

In addition, the overall density of craters is much lower in the northern plains as opposed to the southern uplands. This is consistent with the northern plains being younger. (These regions are also similar in crater density to the Lunar Maria of the Moon.

Using cratering rates, we can approximately age date the Martian terrain.

More exact geologic dating will require the return of Mars rock samples for radioactive age dating.

Volcanoes on Mars

About a dozen large volcanoes have been found on Mars. Most of them are associated with the Tharsis Bulge (and hundreds of smaller ones). Three of the most dramatic lie along the crest of the Tharsis Bulge and all rise to similar heights.

Loction and names of major volcanoes and plains on Mars. Solid black patches represent major volcanic edifices.

The fourth and largest volcano is called Olympus Mons and is on the northwest flank of Tharsis Bulge. It is more than 600 kilometers in diameter and would extend from Boston to Washington, D.C. on Earth.

Viking 1 photograph of the great volcano on Mars called Olympus Mons. This 27 kilometer high (17 miles) mountain stretches about 600 kilometers across (370 miles) and would extend from San Francisco to Los Angeles California or from Boston, Massachusetts to Baltimore, Maryland. NOTE: Complex, multiple vent main crater and the steep cliffs that drop off from the mountain’s flanks to the surrounding plain.

Arsia Mons Volcano

Arsia Mons has a considerably larger caldera than other volcanoes on Mars. The last major collapse event on Arsia Mons was followed by a substantial outflow of lava within the caldera. The caldera rim has been breached on the southwest side while lava on the caldera floor buried parts of the northeast rim. The flanks of the shield volcano have been deeply eroded near the breaks in the caldera rim and lava flows extend away from the volcano at these embayments.

These are great "shield" volcanoes, possibly the biggest in the solar system. Their Earth counterpart would be the Hawaii volcanoes. Presumbly, the Mars volcanoes consists of many overlapping flows of basaltic lava. The summits are collapsed features called "calderas". Since Olympus Mons has very few craters, this suggests that it is geologically young. It is possible that these great volcanoes are active even today.

In addition, there are structures called patera which are large low relief volcanic landform on Mars.

Alba Patera Volcano

Alba Patera volcano has a vertical relief of only 2 km. It is surrounded by a distinctive set of enormous fractures having a north-south orientation. Alba Patera is older than the Tharsis shield volcanoes.

The Tharsis Bulge consists of more than just large volcanoes. There are also tension cracks presumably formed by the same force that raised the Bulge. Photograph displays a tectonic feature in the Tharsis area created by tension in the crust.

One of the most spectacular tension features is the enormous crack called Valles Marineris. It extends nearly ¼ of the way around Mars. It is 100 km wide and over 7 km deep.

Valles Marineris

The enormous canyon Valle Marineris carves into the crust going east-west. North is the top of the photograph. Several sizable landslides happened to the northern walls of the main canyon. A series of branching channels cut into the bottom of the plateau from the south wall. They may have evolved slowly by erosion from groundwater release. The photo shows other branches. Valles Marineris is a pull-apart crack and NOT caused by running water. However, the sides of the canyon are being cut by landslides.

Most of the southern hemisphere is cratered uplands. One of the most curious features is the existence of small twisting channels that appear to be runoff channels. But for what? Could these indicate that there was once running water on these old terrains? There are no such channels in the northern plains - they must be old, about 4 billion years.

A network of runoff channels cut into old upland surface near 48° south latitude. In addition, there are flow features where several outflow channels spread out into the northern plains from the southern highlands. These might also have been carved by water or mud. However, from crater counts nearby, these features also appear to be old; about 3.5 billion years old.

This photograph shows some erosion landforms on Mars near the Viking 1 landing site. It took this picture at a height of 1600 kilometers (992 miles). Knobs and hummocks on the right are erosion remnants of an old crater rim.

Polar Regions

Mars has ice caps at both the North and South poles. These grow and shrink depending on the seasons. Because of the eccentricity of Mars' orbit, the southern hemisphere experiences harsher winters. As a result, the southern seasonal ice cap can reach as high up as 55° South. The northern seasonal ice cap, a half a Martian year later, can reach as far south as 65° North.

Seasonal Changes in Mars North Pole

Both seasonal ice caps are made primarily of carbon dioxide frost or dry ice. These frozen deposits of carbon dioxide condense and freeze directly from the atmosphere when surface temperature drops below 150 K. Carbon dioxide frost varies from a few meters to only several centimeters deep.

During the southern summer, the seasonal ice cap retreats leaving a permanent, smaller residual ice cap. The diameter of the residual southern ice cap is about 350 kilometers. This southern residual ice cap is mostly carbon dioxide as well. The residual northern ice cap is larger at about 1000 kilometers in diameter. Temperature measurements of 200 K suggests that this northern residual ice cap may be primarily water ice. This is also suggested since during the summer atmospheric water vapor increases above the cap. This may be a principle storehouse for water on Mars. Another source of water on Mars may be the permafrost layer in the soil.