Mars

NASA -- larger image and more information

The surface of Mars is cold and bombarded with ultraviolet radiation from
the Sun. As a result, superoxide ions have formed on the surface of mineral
grains, which might decompose organic molecules formed on Mars or
deposited there by meteorites.

Today, what little atmosphere Mars is able to hold on to is composed primarily of carbon dioxide (over 95 percent) with small amounts of other gases (including 2.7 percent nitrogen, 2.7 percent argon, but only 0.13 percent oxygen). Barometric pressure varies slightly between about 7 and 11 millibars in the northern and southern hemispheres as carbon dioxide freezes out to form an immense polar cap at the pole then in Winter, which evaporates in Spring. In contrast, the average atmospheric pressure on Earth is about a hundred times greater at around 1,000 millibars. However, Mars' atmosphere is thick enough to support strong winds and vast dust storms that can blanket the entire planet for months. Its thin atmosphere produces a greenhouse effect that is only enough to raise surface temperature by 5 degrees Celsius (2.8 °F), much less than what occurs on Venus and Earth.

Steve Bartlett, SCIENCE@NASA


Larger illustration.


The planet's atmosphere
is being ripped off in
chunks by the Solar Wind
with assistance by its
"magnetic umbrellas" (more).

Mars once may have had an atmosphere as dense as Earth's. On November 21, 2008, however, researchers announced new evidence that the atmosphere of Mars was and is still being stripped away by the Solar Wind. Although Mars has a weak magnetic field, it is very much unlike Earth's which is like a global bubble. The magnetic field around Mars is in the form of magnetic umbrellas that emanate from the Martian surface and and reach beyond the top of its atmosphere. Numbering in the dozens, these umbrellas about 40 percent of the planet’s surface, mainly in the southern hemisphere. These umbrella fields can link with the magnetic field in the Solar Wind through "magnetic reconnections" so that the joined fields wrapped themselves around a packet of gas at the top of the Martian atmosphere to form a magnetic capsule a thousand kilometers wide with ionized air trapped inside. Subsequently, the pressure of the Solar Wind pressure can "pinch off" the capsule (or "plasmoid") and blow its load of air away into deep space (more).

Mars's rotation produces a day just a little longer than an Earth day. However, it's more distant orbit around the Sun takes almost two Earth years and is also significantly elliptical ("eccentric"). As a result, surface temperature varies by about 30 °C (86 °F) between its closest and farthest approaches to the Sun (perihelion and aphelion).

NASA -- larger image


Blue-white water ice clouds above Tharsis
volcanoes during a northern Summer's day
in April 1999 (more information).

Although the average surface temperature is about -63 °C (-81 °F), it can range widely from as little as -133 °C (-207 F) at the winter pole to almost 27 °C (80 °F) during a local day in summer. Martian air contains only about 1/1,000 as much water as Earth, however, this small amount can condense out to form clouds that ride high in the atmosphere or swirl around the slopes of Mars' towering volcanoes. Local patches of early morning fog can also form in valleys, and a thin layer of water frost can cover the ground in winter (see morning clouds over the canyons of Noctis Labyrinthus at Astronomy Picture of the Day).

NASA -- larger and jumbo images

Mars northern summer at its
North Polar Ice Cap roughly
a Mars year apart, March 13,
1999 and January 26, 2001
(more information).

Mars has permanent ice caps at both poles composed of solid carbon dioxide ("dry ice") over a layer of water ice. The ice caps have alternating layers of ice with varying concentrations of dark dust. Seasonal variations in the size of the polar caps can change Mars atmospheric pressure by as much as 25 percent. In the northern summer, the carbon dioxide completely evaporates and uncovers its lower layer of water ice. The polar layering of water ice on Mars may be caused by climatic changes associated with long-term variation in the planet's axial tilt (its inclination relative to the plane of its orbit).

NASA, JPL, University of Arizona/LPL, Los Alamos National Laboratories

While the uppermost soil layer on Mars may contain very little ice away from
the poles, an ice-rich zone underneath contains 20 to 50 percent ice by mass.
This ice-rich layer is closer to the surface near the Martian South Pole (left of
diagram). While the Mars Odyssey spacecraft can detect hydrogen indicative
of water ice down to a depth of about one meter (three feet), it isn't known
how deep the ice-rich zone may continue below that depth (more).

NASA, JPL, Malin Space Science Systems

Also, see summer and winter images of the
South Polar Cap, and other polar images (more).

Rapid sublimination of carbon dioxide ice at the
South Polar Cap of one to three meters (3-10 feet)
from 1999 to 2001 suggests that Mars may
experience global climate shifts over thousands
of years like Earth's ice age cycles (more).

On March 15, 2007, the ESA announced that its Mars Express (working with NASA’s Mars Global Surveyor) spacecraft used a NASA-Italian Space Agency radar instrument to determine that there appears to be sufficent water ice at Mars' south pole alone to submerge the planet in 36 feet (11 meters) of water, if melted (news release). Ice deposits at the Martian south pole were found to be up to 2.3 miles (3.7 kilometers) thick and widespread enough to cover an area larger than Texas or much of Europe. The ice lies under a polar cap of white, frozen carbon dioxide and water. It appears to be composed of at least 90 percent frozen water, with dust mixed in. Mars Express is also surveying the composition and thickness of the planet's north polar cap. Although some scientists believe that the polar deposits contain most of the planet's known water and can cover the planet with a global layer perhaps tens of meters thick, the amount of water suggested in surface features (such as deeply eroded canyons) indicates that Mars may once have had enough water for an ocean that was hundreds of meters deep.

MOLA Science Team,
NASA


Larger composite image.



Water ice in Mars' south
polar cap alone (area
within the black boundary
line) contains enough
water to submerge the
entire planet to a depth
of 36 feet (11 meters),
if melted (more).

On June 26, 2008, NASA announced that its Phoenix Mars Mission dug up a soil sample from the top inch (2.5 centimeters) that is similar to surface soils found in the upper dry valleys in Antarctica on Earth. The soil is alkaline (pH of 8-9) with trace nutrients (salts of magnesium, sodium, potassium, and chloride) and so may be suitable for using as garden soil, if sufficient organic matter was added, for supporting bacteria and plants -- including vegetables like asparagus and turnips (more). Since its successful landing near the Martian north pole on May 25, 2008, the Phoenix lander has been studying the history of water and habitability potential of the Martian arctic's ice-rich soil. It is designed to search for organic evidence of ancient or current Earth-type life by digging up soil samples down to as much as three feet below the surface if it can scrape through subsurface ice (see the latest news from Phoenix).

Phoenix Team, U. Arizona,
JPL-CalTech, NASA


Larger image.


As on Earth, polygonal ground
fractures are produced by
seasonal expansion and
contraction of ground ice,
and some unexpectedly small
polygons (possibly within
larger polygonal fractures)
have been found around the
Phoenix lander (more).

On December 6, 2006, NASA announced that evidence of liquid water erupting from just below the Martian surface had been imaged by the Mars Global Surveyor before it finally ceased operating in November 2006 (NASA new release and APOD). A number of observations, including the recent light-toned flows in the Centauri Montes Region and Terra Sirenum, indicate that a number of apparent "gully" sites show evidence of recent changes, providing tantalizing evidence for subsurface sources of liquid water. In particular, the two cases of new water flows may indicate the presence of aquifers (subsurface rocks saturated with water) that could be detected by orbiting, ground-penetrating radar systems on the ESA's Mars Express or NASA's Mars Reconnaissance Orbiter, which entered orbit around Sol's outermost rocky planet on March 10, 2006.

MGS, MSSS,
JPL, NASA

Larger image of a
crater in Centauri
Montes.

Evidence of recent
eruptions of liquid
water from just
below the Martian
surface were
announced in late
2006 (more).

At the March 2009, 40th Lunar and Planetary Science Conference, a team of scientists discussed the discovery of ice lying beneath a layer of dust ("desiccated regolith") in Mar's mid-northern plains in images from the HiRISE camera onboard NASA's Mars Reconnaissance Orbiter (MRO). The craters were found at five sites ranging at latitudes of 43° to 56° north and are estimated to be around 10 to 20 feet (3 to 7 meters) across and a foot or two deep (one- to two-thirds of a meter). One cluster of impacts probably occurred between between June and August 2008, while a larger excavation may have been produced between January and September 2008. The whitish substance uncovered by the largest impact was confirmed as water ice by the MRO's CRISM instrument through spectrum analysis. These underlying ice deposits were probably at least two inches (five centimeters) thick, as they were unstable at their mid-latitude locations and so were observed to have subliminated (evaporated) into the Martian atmosphere over a period of months (Bryne et al, 2009; and Kelly Beatty, Sky and Telescope, March 27, 2009).

Bryne et al, 2009;
HiRISE, MRO,
LPL/UA, NASA


Close-up and field images.


Recent meteorite impacts into Mars'
mid-northern plains have uncovered
a of ice lying just below a layer
of dust (more).

At the March 2009, 40th Lunar and Planetary Science Conference, a team of scientists (including Carlton C. Allen, Dorothy Z. Oehler, and David Baker) discussed the possible discovery of mud volcanoes on Mars that may have been created by subsurface eruptions of liquid water. The isolated and overlapping, dome and cone-shaped structures have central craters and have been to shown to overly the surrounding northern plains, and so resemble mud volcanoes seen on Earth in aerial and space images. Infrared images (including color analysis) from space indicate that the Martian mounds are probably composed of fine-grained sedimentary substances (i.e., mud) that cool more quickly at night than surrounding rock, which appear to include minerals such as iron oxides that are formed in the presence of water. If the mounds do turn out to be mud volcanoes, then the clays in their mud may have protected organic molecules (like ammonia and proteins) from any organisms that once lived in watery, surface or underground environments (Allen et al, 2009; David Shiga, New Scientist, March 20, 2009; and Astronomy Picture of the Day).

Allen et al, 2009;
HiRISE, MRO,
LPL/UA, NASA




Larger image.




This suspected mud
volcano, which is
over 100 meters
(300 feet) in
diameter, is located
in Mars' northern
plains (more).

The planet was much more like Earth when it was young and almost all of its carbon dioxide was used up to form carbonate rocks. Without Earth's plate tectonics, however, Mars has been unable to recycle its carbon dioxide back into its atmosphere and so sustain a significant greenhouse effect to retain more solar heat. Hence, the surface of Mars is much colder than the Earth would be at that distance from the Sun.

In the distant past, the planet appears to have had a much denser atmosphere with flowing water on the surface because it has physical features that closely resemble lake and ocean shorelines, the gorges of huge rivers and riverbeds, and islands. Clear evidence of erosion is evident in many places on Mars that suggest large areas of flooding and small river systems, but this probably occurred only briefly and very long ago, as the age of the erosion channels is estimated at about four billion years.

MER Mission, NASA, JPL, Caltech -- larger and jumbo images

Rocks found on a broad plain called Meridiani Planum appear to have been formed underwater
on the shores of an ancient salty sea, that may have been suitable for Earth-type life (more).

In January 2004, two identical robotic geologists (called "rovers") successfully landed on the Martian surface "to search for and characterize a wide range of rocks and soils that hold clues to past water activity" -- particularly those that may provide evidence of environmental conditions suitable for sustaining Earth-type life, such as a persistently wet environment. On March 2, 2004, NASA announced that the rover Opportunity's investigation of rocks on a broad plain called Meridiani Planum has produced "convincing and precise evidence" that the region was once soaked with water (images and press release). On March 23, 2004, Steve Squyres, the mission's principal scientific investigator, announced that the scientific team now believes that Opportunity's findings of ripple patterns and concentrations of salt indicated that the rocky outcropping analyzed was formed underwater, possibly as part of the shoreline of a salty sea (images and press release). If so, conditions on the planet may once have been suitable for Earth-type microbial life (More information and images are available from NASA's Mars Exploration Rover Mission.)

USGS, PSI, NASA


Larger image.


The ancient Martian crater,
Hellas Basin, was once a
giant lake or sea (more).

Scientists working on a geological mapping project of Mars have identified sedimentary deposits in the 1,400 mile (2,300-kilometer) wide and 5-mile (8-km) deep Hellas Basin, a very large and ancient crater. Water drained into Hellas around 4.5 to 3.5 billion years ago during the early to mid-Noachian Period of Mars. The scientists believe that fine-layered outcrops found around the eastern rim of Hellas are likely to be sedimentary deposits, that were formed through the erosion and transport of rock and soil from the Martian highlands into a standing body of water within Hellas (Bob King, Duluth News Tribune, June 9, 2010; BBC News, June 8, 2010; and Bleamaster and Crown, 2010).

ESA, Mars Express,
DLR/FU Berlin (Gerhard Neukum)


Larger pack-ice and area images


Dust-covered, pack ice from a
five-million-year-old sea has been
found in the southern Elysium
region (New Scientist and BBC).

In February 2005, scientists working with the European Space Agency's Mars Express revealed that the orbiting probe had detected a huge frozen sea in the southern portion of Elysium Planitia located near the Martian equator (Murray et al, 2005, in pdf). The pack-ice region in the southern Elysium, five degrees north of the equator, exhibits plated and rutted features across an area measuring roughly 800 by 900 kilometers (497 by 559 miles), which appears to average around 45 meters (nearly 148 feet) in depth. ESA planetary scientists believe that a catastrophic event flooded the region around five million years ago, then pack ice formed on top of that water and broke up, and then the whole area froze rigid. Subsequently, large amounts of dust (possibly volcanic ash) fell over that area, through the water and on top of the pack ice -- protecting it from sublimation and giving it a different hue, while the water evaporated. The water that formed the sea appears to have come from beneath the surface of Mars, erupting from a series of fractures known as the Cerberus Fossae. In addition, there are other regions on Mars with similar plate formations that could also be made of dust-covered pack ice (New Scientist and BBC News; Astronomy Picture of the Day; and Murray et al, 2005, in pdf).

Unknown artist, CU-Boulder


Larger and jumbo images.


An ocean once covered roughly a
third of the Martian surface (more).

A great ocean once covered slightly over a third of the Martian surface some 3.5 billion years ago, according to a 2010 study by scientists at the University of Colorado in Boulder (CU-Boulder). Although previous studies had proposed the existence of a large, ancient ocean on Mars over the past two decades, the available evidence was repeatedly contested. This new study provides further support for the idea of a sustained sea on the Red Planet within and along the margins of the northern lowlands during Mars' wet and warm Noachan epoch (around 4.1 billion to 3.7 billion years ago), based on global databases of known river delta deposits, valley networks, and present-day Martian topography. In a related study, scientists also detected roughly 40,000 river valleys on Mars, around four times the number of river valleys previously identified. These new findings also support the hypothesis that an ocean formed on early Mars as part of a global and active hydrosphere. The ancient ocean likely covered about 36 percent of the planet and contained around 30 million cubic miles, or 124 million cubic kilometers, of water (the equivalent of a 1,800-foot, or 550-meter-deep layer of water spread out over the entire planet, but still about 10 times less than the current volume of Earth's oceans). (CU-Boulder press releases of June 13, 2010; June 17, 2009; and Di Achille and Hynek, 2010).

NASA -- larger image

In late June 2001, a dust storm began that quickly
enveloped Mars for about three months before
showing signs of clearing (more information).

Today, planet-covering dust storms can develop on a very dry Mars. In late June 2001, a small dust cloud erupted inside the Hellas Basin, a 9-km (5.6-mile) deep impact crater on the planet's southern hemisphere. By early July, the storm spilled out of the Basin and wrapped itself around the entire planet within three weeks, shrouding Mars in haze and warming the atmosphere by as much as 30° C.

NASA/Thermal Emission Spectrometer (TES)/Arizona State University
(Growth of Mars dust storm between June 24 and July 8, 2001 -- more information and movie).

One theory is that airborne dust particles warm the local atmosphere by absorbing sunlight. Then, wind is generated as warm air rushes toward colder regions. In a positive feedback loop, strong winds lift even more dust off the ground, which further heats the atmosphere and eventually transforms a small dust cloud into a planet-wide storm. However, once underway, Martian dust storms tend to last last for weeks or months.

Although Mars is much smaller than Earth with a diameter of 6,794 kilometers (4,222 miles), its surface area is about the same as the land surface area of Earth. Its southern hemisphere is predominantly ancient, cratered uplands somewhat similar to those found on the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary between plains and uplands, which may have been caused by a very large meteoric impact shortly after the planet's formation. (See NASA's interactive topographic map of Mars, with more information from Astronomy Picture of the Day, and the Mars Atlas.)

NASA


larger image and 10x
vertical exaggeration


Olympus Mons may be the largest volcano
in the Solar System, at about 27 km (17
miles) tall -- three times the height of
Mount Everest on Earth -- and about 10
times wider than it is high.

The interior of Mars appears to contain a dense core of about 3,400 km (2,200 miles) in diameter, a molten rocky mantle somewhat denser than the Earth's, and a thin crust. The crust is about 80 km (50 miles) thick in the southern hemisphere but only about 35 km (22 miles) thick in the north. Mars' relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (i.e., as iron sulfide).

JPL, NASA


Larger illustration.



Mars has a core of mostly
molten liquid iron that is
about half the planet's
width, with a possibly
solidified inner core (more).

Although scientists now believe that the planet's core still has liquid molten iron in an outer layer, they are uncertain whether the inner core solidified like Earth's (more). Mars appears to lack active plate tectonics and volcanic activity at present like Mercury and Earth's Moon. Large but weak, and not global, magnetic fields exist in various regions of Mars which are probably remnants of an earlier global field that has since disappeared.

Jeffrey C. Andrews-Hanna
et al, 2008; MIT


Larger composite image.


An object up to more than
half of Mercury's diameter
hit the northern hemisphere
of Mars within 200 million
years of its birth and carved
out an elliptical depression
(Borealis Basin) that is
partially obscured by the
Tharsis Bulge (more).

On June 25, 2008, three teams of astronomers announced the publication of separate papers in the June 26, 2008 edition of Nature supporting the hypothesis that Mars has the largest known impact structure (Borealis Basin) in the Solar System, supplanting the Aitken Basin at the Lunar South Pole. First proposed in 1984, the massive elliptical depression in Mars northern hemisphere is partially obscured by the volcanic lava flows of the Tharsis Bulge. It was created by an impact with an object up to over half the size of planet Mercury (between one-tenth and two-thirds the size of Earth's Moon) around 4.4 billion years ago, within 200 million years of planet Mars' formation. Creating a crater four times larger than any other confirmed in the Solar System (around 8,520 by 10,650 kilometers or 5,294 by 6,618 miles across), the impact stripped off 40 percent of Mars' crust and left a surface that is lower, smoother, and less marked by craters than that found in its southern hemisphere (NASA MRO news release; Ashley Yeager, Science News, June 25, 2008; David Shiga, New Scientist, June 25, 2008; Katherine Sanderson, Nature News, June 25, 2008; Andrews-Hanna et al, 2008; and Wilhelms and Squyres, 1984).

William Hartman,
Gerhard Neukum,
HRSC Team,
Mars Express, ESA




Larger illustration





Mars experienced
five major episodes
of global volcanic
activity (more).

At the March 2008 Lunar and Planetary Science Conference, planetary scientists used data collected by the European Space Agency's Mars Express mission announced further evidence that beginning about 3.5 billion years ago, Mars experienced five episodes of major volcanic activity. Each episode released massive eruptions of lava and hot water onto the Martian surface lefted identifiable features. The scientists were able to date each episode by counting the number of small craters subsequently blasted into volcanic terrains created by each episode, by assuming that more craters accumulated on older terrains as meteorites of all sizes bombarded their surfaces over time.

ESA, Mars Express,
DLR/FU Berlin
(Gerhard Neukum)




Larger image




The size and number
of craters on terrain
with lava flows and
water channels have
been used to date
major periods of
global volcanic
activity (more).

Crater analysis allowed the scientists to estimate estimated five major periods of volcanic activity: 3.5 billion years ago, 1.5 billion years ago, between 400 million and 800 million years ago, 200 million years ago, and 100 million years ago. The dates of the earlier episodes have error margins of 100 million to 200 million years, while the later dates are correct to within 20 million to 30 million years. Although the most recent activity on the summit of Olympus Mons, Mars's largest volcano, occurred around 150 million years ago, minor flows in some areas have been detected that are as recent as 2 million years ago. Peaks in eruptions of the volcano match the dates his team found for global volcanic activity. Two large channels, Kasei Valles and Mangala Valles, also provide evidence of episodes of water flow that roughly match the times of high volcanic activity (ESA news release and Hartmann and Neukum, 2001).

ESA, Mars Express,
DLR/FU Berlin (Gerhard Neukum)


Larger image with Stickney Crater at left (more); and
2008 jumbo image (from NASA MRO / HiRISE).


Phobos may have formed from an asteroid colliding
with Mars, it appears to be composed of rocks found
in carbonaceous meteorites as well as water-formed,
phyllosilicate minerals found on the surface of both
Mars as well as Earth (more).

HiRISE, MRO, LPL, NASA


Larger, jumbo, and two-sided images.


Like Phobos, the outer moon, Deimos, has a near-circular and equatorial orbit around Mars, which suggests that it may also have formed from an ancient asteroidal impact that raised a lot of rock from the Martian surface Astronomy Picture of the Day).

Both Deimos and Phobos are saturated with craters. Deimos has a smoother appearance from partial filling of some craters, while Phobos shows striated patterns which are probably cracks resulting from the impact that produced its largest crater, Stickney -- the maiden name of the wife of astronomer Asaph Hall (1829-1907; portrait) who discovered the two satellites in 1877.

Malin Space Science Systems, MGS, JPL, NASA


Larger and jumbo images.



Phobos appears to be covered with loose dust
around a meter, or yard, deep (more).

Based on recent images taken by the Mars Global Surveyor, planetary scientists have determined that Phobos appears to be covered by perhaps a meter (or yard) of loose dust. Phobos has a much closer orbit around Mars than Deimos. The satellite orbits so close to Mars that from some places it would appear to rise and set twice a day, although it would not be visible at all from other Martian locales. As Phobos' orbit is continually decaying, however, the satellite may break up and fall down to the Martian surface within around 50 million years.

Gerhard Neukum, DLR/FU Berlin,
Mars Express, ESA

Larger and jumbo images.

As Phobos's orbit decays, it is may break up
and fall down to the Martian surface within
around 50 million years (more).