Volcanoes
The shield volcano Olympus Mons (Mount
Olympus) is an extinct volcano in the vast upland region Tharsis,
which contains several other large volcanoes. The edifice is over 600
km (370 mi) wide. Because the mountain is so large, with complex
structure at its edges, allocating a height to it is difficult. Its
local relief, from the foot of the cliffs which form its northwest
margin to its peak, is over 21 km (13 mi), a little over twice the
height of Mauna Kea as measured from its base on the ocean floor. The
total elevation change from the plains of Amazonis Planitia, over
1,000 km (620 mi) to the northwest, to the summit approaches 26 km
(16 mi), roughly three times the height of Mount Everest, which in
comparison stands at just over 8.8 kilometers (5.5 mi). Consequently,
Olympus Mons is either the tallest or second-tallest mountain in the
Solar System; the only known mountain which might be taller is the
Rheasilvia peak on the asteroid Vesta, at 20–25 km (12–16 mi).
Impact topography
The dichotomy of Martian topography is
striking: northern plains flattened by lava flows contrast with the
southern highlands, pitted and cratered by ancient impacts. It is
possible that, four billion years ago, the Northern Hemisphere of
Mars was struck by an object one-tenth to two-thirds the size of
Earth's Moon. If this is the case, the Northern Hemisphere of Mars
would be the site of an impact crater 10,600 by 8,500 kilometers
(6,600 by 5,300 mi) in size, or roughly the area of Europe, Asia, and
Australia combined, surpassing Utopia Planitia and the Moon's South
Pole–Aitken basin as the largest impact crater in the Solar System.
Mars is scarred by a number of impact
craters: a total of 43,000 craters with a diameter of 5 kilometers
(3.1 mi) or greater have been found. The largest exposed crater is
Hellas, which is 2,300 kilometers (1,400 mi) wide and 7,000 meters
(23,000 ft) deep, and is a light albedo feature clearly visible from
Earth. There are other notable impact features, such as Argyre, which
is around 1,800 kilometers (1,100 mi) in diameter, and Isidis, which
is around 1,500 kilometers (930 mi) in diameter. Due to the smaller
mass and size of Mars, the probability of an object colliding with
the planet is about half that of Earth. Mars is located closer to the
asteroid belt, so it has an increased chance of being struck by
materials from that source. Mars is more likely to be struck by
short-period comets, i.e., those that lie within the orbit of
Jupiter.
Martian craters can have a morphology
that suggests the ground became wet after the meteor impacted.
Tectonic sites
The large canyon, Valles Marineris
(Latin for "Mariner Valleys", also known as
Agathodaemon in the old canal maps), has a length of 4,000 kilometers
(2,500 mi) and a depth of up to 7 kilometers (4.3 mi). The length of
Valles Marineris is equivalent to the length of Europe and extends
across one-fifth the circumference of Mars. By comparison, the Grand
Canyon on Earth is only 446 kilometers (277 mi) long and nearly 2
kilometers (1.2 mi) deep. Valles Marineris was formed due to the
swelling of the Tharsis area, which caused the crust in the area of
Valles Marineris to collapse. In 2012, it was proposed that Valles
Marineris is not just a graben, but a plate boundary where 150
kilometers (93 mi) of transverse motion has occurred, making Mars a
planet with possibly a two-tectonic plate arrangement.
Holes
Images from the Thermal Emission
Imaging System (THEMIS) aboard NASA's Mars Odyssey orbiter have
revealed seven possible cave entrances on the flanks of the volcano
Arsia Mons. The caves, named after loved ones of their discoverers,
are collectively known as the "seven sisters". Cave
entrances measure from 100 to 252 meters (328 to 827 ft) wide and
they are estimated to be at least 73 to 96 meters (240 to 315 ft)
deep. Because light does not reach the floor of most of the caves, it
is possible that they extend much deeper than these lower estimates
and widen below the surface. "Dena" is the only
exception; its floor is visible and was measured to be 130 metres
(430 ft) deep. The interiors of these caverns may be protected from
micrometeoroids, UV radiation, solar flares and high energy particles
that bombard the planet's surface.
Atmosphere
Mars lost its magnetosphere 4 billion
years ago, possibly because of numerous asteroid strikes, so the
solar wind interacts directly with the Martian ionosphere, lowering
the atmospheric density by stripping away atoms from the outer layer.
Both Mars Global Surveyor and Mars Express have detected ionized
atmospheric particles trailing off into space behind Mars, and this
atmospheric loss is being studied by the MAVEN orbiter. Compared to
Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure
on the surface today ranges from a low of 30 Pa (0.0044 psi) on
Olympus Mons to over 1,155 Pa (0.1675 psi) in Hellas Planitia, with a
mean pressure at the surface level of 600 Pa (0.087 psi). The highest
atmospheric density on Mars is equal to that found 35 kilometers (22
mi) above Earth's surface. The resulting mean surface pressure is
only 0.6% of that of Earth 101.3 kPa (14.69 psi). The scale height of
the atmosphere is about 10.8 kilometers (6.7 mi), which is higher
than Earth's 6 kilometers (3.7 mi), because the surface gravity of
Mars is only about 38% of Earth's.
The atmosphere of Mars consists of
about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with
traces of oxygen and water. The atmosphere is quite dusty, containing
particulates about 1.5 µm in diameter which give the Martian sky a
tawny color when seen from the surface. It may take on a pink hue due
to iron oxide particles suspended in it. The concentration of methane
in the Martian atmosphere fluctuates from about 0.24 ppb during the
northern winter to about 0.65 ppb during the summer. Estimates of its
lifetime range from 0.6 to 4 years, so its presence indicates that an
active source of the gas must be present. Methane could be produced
by non-biological process such as serpentinization involving water,
carbon dioxide, and the mineral olivine, which is known to be common
on Mars, or by Martian life.
Compared to Earth, its higher
concentration of atmospheric CO2 and lower surface pressure may be
why sound is attenuated more on Mars, where natural sources are rare
apart from the wind. Using acoustic recordings collected by the
Perseverance rover, researchers concluded that the speed of sound
there is approximately 240 m/s for frequencies below 240 Hz, and 250
m/s for those above.
Auroras have been detected on Mars.
Because Mars lacks a global magnetic field, the types and
distribution of auroras there differ from those on Earth; rather than
being mostly restricted to polar regions, a Martian aurora can
encompass the planet. In September 2017, NASA reported radiation
levels on the surface of the planet Mars were temporarily doubled,
and were associated with an aurora 25 times brighter than any
observed earlier, due to a massive, and unexpected, solar storm in
the middle of the month.
Climate
Of all the planets in the Solar System,
the seasons of Mars are the most Earth-like, due to the similar tilts
of the two planets' rotational axes. The lengths of the Martian
seasons are about twice those of Earth's because Mars's greater
distance from the Sun leads to the Martian year being about two Earth
years long. Martian surface temperatures vary from lows of about −110
°C (−166 °F) to highs of up to 35 °C (95 °F) in equatorial
summer. The wide range in temperatures is due to the thin atmosphere
which cannot store much solar heat, the low atmospheric pressure, and
the low thermal inertia of Martian soil. The planet is 1.52 times as
far from the Sun as Earth, resulting in just 43% of the amount of
sunlight.
If Mars had an Earth-like orbit, its
seasons would be similar to Earth's because its axial tilt is similar
to Earth's. The comparatively large eccentricity of the Martian orbit
has a significant effect. Mars is near perihelion when it is summer
in the Southern Hemisphere and winter in the north, and near aphelion
when it is winter in the Southern Hemisphere and summer in the north.
As a result, the seasons in the Southern Hemisphere are more extreme
and the seasons in the northern are milder than would otherwise be
the case. The summer temperatures in the south can be warmer than the
equivalent summer temperatures in the north by up to 30 °C (54 °F).
Mars has the largest dust storms in the
Solar System, reaching speeds of over 160 km/h (100 mph). These can
vary from a storm over a small area, to gigantic storms that cover
the entire planet. They tend to occur when Mars is closest to the
Sun, and have been shown to increase the global temperature.
Dust storms on Mars
Orbit and rotation
Mars's average distance from the Sun is
roughly 230 million km (143 million mi), and its orbital period is
687 (Earth) days. The solar day (or sol) on Mars is only slightly
longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. A
Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and
18.2 hours.
The axial tilt of Mars is 25.19°
relative to its orbital plane, which is similar to the axial tilt of
Earth. As a result, Mars has seasons like Earth, though on Mars they
are nearly twice as long because its orbital period is that much
longer. In the present day epoch, the orientation of the north pole
of Mars is close to the star Deneb.
Mars has a relatively pronounced
orbital eccentricity of about 0.09; of the seven other planets in the
Solar System, only Mercury has a larger orbital eccentricity. It is
known that in the past, Mars has had a much more circular orbit. At
one point, 1.35 million Earth years ago, Mars had an eccentricity of
roughly 0.002, much less than that of Earth today. Mars's cycle of
eccentricity is 96,000 Earth years compared to Earth's cycle of
100,000 years.
Habitability and search for life
During the late nineteenth century, it
was widely accepted in the astronomical community that Mars had
life-supporting qualities, including oxygen and water. However, in
1894 W. W. Campbell at Lick Observatory observed the planet and found
that "if water vapor or oxygen occur in the atmosphere of
Mars it is in quantities too small to be detected by spectroscopes
then available". That observation contradicted many of the
measurements of the time and was not widely accepted. Campbell and V.
M. Slipher repeated the study in 1909 using better instruments, but
with the same results. It wasn't until the findings were confirmed by
W. S. Adams in 1925 that the myth of the Earth-like habitability of
Mars was finally broken. However, even in the 1960s, articles were
published on Martian biology, putting aside explanations other than
life for the seasonal changes on Mars. Detailed scenarios for the
metabolism and chemical cycles for a functional ecosystem were being
published as late as 1962.
The current understanding of planetary
habitability – the ability of a world to develop environmental
conditions favorable to the emergence of life – favors planets that
have liquid water on their surface. Most often this requires the
orbit of a planet to lie within the habitable zone, which for the Sun
is estimated to extend from within the orbit of Earth to about that
of Mars. During perihelion, Mars dips inside this region, but Mars's
thin (low-pressure) atmosphere prevents liquid water from existing
over large regions for extended periods. The past flow of liquid
water demonstrates the planet's potential for habitability. Recent
evidence has suggested that any water on the Martian surface may have
been too salty and acidic to support regular terrestrial life.
The lack of a magnetosphere and the
extremely thin atmosphere of Mars are a challenge: the planet has
little heat transfer across its surface, poor insulation against
bombardment of the solar wind and insufficient atmospheric pressure
to retain water in a liquid form (water instead sublimes to a gaseous
state). Mars is nearly, or perhaps totally, geologically dead; the
end of volcanic activity has apparently stopped the recycling of
chemicals and minerals between the surface and interior of the
planet.
Scoop of Mars soil by Curiosity,
October 2012
In situ investigations have been
performed on Mars by the Viking landers, Spirit and Opportunity
rovers, Phoenix lander, and Curiosity rover. Evidence suggests that
the planet was once significantly more habitable than it is today,
but whether living organisms ever existed there remains unknown. The
Viking probes of the mid-1970s carried experiments designed to detect
microorganisms in Martian soil at their respective landing sites and
had positive results, including a temporary increase of CO2
production on exposure to water and nutrients. This sign of life was
later disputed by scientists, resulting in a continuing debate, with
NASA scientist Gilbert Levin asserting that Viking may have found
life. Tests conducted by the Phoenix Mars lander have shown that the
soil has an alkaline pH and it contains magnesium, sodium, potassium
and chloride. The soil nutrients may be able to support life, but
life would still have to be shielded from the intense ultraviolet
light. A 2014 analysis of Martian meteorite EETA79001 found chlorate,
perchlorate, and nitrate ions in sufficiently high concentration to
suggest that they are widespread on Mars. UV and X-ray radiation
would turn chlorate and perchlorate ions into other, highly reactive
oxychlorines, indicating that any organic molecules would have to be
buried under the surface to survive.
Scientists have proposed that carbonate
globules found in meteorite ALH84001, which is thought to have
originated from Mars, could be fossilized microbes extant on Mars
when the meteorite was blasted from the Martian surface by a meteor
strike some 15 million years ago. This proposal has been met with
skepticism, and an exclusively inorganic origin for the shapes has
been proposed. Small quantities of methane and formaldehyde detected
by Mars orbiters are both claimed to be possible evidence for life,
as these chemical compounds would quickly break down in the Martian
atmosphere. Alternatively, these compounds may instead be replenished
by volcanic or other geological means, such as serpentinite. Impact
glass, formed by the impact of meteors, which on Earth can preserve
signs of life, has also been found on the surface of the impact
craters on Mars. Likewise, the glass in impact craters on Mars could
have preserved signs of life, if life existed at the site.
Moons
Mars has two relatively small (compared
to Earth's) natural moons, Phobos (about 22 kilometers (14 mi) in
diameter) and Deimos (about 12 kilometers (7.5 mi) in diameter),
which orbit close to the planet. Asteroid capture is a long-favored
theory, but their origin remains uncertain. Both satellites were
discovered in 1877 by Asaph Hall; they are named after the characters
Phobos (panic/fear) and Deimos (terror/dread), who, in Greek
mythology, accompanied their father Ares, god of war, into battle.
Mars was the Roman equivalent to Ares. In modern Greek, the planet
retains its ancient name Ares (Aris: Άρης).
From the surface of Mars, the motions
of Phobos and Deimos appear different from that of the Moon. Phobos
rises in the west, sets in the east, and rises again in just 11
hours. Deimos, being only just outside synchronous orbit – where
the orbital period would match the planet's period of rotation –
rises as expected in the east but slowly.
Because the orbit of Phobos is below
synchronous altitude, the tidal forces from the planet Mars are
gradually lowering its orbit. In about 50 million years, it could
either crash into Mars's surface or break up into a ring structure
around the planet.
The origin of the two moons is not well
understood. Their low albedo and carbonaceous chondrite composition
have been regarded as similar to asteroids, supporting the capture
theory. The unstable orbit of Phobos would seem to point towards a
relatively recent capture. But both have circular orbits, near the
equator, which is unusual for captured objects and the required
capture dynamics are complex. Accretion early in the history of Mars
is plausible, but would not account for a composition resembling
asteroids rather than Mars itself, if that is confirmed.
A third possibility is the involvement
of a third body or a type of impact disruption. More-recent lines of
evidence for Phobos having a highly porous interior, and suggesting a
composition containing mainly phyllosilicates and other minerals
known from Mars, point toward an origin of Phobos from material
ejected by an impact on Mars that reaccreted in Martian orbit,
similar to the prevailing theory for the origin of Earth's moon.
Although the visible and near-infrared (VNIR) spectra of the moons of
Mars resemble those of outer-belt asteroids, the thermal infrared
spectra of Phobos are reported to be inconsistent with chondrites of
any class. It is also possible that Phobos and Deimos are fragments
of an older moon, formed by debris from a large impact on Mars, and
then destroyed by a more recent impact upon itself.
Mars may have moons smaller than 50 to
100 metres (160 to 330 ft) in diameter, and a dust ring is predicted
to exist between Phobos and Deimos.
Exploration
Dozens of crewless spacecraft,
including orbiters, landers, and rovers, have been sent to Mars by
the Soviet Union, the United States, Europe, India, the United Arab
Emirates, and China to study the planet's surface, climate, and
geology. NASA's Mariner 4 was the first spacecraft to visit Mars;
launched on 28 November 1964, it made its closest approach to the
planet on 15 July 1965. Mariner 4 detected the weak Martian radiation
belt, measured at about 0.1% that of Earth, and captured the first
images of another planet from deep space.
Once spacecraft visited the planet
during NASA's Mariner missions in the 1960s and 1970s, many previous
concepts of Mars were radically broken. After the results of the
Viking life-detection experiments, the hypothesis of a hostile, dead
planet was generally accepted. The data from Mariner 9 and Viking
allowed better maps of Mars to be made, and the Mars Global Surveyor
mission, which launched in 1996 and operated until late 2006,
produced complete, extremely detailed maps of the Martian topography,
magnetic field and surface minerals. These maps are available online
at websites including Google Mars. Both the Mars Reconnaissance
Orbiter and Mars Express continued exploring with new instruments and
supporting lander missions. NASA provides two online tools: Mars
Trek, which provides visualizations of the planet using data from 50
years of exploration, and Experience Curiosity, which simulates
traveling on Mars in 3-D with Curiosity.
As of 2021, Mars is host to fourteen
functioning spacecraft. Eight are in orbit: 2001 Mars Odyssey, Mars
Express, Mars Reconnaissance Orbiter, MAVEN, Mars Orbiter Mission,
ExoMars Trace Gas Orbiter, the Hope orbiter, and the Tianwen-1
orbiter. Another six are on the surface: the InSight lander, the Mars
Science Laboratory Curiosity rover, the Perseverance rover, the
Ingenuity helicopter, the Tianwen-1 lander, and the Zhurong rover.
The Rosalind Franklin rover mission,
designed to search for evidence of past life, was intended to be
launched in 2018 but has been repeatedly delayed, with a launch date
pushed to 2024 at the earliest. The current concept for the Mars
sample-return mission would launch in 2026 and feature hardware built
by NASA and ESA. Several plans for a human mission to Mars have been
proposed throughout the 20th and 21st centuries, but none have come
to fruition. The NASA Authorization Act of 2017 directed NASA to
study the feasibility of a crewed Mars mission in the early 2030s;
the resulting report eventually concluded that this would be
unfeasible. In addition, in 2021, China was planning to send a crewed
Mars mission in 2033.
Astronomy on Mars
With the presence of various orbiters,
landers, and rovers, it is possible to practice astronomy from Mars.
Although Mars's moon Phobos appears about one-third the angular
diameter of the full moon on Earth, Deimos appears more or less
star-like, looking only slightly brighter than Venus does from Earth.
Various phenomena seen from Earth have
also been observed from Mars, such as meteors and auroras. The
apparent sizes of the moons Phobos and Deimos are sufficiently
smaller than that of the Sun; thus, their partial "eclipses"
of the Sun are best considered transits. Transits of Mercury and
Venus have been observed from Mars. A transit of Earth will be seen
from Mars on 10 November 2084.
Viewing
The mean apparent magnitude of Mars is
+0.71 with a standard deviation of 1.05. Because the orbit of Mars is
eccentric, the magnitude at opposition from the Sun can range from
about −3.0 to −1.4. The minimum brightness is magnitude +1.86
when the planet is near aphelion and in conjunction with the Sun. At
its brightest, Mars (along with Jupiter) is second only to Venus in
luminosity. Mars usually appears distinctly yellow, orange, or red.
When farthest away from Earth, it is more than seven times farther
away than when it is closest. Mars is usually close enough for
particularly good viewing once or twice at 15-year or 17-year
intervals. As Mars approaches opposition, it begins a period of
retrograde motion, which means it will appear to move backwards in a
looping curve with respect to the background stars. This retrograde
motion lasts for about 72 days, and Mars reaches its peak luminosity
in the middle of this interval.
The point at which Mars's geocentric
longitude is 180° different from the Sun's is known as opposition,
which is near the time of closest approach to Earth. The time of
opposition can occur as much as 8.5 days away from the closest
approach. The distance at close approach varies between about 54 and
103 million km (34 and 64 million mi) due to the planets' elliptical
orbits, which causes comparable variation in angular size. The most
recent Mars opposition occurred on 13 October 2020, at a distance of
about 63 million km (39 million mi). The average time between the
successive oppositions of Mars, its synodic period, is 780 days; but
the number of days between the dates of successive oppositions can
range from 764 to 812.
Mars comes into opposition from Earth
every 2.1 years. The planets come into opposition near Mars's
perihelion in 2003, 2018 and 2035, with the 2020 and 2033 events
being particularly close to perihelic opposition. Mars made its
closest approach to Earth and maximum apparent brightness in nearly
60,000 years, 55,758,006 km (0.37271925 AU; 34,646,419 mi), magnitude
−2.88, on 27 August 2003, at 09:51:13 UTC. This occurred when Mars
was one day from opposition and about three days from its perihelion,
making it particularly easy to see from Earth. The last time it came
so close is estimated to have been on 12 September 57,617 BC, the
next time being in 2287. This record approach was only slightly
closer than other recent close approaches.
Optical ground-based telescopes are
typically limited to resolving features about 300 kilometers (190 mi)
across when Earth and Mars are closest because of Earth's atmosphere.
In culture
Mars is named after the Roman god of
war. This association between Mars and war dates back at least to
Babylonian astronomy, in which the planet was named for the god
Nergal, deity of war and destruction. It persisted into modern times,
as exemplified by Gustav Holst's orchestral suite The Planets, whose
famous first movement labels Mars "the bringer of war".
The planet's symbol, a circle with a spear pointing out to the
upper right, is also used as a symbol for the male gender. The symbol
dates from at latest the 11th century, though a possible predecessor
has been found in the Greek Oxyrhynchus Papyri.
The idea that Mars was populated by
intelligent Martians became widespread in the late 19th century.
Schiaparelli's "canali" observations combined with
Percival Lowell's books on the subject put forward the standard
notion of a planet that was a drying, cooling, dying world with
ancient civilizations constructing irrigation works. Many other
observations and proclamations by notable personalities added to what
has been termed "Mars Fever". High-resolution
mapping of the surface of Mars revealed no artifacts of habitation,
but pseudoscientific speculation about intelligent life on Mars still
continues. Reminiscent of the canali observations, these speculations
are based on small scale features perceived in the spacecraft images,
such as "pyramids" and the "Face on Mars".
In his book Cosmos, planetary astronomer Carl Sagan wrote: "Mars
has become a kind of mythic arena onto which we have projected our
Earthly hopes and fears."
The depiction of Mars in fiction has
been stimulated by its dramatic red color and by nineteenth-century
scientific speculations that its surface conditions might support not
just life but intelligent life. This gave way to many science fiction
stories involving these concepts, such as H. G. Wells' The War of the
Worlds, in which Martians seek to escape their dying planet by
invading Earth, Ray Bradbury's The Martian Chronicles, in which human
explorers accidentally destroy a Martian civilization, as well as
Edgar Rice Burroughs' Barsoom series, C. S. Lewis' novel Out of the
Silent Planet (1938), and a number of Robert A. Heinlein stories
before the mid-sixties. Since then, depictions of Martians have also
extended to animation. A comic figure of an intelligent Martian,
Marvin the Martian, appeared in Haredevil Hare (1948) as a character
in the Looney Tunes animated cartoons of Warner Brothers, and has
continued as part of popular culture to the present. After the
Mariner and Viking spacecraft had returned pictures of Mars as it
really is, a lifeless and canal-less world, these ideas about Mars
were abandoned; for many science-fiction authors, the new discoveries
initially seemed like a constraint, but eventually the post-Viking
knowledge of Mars became itself a source of inspiration for works
like Kim Stanley Robinson's Mars trilogy.
