Definition of Is There Life on Mars?

For centuries people have gazed at Mars, the red planet, and wondered

if life could exist on this distant world. In the early 1900s, Percival

Lowell saw long, linear features on Mars and suggested that

these marks were canals or ditches carved into the surface by an

advanced or perhaps dying civilization, and the imagination of the

world was captured. Science-fiction novels raised people’s hopes

and expectations that our nearest outward neighbor might have

advanced life-forms, or once have hosted superior civilizations. As

technology and understanding of planetary evolution advanced,

hopes of finding advanced life-forms on Mars diminished and were

replaced by hopes of finding even simple life, answering the question

“Are we alone?” once and for all. As exploration of the solar

system continued, remnants of ice and traces of once-flowing

water were discovered on Mars, increasing the chances for life.

Excitement was generated twice in the late 20th century, first when

meteorites thought to be from Mars had organic remains identified

in them, and then again when samples taken from Mars yielded

what appeared to be fossil organisms. Unfortunately, both claims

were proven false, but hopes of finding life continue, and a new

generation of Mars orbiters, landers, and rovers are currently

exploring the planet with the hopes of finding signs of life.

Like Earth, Mars experiences cyclic climate fluctuations in

response to orbital fluctuations analogous to Milankovitch cycles on

Earth. However, Mars experiences much more severe fluctuations

in temperature in response to the huge variations in orbital tilt of the

planet. The orientation of the orbital axis of Mars with respect to the

ecliptic plane of the solar system has fluctuated from 15° to 35° at

least 50 times in the last 5 million years, and has swung from 0° to

60° less frequently. These dramatic changes in tilt cause huge variations

in solar insolation and correspondingly large variations in temperature

on the planet. When the tilt axis is most inclined, the polar

regions get the most solar radiation, causing ice to vaporize and

crystallize in equatorial regions, where evidence of glacial cycles

have recently been documented. When Mars has a low obliquity

(not tilted much), the equatorial regions get the most sunlight, and

the ice in low latitudes vaporizes and condenses at the poles. Most

life-forms can not tolerate such large variations in environmental

conditions, making it much less likely that life will be found on the

red planet, unless the life-forms have found some mechanism to

minimize or tolerate the astronomically large variations in climate.

However, fleeting traces of ice and flowing water offer such hope

and potential for hosting life that scientists have not given up hope

of finding some adaptable forms that may hide in the nearly constant

temperatures of ice, deep in the soil, or elsewhere. Some models for

the origin of life on Earth suggest that primitive organic compounds

may have been delivered to Earth by cometary or asteroid impacts,

and if this is true, it is likely that the same compounds were delivered

to Mars as well. If so, we may not be alone.

surface, with surface temperatures on average about 370°F

(50°K) cooler than on Earth.

Many early speculations centered on the possibility of

life on Mars, and several spectacular claims of evidence for

life have been later found to be invalid. To date, no evidence

for life, either present or ancient, has been found on Mars.

mass extinctions Most species are present on Earth for

about 4 million years. Many species come and go during a

typically low rate of background level extinctions and evolution

of new species from old, but the majority of changes

occur during distinct mass-death and repopulations of the

environment. The Earth’s biosphere has experienced five

major and numerous less-significant mass extinctions in the

past 500 million years (in the Phanerozoic Era). These events

occurred at the end of the Ordovician, in the Late Devonian,

at the Permian-Triassic boundary, the Triassic-Jurassic boundary,

and at the Cretaceous-Tertiary (K-T) boundary.

The Early Paleozoic saw many new life-forms emerge in

new environments for the first time. The Cambrian explosion

led to the development of trilobites, brachiopods, conodonts,

mollusks, echinoderms, and ostracods. Bryozoans, crinoids,

and rugose corals joined the biosphere in the Ordovician, and

reef-building stromatoporoids flourished in shallow seas. The

end-Ordovician extinction is one of the greatest of all

Phanerozoic time. About half of all species of brachiopods

and bryozoans died off, and more than 100 other families of

marine organisms disappeared forever.

The cause of the mass extinction at the end of the

Ordovician appears to have been largely tectonic. The major

landmass of Gondwana had been resting in equatorial

regions for much of the Middle Ordovician but migrated

toward the South Pole at the end of the Ordovician. This

caused global cooling and glaciation, lowering sea levels from

the high stand where they had been resting for most of the

Cambrian and Ordovician. The combination of cold climates

with lower sea levels, leading to a loss of shallow shelf environments

for habitation, probably were enough to cause the

mass extinction at the end of the Ordovician.

The largest mass extinction in Earth history occurred at

the Permian-Triassic boundary, over a period of about 5 million

years. The Permian world included abundant corals,

crinoids, bryozoans, and bivalves in the oceans, and on land,

amphibians wandered about amid lush plant life. Of all

oceanic species, 90 percent were to become extinct, and 70

percent of land vertebrates died off at the end of the Permian.

This greatest catastrophe of Earth history did not have a single

cause but reflects the combination of various elements.

Before the extinction event began, plate tectonics was

again bringing many of the planet’s landmasses together in a

supercontinent (this time, Pangea), causing greater competition

for fewer environmental niches by Permian life-forms.

Drastically reduced were the rich continental shelf areas. As

the continents collided mountains were pushed up, reducing

the effective volume of the continents available to displace

the sea, so sea levels fell, putting additional stress on life by

further limiting the availability of favorable environmental

niches. The global climate became dry and dusty, and the

supercontinent formation led to widespread glaciation. This

lowered sea level even more, lowered global temperatures,

and put many life-forms on the planet in a very uncomfortable

position, and many perished.

In the final million years of the Permian, the Northern

Siberian plains let loose a final devastating blow. The Siberian

flood basalts began erupting at 250 million years ago,

becoming the largest known outpouring of continental flood

basalts ever. Carbon dioxide was released in hitherto

unknown abundance, warming the atmosphere and melting

the glaciers. Other gases were also released, perhaps also

including methane, as the basalts probably melted permafrost

and vaporized thick accumulations of organic matter that

accumulate in high latitudes like that at which Siberia was

located 250 million years ago.

The global biosphere collapsed, and evidence suggests

that the final collapse happened in less than 200,000 years,

and perhaps in less than 30,000 years. Entirely internal processes

may have caused the end-Permian extinction, although

some scientists now argue that an impact may have dealt the

final death blow. After it was over, new life-forms populated

the seas and land, and these Mesozoic organisms tended to be

more mobile and adept than their Paleozoic counterparts.

The great Permian extinction created opportunities for new

life-forms to occupy now empty niches, and the most adaptable

and efficient organisms took control. The toughest of the

marine organisms survived, and a new class of land animals

grew to new proportions and occupied the land and skies.

The Mesozoic, time of the great dinosaurs, had begun.

The Triassic-Jurassic extinction is not as significant as the

Permian-Triassic extinction. Mollusks were abundant in the

Triassic shallow marine realm, with fewer brachiopods, and

ammonoids recovered from near total extinction at the Permian-

Triassic boundary. Sea urchins became abundant, and new

groups of hexacorals replaced the rugose corals. Many land

plants survived the end-Permian extinction, including the ferns

and seed ferns that became abundant in the Jurassic. Small

mammals that survived the end-Permian extinction re-diversified

in the Triassic, many only to become extinct at the close of

the Triassic. Dinosaurs evolved quickly in the late Triassic,

starting off small, and attaining sizes approaching 20 feet (6

m) by the end of the Triassic. The giant pterosaurs were the

first known flying vertebrate, appearing late in the Triassic.

Crocodiles, frogs, and turtles lived along with the dinosaurs.

The end of the Triassic is marked by a major extinction in the

marine realm, including total extinction of the conodonts, and

a mass extinction of the mammal-like reptiles known as therapsids,

and the placodont marine reptiles. Although the causes

of this major extinction event are poorly understood, the timing

is coincident with the breakup of Pangea and the formation

of major evaporite and salt deposits. It is likely that this was a

tectonic-induced extinction, with supercontinent breakup initiating

new oceanic circulation patterns, and new temperature

and salinity distributions.

After the Triassic-Jurassic extinction, dinosaurs became

extremely diverse and many quite large. Birds first appeared

at the end of the Jurassic. The Jurassic was the time of the

giant dinosaurs, which experienced a partial extinction affecting

the largest varieties of Stegosauroids, Sauropods, and the

marine Ichthyosaurs and Plesiosaurs. This major extinction is

also poorly explained but may be related to global cooling.

The other abundant varieties of dinosaurs continued to thrive

through the Cretaceous.

The Cretaceous-Tertiary (K-T) extinction is perhaps the

most famous of mass extinctions because the dinosaurs perished

during this event. The Cretaceous land surface of North

America was occupied by bountiful species, including herds

of dinosaurs both large and small, some herbivores, and

other carnivores. Other vertebrates included crocodiles, turtles,

frogs, and several types of small mammals. The sky had

flying dinosaurs including the vulture-like pterosaurs, and

insects including giant dragonflies. The dinosaurs had dense

vegetation to feed on, including the flowing angiosperm trees,

tall grasses, and many other types of trees and flowers. Life in

the ocean had evolved to include abundant bivalves including

clams and oysters, ammonoids, and corals that built large

reef complexes.

Near the end of the Cretaceous, though the dinosaurs

and other life-forms did not know it, things were about to

change. High sea levels produced by mid-Cretaceous rapid

seafloor spreading were falling, decreasing environmental

diversity, cooling global climates, and creating environmental

stress. Massive volcanic outpourings in the Deccan traps and

the Seychelles formed as the Indian Ocean rifted apart and

magma rose from an underlying mantle plume. Massive

amounts of greenhouse gases were released, raising temperatures

and stressing the environment. Many marine species

were going extinct, and others became severely stressed.

Then, one bright day, a visitor from space about six miles (10

km) across slammed into the Yucatán Peninsula of Mexico,

instantly forming a fireball 1,200 miles (1,931 km) across,

followed by giant tsunamis perhaps thousands of feet tall.

The dust from the fireball plunged the world into a dusty

fiery darkness, months or years of freezing temperatures, followed

by an intense global warming. Few species handled the

environmental stress well, and more than a quarter of all the

plant and animal kingdom families, including 65 percent of

all species on the planet, became extinct forever. Gone were

dinosaurs, mighty rulers of the Triassic, Jurassic, and Cretaceous.

Oceanic reptiles and ammonoids died off, and 60 percent

of marine planktonic organisms went extinct. The great

K-T deaths affected not only the numbers of species but also

the living biomass—the death of so many marine plankton

alone amounted to 40 percent of all living matter on Earth at

the time. Similar punches to land-based organisms decreased

the overall living biomass on the planet to a small fraction of

what it was before the K-T 1–2–3 knockout blows.

Some evidence suggests that the planet is undergoing the

first stages of a new mass extinction. In the past 100,000

years, the ice ages have led to glacial advances and retreats,

sea-level rises and falls, the appearance and rapid explosion

of human (Homo sapiens) populations, and the mass extinction

of many large mammals. In Australia 86 percent of large

(greater than 100 pounds) animals have become extinct in the

past 100,000 years, and in South America, North America,

and Africa the extinction is an alarming 79 percent, 73 percent,

and 14 percent. This ongoing mass extinction appears

to be the result of cold climates and, more important, predation

and environmental destruction by humans. The loss of

large-bodied species in many cases has immediately followed

the arrival of humans in the region, with the clearest examples

being found in Australia, Madagascar, and New

Zealand. Similar loss of races through disease and famine has

accompanied many invasions and explorations of new lands

by humans, suggesting we are causing a new mass extinction.

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