Evidence of Ancient Martian Life in Meteorite ALH84001?
A group of scientists led by David McKay of NASA's
Johnson Space Center published an article in
the 16 August 1996 issue of
Science
magazine announcing
the discovery of evidence for primitive bacterial
life on Mars. An examination of a meteorite found
in Antarctica and believed to be from Mars shows:
1) hydrocarbons which are the same as breakdown
products of dead micro-organisms on Earth,
2) mineral phases consistent with by-products
of bacterial activity, and 3) tiny carbonate
globules which may be microfossils of the primitive
bacteria, all within a few hundred-thousandths of
an inch of each other. Based on age dating of the
meteorite, the following scenario has been proposed:
- The original igneous rock solidified within Mars
about 4.5 billion years ago, about 100 million years
after the formation of the planet. (Based on isotope
ages of the igneous component of the meteorite)
- Between 3.6 and 4 billion years ago the rock
was fractured, presumably by meteorite impacts.
Water then permeated the cracks, depositing
carbonate minerals and allowing primitive bacteria
to live in the fractures.
- About 3.6 billion years ago, the bacteria and
their by-products became fossilized in the fractures.
(Based on isotope ages of the minerals in the fractures)
- 16 million years ago, a large meteorite struck
Mars, dislodging a large chunk of this rock and
ejecting it into space. (Based on the cosmic ray
exposure age of the meteorite)
- 13,000 years ago, the meteorite landed in Antarctica.
- The meteorite, ALH84001, was discovered in 1984 in
the Allan Hills region of Antarctica.
How do we know the meteorite came from Mars?
Meteorite ALH84001 is a softball-sized igneous rock
weighing 1.9 kilograms (4.2 pounds). It is one of twelve
meteorites discovered on Earth which are thought to be
from Mars. Most meteorites formed early in the history
of the solar system, some 4.6 billion years ago. Eleven
of the twelve martian meteorites have ages less than
1.3 billion years, ALH84001 at 4.5 billion years old
being the only exception. All twelve are igneous rocks
crystallized from molten magma in a way which suggests
they formed in a planetary-sized body, not an asteroid.
They have similar oxygen isotope characteristics to each
other and higher concentrations of ferric iron, water,
and other volatiles than other meteorites. All twelve
also show evidence of shock heating, presumably as a
result of the impact which ejected them into space.
Gas bubbles trapped in one meteorite, EETA79001, have a
composition which matches the current martian atmosphere
as measured by the Viking Landers, compelling evidence
that this meteorite and by association the others,
including ALH84001, came from Mars.
The evidence for life
The indication of life hinges on three important
pieces of evidence, all discovered within mineralized
fractures in the meteorite in close proximity to each
other. One is the discovery of abundant polycyclic
aromatic hydrocarbons (PAHs) on the fracture surfaces.
These are a family of complex organic molecules which
are commonly found on dust grains and certain types of
meteorites in outer space, presumably formed by
non-biological chemical reactions. However, when
micro-organisms die they break down into PAHs as well.
The mixture of PAHs found on ALH84001 is very different
from that found on dust grains and other meteorites,
suggesting the possibility of a biological origin.
Thousands of different types of PAHs are found all
over the Earth, but those in ALH84001 do not appear
to be contaminants which have leaked into the meteorite.
Another line of evidence involves unusual mineral phases
found beside the PAHs. These carbonate minerals form
"globules" about 50 micrometers across, some of which
have cores containing manganese and rings of iron
carbonate and iron sulfides, and also contain magnetite
and pyrrhotite. These minerals bear strong resemblance
to mineral alterations caused by primitive bacteria on
Earth. This diversity of minerals in such a small area,
formed under the presumed conditions, seem to make a
non-biological origin unlikely.
Finally, high-resolution scanning electron microscopy has
revealed the presence of tiny "ovoids" which may actually
be fossil remnants of tiny (20 to 100 nanometer) bacteria.
If so, they are 100 times smaller than any bacteria
microfossils found on Earth, except for some supposed
"nanofossils" recently discovered in very young
terrestrial rocks, a finding currently not generally
accepted as fossil organisms.
Taken together, the findings are thought to be strong
evidence pointing to primitive bacterial life on Mars.
The PAHs, unusual mineral phases, and "microfossils"
were all located within a few micrometers of one another,
indicating a relationship which may require a biological
explanation. However, much work will be done on this
in the future, including searching for amino acids,
other fossil structures such as cell walls, other
types of fossils, and fossils of bacteria frozen in the
act of reproducing. Mars is almost certain to have
been warmer and wetter in its distant past, so the
existence of primitive life has been a tantalizing
possibility for some time, but the real search may be
just beginning.
Other Sources of Information
McKay et al. Science Article - Summary of the article that started it all
Images of Mars - from the NSSDC Catalog of Spaceborne Imaging
Images of Mars - from the NSSDC Photo Gallery
NASA Press Release on the Discovery
Mars Fact Sheet
NSSDC Mars Home Page
Life on Mars Press Briefing on 19 March 1997
- NASA Press Release (97-03-14)
AAAS Symposium on Mars life and sample return mission
- NASA Press Release (98-02-09)
Possible Source Craters for Mars Meteorite Found - Press Release - 12 August 1996
Summary Page on the Mars Meteorite - Including images of ALH84001
Mars Meteorite Compendium - Johnson Space Center
News and scientific papers on the Mars Meteorite - Lunar and Planetary Institute
Photos of the meteorite - The electron microscopy which led to the discovery
SNC Meteorites - General information on meteorites from Mars
American Association for the Advancement of Science - News Release
CNN On-Line Information - Special section on the discovery
Mars Missions
Viking - NASA Orbiter
and Lander Missions to Mars, 1976
Mars Global Surveyor - NASA Global Orbiter
Mission to Mars, November 1996
Mars Pathfinder - NASA Environmental Survey
Lander Mission to Mars, December 1996
2001 Mars Odyssey -
NASA Mars Orbiter, April 2001
Spirit and Opportunity -
NASA Mars Exploration Rovers, 2003
Author/Curator:
Dr. David R. Williams, dave.williams@nasa.gov
NSSDC, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
+1-301-286-1258
NASA Official: Ed Grayzeck, edwin.j.grayzeck@nasa.gov
Last Updated: 09 January 2005, DRW