[meteorite-list] Martian Meteorites Record Surface Temperatures on Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Jul 29 21:10:15 2005
Message-ID: <200507300037.j6U0bhJ25520_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/July05/Mars_paleotemp.html

Martian Meteorites Record Surface Temperatures on Mars
Planetary Science Research Discoveries
July 29, 2005

--- Gases trapped in Martian meteorites indicate that Mars has been a
cold desert for a long, long time.

Written by G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

Using published data for argon (Ar) released when Martian meteorites are
heated, David Shuster (California Institute of Technology, now at
Berkeley Geochronology Center, Berkeley, CA) and Benjamin Weiss
(Massachusetts Institute of Technology) show that the nakhlite group of
Martian meteorites <../PSRDglossary.html#snc> and unique Martian
meteorite ALH 84001 were probably not heated above about 0 oC
<../PSRDglossary.html#celsius> for most of their histories. This
indicates that the surface of Mars has been cold for almost four billion
years. If a warm, wet environment existed on Mars (inferred from
previous studies of surface features and geochemical parameters), it
occurred before four billion years ago.

Reference:

    * Shuster, David L. and Benjamin P. Weiss (2005) Martian surface
      paleotemperatures from thermochronology of Meteorites. Science,
      vol. 309, p. 594-597.

------------------------------------------------------------------------

Soaking Wet, Bone Dry Mars

Climate change on Earth is often in the news. Climate specialists worry
about swings in global temperatures of several degrees Celsius. This
does not sound like much, but it is enough to cause ice ages sometimes
and widespread shallow seas at other times. But those changes are
nothing compared to what the planet Mars seems to have experienced. Mars
is decorated with huge channels eroded when vast quantities of water
flowed through them. Oceans may have existed in the northern plains.
Valley networks decorate the planet's surface. Yet now it is a dry, cold
place. The daily average temperature at the equator is an ultra-nippy 60
oC below zero. Its monotonous dry climate has been enlivened
occasionally by water seeping from the sides of impact craters, and
changes in the planet's tilt may have moved glaciers from the current
poles to more equatorial regions, but basically it has been colder and
drier than anyplace on Earth. Yet at some time in the past, probably
billions of years ago, Mars was a much warmer and wetter place.

 
Mars flaunts strong evidence for vigorous water activity in the past
(see images below from left to right), such as immense, water-carved
outflow channels, valley networks, possibly an extensive northern ocean,
and presence of layered deposits whose origin involved evaporation of
salty water.
 
Martian water features
 
On the other hand (see images below), it appears today to be extremely
dry, a vast desert shaped mostly by wind, except in a limited number of
locales where water has recently formed gullies on the walls of impact
craters.

[Images]

[Image]
Columbia Hills, Gusev crater-wall gullies


David Shuster and Benjamin Weiss wanted to determine past temperatures
during this impressively long Martian cold, dry spell. Experts in
determining the ages of rocks using potassium-argon dating and its
advanced cousin, 40Ar/39Ar dating, they reckoned that Martian meteorites
contained a record of surface temperatures. This is possible because Ar
leaks out of rocks unless they are kept cool enough. They chose to study
the nakhlite group of Martian meteorites because they do not have the
same level of shock damage by meteoroid impact as do other types of
Martian meteorites, thereby minimizing one form of heating besides
surface temperature. They also studied data from Allan Hills (ALH) 84001
because it is by far the oldest in our collection of Martian meteorites.
(For evidence that Martian meteorites actually do come from Mars, go to
the curatorial office
<http://www-curator.jsc.nasa.gov/antmet/marsmets/indepth.htm> at the
Johnson Space Center.)

Nakhlites have already proven to be useful in assessing the timing of
relatively recent aqueous events on Mars (see PSRD article: Liquid Water
on Mars: The Story from Meteorites
<http://www.psrd.hawaii.edu/May00/wetMars.html>). The nakhlites contain
mineral grains formed by the reaction of water with original minerals
and deposition of others as the solutions dried up (see images below).
Tim Swindle and his colleagues at the University of Arizona determined
from potassium-argon dating that these water-based alteration events
were of short duration and took place intermittently during the past 600
million years or so. Shuster and Weiss hoped to look at a broader time
scale and to set limits on the temperature during the past 4 billion years.

olivine and sulfates in MIL03346

The nakhlite group of Martian meteorites show that small amounts of
water have flowed on Mars since the nakhlites formed in lava flows 1.3
billion years ago. On the left is a transmitted light photograph of red
staining in an olivine crystal in the MIL 03346 nakhlite. The staining
is composed of a complex mixture of weathering products. On the right is
another transmitted light photograph of the same meteorite showing
sulfate crystals deposited from evaporating salty water.

------------------------------------------------------------------------

Using Ages to Deduce Temperatures

The startling thing about nakhlites is that all age dating techniques
give the same age for their origin as igneous rocks, 1.3 billion years.
You'd think that the concordance of ages by potassium-argon,
rubidium-strontium, uranium-lead, and samarium-neodymium would not be
surprising. If these techniques really work to date a rock and the rock
formed at a given time, shouldn't they all yield the same result? They
would, if nothing happened to a rock after it was formed, but it can be
heated, metamorphosed, shocked (by impact), and altered by water.
Because each age-dating technique is affected by these events
differently, they tend to yield different apparent ages. The fact that
the nakhlites give the same age for all systems indicates that they have
had a relatively simple history. Even uranium-thorium-helium dating,
which is easy to alter because helium leaks out of minerals like sand
through a sieve (even at low temperatures), gives about the same age
(0.8 to 1.2 billion years).

40Ar/39Ar age dating involves irradiating a sample with fast neutrons in
a nuclear reactor and then measuring the ratio as the sample is heated
progressively from low temperature (about 250 oC) to higher temperature
(up to 1200 oC). Because 39Ar is produced from nuclear reactions with
39K, the experiment measures both potassium and 40Ar at the same time.
This progressive heating causes release of the gases from different
minerals sequentially, providing information about the temperature-time
history of the rock. This can be quantified by knowing the rates at
which argon diffuses out of mineral grains and the sizes of the mineral
grains. The result is that the nakhlites appear to have lost only 1% of
the 40Ar produced by the decay of 40K since they formed 1.3 billion
years ago.

The problem is that Shuster and Weiss did not know how hot the nakhlites
got or for how long. Nevertheless, they could test different intensities
of heating events to produce a set of solutions that result in loss of
only 1% of the 40Ar from the nakhlites. The calculations are shown in
the diagram below. If the nakhlites were never heated after they formed,
they would preserve their age if held at a temperature of about minus 15
oC. If heated for a period of 10 million years, they could have reached
as high as about 90 oC if the heating happened soon after they formed,
but much less if more recent. A 10-million-year heating event that
occurred during the past one billion years would not have heated the
nakhlites to more than about 20 oC. Longer duration heating events must
have been much cooler than 20 oC, and most likely not much higher than
zero oC (see graph below).

heating Nakhla

Maximum temperatures reached in long-duration heating events of the
nakhlite lava flows on Mars. The curves are calculated from gas-release
data from the Nakhla meteorite. They show the maximum temperature
reached for a temperature increase lasting for 10, 100, 200, and 500
million years, and beginning at any point along the curve. The case for
no heating (isothermal) is also shown. A sustained period of warm
temperature is possible, but it is more likely that the nakhlites were
not heated to more than about zero oC during the past billion years or so.

A similar analysis can be done for the ancient ALH 84001 meteorite. The
calculations in that case indicate that in all likelihood that
meteorite, which was shock-heated 3.9 billion years ago, was not since
heated above 0 oC for longer than a million years. In fact, it is likely
that it was never hotter than about 7 oC for more than a million years
during the past 3.9 billion years (see graph below). This suggests that
Mars has been mostly a dry desert for all that time.

heating ALH84001

The ancient meteorite ALH 84001 formed about 4.5 billion years ago, but
was shocked (probably when excavated from great depth) 3.9 billion years
ago. Since then, it could not have been heated to much more than zero oC
for more than a million years.

One worry is that the nakhlites and ALH 84001 could have been heated
when a big impact launched them from Mars. Shuster and Weiss address
this problem. Using the same techniques to calculate gas loss, and
knowing from the cosmic ray exposure ages of nakhlites that they were
launched 11 million years ago, Shuster and Weiss calculate that the
meteorites could not have been heated to more than 350 oC for more than
a few hours.

------------------------------------------------------------------------

Mars/NASA The Long Drought

If Shuster and Weiss' analysis is correct, the areas on Mars that were
home to the nakhlites and ALH 84001 got neither warm nor wet for very
long. Short increases in temperature and brief wet spells are certainly
allowable, and even required by the presence of weathering products in
the nakhlites. Mars appears to have been a desert for billions of years.
This implies that if life arose on the Red Planet, it is likely to be
hidden underground. Places with groundwater beneath a permanently frozen
underground cryosphere may be teeming with life. Or not. We can search
for this life by drilling deep into the crust, or by choosing the right
spots to sample, such as the terminations of young gullies and other
apparently youthful features shaped by flowing water.

Shuster and Weiss also point out that the lack of heating of the
nakhlites and ALH 84001 when they were blasted off Mars indicates how
easily undamaged materials can be lifted off Mars and sent to Earth. The
inner planets might not be biologically isolated from each other. Life
on Mars (if there is life on Mars) might be related to life on Earth. We
may all be one big, solar system family.

------------------------------------------------------------------------

ADDITIONAL RESOURCES LINKS OPEN IN A NEW WINDOW.

    * Mars meteorites <http://www2.jpl.nasa.gov/snc/index.html>
      comprehensive page from Ron Baalke, Jet Propulsion Lab.
    * Martian meteorites
      <http://www.amnh.org/exhibitions/permanent/meteorites/planets/mars.php>
      from the American Museum of Natural History.
    * MIL 03346: New Martian meteorite found in Antarctica, News release
      <http://www.case.edu/news/2004/7-04/meteorite.htm> from Case
      Western Reserve University (2004).
    * Shuster, David L. and Benjamin P. Weiss (2005) Martian surface
      paleotemperatures from thermochronology of Meteorites. Science,
      vol. 309, p. 594-597.
    * Taylor, G. J. (2000) Liquid Water on Mars: The Story from
      Meteorites. Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/May00/wetMars.html
Received on Fri 29 Jul 2005 08:37:42 PM PDT