[meteorite-list] Meteoroids Change Atmospheres of Earth, Mars, Venus

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 4 Sep 2012 14:44:10 -0700 (PDT)
Message-ID: <201209042144.q84LiARW022422_at_zagami.jpl.nasa.gov>

http://www.space.com/17440-meteoroids-mars-venus-atmospheres.html

Meteoroids Change Atmospheres of Earth, Mars, Venus
by Nola Taylor Redd
space.com
04 September 2012

Meteoroids streaking through the atmospheres of planets such as Earth,
Mars and Venus can change these worlds' air, in ways that researchers
are just now beginning to understand.

Most planetary atmospheres are made up of simple, low-mass elements and
compounds such as carbon dioxide, oxygen and nitrogen. But when a debris
particle, or meteoroid passes through, it can shed heavier, more exotic
elements such as magnesium, silicon and iron.

Such elements can have a significant impact on t
dynamics of winds in the atmosphere, researchers say.

"That opens up a whole new network of chemical pathways not usually
there," said Paul Withers of Boston University.

Contaminating the outer layers

Part of a planet's upper atmosphere, the ionosphere contains plasma - a
mixture of positively charged (ionized) atoms or molecules and the
negatively charged electrons stripped from them. When simple elements
such as oxygen move into this outer shell, they break apart easily,
decaying in a matter of minutes.

But meteoroids streaking toward a planet's surface carry heavier metals
that can be removed in a variety of ways. A grain of dust, for instance,
may rapidly burn up, shedding already-ionized magnesium as it falls. Or,
neutral magnesium may be torn from the small rock, then receive a charge
from sunlight or from stripping an electron from another particle. The
newly charged elements can take as much as a full day to decay.

Meteoroids that blaze a trail through the atmosphere are called meteors,
or shooting stars, Only those that make it to the ground are meteorites.

"When we add metal ions to the ionosphere as a result of this meteoroid
input, we create plasma in regions where there wasn't any plasma there
to start out with," Withers told SPACE.com.

In a recent article for Eos, the American Geophysical Union's newspaper
covering Earth and space sciences, Withers discusses important questions
raised by the recent wealth of research on the upper atmosphere of Mars
and Venus.

Shocking similarities, strange differences

Over the last decade, scientists have collected more and more
information about the ionospheres of Mars and Venus. Though one might
envision the composition and location of the two planets would create
different interactions in the ionosphere, the two are actually very
similar, scientists say.

"If you stand at the surface of the two planets, they are very
different," Withers said. "But up at about 100 kilometers (62 miles),
conditions are surprisingly similar."

The pressures, temperatures, and chemistry at high altitudes are
comparable for the two planets. So too are many of the properties of the
layers of charged particles shed by meteoroids.

"The plasma densities are quite similar on average on all three planets,
which is not what you might expect on the first impression," Withers
said, referring to Earth, Mars and Venus.

Since the sun is the ultimate driving force for most ionization
processes, it's tempting to assume that Venus has more particles in a
given area than Mars does because it orbits twice as closely to our
star. Instead, the two planets have similar densities, which differ from
Earth's measurements by only a factor of ten.

At the same time, the layers affected by the meteoroids on Earth are
very narrow, maybe only a mile or two wide, while Venus and Mars both
have layers stretching six to eight miles.

According to Withers, the difference may come from the presence of
Earth's strong magnetic field, a feature lacking on the other two
planets. But scientists aren't certain how much of a role the field
actually plays.

Finding the source

To study Earth's ionosphere, scientists can launch rockets to
take measurements in the region. But the process is more complicated for
other planets.

As a spacecraft travels through the solar system, a targeted radio
signal sent back to Earth can be aimed through the ionosphere of a
nearby planet. Plasma in the ionosphere causes small but detectable
changes in the signal that allow scientists to learn about the upper
atmosphere.

This process - known as radio occultation - doesn't require any fancy
equipment, only the radio the craft already uses to communicate with
scientists on Earth.

"It's really one of the workhorse planetary science instruments,"
Withers said.

Because it is so simple, the process has been applied to every planet
ever visited by spacecraft.

Only in recent years has enough data come back on Venus and Mars to
seriously examine their upper atmospheres. As of yet, no numerical
simulations have been created to explain some of the differences, but
Withers expressed hope that this would change in the near future. Such
simulations could help answer some of the questions that the
observations have raised.

Withers also hopes that, in time, a detailed understanding of the
ionosphere could even help scientists engage in a kind of "atmospheric
archeology" for Venus and Mars.

One day, scientists may be able to track the history of comets in the
solar system by measuring how planetary atmospheres have been affected
by the icy wanderers' shed dust and gas. But conclusions drawn by this
sort of sleuthing are probably a ways down the road, Withers said.
Received on Tue 04 Sep 2012 05:44:10 PM PDT