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density or surface pressure.<br />
(Although over time it has<br />
likely affected the atmosphere’s<br />
composition.)<br />
Earth’s atmosphere also had<br />
a lot of carbon dioxide (CO 2<br />
),<br />
which makes up Venus’ dense<br />
atmosphere. But on Earth,<br />
most of the atmospheric CO 2<br />
was removed. Earth has liquid<br />
water oceans. Water takes CO 2<br />
out of an atmosphere and turns<br />
it into limestone (carbonate<br />
rocks), which is plentiful in<br />
Earth’s crust. Venus’ CO 2<br />
stays<br />
in vapor form in its atmosphere<br />
because Venus hasn’t<br />
had surface water for a long<br />
time. The resulting greenhouse,<br />
coupled with its solar<br />
proximity, evidently helps keep<br />
it that way.<br />
Still, Venus’ missing magnetic<br />
field does influence the<br />
physical processes of atmospheric<br />
escape. Scientists are<br />
still investigating the escape<br />
processes at a magnetized<br />
planet like Earth and those<br />
affecting Venus. The sister<br />
planets also likely have different<br />
impact and volcanic outgassing<br />
histories, as well as distinct<br />
magnetic and solar histories.<br />
Those differences are critical,<br />
too. A lot of detective work on<br />
the details remains to be done.<br />
Janet Luhman<br />
Senior fellow<br />
Space Sciences Laboratory<br />
University of California, Berkeley<br />
As the Milky Way and Andromeda galaxies approach for their imminent merger, as shown in this simulation of the<br />
coming several billion years, star formation runs rampant and lights Earth’s night sky. Eventually, the two form<br />
one enormous elliptical galaxy. NASA; ESA; Z. LEVAY AND R. VAN DER MAREL, STSCI; T. HALLAS; AND A. MELLINGER<br />
Q: THE MILKY WAY AND<br />
ANDROMEDA GALAXIES<br />
ARE APPROACHING EACH<br />
OTHER. WITH CURRENT<br />
TECHNOLOGY, HOW LONG<br />
WOULD IT TAKE BEFORE WE<br />
COULD DIRECTLY MEASURE<br />
THE APPARENT INCREASE IN<br />
SIZE OF ANDROMEDA?<br />
Allan Burger<br />
Passaic, New Jersey<br />
A: In the next several billion<br />
years, our Milky Way Galaxy<br />
will merge with the neighboring<br />
Andromeda Galaxy, which<br />
is now some 2.5 million lightyears<br />
away. Currently,<br />
Andromeda’s disk of stars is a<br />
few times bigger than the<br />
apparent size of the Full Moon,<br />
covering a few degrees on the<br />
sky (depending exactly on<br />
where you draw its edge).<br />
As Andromeda approaches<br />
us, the apparent size of its disk<br />
will increase by about one arcsecond<br />
per million years.<br />
The Very Large Baseline<br />
Array of radio telescopes could<br />
measure this apparent separation<br />
over a century by measuring<br />
the distance between<br />
radio-bright star forming<br />
regions within Andromeda<br />
called masers. There is an<br />
ongoing search for such<br />
masers, since we also want to<br />
measure precisely (on a shorter<br />
timescale) the unknown internal<br />
motion of Andromeda.<br />
Avi Loeb<br />
<strong>Astronomy</strong> department chair<br />
Harvard University<br />
Cambridge, Massachusetts<br />
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askastro@astronomy.com,<br />
or write to Ask Astro,<br />
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