When we are searching exoplanets for
life, one of the premier signals is supposed to be the presence of
oxygen. Oxygen typically is chemically combined unless it is
renewed, and vegetative life provides this renewal. As all
schoolchildren know, photosynthesis involves chlorophyll acting as a
catalyst to break carbon dioxide into oxygen, which is released, and
carbon which is utilized. But photosynthesis can be done in the
oceans by near-surface plants, and while life in oceans would be
tremendously interesting, life on land would be even more
tremendously interesting. It is very hard to see how intelligent
civilizations could evolve underwater, but on land, there is the
possibility.
Thus, oxygen might be a great signature
for life, vegetative specifically, but not so certain an indicator
for alien civilizations. What might be used in addition?
It is certainly worth asking the
question. Just consider that a hundred years from now we find
ourselves in a galaxy with thousands of planets with life, but all of
it wet. Now consider instead of that situation, we are in a galaxy
with even hundreds of planets with alien civilizations. Orders of
magnitude more interesting.
So, what is beyond oxygen as a
signature of life on land? Trivially, there must be dry land, and
that would have some reflective signature. Rock doesn't look like
water when reflecting sunlight. But just having rock doesn't imply
that there is anything living on it. There needs to be some
preconditions before life can crawl out of the oceans and take up
habitation on dry land. One is rain.
Life needs water. It doesn't need to
be immersed in it, but it needs to have it to drink. Water
evaporates, and water on land needs to be renewed. That means rain.
Water evaporates from the oceans, drifts over the land, condenses
into droplets and falls to the ground. There aren't too many other
possible mechanisms. One could consider tidal flooding, which might
produce wet areas, but if the ocean has dissolved some minerals from
the rock, like salt, it might not be drinkable. If there was a lot
of volcanic action, possibly someone could come up with a process
that, on some suitable planet, might pump water, distill it in the
volcanic heat, let it condense elsewhere, and expel it into a river.
This complicated a mechanism doesn't seem likely, at least during the
later life of the planet. So it is rain that is the mandatory
precondition for an alien civilization and for animal life on land as
well.
Detecting water vapor in the atmosphere
might be the first surrogate for detecting rain. On Earth, water
vapor is at a much lower concentration than oxygen, and therefore
more difficult to detect; but it is not impossible to foresee that
that would be a further step in astronomical capability, once oxygen
was detectable. Carbon dioxide might be detectable first, or some
other compound or element, such as argon, but eventually water vapor
would succumb to astrophysical technology. These gases would likely
first be detected for a transiting planet, where the light of the
parent star shines through the atmosphere and gets spectrally
absorbed. Later they might be detected from reflected light.
Rain would have to be either a local
phenomenon, evaporating over most of the ocean area, and then
precipitating on some region, or else a seasonal phenomenon,
evaporating during one season of the year, and precipitating during
another. Wet ground has little difference in albedo than dry ground,
however, so even if telescopes
grew sufficiently in aperture to see different parts of the
exoplanet, seeing wet areas would be quite difficult. But if there
was sufficient temperature range on the planet, and there was snow,
then a significant albedo change might be detected. This would be
even easier in the seasonal case, where evaporation occurred all
summer and then all winter, snow happened. A planet with an elliptic
orbit might produce this situation.
Another option is clouds. Rain and
clouds are not the same thing, but there must be clouds to produce
rain. Searching for clouds might be considerably easier that
searching for rain. Clouds do change the global albedo, and
monitoring for these changes would be an indicator of cloudiness, and
by implication, rain.
Another variable which affects the
detectability of atmospheric gases is the thickness of the
atmosphere. Earth has a very thin atmosphere, about 10 km thick
compared to 6000 km radius of the planet. This is to be compared to
Venus, with approximately the same radius, but an atmospheric
thickness of 250 km. Probably the components of Venus' atmosphere
would be much easier to detect on a transiting planet. Clearly one
must compare the loss of signal due to the longer transmission path
with the larger cross-section of the atmosphere and the lengthened
time for absorption. For a reflective signal, the loss of albedo may
make the comparison go the opposite way, with thicker atmospheres
being more difficult to break down into components.
One interesting question is, if there
was an exoplanet
with an atmosphere as thick as Venus' atmosphere but of the same
composition as Earth's, would rain be possible? One might also make
an assumption that the rotation rate was identical to Earth's as
well. Similarly the average temperature would be assumed to be the
same as Earth's. The vapor pressure of water is the same no matter
where it is, so the amount of water in the atmosphere would be the
same as Earth's, maybe a half percent on Earth on the average, with
more of it at lower altitudes. On the exo-planet it would be a half
a hundredth of a percent. Rain forms when the temperature of the
atmosphere drops sufficiently that liquid water can form. Would the
thermal inertia of a large atmosphere, with 100 times the atmospheric
mass of Earth, prevent this temperature drop? Temperature drop
comes from heating or cooling of the atmosphere, and a portion of the
lower atmosphere gets a certain amount of heating from the solar
energy passing through it, which is largely identical and a certain
amount from the reflected heat from the planet. On the exoplanet,
much more solar energy would be absorbed by the atmosphere, leaving
the surface much darker and receiving less energy. Solar energy
incident on the atmosphere would be largely identical for different
longitudes, meaning much less opportunity for the temperature change
that is required for rain. This possibly means that finding an
exoplanet with a thick atmosphere would imply no rain, and no land
lifeforms, and no alien civilization.
One hypothesis about the reason for the
thinness of Earth's atmosphere is that atmospheric mass is almost
unaffected by the aging of the planet, and once thin, it stays thin.
If the formation of the Earth was mediated by the impact of a
protoplanet, which led to the formation of our large moon, and the
legacy atmospheric hypothesis is true, looking for a large moon might
be the fastest way to find land life and the possibility of a
civilization.
