Why don't Venus and Mars have life if they are both in the habitable zone? -
Perché su Venere e Marte non c'è vita se entrambi si trovano nella zona abitabile?
The habitable zone is a region of the solar system where temperatures allow
liquid water to exist on a planet, that is, a region that is neither too hot nor too cold.
Did you know that there are three planets within the habitable zone in our solar system?
Yes! Believe it or not, Earth is not the only planet in the habitable zone. Mars and Venus
are also within this region. But... If these two planets are within the habitable zone,
why do neither have the necessary conditions to host living beings?
Ready? Let's get started! "
The habitable zone The habitable zone is
an area around a star within which temperatures between 32°F (0°C or 273K) and 212°F (100°C or
373K) are reached. That is, the habitable zone corresponds to the area around a star
where the average temperature of the planets there would allow liquid water on its surface.
To calculate the habitable zone around a star, the star is generally considered the planet's
only energy source. The average temperature the planet should have is calculated by measuring
the star's radiation and the distance the planet is at. However, this definition is too simple and
does not consider various other factors that may be affecting the planet's temperature.
Other factors for habitability The existence of liquid water on
a planet does not depend only on the amount of energy it receives from its star. Atmospheric
pressure also plays a vital role in the presence of water. Some of these other factors are:
Greenhouse effect: carbon dioxide in a planet's atmosphere generates the well-known greenhouse
effect, due to which much of the radiation from the star cannot escape from the planet's surface.
This makes the temperature of such a planet much higher than that obtained by calculating
only the energy it receives from its star. It is this effect that prevents the presence
of liquid water on Venus. If Earth had more carbon dioxide in its atmosphere,
we would have to move it to the position of Mars so that it could harbor liquid water on
its surface since otherwise; the intense heat would evaporate all the water on our planet.
Albedo: is called albedo to the level of reflectivity of the surface of the planets or
celestial bodies. A body that reflects much of the light it receives from its star has a high albedo;
In contrast, bodies that absorb most of the energy they receive have a low albedo.
A planet with a low albedo can have a much higher temperature than one with a high albedo, even if
both receive the same energy from their star. If Earth had a lower albedo, it would absorb
more energy from the Sun so that the global climate would be warmer. So to compensate
for those high temperatures, we would have to move the planet away to the outermost limit
of the habitable zone, where Mars is located. On the contrary, if the Earth had a higher albedo,
it would reflect more light, and this would cause an ice age, so to compensate, we would have to
move the Earth to the limit of the habitable zone closest to the Sun, that is, the area where Venus
is located so that the planet does not freeze. Currently, the most accepted measure by
astronomers about the measurements of the habitable zone of our solar system sets
its limits at a distance of between 0.84 and 1.67 Astronomical Units concerning the Sun.
The habitable zone of other stars The main factor that determines the
extent and characteristics of the habitable zone is stellar luminosity. As such, it varies
significantly depending on the type of star and its life cycle, being broader and more variable
the more luminous the star to which it belongs. Stars more excellent and smaller than the Sun
(type M and K) remain much longer in the main sequence. Their reduced habitable zone varies
very little throughout their lifetime. In comparison, more massive stars (type F, A,
B, and O) have a much longer habitable zone, which varies significantly throughout their short life.
The M-type and K-late stars, better known as red dwarfs, are the smallest of the main sequence and
most common in the galaxy, accounting for 75% of the stars in the Milky Way.
Since the life expectancy of a star is inversely proportional to its mass,
these stars are the longest-lived of all, being able to remain on the main sequence
for up to 10,000 billion years compared to approximately 10,000 million of the Sun.
However, the potential for a life of red dwarfs is the subject of debate among the scientific
community since they present serious problems for life to arise on any rocky planet that
orbits one of them in its habitable zone. In their first billions of years of life,
they are highly active, increasing their levels of ultraviolet radiation up to
10,000 times repeatedly throughout an Earth day due to their violent flares.
Models suggest that an Earth-like planet orbiting one of these stars in its habitable zone would
gradually lose its atmosphere, even if its magnetosphere were similar to our planet.
Due to their low luminosity, the habitable zone of M-type and late
K-type stars comprise a tiny and close region compared to that of larger stellar bodies.
This closeness also translates into a tremendous gravitational influence of the star on its
potentially habitable planets, causing them to have tidal coupling; that is, one of its faces
is always pointing at the star, just as the Moon always points the same face to the Earth.
That absence of rotation can seriously damage a planet's magnetic field, leaving
it unprotected from the effects of its star. In summary, although M-type and late-K stars are
very stable for a long time, being so small, their habitable zone is too close to their
gravitational influence; even if a planet is in the habitable zone, this will not guarantee that
it has the necessary conditions for life. Then we have the orange-intermediate to
white-yellow stars, representing the best type of star for life because of their stability,
the distance of the habitable zone from the star, and the life cycle.
Unlike red dwarfs, orange, yellow and white-yellow dwarfs have a phase of intense stellar activity
after their much shorter formation of about 500 million years for a Sun-like star.
A concise period if we compare it to the 2000 or 3000 million of an M-type star)
Thanks to this separation between the habitable zone and the star,
potentially habitable planets do not run the risk of losing their atmospheres in their
first billions of years of existence and are not prone to tidal coupling since the gravitational
influence of their star is not so great when located in a more distant area. Which means that
a planet in these types of stars is much more likely to have a magnetic field like Earth's.
Venus and Mars: habitable or not? As we have already seen, the habitable
zone of our solar system hosts three planets, Venus, Earth, and Mars; however, of those three,
only Earth has life. Not only that but Venus and Mars are a long way from habitable planets.
This is because it is not enough to be within the habitable zone to host living beings;
as we have already seen, several factors can determine whether a
planet will be habitable or not, regardless of whether it is in the habitable zone.
In addition to the ones we already mentioned, we also have these factors:
1. Dough Low-mass planets are
bad candidates for life for two reasons. First, its low gravity makes conserving
the atmosphere difficult. The constituent molecules are more likely to reach escape
velocity and be lost to space when bombarded with solar wind or churned by a collision.
Planets that do not have a thick atmosphere lack the material needed for primary biochemistry, have
little insulation and little heat transfer between their surface (for example, Mars, with its thin
atmosphere, is colder than Earth would be at a similar distance), a smaller atmosphere also means
less protection against radiation and meteorites. In addition, if the atmosphere is less than
0.006 Earth's atmosphere, no water can exist in liquid form because it does not
reach the required atmospheric pressure. The second reason why a planet with little
mass is a lousy candidate to host living beings is that they have small diameters and therefore
tends to quickly lose the energy left over after its formation and end up geologically dead,
lacking volcanoes, earthquakes, and tectonic activity, which provide the surface with
materials necessary for life and the atmosphere temperature moderators such as carbon dioxide.
Venus meets the size requirement as it has a diameter similar to Earth, but Mars does not. Mars
is a tiny planet on which it will be difficult to trap an atmosphere due to its low gravitational
attractive force. Not to mention that it has very little or almost no seismic activity
and lacks a magnetic field, three essential requirements to be able to house living beings.
2. Orbit and rotation of the planet As in the other criteria, stability is critical in
determining the effect of orbital and rotational characteristics on planetary habitability.
Orbital eccentricity is the difference between the greatest and smallest distances to the primary
object. The greater the eccentricity, the greater the temperature fluctuation on a planet's surface.
Although they are adaptive, living things can only withstand a particular variation,
if the fluctuations exceed the freezing and boiling points of the planet's primary
biotic solvent (for example, water on Earth). If, for example, Earth's oceans alternately
evaporated and froze, it's hard to imagine how life as we know it could have evolved.
Other habitability considerations about a planet's
orbit and rotation are the length of days and years.
For a planet to be habitable, it must rotate relatively quickly so that the day-night cycle
is not too long. If a day lasts for years, the differential temperature between the day and night
sides will be pronounced, and problems similar to those of extreme orbital eccentricity will appear.
In turn, its speed of translation around the sun must also be moderate, if it orbits too fast,
it could cause a tidal coupling effect that severely affects its magnetic field, and if it
rotates too slow, the duration of the seasons will be too long for a living being to adapt.
Venus has this problem; although its years last 225 Earth days, its days last 243 days;
that is, a day on Venus lasts more than a year; this causes drastic effects on its
global climate since one of the faces of Venus receives 121 days of uninterrupted
sunlight while on the other side, there are 121 days of complete darkness. This produces
enormous temperature changes all over the planet. On the other hand, Mars has a year that lasts
twice that of Earth but a rotation period that is identical to that of the Earth,
of almost 24 hours, so in this aspect, it is very similar to our planet.
In short, although Mars and Venus are in the habitable zone of our solar system,
that is not enough to have the conditions needed to have liquid water and living things.
In addition, they must meet many other requirements that they do not have. Earth is the
only planet in the known universe that meets all the necessary conditions to host living beings.