Why do Mars explorations outnumber Venus explorations?

SnT : Why-do-Mars-explorations-outnumber-Venus-exploration

Answer by Sandhya Ramesh:

This is a great question. There have been 41 missions to explore Venus and 55 missions to explore  Mars so far, including ongoing missions as well as missions to Martian  moons in the form of orbiters, landers, rovers, and flybys.

1. Exploration of Venus is cheaper, since its much closer to Earth (this in itself outweighs other reasons). The closest Mars has been in human history is 56 million km (2003), while Venus, on average, is 40 million km away. Mars, on average, is 250 million miles away.

This  was the primary reason we sent the first spacecraft to Venus after Moon. Venus  was the second body outside of Earth to have non-human visitors from our  planet. But, to obtain decent information from a planet, you need to be  able to see its surface. Venus's atmosphere is almost completely opaque and reflects all light away from the surface due to a thick layer of clouds of toxic gases that stretch up for  kilometers.

(Venus, through a telescope. We cannot see even an inch of the surface. Image from Scientific American)

Because of this, landing and getting an explorer to survive  would cost way more than sending a probe to Mars. Traveling is not the biggest issue, although landing is. Distance just means more time (in case of Mars vs Venus) because bodies in space move in inertia after an initial (and the occasional correctional) impetus. There's no friction, so once launched in a direction, your spacecraft can keep going, consuming power only for heating and communications. However, the Sun's gravity plays a huge role at interplanetary distances. So in fact, it is easier to travel to Venus because the Sun would pull the craft, but Sun would also make it harder to slow down. Venus's gravity itself is very high as compared to Mars, making descent through the atmosphere a thousand times harder. Venus's gravity is very close to that of Earth's, and almost thrice as high as Mars's. The planet's gravity, in conjunction with the already superheated atmosphere and high atmospheric pressure, requires extremely powerful heat shields; the most powerful ever built, every time.

There is acid rain (albeit not on the surface itself), and thick clouds with violent thunderstorms all over the surface that cloud visuals as well as communications. Due to the clouds, we don't know much about anything about Venus through direct contact. We know what we know mostly through radar data.

The extreme heat and lack of visibility also make landing very very difficult. Out of the 18 lander missions, only 8 were successful. Ok, actually, only 15 made it out of Earth's orbit, and 2  more partially failed in deploying all components. The most any lander survived was for 127 minutes before losing signal or getting destroyed (Venera 13). So we'll put the success rate at 10/15, with a very low shelf life. Even the future missions to Venus estimate a functioning span of one hour (Venera D). The Russians aren't giving up on Venus.

Contrast  this with Mars — clear sky and easier landings aided by the low gravity because we can study the topology from here.

(Composite image of the Martian surface. Image from Space Facts)

There have only been 14 Mars landers/rovers that have left the Earth successfully, and out of them, 10 successfully landed on Mars without crashing, and  two rovers are still functional. Opportunity is eleven years old while Curiosity is three. In all probability, Phoenix, and Pathfinder's rover, Sojourner are still functional, but have solar panels covered with the notorious Martian dust which prevents them from powering up and communicating. So that's 10/14. Considering the billions of dollars that go into one mission, that's a helluva lot more successful compared to Venusian probes.

2.   It is more Earth-like (in size, composition), etc. so setting up a   human colony may be feasible (assuming solutions are worked out for the   high atmospheric pressure and temperature)

It's  definitely more Earth-like in size and that is the reason Venus is  called the Earth's twin. But, on the surface, the pressure is over 90 times that of the Earth and the average  surface temperature is 462 deg C. Four hundred and sixty two degree celsius. Highest temperature ever recorded on Earth was 56.7 °C. The atmosphere  is filled with carbon dioxide that stays close to the planet's surface due to strong gravity and the sheer magnitude of greenhouse  effect in play on Venus makes it the perfect example to compare the Earth's future to if we don't stop global warming.

Mars  reaches a maximum high of 35 °C, but is very cold despite similar amounts of carbon dioxide in the atmosphere because of lack of strong gravity. Lesser energy is  expended to heat something in a cold atmosphere than to cool it down in a  hot atmosphere. It's more practical to have heated pods on the Martian  surface than have super cooled pods on Venusian surface. The main natural disadvantage any human/rover have for exploration on Mars are dust storms, whereas in Venus, there are far too many to even count. That's why we've lost so many probes on the surface of Venus.

Venus's composition might be similar to that of Earth's inside the planet, but on the surface, it's a very different story. Venus's surface is comprised of rocks that are mostly igneous in nature due to volcanic activity and extremely alkaline, and can support no life.

(Surface of Venus as taken by Venera 14)

Mars has a soil that is capable (to an extent) of supporting life, if life were to be introduced to it.

(Martian soil. Image taken by Curiosity rover.)

3. It has a higher chance of life (specially in the upper atmosphere – Possibility of life on Venus)

Quite true. In fact, many scientists believe that life may once have existed on Venus, along with plenty of water, but due to the exponential Greenhouse effect, might have destroyed almost entirely and the surviving life forms could have retreated to favorable conditions. Such conditions seem to be present in the upper atmosphere. However, there are multiple things that go into sustenance of life. Life needs an abundant quantity of water to survive, not just a few droplets floating around haphazardly. Venus also purpotedly undergoes a unique phenomenon of 'resurfacing'.

There is an unverified, but accepted theory, that every 300 million years or so, Venus undergoes a complete change in the rock that makes up its surface. This is because, due to the lack of water, the inner core of Venus is much harder as compared to Earth and therefore, there are no plate tectonics. On Earth, tectonic shifts and earthquakes help the heat from the mantle escape out.

Inside Venus, the mantle gets superheated and forces its way to the surface in the form of violent volcanic activity, among other things. This covers the entire planet's surface with lava which later cools down to form the new surface of the planet.

This theory is accepted because it provides a likely explanation for many inexplicable things about Venus, such as the planet's retrograde rotation and lack of magnetic field. But it also as good as rules out any possibility of survival of life for obvious reasons. Such a giant upheaval would destroy any residual life surviving in the upper atmosphere and make it non conducive for growth.

That said, we can't tell for sure if at the moment there isn't life in form of bacteria in the upper atmosphere of Venus. But we know that there isn't sophisticated life on Mars.

(Polar ice caps and presence of water clouds on Mars. Image taken from Space Today Online)

However, Mars still shows the presence of water, in the form of ice. We can see the polar ice caps from Earth, while  we can tell only by deduction that all the water on Venus must have  evaporated away millions of years ago.  This offers a huge advantage in favor of Mars for us in understanding conditions for creation and survival of life, tracing our own origins, and understanding the future of human life (colonization and survival conditions outside of our planet).

Venus's axial tilt is a whopping 177 degrees, while Mars's is 25 degrees; Earth's is 23 degrees. This enables Mars to have seasons very much like Earth's, only way more extreme.

(Image from Universe Today)

4. It has access to way more (solar) energy. Looking ahead several centuries, it should easier to block out excess energy on Venus, than to manufacture it on Mars.

Nope, not at all. It has a lot of heat because of the greenhouse effect, but the Venusian clouds completely cover the surface of the planet, cutting off all access to sunlight. Remember, to harness solar energy, we need direct exposure to sunlight. Venus doesn't hold a candle to Mars, where the sun is absent only during a dust storm or in the night. Solar energy is present in overabundance on Mars.

(Dust on the solar cells of Spirit, before and after a Cleaning event.)

The length of a day on Venus is a 116 Earth days, while on Mars, it is 24 hours, 40 minutes, providing more quantity of sunlight. This provides a larger scope for using Mars as a station for further space missions (similar to the Moon, and interestingly, Mercury). Such near constant free source of power makes Mars much more appealing to explore and find out more about than Venus. On Mars: use solar cells, prepare for dust storms, wipe dust off when storm's passed, continue as before. On Venus: use solar cells, wait for a thousand years for the sun to show. (Have you read The Martian (2014 novel) by Andy Weir? The book very accurately describes Martian ability and problems when it comes to sustenance of life. I think the movie comes out next year.)

At  the end of the day, there are five things that work greatly in Mars's  favor — presence of water, composition of soil, length of one day, axial tilt of the planet, and abundance of sunlight. All these  factors suggest that despite the size difference between the two  planets, Mars is closer to Earth in terms of ability to support and sustain life. Exploration of Mars and exploration of Venus are two completely different missions with different objectives. For humans, who are just on the throes of a budding space exploration and on a perennial quest to find out about the past and future, exploration of Mars provides much more return on investment and makes infinitely more sense than exploration of Venus does.

References:
Planetary Missions
Venus images
Venera: The Soviet Exploration of Venus
Murphy's Law, Part two
NASA – Mars Pathfinder and Sojourner
Page on wired.com
Venera-D mission
Gabbro
Sojourner: Roving on Mars for the First Time
Acid clouds and lightning
Clouds on Venus
 Stereo-Derived Topography home page
Status of Venus Geologic Mapping (Table)
Page on usra.edu
Press Release Images: Opportunity
Page on usra.edu
История Земли. планет, Вселенной….
Martian soil 'could support life'

Why do Mars explorations outnumber Venus explorations?

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