For me, the top reason is the risk of contaminating Mars with Earth microbes. That’s especially important because in the last eight years we have found many possible habitats for life on the surface of Mars. We don’t know if they are habitable or not. But with the plans of Elon Musk or NASA they would send humans to Mars before we have had a chance to send our robots to look at them close up at all.

Many of these may be seeps of liquid brine that are either too salty for life or too cold. But some may not be. For a particularly intriguing one, see my Does Ice Act As Greenhouse On Mars - Fresh Liquid Water Habitats In Spring 10-20 Cms Below Polar Ice?

One of many types of seasonal process on Mars - these seeps though might be the result of liquid fresh water forming underneath transparent ice, then flowing out onto the surface mixed with salts as a salty brine.

This shows the location of Richardson Crater where these seeps occur. Mars has similar surface area to Earth’s continents, and it would take a fair number of rover missions to get even a first basic biological survey. This might be one of the more intriguing places to visit first. For details see:

That one might even have water at 0 C on the surface of Mars protected from evaporating by a thin sheet of transparent blue ice.

In total there are about eight suggested types of potential habitat. And the thing is we can’t send a spacecraft up to look at them until we are able to sterilize them 100%. But the spacecraft missions planned to Mars for the near future are not sufficiently sterilized and will have to avoid these potential habitats. They are looking mainly for past life. That includes ExoMars and Curiosity2020 - though ExoMars does have a mission to search for both present and past life it wouldn’t be able to go up to a potential habitat for present day life and inspect it directly.

So they can’t explore them and we can’t spot microbes or even lichens or small microscopic multicellular lifeforms if they exist, from orbit. If these habitats exist, they are just on the edge of habitability. But life may be very tenacious once evolved, so if there ever was life on Mars in the past, perhaps it is still there. These habitats are just on the edge of habitability but as Nilton Renno said, for a microbe a tiny droplet of salty water is like a huge swimming pool. A few microbes in droplets of water, photosynthetic life beneath the skin of rocks, lichens crouching in shadows to protect themselves from direct UV light - these are not going to be easy to see from orbit and chances are they are impossible to see until you can send a rover right up close to look at them.

You might think microbes are boring. But these could be extra terrestrial microbes, which turns them into something that could be not only interesting, but the next breakthrough in biology. The amount we could learn from them is staggering if they are indeed different from Earth life in some essential way. They could be

• Earlier forms of life, e.g. RNA world - this is especially vulnerable to Earth microbes, whatever came before DNA life which is extinct on Earth.
• Parallel life - evolved separately on Mars - maybe based on completely different principles
• Related life, evolved separately on Mars for anything from tens of millions of years (most recent time life could be transferred from Earth to Mars on a meteorite) up to many billions of years.
• No life at all, but must have organics from the influx of meteorites and comets - so what happens on a planet with organics and conditions for life, but without life? Although this may seem boring at first, we could learn a lot from that too.

So, the colonists want to just go ahead and land humans on Mars anyway before we know what is there. They tell us they think the chance that there is anything there that can be harmed by Earth life is tiny. But how can they know without looking?

If we send humans there without looking first, we have no idea what the effect would be of Earth microbes on Mars. And that’s especially so since the landing on Mars is the most risky in our inner solar system. See my Why Spacecraft Crash On Mars

A crash on Mars of a human occupied spacecraft would be the end of planetary protection of the planet. A safe human landing would probably be also, in the sense that it would introduce trillions of spores from the planet that would gradually spread in the dust storms. They would do their best to slow down the inevitable but would not be able to guarantee to contain it.

I think it is just far far too soon to do something irreversible like this. After introducing Earth microbes to Mars, irreversibly - that means - that we have changed Mars for all future time. No future civilization in our solar system, not just ourselves and our descendants but nobody ever would be able to study Mars as it was before the introduced Earth life.

Is it right to take the chance that perhaps this might have no harmful consequences, when the positive consequences of keeping Mars pristine coudl be so great?

Note it is not about whether we exploit Mars or not. If we can mine it, make fuel there, grow crops even using hydroponics or aerponics tended telerobotically - if it doesn’t introduce Earth microbes irreversibly, then that’s different.

But how can anyone, even if they are CEO of a large company and very wealthy, or director of a space agency - how can they make such a decision for the rest of humanity?

Well - they have all said they will comply with the requirements for planetary protection. So it’s a matter of what those requirements are. I think that we should have the Moon as our first objective for humans, do our experiments and tests there. The Moon is of great interest too. Then have humans to Mars orbit - but also to Venus, Callisto, Mercury etc as the longer term goal. But not have humans to the Mars surface as a fixed term goal at all. Let’s first find out if there are habitats on the surface, find out what is in those habitats, find out if Earth microbes will impact on Mars and if so how. Let’s decide what to do next only after we have that understanding of Mars.

That may take some time. Carl Sagan used a figure of 54 landers and 30 orbiters for a biological exploration. If we sent two landers and one orbiter to Mars every two year opportunity that would take more than half a century and that is a very sketchy first exploration given that the surface area is similar to Earth’s and there are many different potential types of habitat there to study. Or we might find a habitat for life on our first mission sufficiently sterlized to explore them. We might speed it up a lot with telerobotic exploration from orbit, or miniaturized robotic landers or both.

But anyway I think that needs to be our near term objective for Mars - robotic study from Earth then telerobotic study from orbit. It can be an exciting mission. For more about this see my

MOON FIRST Why Humans on Mars Right Now Are Bad for Science

Also available on Kindle MOON FIRST Why Humans on Mars Right Now Are Bad for Science: