It is hard to imagine a better frontier for humanity than Mars. Why? For a number of reasons which boil down to one: it is an appropriately sized challenge for current human capabilities. The challenge of not just surviving on Mars, but of living and thriving there, is a good balance between a challenging problem and a tractable one.
It is a challenging problem because it is a challenging place for humans to survive:
the weather is harsh to the extreme (it is usually very cold, there is low air pressure, there is not enough oxygen to breathe)
there is no obvious life on the surface
there are no grocery stores
there are no roads
and there is no arid or farmable land (the soil is poisoned with perchlorates).
But it is tractable place to colonize because it has everything we need, if we only know where to look:
it can be warm on some days (even up to 70 degrees Fahrenheit!)
there is enough air pressure to make cheap, lightweight, pressure sealed living spaces and greenhouses out of thin plastic
there are plenty of natural resources to sustain life (water, oxygen, land for growing food, carbon to make plastic products, raw ores to make steel, etc.)
and the kicker is that Mars is practically waiting for humanity to help kickstart its generation of its own atmosphere.
When Mars lost its atmosphere billions of years ago, it did so through a runaway cooling effect which froze its atmosphere into the soil and ice sheets. As more atmosphere froze into solid, the atmosphere shrank and the planet cooled. But the sun is now 30% warmer than back then, and it is estimated that raising the global temperature just 8 degrees would reverse this process.
There is enough CO2 absorbed in the soil and stored in the ice sheets that with a little effort (little on a planetary scale - think like a 250 km^2 reflector in space), a runaway greenhouse effect can be kicked off by humans which would increase the atmospheric pressure enough in just a few decades for hardy plants to live on the surface. And in a handful more decades there cold be more Earth-like vegetation there.
Building a 250 km^2 solar reflector in space near Mars might sounds crazy hard, given that no one lives on Mars and it is a 6 month journey from Earth. But it can be a very thin reflector and the total mass is only on order of the weight of an oceangoing cruise liner here on Earth. With local Martian industry (which will require tens of thousands, or perhaps hundreds of thousands of Martian inhabitants) this kind of thing is not only possible but eminently feasible.
And once the vast amounts of CO2 and water are released from their frozen prisons on Mars, the natural water cycle of rain and evaporation will wash the poisonous perchlorates from the soil. This in combination with a thicker atmosphere and a warmer average temperature will enable the growth of crops on wide swaths of the Martian surface without greenhouses. People will still need to wear oxygen masks, but they won't need pressurized suits and might not even need a heated suit much of the year in many locations. Depending on how many people we send to Mars, and how quickly, this kind of future could be a reality for the grandkids of today's children.
Mars is like a fixer upper house with good bones. It's got what you need, if only you put a little work in at the beginning to build yourself a home. And by 'little' we mean the combined efforts of tens of thousands, hundreds of thousands, or perhaps millions of people over decades. But for a new planet for humanity, that seems like a worthwhile trade.
For further reading:
Much of the information about terraforming Mars is sourced from The New World On Mars: What we can create on the red planet by Robert Zubrin (It is a fantastic book with lots of detail as to how colonizing Mars will work. As an Amazon Associate we earn from qualifying purchases. We hope you enjoy the book as much as we have.)
Map of water in the soil on Mars: https://ammos.nasa.gov/marswatermaps/?mission=MWR