During my thesis defense, one of the questions that caught me most off guard was: “What would you say to a member of the public who asked you why we’re spending more than $2 billion on the Mars Science Laboratory rover?”

I’m ashamed to say that I didn’t answer very well. I was all prepared for my research methods and results to be picked apart, and so I went sort of brain-dead when this question came up. I managed to basically repeat the mission objective of assessing habitability and then my adviser came to my rescue by referring to this blog, where I have talked before about the reasons for space exploration.

My failure to answer well has bothered me ever since, and now, with the brutal cuts to the planetary science budget in the president’s proposed budget for fiscal year 2013, I think it’s time I gave a proper answer.

So, why explore Mars?

We don’t explore Mars to provide jobs. And yet, most of the money spent on missions to Mars, including MSL, goes toward paying the thousands of people who are involved in making a cutting-edge rover a reality. Everyone from the lead scientists to the rover drivers to the person who soldered the circuit boards has a family to feed and bills to pay. That’s where the money goes.

We don’t explore Mars so that we can develop spinoff technologies. But the same technique used by ChemCam to analyze rocks and soils from a distance — laser-induced breakdown spectroscopy — is also used to analyze everything from artwork to nuclear waste to food products to iron ore, and the advances in data analysis for Mars exploration are improving the method’s utility here on Earth. The CheMin x-ray diffraction instrument on MSL will be used to probe the crystal structure of rock samples, but commercial versions are also available. By making the instrument extremely compact and energy efficient, it can be taken to remote locations, where it is being used to identify counterfeit malaria drugs and save lives.

We don’t explore Mars to improve our diplomatic ties, but we can’t do it without the help of other countries. Half of the ChemCam team is French, and they built the half of the instrument on the mast. The Sample Analysis at Mars instrument also is partially developed by French team members. The alpha particle x-ray spectrometer (APXS) is being built by the same Canadian aerospace company that built the Canadarm on the international space station. The Radiation Assessment Detector (RAD) is partially funded and developed by the German Space Agency, and the rover environmental monitoring station, which will provide daily weather reports from Mars, is being provided by Spain. The Dynamic Albedo of Neutrons (DAN) instrument, which will probe the subsurface and search for buried ice, is being provided by the Russian Federal Space Agency.

Unfortunately, with the proposed budget cuts to Mars exploration, NASA had to cancel its involvement in a joint European/American rover that was being planned for 2016 or 2018. That mission is proceeding as a partnership between Europe and Russia.

We don’t explore Mars to inspire the next generation of scientists and engineers, but nothing captures people’s imaginations like space exploration. MSL is a nuclear-powered rover the size of a small car that will autonomously land with unbelievable precision on the surface of Mars by using a jetpack, and then start zapping the rocks with a laser beam. This mission sounds like science fiction but it’s even better because it is real. NASA shows what humans can do when they put their minds to something. If we want a new generation of scientists and engineers to solve the problems that we face here at home, then we need high-profile technical achievements like missions to Mars to inspire them and show that anything is possible.

So, if thousands of jobs, money- and life-saving spinoff technologies, strong diplomatic ties with other countries, and inspiring a new generation of scientists and engineers are all just side effects, then why do we explore Mars?

We explore Mars because we are human, and to be human is to ask questions. There’s a reason the rover is named Curiosity! There are thousands of questions that make up the bread and butter of our scientific investigations: why does that crater have a mountain of layered rocks in it? How did those minerals form? Why did the climate on Mars change? How did the planets form?

But there is one fundamental question that underlies Mars exploration: Are we alone in the universe?

We live on a planet that is overrun with life and we look up at the night sky and wonder if there is anywhere else in the universe that is alive. Mars, our next-door neighbor, is tantalizingly similar to the earth in many ways, and so we want to know if life ever arose there. Were the conditions ever right? And if so, what happened? Did life arise and die out, leaving telltale signatures in the ancient rocks, or did life miss its chance on Mars? Is it difficult for life to arise if the conditions are just right, or is it commonplace?

If we can understand what Mars used to be like, then we can say whether life ever had a chance. If we find that life had the chance but never got started, then we learn that the universe is likely a lonely place, with only the rarest special places like the Earth getting lucky enough to support living things. If we find that life arose quickly on early Mars, then we learn that wherever the conditions are right, life is inevitable: the universe wants to be alive. Then we can look to the icy oceans of the outer solar system, and the thousands of planets that we are discovering around other stars, and know that the cosmos is teeming with life.

We embark on big scientific endavors — exploring Mars, and colliding particles, and sequencing the genome, and peering back into the depths of time — because we want to answer the big questions. Every small step toward those answers is worth the investment, just for the knowledge gained. But even as we pursue the lofty ideals of pure science, the benefits of the pursuit are apparent in more concrete terms.

I often hear the criticism that we shouldn’t spend money on NASA, we should spend it on making things better here on Earth. I reject that false choice. As a great nation we can do both. And besides, last time I checked, NASA is on Earth. The people who work on space exploration are here on Earth. The new technologies that save lives and spur new industries are here on Earth. The diplomatic ties that we build are with other nations are here on earth. The child who is inspired to study science and technology and change the world is here, on Earth.

Spending money on space exploration does make things better here on earth, even as it answers our most fundamental questions about the universe.

That is why it’s worth it to explore Mars.