Humanity has gazed at Mars for centuries, dreaming of what might lie on its dust-orange surface. As our telescopes improved, so did our picture of the Red Planet, though scientists' interpretation of that growing detail has not always been accurate (see: Mars canals). The first mission to successfully view Mars up close, Mariner 4, beamed home a handful of blurry crater views, but the missions that followed — those that succeeded; the overall failure rate is upward of 50 percent for Mars-bound spacecraft — painted a clearer picture of the planet's dusty, bouldery present.
More recently, scientists have found evidence of a warmer, wetter ancient past for the planet that could have hosted Earth-like life, and so the search for life continues, though it now stretches to the past. And the planet has less scrambling of material from tectonic movement than Earth, so its makeup can tell scientists about the formation of the solar system.
NASA's InSight Mars mission, set to touch down on Monday (Nov. 26), will delve deeper below the planet's surface than any mission before it, learning about the planet's interior. InSight is a smaller mission than many of the rovers and orbiters preceding it, but it's the latest in a long line of robotic emissaries to explore our planetary neighbor. [Missions to Mars: A Robot Red Planet Invasion History (Infographic)]
"Mars is an incredible natural laboratory right next door to Earth," Lori Glaze, the acting director of NASA's planetary sciences division, said during a news briefing for InSight on Nov. 21. "We really want to understand how we came up with this diversity of rocky planets in our solar system — they're all very different, each one of them is unique in its own way, and trying to understand how they ended up so differently is a really important question."
Time capsule of early planets...
Earth, Mars and the other rocky planets in our solar system glommed together from a dusty disk surrounding the young sun, getting hotter and hotter as material was added and melting into bodies with distinct mantles and cores. Yet we don't know much about that early time in the planets' history.
"On Mars, that structure's been preserved over the last 4.5 billion years, whereas on the Earth, where we actually can study it pretty easily, that structure's all been scrambled up both by plate tectonics, by mantle convection, and so the evidence of the very earliest processes has been wiped away on the Earth," Bruce Banerdt, principal investigator of the InSight mission and a researcher at NASA's Jet Propulsion Laboratory in California, said during the briefing.
So just like studying comets — the leftovers of this formation process — tells researchers about the solar system's earliest days, probing Mars' structure by measuring the planet's temperature and marsquakes can tell scientists about the next step in planetary evolution.
And knowing more about Mars' current conditions can also help researchers understand what it might have been like in the past. [Why We're Obsessed with Mars]
...and early Earth
"Mars is a very unique place in our solar system because it's one of the few other planets that we think has really ever been truly Earth-like," Briony Horgan, a planetary scientist from Purdue University who focuses on the geologic history of the moon and Mars, told Space.com. "Today, it's this cold, inhospitable place with very thin atmosphere...low pressure, all the radiation that's bathing the surface. But when we look at the geologic record of Mars, we see huge amounts of things like dried-up river channels, dried-up lake deltas and lake sediments; we see minerals all over the planet that can only form in the presence of water." [Water on Mars: Curiosity Rover Uncovers a Flood of Evidence]
Mars 3 or 4 billion years ago may have looked very similar to early Earth, Horgan said, and while on our planet erosion, plate tectonics and other processes have swept away rocks dating from that time, Mars offers another chance to see them.
"The geology of Mars has just been so much less active on the kind of scale the Earth is that the rocks 4 billion years ago are just sitting on the surface," Horgan said. "They haven't been subducted, they haven't been buried, they haven't been eroded — they're just sitting there, basically waiting for us to go look at them and to try to understand what these ancient, 4-billion-year-old environments might have looked like, and did they support life."
Horgan is a scientist on NASA's upcoming Mars 2020 rover mission, whose landing site the agency announced on Nov. 19. The Mars 2020 rover follows in the footsteps of 1976's twin Viking landers, which landed on the Red Planet to search for life according to scientists' best understanding of the planet's conditions, and the Curiosity rover, which touched down in 2012 to investigate Mars' past habitability.
As our views of the planet evolve, our life-searching tools have, too, Horgan said — after the Spirit and Opportunity rovers presented evidence of past water, Curiosity brought a vast suite of science instruments to try to find organics or other evidence of habitability near those ancient water beds. Mars 2020 will build on Curiosity's ongoing work by bringing even finer analysis tools to, for instance, image organics in rocks, looking for microfossils or textures that suggest ancient biology. Scientists still don't know if flowing water was frequently on the surface, or if it might have been mostly frozen with occasional melts due to volcanic activity.
"Some of the instruments on 2020 are really going to zero in on the finer detail of the rocks, the kind of things that we can't see with the instruments that we currently have, and cache samples that eventually, if they were returned to Earth, could provide us long-term, fundamental information," said John Grant, a geologist at the Smithsonian Institution who has been on the science teams for the Spirit and Opportunity rovers, Curiosity and the Mars Reconnaissance Orbiter. He also co-led the process to choose Mars 2020's landing site.
“InSight is a very important part of that, because none of [NASA's previous missions] really — pun intended — scratch below the surface in terms of the planetary evolution and understanding how the planet evolved over time,” Grant added.
"If we know something about its internal structure and evolution, we can say something about how long it has been active, whether or not or how active it is today, and all of those have implications for the changing conditions … as related to things like habitability and whether there was past life," he told Space.com. So although InSight isn't directly addressing Mars life, "it's all interwoven," he added.
And, of course, with NASA's evolving plans to eventually send humans to Mars, anything we learn will help prepare for that moment.
"Yes, we're going back to the moon, but we're also on the way to Mars, and science [helps] make sure that we understand the resources, and to explore that we understand the conditions of life and we understand what is to be researched there," Thomas Zurbuchen, associate administrator of NASA's science mission directorate, said during the Mars 2020 landing site announcement.
"That's, I would argue, an additional over-and-above argument for why Mars is so exciting to us,” he added. “We're not going to go to any other place anytime soon of our terrestrial planets for all the obvious reasons. Mars is really the obvious place after the moon to go back and expand our presence into deeper and deeper space."
So why do we keep going to Mars? To learn about our solar system, to learn about early Earth, to search for life and simply to learn about our neighbor before we visit.
"Science drives our understanding and allows us to get humans to a place like Mars," Glaze said during the briefing. "The more exploration we have, the better we understand that environment, the better prepared we'll be to send humans to Mars in the future."