海角大神

Nearly 20 years after Pathfinder rolled onto an ancient Martian flood plain called Ares Vallis, NASA鈥檚 four Mars rovers have only covered about 38 miles of the Red Planet. That leaves plenty of territory for the next lander, Mars 2020, to explore.

At a conference last week, scientists for the rover: Columbia Hills, Northeast Syrtis, and Jezero Crater. Orbital observations and previous rovers have found that the first two sites were likely once home to hot springs; Jezero Crater may have held a large lake.

鈥淚f you find where the liquid water was,鈥� Bruce Betts, director of science and technology for the Planetary Society, tells 海角大神, 鈥渋f there were ever life on Mars, that would be a good place to look.鈥�

This 鈥溾�� paradigm has guided NASA's missions to Mars since the 1990s. The Mars 2020 mission, scheduled for launch in three years, continues this approach and adds a new goal: returning samples for Earth-based study.

鈥淎ll of the recent landings, going back to , have been driven by this liquid water interest,鈥� Dr. Betts says. With Mars 2020, he continues, 鈥渢hey've upped the ante a little, particularly with the collection of samples where you can bring them back to Earth and do detailed studies.鈥�

Past rovers have hammered Mars rocks, probed them with microscopes, and , but they've all been limited by the need to weigh as little as possible. Some scientific instruments, notes Betts, 鈥渁re larger than you could ever hope to fly.鈥�

We also have Mars rocks that fell to Earth as meteorites after getting kicked up by ancient impacts. But those have been stripped from their geological context, Betts explains: 鈥渆xactly where it came from, the layer of the rocks, how it fits into the overall history.鈥�

Engineers at NASA鈥檚 Jet Propulsion Laboratory are preparing Mars 2020 to gather both samples and context. As reported , the rover will fill 37 14-centimeter-long titanium tubes with air, rock, and soil samples, then leave them in a cache on the planet鈥檚 surface.

At some unknown future date, another vehicle will land, deploy a 鈥渇etch鈥� rover to retrieve the cache, and return the samples to Earth.

In 2013, the authors of a National Research Council report titled 鈥溾�� labeled a sample-return mission like this as a top priority. After considering one alternative 鈥� sending more rovers 鈥� the study鈥檚 authors concluded that 鈥渟ample return would have significantly higher science return and a much higher science-to-dollar ratio.鈥�

That doesn鈥檛 mean it will be cheap. , performed by NASA鈥檚 inspector general, pegs the cost of Mars 2020 at $2.4 billion. Dr. Firouz Naderi, JPL鈥檚 associate director, of sample collection and a second pickup mission at between $6 and $7 billion. 聽

By working with SpaceX, the cost of that second mission. that its Red Dragon capsules could get to Mars and back for as little as $300 million.

But Dr. Betts warns that 鈥渋t's going to be challenging for anyone to go get the samples and return them. NASA so far is the only one who's demonstrated experience at being able to get [a Mars lander] into a relatively small location and go where they want to go.鈥�

Despite the costs and challenges of bringing back Mars rocks, the rewards could be enormous, says Timothy Goudge, a postdoctoral fellow at the University of Texas at Austin who attended last week鈥檚 site selection meeting.

鈥淗aving returned samples from a known context/location on Mars would open up a huge suite of analysis techniques that the rocks could be subjected to,鈥� he tells 海角大神 in an email. Analyzing the rocks in an Earth-based laboratory, he explains, 鈥渨ill allow us to understand the geologic history of the samples (e.g., age, temperature of formation, interaction with fluids and the atmosphere, etc.) in a way that we couldn鈥檛 accomplish in situ on Mars.鈥�

That bring us closer to answering the burning question of whether or not life existed on Mars, by applying our best techniques to "any potential organic matter found by the rover, and determining if that material has a biologic origin or not.鈥�

It also accomplishes all this while risking , and avoiding the costly business of keeping astronauts alive during a years-long mission in deep space.

But aspiring Mark Watneys shouldn鈥檛 change their career plans just yet.聽Dr. Michael Meyer, lead scientist for NASA鈥檚 Mars Exploration Program, tells the Monitor in an email that a sample return 鈥渨ould help humans get to Mars.鈥�

The Planetary Society's Bruce Betts agrees.

鈥淵ou can do human missions without a sample return," he says. But because such a mission would involve geological analysis, landing, and taking off from the Martian surface, a "sample return is a logical precursor to a human mission.鈥�

[Editor's note:聽This article has been updated to include a comment from Dr. Michael Meyer.]