Good news: Mars was once habitable. Bad news: it will kill you now.

NASA sent out a press release with two Mars-related news items yesterday. Whether this was an attempt to bury the bad news, you decide.

First, the good news: An erstwhile freshwater lake in what’s now Gale Crater, whose rocks the Curiosity rover sampled, wasn’t too acidic to have harbored life maybe 4 billion years ago. If Mars could have spawned life in the short period of time it was habitable, it stands to reason that it also happened somewhere among the billions of habitable-zone planets elsewhere in the Milky Way galaxy alone. Or better, that life most certainly happened, and is happening, almost everywhere.

The bad news: Curiosity did radiation measurements, too, and the place has devolved into a rusty killer. From NASA:

[blockquote source=”NASA RELEASE 13-366 “]

Cosmic rays from outside our solar system and energetic particles from the sun bombarded the surface at Gale Crater with an average of 0.67 millisieverts per day from August 2012 to June 2013, according to a report by Don Hassler of Southwest Research Institute in Boulder, Colo., and co-authors. For comparison, radiation exposure from a typical chest X-ray is about 0.02 millisievert. That 10-month measurement period did not include any major solar storms affecting Mars, and more than 95 percent of the total came from cosmic rays.

Results from the surface-radiation monitoring provide an additional piece of the puzzle for projecting the total round-trip radiation dose for a future human mission to Mars. Added to dose rates Curiosity measured during its flight to Mars, the Mars surface results project a total round-trip dose rate for a future human mission at the same period in the solar cycle to be on the order of 1,000 millisieverts.

Long-term population studies have shown exposure to radiation increases a person’s lifetime cancer risk. Exposure to a dose of 1,000 millisieverts is associated with a 5 percent increase in risk for developing fatal cancer. NASA’s current career limit for increased risk for its astronauts currently operating in low-Earth orbit is 3 percent. The agency is working with the Institute of Medicine of the National Academies to address the ethics, principles and guidelines for health standards for long duration and exploration spaceflight missions.

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So you’re getting, on average, the equivalent of about 33 chest X-rays a day you spend on Mars, and that’s during a quiet sun cycle (cosmic rays are from distant stars). A good solar storm would pile it on.

I don’t see this as a major issue for human jaunts to Mars. Lots of folks would still sign up. Hell, were my kids older, I would sign up. But for those with dreams of long-term stays — not to speak of geoengineering the Red Planet into another blue one — game over.

You need a magnetosphere to redirect cosmic rays and space weather (the official term for the dangerous stuff the the sun kicks out with coronal mass ejections and the like). The magnetosphere is why, rather than us taking cosmic-ray and solar-storm shots to the corneas and chromosomes, we get the Northern Lights instead (sweet deal!). You need a molten metallic dynamo to act as a gigantic bar magnet to generate a magnetosphere. Mars cooled off and the bar magnet lost its mojo, leaving the poor thing exposed to stellar bullying. Mars is broke and we can’t fix it.

This takes nothing away from Mars as an object of study or exploration destination (though I would argue it strengthens the hands of who prefer to have this sort of thing done by robots, which need no heavy shielding and rarely die of carcinomas). But without lead blankets, it’s a death sentence for settlers.