More than a century ago, telescopic observers often reported seeing a "wave of darkening" that spread toward the Martian equator from its polar regions during local spring and summer. This changeover, they assumed, resulted from the annual green-up of alien vegetation as the icy polar caps melted, and the notion of a seasonally verdant Red Planet persisted well into the 1960s, until spacecraft images showed us otherwise.
There's water on Mars, to be sure. Billions of years ago torrents of water flooded parts of the surface and gouged huge, nasty-looking flood channels in the landscape. But all that water now lies frozen in thick polar slabs or buried out of sight.
Today Mars is a bitterly cold and dessicated place, at least at ground level. But admit it: aren't you still captivated by the idea that liquid water is flowing somewhere across those ruddy plains? Is it even remotely possible to find a few drops here and there given the current Martian climate?
A decade ago, the high-resolution camera on NASA's Mars Global Surveyor spotted sets of narrow gullies snaking down the walls of some craters. At the time many researchers thought water must be periodically oozing out of cracks in the rock and trickling downslope. It's still not clear how those gullies form, but the minds of most it seems that they're not active now.
More recently, some scientists on the polar-probing Phoenix mission noticed little blobs clinging to the craft's landing struts and suggested they might water droplets. Unfortunately, they're hard to see in Phoenix's self-portraits, and not everyone is convinced.
But there's new indications that liquid water is indeed flowing in small amounts during the Martian summer. The evidence comes from the super-resolving HiRISE camera aboard the Mars Reconnaissance Orbiter, which has seen dark rivulets form, grow, and then fade in the planet's southern hemisphere. These transient slope lineae, as they've been dubbed by Alfred McEwen (University of Arizona) and his colleagues, could be formed by brines containing enough salt to depress their freezing points by more than 100°F (50° to 60°C). He described them yesterday at the annual Lunar & Planetary Science Conference in Texas.
What makes the case convincing is that many of the flowlike features have come and gone between HiRISE's successive passes. According to Lujendra Ojha, an undergrad working with the HiRISE team, at least 15 sets of TSLs have been observed to change over time. They darken the surface when they appear and then gradually lighten.
"These flows may advance a little near the warmest time of each day," McEwen writes, "or they may advance by greater amounts on some days but not others."
Unlike the gullies seen by Mars Global Surveyor, these new finds occur only along Sunward-facing slopes, and they form only during the relative warmth of midsummer. Why they're found just in the southern hemisphere, at latitudes from -32° to -48°, is unclear. It's also a puzzle where and how brines can be stored so near the surface.
McEwen, Ojha, and others hope to get more answers soon. They're targeted particularly rich troves of TSLs for repeated looks with HiRISE and a sensitive spectrometer called CRISM during the coming southern summer.