1 DECEMBER 2017 • VOL 358 ISSUE 6367 1119 SCIENCE sciencemag.org
Few people have inspected the can- yons and craters of Mars like Alfred McEwen, a planetary geologist at the University of Arizona in Tucson. He is the principal investigator for the high- resolution camera on the Mars Reconnaissance Orbiter (MRO), which entered
orbit in 2006, revealing features as small as
a loaf of bread. With more than 55,000 observations so far, it has exposed about 2% of
the planet to spy-camera scrutiny.
One of McEwen’s most important finds
came in 2011, with the discovery of recurring slope lineae (RSL), thousands of
streaks along steep slopes, mostly near the
equator, that gradually grow and darken as
spring turns to summer, as if fed by seeps
of water, then fade. They soon became cited
as the best evidence for liquid water on the
surface of Mars today—and also one of the
best places to search for microbial life (see
But McEwen has now dowsed some of the
excitement ignited by his initial finding. In
a study published online on 20 November
in Nature Geoscience, he and his colleagues
analyzed 151 of the streaks, finding that the
streaks only occur on slopes steeper than
27° and always peter out when the angle
Q: You’ve actually been cautious from the
drops below that. The researchers interpret
this as a sign that the RSL are not formed by
water—which would flow down shallower
slopes—but rather are dry flows of sand and
dust seeking their natural angle of repose.
Science recently caught up with McEwen.
This interview has been edited for brevity
beginning. Did you come up with this strange
name—recurring slope lineae—so as to avoid
presuming any origin?
A: Yes. We wanted a name that was purely
descriptive, even though it’s a horrible
name from a press outreach perspective.
Q: Are you frustrated when you see loud
“water on Mars” headlines?
A: I’m bemused by it. 99.99% of science
results are incremental. You read the news
media and you think it’s all breakthroughs.
In our first Science paper, the title was,
“Seasonal flows on warm martian slopes.”
They are still seasonal flows on warm martian slopes—that has not gone away.
Q: You already had reasons to doubt that
RSL are watery seeps.
A: We’ve always stated it’s difficult to
Q: In 2015, scientists found that the RSL
explain where this water comes from in the
current martian environment. The atmo-
sphere is so dry—you’d have to create a
bizarre mechanism that sucks water out and
concentrates it in these areas. There may be
deep pockets of groundwater in Mars’s crust
today, I don’t doubt that. But how do you
get that to the surface? It would be expected
to come out in specific locations: near a
fault at the bottom of Valles Marineris, for
instance. Instead, we see these coming out
all over the slopes, even at the top of ridges
and isolated peaks. That really does not
make sense at all for groundwater.
contained hydrated salts. Isn’t that evidence
that water is involved somehow?
A: It doesn’t mean flowing water. Which,
unfortunately, is the way the NASA press
release played it. It was just before the
movie The Martian came out.
One hypothesis would be that there is
water being trapped from the atmosphere
into a thin subsurface layer that is destabilizing the overlying grains. Then when
they slip, you expose this damp soil, which
is dark. Once it’s actively exposed, it dries
and fades. Whether that works in detail
hasn’t been explained. Overall, these things
are still mysterious.
Q: So, you’re still leaving open the possibility
of thin films of water?
A: That’s correct. They’re not only possible,
but likely somewhere on Mars. Highly
deliquescent [water-absorbing] salts are
known to be abundant on Mars. It’s known
that a small amount of water is sufficient
to darken the surface. But it’s not enough
water to flow downhill. It’s not enough to
fill the pore space.
Q: How good of a microbial environment
would these thin films be?
A: It’s very salty water. That makes it challenging for life as we know it. Honey is a
good example of that. Honey is a liquid—it
contains water and yet it never spoils. It’s a
liquid that isn’t habitable because it holds
the water too tightly, in a sense.
Q: What will resolve the mystery of the RSL?
A: We’re continuing to learn from monitoring in orbit. If this is related to deliquescence from the atmosphere, we should see
a difference in brightness and darkness,
with time of day. To really understand
these, you’d probably need to go there and
really examine it up close. There’s been a
lot of talk about this, but there’s no definite
plans. These are difficult places to access.
Q: Do you think of Mars as a hospitable place?
A: It’s inconceivable to me that there aren’t
places where there’s liquid water today
within Mars. If there was ever life on Mars,
why wouldn’t there still be life today in
these underground pockets? The surface,
on the other hand, is a very harsh environment for life. j
On Mars, dark streaks
emerge seasonally on steep
slopes, but seeps of
water may not explain them.
By Eric Hand
A wet blanket for theories
of liquid water on Mars
The mystery of dark streaks endures for Alfred McEwen