O
NE OF THE biggest space
news stories of late 2015 was
the announcement of
compelling evidence for active water
flows on Mars. It’s a story that goes
back almost a decade, with images
from orbiting spacecraft revealing
ribbon-like features that hinted at the
flow of liquid down Martian hillsides
and sand dunes. Tantalisingly, these
features showed changes over time,
with some of them – known as
‘recurrent slope lineae’ – temporarily
assuming a darker appearance during
the Martian summer.
What could be causing the
features? One idea was that bubbles
of carbon dioxide might be escaping
from their frozen state beneath the
Martian surface near the hilltops –
these would disturb the soil as they
careered downhill in a process
reminiscent of the dangerous hot gas
and lava flows that sometimes accom-
pany volcanic eruptions on Earth.
But the most favoured explanation
was that the flows were due to a
periodic wetting of the surface by
liquid water, with the soil darkening
as the water flowed.
How could this be? The average
surface temperature on Mars is
-63°C, and – even in summertime on
the Martian equator (where most ofthese phenomena have been
observed) – the temperature
seldom exceeds -20°C. Any water,
therefore, should be frozen solid.
But observations made with NASA’s
Mars Reconnaissance Orbiter (MRO)
have shown that the lineae are rich in
chemicals that effectively lower
water’s freezing point, sometimes by
as much as 70°C. This natural
antifreeze has already been observed
in analyses carried out by NASA’s
Phoenix lander back in 2008. MRO’s
remote sensing has now confirmed its
presence in the lineae.
The discovery excited astrobiol-
ogists – the scientists exploring the
possibilities for life beyond Earth –
and raises an immediate question.
Could we send one of the two
operational rovers currently on Mars
(Opportunity and Curiosity) to take a
closer look at the recurrent slope
lineae? Sadly, the answer is no,
because the distances are too great.
There are also international protocols
that forbid spacecraft to approach sites
that may harbour living organisms for
fear of contamination (see AG 118).
We’ll just have to wait to confirm
precisely what the streaks are.The Moon’s path through
the sky is much the same as
the Sun’s. So, on the equator, the
Moon rises and sets vertically
in relation to the local horizon,
passing more or less overhead,
depending on the season – just
like the Sun. It also means the
crescent Moon lies on its back
in the morning or evening
equatorial sky – giving the
world a celestial smile!
If you have a space question for Fred,
email it to [email protected]FR ED ANSW ERS
YOU R QU E S T IONS
Melting point
SPACE
buzz
Mars may be frozen, but the evidence of water flow is easily explained, says Fred Watson.
Why does the Moon look
different when seen from the
Earth’s equator?
Anne Spencer, Narrabeen, NSWFRED WATSON
Fred Watson is an
astronomer at the
Australian Astronomical
Observatory.In this false-colour image of
the surface of Mars, enhanced
with topographic data, dark
100m-long streaks (left) hint at
the presence of flowing water.NASA/JPL/UNIVERSITY OF ARIZONA