Sky and Telescope - July 2015 - 14

News Notes

MARS I MAVEN Spots Dust Cloud, Aurora
NASA's bat-winged MAVEN spacecraft has detected dust high in the Red
Planet's atmosphere and auroras across
its northern hemisphere, team members
announced in March at the Lunar and
Planetary Science Conference.
MAVEN found the dust indirectly, by
what grains do to the spacecraft when
they strike it, explains mission principal
investigator Bruce Jakosky (University
of Colorado, Boulder). When a speedy
dust mote hits hard enough to vaporize
and ionize its bits, the aftermath aﬀects
the spacecraft's electrical potential and
creates a milliseconds-long signature that
the Langmuir Probe and Waves (LPW)
instrument detects.
LPW has "seen" the dust since turning
on last fall. But LPW doesn't encounter
it everywhere. MAVEN's orbit precesses
around Mars, meaning that over time the
spacecraft's closest point to the surface
moves around the planet. LPW detected
dust near local dawn and dusk, but not at
night. "That's an important clue, but we
haven't yet ﬁgured out how to interpret
it," Jakosky says.
The dust is generally concentrated
between 150 and 500 kilometers (90 and
300 miles) above the surface; it also occa-

sionally shows up as high as 1,000 km.
It's hard to explain how dust would get to
this part of the atmosphere. Raising it up
from far below doesn't make sense, physics-wise. Oﬀ-planet alternatives include
dust from Mars's two moons, Phobos and
Deimos, or dust carried by the solar wind.
It might also be comet debris the planet
picks up along its orbit, although it seems
unrelated to Comet Siding Spring, which
brushed past Mars in October.
The low point of MAVEN's orbit has
now swept around to the planet's dayside.
If LPW detects dust during local daytime,
too, that will help scientists narrow in on
where the dust is coming from.
MAVEN's second discovery, the aurora,
looks diﬀerent from its counterpart on
Earth. On our planet, auroras happen
when the solar wind pours charged particles down the magnetic-ﬁeld highways
near the poles and into the atmosphere.
But Mars doesn't have a global magnetic
ﬁeld; instead, it has remanent ﬁelds, ghost
ﬁelds locked into the surface when molten, magnetizable rocks solidiﬁed during
the brief era that the planet did have a
global ﬁeld, more than 4 billion years ago.
The European Mars Express orbiter
detected an aurora in the southern hemi-

sphere in 2005, where these magnetic
anomalies exist. MAVEN has now seen
an aurora across much of the northern
hemisphere, too. The northern aurora
persisted for ﬁve days. MAVEN didn't
observe the whole hemisphere simultaneously - only a region about the size of
Africa - so the team doesn't know for
sure that the aurora covered the whole
northern nightside, but it's likely.
The aurora might have spread so far
because there are essentially no magnetic
ﬁelds in the northern hemisphere: with
nothing to channel the energetic particles, they just dump into the atmosphere.
The team will need further observations
to conﬁrm this scenario.
The electrons that created the northern aurora dove deep into Mars's atmosphere, 50 to 100 km above the surface.
That's deeper than those spotted in the
south, which made it to 120 km or so. The
northern electrons penetrated so much
deeper because of their high energies:
tens of thousands of electron volts, ten
times higher than the ones that triggered
the southern aurora. They seem to have
come from a solar temper tantrum that
happened around that time.

releases in energy the equivalent of billions
of megatons of TNT. When Earth's ﬁeld lines
reconnect, they can hurl charged particles
into our atmosphere, spurring auroras.
Reconnection is also the process by which the
Sun unleashes massive ﬂares and that causes
so-called sawtooth crashes, an active topic of
investigation for plasma physicists trying to
harness nuclear fusion in the lab. Find videos
explaining reconnection and the mission at
http://is.gd/mmslaunch.

launch December 2020. The boulder will be
up to 4 meters wide and brought back to
lunar orbit for two astronauts to have a looksee mid-decade. Sans launch vehicle, the
total mission cost is $1.25 billion. This cost is
slightly more than that of Option A (bagging
a small asteroid and toting it home). But the
boulder option provides more time to assess
the asteroid during approach, more targets
(such as boulders) to pick from, and multiple
opportunities for the actual pickup. The mission will also develop capabilities for human
space exploration and defense, including the
gravity tractor technique, in which the spacecraft will attempt to redirect the asteroid's

■ CAMILLE M. CARLISLE

IN BRIEF
Magnetosphere Mission Launches. With
the goal of better understanding Earth's
space weather environment, NASA's Magnetospheric Multiscale (MMS) mission launched
on March 12th from Cape Canaveral. MMS
is a quartet of four, 3.5-meter-wide (11-footwide) octagons each decked out with 11
instruments. With a nominal 2-year mission,
the quartet will ﬂy 10 km (6.2 miles) apart
from one another in a tetrahedron formation,
with positions accurate to 100 meters. The
mission aims to provide a high-resolution,
3D view of magnetic reconnection in Earth's
magnetosphere, with measurements taken
100 times faster than before. Reconnection is essentially a magnetic explosion, the
rapid-ﬁre splicing of magnetic ﬁeld lines that

July 2015 sky & telescope

■ CAMILLE M. CARLISLE

NASA Selects Asteroid Mission Concept.
NASA has opted to retrieve a boulder from
an asteroid for its Asteroid Redirect Mission
(S&T: Oct. 2014, p. 16), tentatively slated to

orbit with its own gravitational inﬂuence.
■ MONICA YOUNG

http://www.is.gd/mmslaunch

Sky and Telescope - July 2015

Table of Contents for the Digital Edition of Sky and Telescope - July 2015