http://www.skyandtelescope.com.au 9Combining these four images into
a single one took a long, long time,
Johnson says. The researchers wrestled
with how to convey what was sure
versus what might be the by-product
of a single algorithm’s favourite bits.
They finally decided to combine the four
images and blur the result to match
their instruments’ resolution. By doing
so, they only showed the structure that
appears using all four methods. “We
stand behind basically every element”
of this conservative image, he says.
And what an image it is. The
width of the silhouette is about 40
microarcseconds — the size of a
thumbnail seen from 64,000 kilometres
away. “This is the first time that I saw
this image,” said NSF Director and
astrophysicist France Córdova, “and it
did bring tears to my eyes.”
The team’s primary focus for April’s
announcement was creating the image.
But they have ascertained some of the
underlying physics, too. Based on the
size of the shadow, the researchers
calculated the black hole contains 6.
billion solar masses, a figure close to the
larger of two contested values.
The black hole is spinning clockwise
from our perspective; the bright
crescent to the south is the boosted
beam of gas moving toward us, while
the dimmer north is where gas recedes
from us. The data do not, however,
reveal how quickly the black hole spins,
because the shadow’s shape and size are
independent of the spin except for the
most extreme rotations.
The image also gives astrophysicists
a huge boost of confidence in their
theoriesaboutwhathappensnearan
accretingblackhole.“I havetoadmitI
wasa littlestunnedthatit matchedso
closelythepredictionswehadmade,”
saidAveryBroderick(PerimeterInstitute
andUniversityofWaterloo,Canada).
“Justthefactthatour[simulations]
camesoclosetoimagesliketheonewe
endedupgettingforM87alreadytellsus
thatwe’reontherighttrackforunder-
standingaccretionphysics,”saysÖzel.
“Wecouldhavebeencompletelyoff.”
■ CAMILLEM.CARLISLETWO STUDIES ARE FINDING
conflicting results about whether
methane exists on Mars. Its
presence would point to certain
geochemical processes or, less likely,
biological activity.
Researchers using the Planetary
Fourier Spectrometer (PFS) aboard
Europe’s Mars Express orbiter
spotted 15.5 parts per billion by
volume (ppbv) of methane in 2013,
a day after NASA’s Curiosity rover
measured a spike of 5.78 ppbv.
Atmospheric simulations and
geological analysis helped track the
emission’s origin to a fault area
southeast of Gale Crater, Marco
Giuranna (National Institute of
Astrophysics, Italy) and colleagues
report April 1 in Nature Geoscience.
However, the European-Russian
ExoMars Trace Gas Orbiter (TGO),
a spacecraft designed to measure
vanishingly small amounts of
gases in the carbon dioxide–based
atmosphere, has failed to find any
methane during the first months
of its science operations. The TGO
team reported the result April
10 at the European Geosciences
Union General Assembly in Vienna,
Austria, and in Nature.
TGO’s lack of detection limits
methane abundance to less than
0.05 ppbv. However, TGO makesits measurements using the solar
occultation method, looking through
the atmosphere at sunset from
orbit. This method limits TGO to
detections at altitudes above about
3km; methane could still lurk lower
down. Conversely, PFS looks down at
Mars, so its observations include lower
altitudes.
Nevertheless, Oleg Korablev
(Russian Academy of Sciences)
says it’s difficult to reconcile TGO
measurements with the spikes of
methane emission other missions
have reported, because atmospheric
circulation should spread the gas to
higher altitudes. Giuranna and others
have suggested that methane might
not always be present in the Martian
atmosphere. But even if marsquakes or
small meteorite impacts release small
puffs of methane from underground
reservoirs, these clouds should still
survive in the Martian atmosphere for
at least 300 years before breaking apart
under the Sun’s ultraviolet radiation.
An unknown mechanism might
remove the gas, but if so, it must
work efficiently, at low altitudes,
and only on methane, as current
models successfully reproduce other
components of Mars’s atmosphere.
Another option is that some
mechanism may keep the gas near the
surface. ■ JAVIER BARBUZANOMars ExpressIs there methane on Mars?
2020 NASA budget boosts exploration, not science
The US president’s FY2020 budget request puts US$21.019 billion toward NASA — a drop
of a half billion dollars from the just-approved 2019 budget. The proposal doubles funding for
NASA’s return to the Moon via the Lunar Gateway, which aims to put a crewed outpost in lunar
orbit. The budget request also initiates a Mars sample-return mission. At the same time, the
proposal decreases funding for astrophysics (-20%), planetary science (-5%) and Earth science
(-8%). And while the James Webb Space Telescope gains $46.6 million in funding, the Wide-
Field Infrared Space Telescope (WFIRST) is cancelled. The request also zeroes out funding for
NASA’s Office of STEM Engagement. However, the FY2019 budget request likewise cancelled
WFIRST, STEM outreach and several Earth science missions, but instead Congress pushed
through a budget increase. The question is, will Congress step in again? ■ DAVID DICKINSON
MARS EXPRESS: NASA / JPL-CALTECH / CORBY WASTE
IN BRIEF