24 September 2008

Rain on Mars?

Posted by Ryan Anderson

Universe Today is reporting that scientists have found evidence of rainfall and stable surface water on Mars. Unfortunately, the paper is published in a journal that Cornell does not subscribe to, so all I have to go on is the press release and the abstract for the paper. Still, it doesn’t sound to me like there is anything really ground-breaking in this study. They are reporting evidence for fan-shaped delta deposits in some craters, fed by sinuous channels, but these have been well known for a very long time! The paper’s abstract says that they used high-resolution topography to calculate that ~30% of the eroded material from the channels ended up in the crater-lake fans. They did some crater counting on the fans to show that they are from early in martian history, the period when most scientists expect some water activity. Hydrologic models indicate that the observed channels formed in a short period of time, which seems to be their main reason for invoking rainfall.

Don’t get me wrong, it’s awesome to find evidence of stable water on the surface of Mars, but I don’t see what this study shows that isn’t something we already knew. I’m probably being unfair since I can’t read the actual paper, but their evidence for rainfall sounds sketchy to me, and their discovery of crater lakes and rivers is not really news. Heck, the top three potential MSL sites all have some evidence that they were lakes, and Eberswalde has the most spectacular delta on the planet.

Within the Mars community, I have heard people joke that water is discovered on Mars every few years, referring to the fact that these sorts of announcements keep making the news even though we’ve known for a long time that there was once water. Now, show me evidence of stable water in the recent past, and I’ll get more excited!

Update: A reader whose institution does have access to the full paper sent me a copy! Here’s the key paragraph from the end of the paper:

Our study suggests that a warmer climate conducive of
fluvial activity and limited surface runoff was required at
the end of the Noachian period and/or the beginning of the
Hesperian period. The timing of the fluvial activity as
determined in this study agrees with independent work of
Hoke and Hynek (2008) and Fassett and Head (2008b),
who both report Late Noachian to Early Hesperian ages
for fluvial activity in the cratered highlands. The short time
scales needed for the sedimentation of the deposits would
be in agreement with a relatively short period of intense
precipitation and fluvial activity at the end of the Noachian
period or the early Hesperian period, a scenario that was
suggested by Howard et al. (2005) and Irwin et al. (2005a).
This episode would have been different than the previous
moderate fluvial activity during most of the Noachian
period, which was responsible for the development of the
smaller and shallower older valleys, also observed in
Xanthe Terra. An increase of fluvial activity toward the
end of the Noachian might have been caused by Tharsis
formation in the Noachian, associated outgassing of
volatiles and their accumulation in the atmosphere (Phillips
et al., 2001). The intense fluvial activity would have been
accompanied by at least some surface runoff, as indicated
by the upstream tributaries on the plateaus that lead to the
theater-shaped valleys (Fig. 2b). Sapping may have been an
equally important process in Xanthe Terra, since such
small upstream tributaries can only be identified in a few
cases in the available images. Late-stage entrenchment
of valleys with morphologically different along-stream
sections was attributed to sapping by Baker and Partridge
(1986). A transition from surface runoff to sapping and a
corresponding change in the style of erosion was also
suggested by Harrison and Grimm (2005). Hydrologic
modeling by Kraal et al. (2008b) indicates a discrepancy
between the massive discharges required to form the
stepped topography of some Martian deltas and the
comparatively small dimensions of the feeder channels,
and suggests rapid release from subsurface reservoirs
rather than surface precipitation. On the other hand,
Howard et al. (2005) and Luo and Howard (2008) question
the efficiency of sapping to create the deeply incised valleys
by headward migration of knickpoints and offer alternative
mechanisms such as erosion through indurated rock into
underlying weaker sediments or the development of a
duricrust that focused erosion. In summary, we observe
evidence for both surface runoff and sapping, in accordance
with findings by Luo and Howard (2008), but it is
beyond the scope of this study to quantify the relative

So basically, the landforms they looked at probably formed late in the noachian (early) period of Mars history and may have been formed by erosion due to rainfall or by groundwater sapping. Basically I stand by my earlier stance: there’s nothing wrong with the science being done here (in fact I think it is quite interesting), but it is not as ground-breaking as the press release suggests.